Optically Mounted Analog MUX Accessory for Test and Measurement Equipment

Abstract

The test and measurement system includes one or more remote heads, each configured to be coupled to a respective device under test (DUT) to receive electrical test signals from the DUT, and each of the one or more remote heads includes an electro-optical modulator (EOM) configured to convert the received electrical test signals into optical test signals. An optical interconnect circuit receives the optical test signals from the EOMs of the one or more remote heads and, in response to a control signal, selects one optical test signal from among a plurality of optical test signals to provide to the test and measurement system. The optical interconnect circuit further converts the selected optical test signal into an electrical test signal and provides it to a test port of the test and measurement instrument. A method is also described.

Description

[Technical Field]

[0001] The present disclosure relates generally to systems and methods relating to test and measurement systems, and more particularly to test and measurement systems that include optical interconnects for measuring multiple signals from a device under test (DUT). [Background technology]

[0002] Many DUTs contain multiple similarly designed signal paths. For example, a PCI Express (PCIe or PCI-e) plug-in card or PCIe motherboard slot typically contains up to 16 electrical signal "lanes." The electrical signal lanes are typically differential signal lanes, and a complete measurement of the differential signaling across a lane requires two electrical measurement ports per differential lane. Therefore, a complete measurement of a 16-lane PCIe device requires 32 electrical measurement ports. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2022 / 0187357 [Patent Document 2] US Patent Application Publication No. 2018 / 0367225 [Patent Document 3] US Patent Application Publication No. 2015 / 0035546 [Patent Document 4] US Patent Application Publication No. 2007 / 0002960 [Patent Document 5] Japanese Patent Application Laid-Open No. 2015-118087 Summary of the Invention [Problem to be solved by the invention]

[0004] However, test and measurement equipment commonly used to test such DUTs, such as oscilloscopes and bit error rate testers (BERTs), typically have one, two, or four input ports. While some oscilloscopes have as many as eight input channels or ports, high-performance instruments typically have fewer input channels due to the increased hardware costs associated with each channel. Physical channel density limitations for thermal and throughput reasons can also dictate a lower channel count. For this reason, test and measurement equipment configurations typically connect to only a portion of the electrical signal lanes under test.

[0005] Because only a portion of the lanes are coupled to the test and measurement equipment, if a user wants to test all signal lanes, they must manually move connections (e.g., test cables and probes) between the test and measurement equipment and the DUT from lane to lane. Manually moving cables and probes is an error-prone, time-consuming, and labor-intensive process. Alternatively, in a two-channel test and measurement environment, radio frequency (RF) switches can be built and maintained to automate testing of all signal lanes of the DUT. However, identifying the appropriate switches and properly de-embedding their effects from the signal path (which is done manually) is difficult at higher frequencies. For this reason, many users do not believe they can de-embed switches without significant error, especially above 25 GHz. De-embedding, or "calibrating," the effects of switches and other components in a signal path means compensating for the adverse effects of the switches on the signals propagating through the signal path. In addition to the need for de-embedding, RF switch matrices are physically large, making RF switch matrix solutions particularly unattractive for users with limited physical test space.

[0006] When testing a DUT, it is necessary to place the DUT as close as possible to the input of the test and measurement equipment to minimize degradation of signal integrity (e.g., signal distortion). When using RF cables for communication of signals above 60 GHz, the cables are expensive and have high insertion loss (>6 dB / meter). Insertion loss is the energy lost as the signal propagates along the cable, and insertion loss limits the maximum cable length between the DUT and the test and measurement equipment. Current approaches to correct for the adverse effects of cables utilize deembedding, which requires expensive equipment such as vector network analyzers (VNAs), as well as skilled technicians / engineers, specialized software such as serial data link analysis (SDLA) software, and time to achieve the required deembedding. Furthermore, this deembedding approach has limitations. For example, deembedding can reduce the signal-to-noise ratio of signals propagating through the cable, potentially affecting the accuracy of signal measurements.

[0007] Embodiments of the present disclosure address these and other deficiencies in the prior art. [Means for solving the problem]

[0008] Embodiments of the disclosed technology utilize one or more remote heads coupled to a test measurement system via multiplexing circuitry and optical interconnection circuitry. The remote head may include front-end circuitry (e.g., amplifier circuitry) for the test channel of the test measurement instrument. This architecture, including the remote head and optical interconnection circuitry, reduces the need for de-embedding to compensate for cable signal loss by allowing the remote head to be located close to each DUT under test. The optical interconnection to the test measurement system also reduces cable insertion loss. This architecture also allows the test measurement system to remain stationary and connect to the test measurement system over longer distances due to the optical interconnection between the remote head and the test measurement system. Converting the electrical signal from the DUT to an optical signal allows the DUT to be located much farther from the test measurement system while maintaining the signal-to-noise ratio of the electrical signal.

[0009] The remote or distant location of the remote head in embodiments of the disclosed technology also means that the test measurement system does not need to occupy a large amount of space on the benchtop where it is located, when attempting to place the inputs of a test measurement system in accordance with embodiments of the disclosed technology as close as possible to the test points of the DUT. The use of a remote head also allows for closer connections between the test points of the DUT and the front-end circuitry that processes the signals from those test points. This is true even when the DUT is located close to the test measurement system on the benchtop and direct connections are made between the test points on the DUT and the inputs of the test measurement system. Furthermore, the multiplexing of DUT signals coupled to one or more remote probe heads, either before or after electro-optical modulation or conversion, in test measurement systems in accordance with embodiments of the disclosed technology simplifies customer setup, allowing customers to use one channel of the test measurement system without having to reconfigure their setup to measure signals from multiple DUTs.

[0010] Aspects, features, and advantages of embodiments of the disclosed technology will become apparent from the following description of examples, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram of a test and measurement system including multiple remote heads, each including an electro-optic modulator optically coupled to the test and measurement system via an optical multiplexing circuit, in accordance with some embodiments of the present disclosure. [Figure 2] FIG. 2 is a block diagram of a test and measurement system including multiple remote heads according to some embodiments of the present disclosure, each remote head including an RF switch and an electro-optical modulator optically coupled to one of two synchronized test and measurement instruments via an optical interconnect circuit to enable pseudo-differential measurements. [Figure 3]FIG. 3 is a block diagram of a test and measurement system including multiple remote heads according to some embodiments of the present disclosure, each remote head including an RF switch and an electro-optical modulator, each coupled to a corresponding test port of a test and measurement instrument via an optical interconnection circuit, enabling pseudo-differential measurements. [Figure 4] FIG. 4 is a block diagram of a test and measurement instrument according to some embodiments of the present disclosure, including a remote head coupled to receive a single-ended electrical test signal from a DUT, and including an electro-optical modulator and optical multiplexing circuit, and providing a corresponding optical test signal to the test and measurement instrument via an optical interconnect circuit. [Figure 5] FIG. 5 is a block diagram of a test and measurement instrument according to some embodiments of the present disclosure, including a remote head coupled to receive differential electrical test signals from a DUT, and including an electro-optical modulator and optical multiplexing circuit, and providing corresponding optical test signals to the test and measurement instrument via an optical interconnect circuit. [Figure 6] FIG. 6 is a block diagram of a test and measurement instrument including a remote head and a feedback compensation circuit, according to some embodiments of the present disclosure, where the feedback compensation circuit compensates for variations in the optical interconnect circuitry that may adversely affect the characteristics of the electrical test signals provided to the test and measurement instrument. [Figure 7] FIG. 7 is a block diagram of a test and measurement instrument including a remote head and a feedforward correction circuit, according to some embodiments of the present disclosure, that corrects for variations in optical interconnection circuits that may adversely affect the characteristics of electrical test signals provided at test ports of the test and measurement instrument. DETAILED DESCRIPTION OF THE INVENTION

[0012] 1 is a block diagram of a test and measurement system 100 including multiple remote heads 102A and 102B, each including an electrical-to-optical modulator (EOM) 104A and 104B, respectively, optically coupled to a test and measurement instrument 108 through an optical interconnect circuit 106. Each EOM 104A and 104B converts electrical test signals from a corresponding device under test (DUT) 110A and 110B into optical test signals for communication to the test and measurement instrument 108, which is physically remote or located at a distance from the remote heads 102A and 102B, according to some embodiments of the present disclosure. Each of the DUTs 110A and 110B is, for example, a test board, and is connected to a test and measurement device 108 via a corresponding one of the remote heads 102A and 102B and an optical interconnection circuit 106. The optical interconnection circuit 106 includes an optical switch 112, also known as a data selector, which has multiple optical input ports 114 and one optical output port 116. Each of the multiple optical input ports 114 receives an optical signal corresponding to an electrical test signal from one of the DUTs 110A and 110B.

[0013] An electrical control signal (CS) is applied to the optical switch 112, which, in response to the CS signal, provides an optical signal on a selected one of the multiple optical input ports 114 to the optical output port 116. In this manner, the optical switch 112 functions as a multiplexer, allowing a user to select any test signal provided by the DUTs 110A and 110B for testing via the CS signal. This configuration minimizes the number of required connections between the test and measurement instrument 108 and the DUTs 110A and 110B. While the example embodiment of FIG. 1 shows two DUTs 110A and 110B, additional DUTs may be coupled to the system 100 for testing. For example, in the system 100, the optical switch 112 has four optical input ports 114, and therefore, a differential signal from one of four DUTs or one of four differential signals from a single DUT can be coupled to the test and measurement instrument 108 through the optical switch 112.

[0014] The control signal CS applied to the optical switch 112 may be provided by circuitry (not shown) external to the test and measurement system 100, allowing a user to control the optical switch 112 via this circuitry to provide the optical signal on a selected one of the plurality of optical input ports 114 to the test and measurement instrument 108. In another exemplary embodiment, the processor 118 of the test and measurement instrument 108 can apply the control signal CS to the optical switch 112 to select the optical signal on one of the plurality of optical input ports 114 to be provided to the optical output port 116 for testing by the test and measurement instrument 108. This is represented in FIG. 1 by the dashed line 119 between the test and measurement instrument 108 and the optical switch 112.

[0015] The optical interconnect circuit 106 further includes an optical-to-electrical modulator (OEM) 120 having an optical input port 122 coupled to the optical output port 116 of the optical switch 112 via a suitable optical waveguide, such as a fiber optic cable 124. An optical signal on a selected one of the optical input ports 114 of the optical switch 112 is provided to the optical output port 116 and propagates via the fiber optic cable 120 to the optical input port 122 of the OEM 120. The OEM 120 modulates or converts the optical signal received at the optical input port 122 to provide a corresponding electrical test signal. A test port 126 of the test and measurement instrument 108 is coupled to the OEM 120 to receive the electrical test signal from the OEM 120 and provide the electrical test signal to the processor 118 and other circuitry within the test and measurement instrument for testing. The processor 118 of the test and measurement instrument 108 is coupled to a memory 128, which may store test environment configuration data for generating the CS signal applied to the optical switch 112 and for controlling the selection of the signal from the desired one of the DUTs 110A and 110B under test.

[0016] Each of the DUTs 110A and 110B has a single electrical signal lane, “Lane 0,” which is coupled to a corresponding differential output amplifier 130A and 130B. In some embodiments, each of the DUTs 110A and 110B may be a standalone device and may have one or more signal lanes. Furthermore, each of the DUTs 110A and 110B may refer to a particular output component of the DUT, which may itself have multiple signal lanes. For example, a modern PCIe device may contain 16 lanes, each lane having a differential signal pair. Embodiments according to the present disclosure provide users with the ability to easily select any signal from such a DUT for testing while also eliminating the noise inevitably added by using one or more cascaded RF switches, such as in the present system.

[0017] In the example embodiment of FIG. 1 , each signal lane “Lane 0” of DUTs 110A and 110B is a differential signal lane including a pair of differential output signals. While the electrical signals are depicted as differential here, single-ended electrical implementations are also possible. The output signal of “Lane 0” of each DUT 110A and 110B is transmitted as the difference between the two differential output signals. The differential output signals of amplifiers 130A and 130B, respectively, are coupled to the inputs of remote heads 102A and 102B. Specifically, for “Lane 0” of DUT 110A, the first of the two differential output signals is coupled from RF connector 132A on the DUT to RF connector 136A on remote head 102A via RF cable 134A. RF cable 134A may be, for example, a coaxial cable to minimize signal loss between DUT 110A and remote head 102A. Similarly, the second of the two differential output signals from Lane 0 is coupled from RF connector 138A on DUT 110A via RF cable 140A to RF connector 142A on remote head 102A. Similarly, amplifier 130B of DUT 110B is coupled to RF connectors 136B and 142B on remote head 102B via RF connectors 132B and 138B and RF cables 134B and 140B.

[0018] Differential preamplifiers 144A and 144B in each remote head 102A and 102B have a pair of input terminals coupled to corresponding RF connectors 136A and 142A and 136B and 142B on the remote head. Each preamplifier 144A and 144B generates an amplified electrical output signal based on the difference between the two differential output signals from lane "Lane 0" of each DUT 110A and 110B, and this amplified output signal is provided to electro-optical modulators (EOMs) 104A and 104B in the remote heads 102A and 102B. In some embodiments, amplifiers 144A and 144B may be omitted, and electrical signals from DUTs 110A and 110B may directly drive EOMs 104A and 104B. Locating the test signals as close as possible to their source (i.e., the corresponding DUTs), including locating the preamplifiers 144A and 144B and EOMs 104A and 104B as close as possible to the test signals of the corresponding DUTs 110A and 110B, increases the signal-to-noise ratio of the test signals in the test system 100. In the remote heads 102A and 102B, each EOM 104A and 104B modulates or converts the amplified electrical output signal from the preamplifier 144A and 144B into an optical signal and provides it to the optical output ports 146A and 146B of the remote heads 102A and 102B. The optical signals on the output ports 146A and 146B of the remote heads 102A and 102B are provided via appropriate fiber optic cables 148A and 148B to a corresponding one of the multiple optical input ports 114 of the optical switch 112.

[0019] In operation, each DUT 110A and 110B generates a test signal that is measured by the test and measurement equipment 108 in the form of a differential output signal for lane "Lane 0" provided by differential amplifiers 130A and 130B within the DUT. These differential output signals from each DUT 110A and 110B are provided to a corresponding remote head 102A and 102B via RF cables 134A and 134B and 140A and 140B. Each remote head 102A and 102B may be located in close proximity to the DUT 110A and 110B that is coupled to it. This reduces the length of the RF cables 134A and 134B and 140A and 140B required to couple the remote head to the DUT 110A and 110B, thereby reducing signal loss and improving the signal integrity of the test signal received at each remote head.

[0020] In each remote head 102A and 102B, a corresponding preamplifier 144A and 144B generates an amplified electrical output signal based on the differential output signal received at the remote head's RF connectors 136A and 136B and 142A and 142B. Each EOM 104A and 104B modulates or converts the amplified electrical output signal to an optical signal and provides it to the optical output port 146A and 146B of the remote head 102A and 102B. In this manner, the test signals of the DUTs 110A and 110B are converted to optical signals and then transmitted to the test and measurement instrument 108 for testing. Optical signals can communicate over much longer distances than electrical signals without suffering adverse effects or distortions that could impair the test and measurement instrument 108's ability to reliably measure the test signals. This architecture reduces the need for de-embedding or calibration of components in the connection path between the remote heads 102A and 102B and the test and measurement instrument 108.

[0021] The optical signals from EOMs 104A and 104B are provided to optical output ports 146A and 146B of remote heads 102A and 102B and provided through optical interconnect circuit 106 to test port 126 of test and measurement instrument 108. A control signal CS is applied to optical switch 112 of optical interconnect circuit 106 in response to user input, such as via an external circuit, to select the test signal provided to test port 126 for testing. Optical switch 112 performs a multiplexing function and provides the optical signal on one of several optical input ports 114 to optical output port 116, which is provided to test and measurement instrument 108. The optical signal provided to output port 116 of optical switch 112 is provided through fiber optic cable 124 to optical input port 122 of OEM 120, which modulates or converts the optical signal into an electrical test signal that is provided to test port 126 of test and measurement instrument 108. In this configuration, test signals from selected DUTs 110A and 110B are converted to optical signals at remote heads 102A and 102B, which are then transmitted to OEM 120, which is coupled to a test port of test and measurement equipment 108.

[0022] The distance between the remote heads 102A and 102B and the corresponding DUTs 110A and 110B will typically be much smaller than the distance between each remote head and the test and measurement instrument 108. Therefore, the architecture of system 100 conveys electrical test signals from the DUTs 110A and 110B to the remote heads 102A and 102B located near the DUTs, where the test signals are then converted to optical signals for communication over much longer distances between the remote heads and the test and measurement instrument 108. The optical signals are not converted to electrical test signals until they reach the EOM 120, which is directly connected to the test port 126 of the test and measurement instrument 108. This reduces signal degradation and the need for de-embedding in the propagation of the test signals from the remote heads 102A and 102B to the test and measurement instrument 108. Additionally, test and measurement system 100 allows a user to select the desired signals from DUTs 110A and 110B via a CS signal provided to optical switch 112 without manually moving cables or probe heads, which is an error-prone, time-consuming, and labor-intensive process.

[0023] Those skilled in the art will appreciate suitable structures that can be used for the optical interconnect circuit 106 and the EOMs 104A and 104B components included in the remote heads 102A and 102B. For example, the OEM 120 may include a photodiode for converting the optical signal received from the optical switch 112 into an electrical test signal that is provided to the test port 126 of the test and measurement instrument 108. In some embodiments of the present disclosure, each of the EOMs 104A and 104B may be a Mach-Zehnder modulator (MZM) that uses the electrical signal of the corresponding DUT 110A or 110B to modulate an optical signal (which would typically be provided by a laser, considered part of the MZM). However, embodiments of the present disclosure are not limited to using Mach-Zehnder modulators for the EOMs 104A and 104B. The theory and operation of Mach-Zehnder modulators will be understood by those skilled in the art and will not be described in detail herein (see U.S. Patent No. 5,649,399). Regardless of the specific configuration of each EOM 104A and 104B, the EOM functions to modulate some characteristic of an optical carrier signal with an electrical signal, e.g., an RF electrical test signal. Similarly, regardless of the specific configuration of OEM 120, the OEM functions to recover the original electrical signal from the modulated optical carrier signal it receives.

[0024] Figure 2 is a block diagram illustrating another test measurement system 200 in accordance with some embodiments of the present disclosure. In test measurement system 200, many components are identical or similar to components of test measurement system 100 described above with respect to Figure 1. Accordingly, these identical or similar components in test measurement system 200 are given the same 200-series numbers as the corresponding 100-series components in test measurement system 100 of Figure 1 and will not be described again in detail with respect to Figure 2.

[0025] Test and measurement system 200 includes multiple remote heads 202A and 202B, each containing an RF switch 203A and 203B and an electro-optical modulator (EOM) 204A and 204B. Each of EOMs 204A and 204B is optically coupled to one of two synchronized test and measurement instruments 208A and 208B via optical interconnection circuits 206A and 206B. This structure of test and measurement system 200 enables pseudo-differential measurements to be made on differential signals from DUTs 210A and 210B, as described in more detail below.

[0026] Unlike the test and measurement system 100 of FIG. 1, each remote head 202A and 202B in the test and measurement system 200 of FIG. 2 includes an RF switch 203A and 203B that operate in conjunction to provide multiplexing of differential output signals from the DUTs 210A and 210B. The RF switches 203A and 203B provide the signal switching function in place of the optical switch 112 in the test and measurement system 100. Each RF switch 203A and 203B receives an RF control signal RFCS to select one of multiple RF input ports 205A and 205B to be coupled to an RF output port. The RF output port of each RF switch 203A and 203B is coupled to a corresponding single-ended preamplifier 207A and 207B in the remote head 202A and 202B. Each preamplifier 207A and 207B amplifies the corresponding RF signal received from RF switch 203A and 203B to improve the signal-to-noise ratio of the RF signal, and the amplified RF signal from each preamplifier is provided to a corresponding EOM 204A and 204B in remote head 202A and 202B.

[0027] RF input ports 205A and 205B of RF switches 203A and 203B are coupled to differential output amplifiers 230A and 230B of DUTs 210A and 210B, respectively, so that a differential output signal from one of these differential output amplifiers may be selected for test by an RFCS signal applied to the RF switches. For example, when RF switches 203A and 203B are in the state shown in FIG. 2, the differential output signal from differential output amplifier 230B of DUT 210B is selected. The first of the two differential output signals from differential output amplifier 230B is provided to preamplifier 207A of remote head 202A via RF switch 203A, and the second of the two differential output signals from differential output amplifier 230B is provided to preamplifier 207B of remote head 202B via RF switch 203B. By alternately applying the RFCS signal to the RF switches 203A and 203B, the differential output signal from the differential output amplifier 230A of the DUT 210A can be selected.

[0028] EOMs 204A and 204B operate in a similar manner as described above for components 104A and 104B, modulating optical signals with RF signals from corresponding amplifiers 207A and 207B. These modulated optical signals from EOMs 204A and 204B are then transmitted to corresponding test and measurement instruments 208A and 208B through corresponding optical interconnection circuits 206A and 206B. Optical interconnection circuits 206A and 206B include fiber optic cables 224A and 224B and OEMs 220A and 220B, which function similarly to the corresponding components described above for system 100 of FIG. 1. Optical interconnection circuits 206A and 206B provide the same benefits for system 200 as described above for system 100: reduced signal degradation and the need for de-embedding in the propagation of test signals from remote heads 202A and 202B to test and measurement instruments 208A and 208B.

[0029] 1, the RFCS signals applied to RF switches 203A and 203B may be provided by circuitry (not shown) external to test and measurement system 200, thereby allowing a user to control the RFCS switches to select the differential output signal from either of DUTs 210A and 210B for testing. Alternatively, processors 218A and 218B of either of test and measurement instruments 208A and 208B can apply RFCS signals to RF switches 203A and 203B to select the desired one of the differential output signals in response to user input provided through the instrument.

[0030] In operation, a user provides an input to generate an RFCS signal to select a differential output signal from a desired one of the DUTs under test 210A and 210B. In response to the RFCS signal, one of the selected differential output signals is provided to device 208A through the corresponding remote head 202A and optical interconnect circuitry 206A. The other of the selected differential output signals is provided to device 208B through the corresponding remote head 202B and optical interconnect circuitry 206B. OEMs 220A and 220B convert the received optical signals into corresponding electrical test signals, and test and measurement devices 208A and 208B each receive one electrical test signal at the corresponding test port 226A and 226B.

[0031] Synchronization circuitry 209 is coupled to test and measurement instruments 208A and 208B and synchronizes the capture of electrical test signals received at test ports 226A and 226B of these two separate (independent) instruments. Synchronization circuitry 209 controls test and measurement instruments 208A and 208B to synchronize the sample clocks used by each instrument to capture and digitize the electrical test signals on test ports 222A and 226B. Synchronization circuitry 209 may provide the necessary synchronization for the two test and measurement instruments 208A and 208B by implementing, for example, an UltraSync multi-unit timing synchronization bus by Tektronix, Inc. of Beaverton, Oregon, USA.

[0032] The test and measurement system 200 of FIG. 2 enables pseudo-differential measurement of differential output signals from DUTs 210A and 210B. The system 200 delivers one of the differential output signals from either DUT 210A or 210B to one channel (e.g., test port 226A) of a first test and measurement instrument 208A and delivers a second of the differential output signals to one channel (e.g., test port 226B) of a second test and measurement instrument 208B. A synchronization circuit 209 synchronizes the two instruments 208A and 208B to synchronize the capture of the two differential output signals under test. The architecture of the system 200 also requires the use of only one test port 226A or 226B on each of the test and measurement instruments 208A and 208B, making it advantageous for use in situations where there are limited ports available or present on the test and measurement instruments.

[0033] Figure 3 is a block diagram illustrating yet another test and measurement system 300 in accordance with some embodiments of the present disclosure. In test and measurement system 300, many components are the same as or similar to components of test and measurement systems 100 and 20 described above with respect to Figures 1 and 2. Accordingly, these same or similar components in test and measurement system 300 are given the same 300-series numbers as the corresponding 100-series or 200-series components in test and measurement systems 100 and 200 of Figures 1 and 2, and will not be described again in detail.

[0034] Test and measurement system 300 of Figure 3 includes multiple remote heads 302A and 302B, each including an RF switch 303A and 303B and an electro-optic modulator 304A and 304B, which are optically coupled to corresponding test ports 226A and 226B of test and measurement instrument 308 via optical interconnection circuitry 306A and 306B. In contrast to system 200 of Figure 2, test and measurement system 300 includes a single test and measurement instrument 308, compared to two instruments 208A and 208B and synchronization circuitry 209 in system 200. The operation of test and measurement system 300 is understood from the above description of systems 100 and 200, and therefore this operation will not be described again with reference to Figure 3.

[0035] In system 300, remote heads 302A and 302B allow a user to select, via RFCS signals, the desired one of the differential output signals from DUTs 310A and 310B to couple to test ports 326A and 326B of test and measurement instrument 308. Each of the selected first and second differential output signals from DUTs 310A and 310B is thus delivered to a separate channel of test and measurement instrument 308, where test ports 326A and 326B are input ports to a channel of the test and measurement instrument. This allows pseudo-differential measurement of the differential output signals from DUTs 310A and 310B by test and measurement instrument 308, eliminating the need for external synchronization circuitry 209 of FIG. 2.

[0036] 4 is a block diagram of a test and measurement system 400 having a remote head 402 according to some embodiments of the present disclosure, the remote head 402 containing multiple electro-optical modulators (EOMs) 404A1-404AN and 404B1-404BN coupled to a test and measurement instrument 408 via an optical switch (optical multiplexer) 405 and optical interconnection circuitry 406 for testing single-ended electrical test signals received from output terminals OUT1-OUTN of a DUT 410A. The remote head 404 provides optical multiplexing of the test signals from DUTs 410A and 410B, and an optical transmission of a selected one of these test signals is provided to the test and measurement instrument 408 via the optical interconnection circuitry 406. In system 400, electrical test signals from DUTs 410A and 410B provided at output terminals OUT0-OUTN are converted to corresponding optical test signals by EOMs 404A1-404AN and 404B1-404BN in remote test head 402. A control signal CS is applied to optical multiplexer 405 to select one optical test signal from the plurality of optical test signals from EOMs 404A1-404AN and 404B1-404BN, and the selected optical test signal is transmitted to test and measurement equipment 408 via fiber optic cable 424 and OEM 420, which form optical interconnection circuit 406.

[0037] Remote head 402 may optionally include amplifiers 444A1-444AN and 444B1-444BN for amplifying electrical test signals received at output terminals OUT1-OUTN of DUTs 410A and 410B, and the amplified electrical test signals are provided to EOMs 404A1-404AN and 404B1-404BN. In some embodiments, EOMs 404A1-404AN and 404B1-404BN may convert the received electrical test signals to optical signals without additional amplification, thus omitting amplifiers 444A1-444AN and 444B1-444BN. In these embodiments, the electrical test signals on output terminals OUT1-OUTN of DUTs 410A and 410B are applied to input port 414 of remote head 402 through output ports 432A and 432B of the DUTs and cables 434A and 434B, and then applied directly to EOMs 404A1-404AN and 404B1-404BN in the remote head.

[0038] The architecture of system 400 allows 2N single-ended electrical test signals from DUTs under test 410A and 410B to be tested using a single test port 426 of test and measurement instrument 408. These electrical test signals are converted to optical signals before being transmitted to test and measurement instrument 408, allowing them to be communicated over much longer distances than electrical signals without adverse effects or distortions that could impair the ability of test and measurement instrument 408 to reliably measure the test signals. In some embodiments, the components of remote head 402 are formed in a silicon optical integrated circuit that integrates the electrical and optical components of the remote head. The remainder of test system 400 not specifically described operates as described above with reference to the previously described embodiments.

[0039] 4 illustrates electronic test signals originating from two DUTs 410A and 410B, the remote head 402 may be coupled to only a single DUT or any number of DUTs. In other words, the input port 414 may be coupled to an electrical test signal from any source.

[0040] 5 is a block diagram of a test and measurement instrument 500 having a remote head 502 according to some embodiments of the present disclosure, the remote head 502 containing multiple electro-optical modulators (EOMs) 504-1 through 504-N, which are coupled to a test and measurement instrument 508 via optical switches (optical multiplexers) 505A and 505B and optical interconnect circuits 506A and 506B for testing differential electrical test signals received from lanes "Lane 0" through "Lane N" of a DUT 510. In the system 500, one signal from each of the differential signals on lanes "Lane 0" through "Lane N" of the DUT 510 is coupled to a corresponding EOM 504 in the remote head 502. In response to the received differential electrical test signals, corresponding optical test signals generated by EOM 504 are applied to optical multiplexers 505A and 505B, with one optical test signal of each differential signal pair from each lane of DUT 510 being provided to the optical input port of optical multiplexer 505A and the other optical test signal being provided to the optical input port of optical multiplexer 505B. This allows optical test signals corresponding to one differential signal pair of the electrical test signals to be provided by optical multiplexers 505A and 505B to optical interconnect circuits 506A and 506B and then to test ports 526A and 526B of test and measurement instrument 508. This enables test and measurement instrument 508 to perform pseudo-differential measurements on the differential electronic test signals from lanes "Lane 0" through "Lane N" of DUT 510, as described above with respect to system 200 of FIG. 2. Remote head 502 may optionally include amplifiers 544-1 through 544-N for amplifying each of the differential electrical test signals received from lanes "LANE 0" through "LANE N" of DUT 510, and the amplified differential electrical test signals are provided to EOMs 504-1 through 504-N.

[0041] 6 is a block diagram of a test and measurement instrument 600 having a remote head 602 and a feedforward correction circuit 611, in accordance with some embodiments of the present disclosure, that corrects for variations in optical interconnect circuitry 606 that may adversely affect the characteristics of electrical test signals provided to test ports 626 of a test and measurement instrument 608. The remote head 602 includes multiple electrical input ports 603 configured to receive electrical test signals from a DUT (not shown). Each electrical test signal on one of the electrical input ports 603 is provided to a corresponding electrical-to-optical converter (EOM) 604, and a corresponding optical test signal generated by each EOM is provided to a corresponding optical input port of an optical switch (optical multiplexer) 605. The optical test signal from the EOM 604 is selected by the optical multiplexer 605 in response to a control signal CS and provided to an optical output port of the optical multiplexer and then to the optical interconnect circuitry 606 coupled to the test and measurement instrument 608.

[0042] In the embodiment of FIG. 6, optical interconnect circuitry 606 includes fiber optic cable 624 and optical-to-electrical modulator (OEM) 620. The fiber optic cable 624 and OEM 620 may experience changes in their operating characteristics during operation of system 600. For example, changes in the temperature of fiber optic cable 624 may cause the cable's length to change, which is significant for the short wavelengths of the optical test signals propagating therethrough. Correction circuitry 611 corrects for such variations. Correction circuitry 611 includes a comparator 613 that compares the low frequency (LF) or DC component of the electrical test signal output by OEM 620 with the low frequency or DC component of the electrical test signal provided to the corresponding EOM 604 in test head 602. The electrical test signal provided to the corresponding EOM 604 is provided to one input terminal of comparator 613 via multiplexer 615 in response to a control signal CS. Comparator 613 generates an output signal based on a comparison of the low frequency components of the two electrical test signals and provides this output signal to the bias input of EOM 604 to correct for variations in the electrical test signal output by OEM 620 due to variations in the characteristics of optical interconnect circuit 606.

[0043] 7 is a block diagram of a test and measurement instrument 700 including a remote head 702 and a feedback compensation circuit 711, in accordance with some embodiments of the present disclosure. The feedback compensation circuit 711 compensates for variations in the optical interconnect circuit 706 that may adversely affect the characteristics of the electrical test signals provided to the test ports 726 of the test and measurement instrument 708. The compensation circuit 711 includes a comparator 713 that compares the low frequency (LF) or DC component of the electrical test signal output by the OEM 720 with the low frequency or DC component of the electrical test signal provided to the corresponding EOM 704 in the test head 702. The electrical test signal provided to the corresponding EOM 704 is provided to one input terminal of the comparator 713 via a multiplexer 715 in response to a control signal CS. The comparator 713 generates an output signal based on a comparison of the low frequency components of the two electrical test signals and provides the output signal to a summer 717 coupled to the output of the OEM 720. In this manner, summer 717 adds a low frequency or DC component to the electrical test signal output by OEM 720 to compensate for variations in optical interconnect circuit 706 that may occur during operation of system 700, as described above with reference to system 600 of FIG. 6.

[0044] In test and measurement systems 100-700, components interconnecting the DUTs and test and measurement instruments within these systems can receive power in a variety of ways. Power delivery will now be described in more detail with reference to the test and measurement system of FIG. 1, but this description also applies to the embodiments of FIGS. 2 and 3. In some embodiments, test and measurement instrument 108 may provide power via appropriate power lines (not shown) to power components within remote heads 102A and 102B, as well as other components, such as OEM 120 and optical switch 112. In other embodiments, remote heads 102A and 102B may be coupled to a power source independent of test and measurement instrument 108, such as by plugging directly into a wall outlet. In yet other embodiments, remote head 102A may be able to draw power from DUTs 110A and 110B without affecting the signal quality of the DUTs under test. For example, because USB (Universal Serial Bus) transmits power and data over its lines, remote heads 102A and 102B may be able to use a small amount of power from the USB cable.

[0045] In yet another embodiment, the power needs of remote heads 102A and 102B may be small enough to be met by a rechargeable or replaceable battery (not shown) housed within each remote head. This may be possible in certain embodiments of remote heads 102A and 102B in which circuitry is embodied as application specific integrated circuits (ASICs) or microelectromechanical systems (MEMS) switches. In another embodiment, the components housed in remote heads 102A and 102B and optical interconnect circuitry 106 may include multiple discrete components mounted on one or more printed circuit boards (PCBs).

[0046] As described above, the control signals CS and RFCS may be provided by the test and measurement instrument 108, 208A, 208B, 308. In such embodiments, a user or operator can operate the test and measurement instrument to select one signal for testing from among multiple signals from DUTs 110A and B, 210A and B, and 310A and B. The user may make the selection on a user interface (not shown) of the test and measurement instrument 108, 208A, 208B, 308, or may instruct the test and measurement instrument using programmatic control, such as using the PI programming interface available in Tektronix measurement instruments of Beaverton, Oregon, USA. For example, a user may select a differential output signal, e.g., from DUT 110A, for testing and then program the test and measurement instrument 108 to test desired parameters of the selected differential output signal for a period of time. After the first test period, the test program on the test and measurement instrument 108 causes the optical switch 112 to select the second differential output signal from DUT 110B for testing. In this manner, all signals from DUTs 110A and 110B, and possibly additional DUTs, can be scripted to be tested in sequence, without the user having to physically change the cables between the DUTs and the test and measurement instrument 108. Instead, switching to connect the desired signals of DUTs 110A and 110B to the test and measurement instrument 108 is performed by controlling the optical switch 112. This operation of the optical switch 112 and the RF switches 203A and 203B and 303A and 303B can be programmatically controlled in this manner, allowing the user to program the system 100, 200, 300 to step through all of the connected signal lanes of all of DUTs 110A and B, 210A and B, and 310A and B without having to disconnect or rearrange cables. This saves hours of manual work during the testing session.

[0047] Aspects of the disclosed technology may operate on specially created hardware, firmware, digital signal processors, or specially programmed general-purpose computers, including processors that operate according to programmed instructions. The terms "controller" or "processor" herein contemplate microprocessors, microcomputers, ASICs, and dedicated hardware controllers, among others. Aspects of the disclosed technology may be implemented with computer-usable data and computer-executable instructions, such as one or more program modules, executed by one or more computers (including a monitoring module) or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc., which, when executed by a processor in a computer or other device, perform particular tasks or implement particular abstract data types. Computer-executable instructions may be stored in computer-readable storage media, such as hard disks, optical disks, removable storage media, solid-state memory, RAM, etc. Those skilled in the art will appreciate that the functionality of the program modules may be combined or distributed as desired in various embodiments. Furthermore, such functionality may be embodied in whole or in part in firmware or hardware equivalents, such as integrated circuits, field programmable gate arrays (FPGAs), etc. Certain data structures may be used to more effectively implement one or more aspects of the disclosed technology, and such data structures are considered within the scope of the computer-executable instructions and computer-usable data described herein.

[0048] The disclosed aspects may, in some cases, be implemented in hardware, firmware, software, or any combination thereof. The disclosed aspects may also be implemented as instructions carried by or stored on one or more computer-readable media, which may be read and executed by one or more processors. Such instructions may be referred to as a computer program product. As used herein, computer-readable media refers to any medium that can be accessed by a computing device. By way of example, and not limitation, computer-readable media may include computer storage media and communication media.

[0049] "Computer storage media" means any medium that can be used to store computer-readable information. By way of example and not limitation, computer storage media may include random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory and other memory technologies, compact disc read-only memory (CD-ROM), digital video disc (DVD) and other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage and other magnetic storage devices, and any other volatile or nonvolatile, removable or non-removable medium implemented in any technology. "Computer storage media" excludes signals themselves and transitory forms of signal transmission.

[0050] A communication medium means any medium usable for communicating computer-readable information. By way of example, and not limitation, communication media may include coaxial cable, fiber optic cable, air, or any other medium suitable for communicating electrical, optical, radio frequency (RF), infrared, acoustic, or other types of signals. Example

[0051] The following examples are provided to aid in understanding the technology disclosed in this application. Embodiments of the technology may include one or more of the examples described below, and any combination thereof.

[0052] Example 1 is a test measurement system, one or more remote heads each configured to be coupled to a corresponding device under test (DUT) and to receive electrical test signals from the DUT, each remote head having an electro-optical modulator (EOM) configured to convert the received electrical test signals into optical test signals; an optical interconnection circuit coupled to receive optical test signals from the EOM of the one or more remote heads, the optical interconnection circuit responsive to a control signal to select one optical test signal from among the plurality of optical test signals, convert the selected optical test signal into an electrical test signal, and provide the electrical test signal to a test port of a test and measurement instrument; It is equipped with.

[0053] A second embodiment is the test and measurement system according to the first embodiment, wherein the optical interconnection circuit comprises: an optical switch including a plurality of optical input ports each coupled to receive the optical test signal from the EOM of one of the one or more remote heads, the optical switch configured to select the optical test signal on one of the plurality of optical input ports in response to the control signal and provide the optical test signal on an optical output port; an optical waveguide coupled to the optical output port of the optical switch; an optical-to-electrical modulator (OEM) coupled to the optical waveguide for receiving the selected optical test signal, converting it into the electrical test signal, and providing it to the test port of the test and measurement instrument; It has.

[0054] Example 3 is the test and measurement system according to Example 2, wherein the optical waveguide comprises a fiber optic cable.

[0055] Example 4 is the test and measurement system according to example 2, wherein the OEM comprises a photodiode.

[0056] Example 5 is the test and measurement system according to Example 2, wherein each of the EOMs includes a Mach-Zehnder modulator.

[0057] Example 6 is a test and measurement system according to any of the above-described examples, further comprising a pair of coaxial cables coupled between each of the DUTs and a corresponding remote head among the one or more remote heads, the pair of coaxial cables supplying a differential electrical test signal from the DUT to the corresponding remote head.

[0058] Example 7 is the test and measurement system according to Example 6, wherein each of the remote heads further comprises a differential input amplifier configured to amplify the electrical test signal from a corresponding DUT.

[0059] Example 8 is a test measurement system, comprising: a pair of remote heads, each remote head coupled to a corresponding device under test (DUT) and configured to receive one of a set of differential electrical test signals from the DUT; an RF switch having a plurality of RF input ports each coupled to receive a corresponding electrical test signal from one of the plurality of DUTs, and configured to select one RF input port from the plurality of RF input ports in response to a control signal and provide the electrical test signal of the selected RF input port to an RF output port; an electro-optical modulator (EOM) configured to convert the electrical test signal on the RF output port into an optical test signal; The pair of remote heads having an optical interconnection circuit coupled to receive the optical test signals from the EOMs in the pair of remote heads, and configured to convert each of the optical test signals to a corresponding electrical test signal and provide it to a test port of a test and measurement instrument; It is equipped with.

[0060] Example 9 is the test and measurement system according to Example 8, wherein the optical interconnection circuit comprises: a pair of optical waveguides each coupled to the EOM of one of the pair of remote heads; a pair of optical-to-electrical modulators (OEMs), each coupled to one of the pair of optical waveguides, for receiving the corresponding optical test signal, converting it into an electrical test signal, and providing it to the test port of the test and measurement instrument; It has.

[0061] Example 10 is a test and measurement system according to Example 9, wherein the test and measurement system includes first and second separate test and measurement devices, and the test and measurement system further includes a synchronization circuit coupled to the first and second test and measurement devices and configured to synchronize capture of electrical test signals received from the pair of OEMs at test ports of the first and second separate test and measurement devices.

[0062] Example 11 is a test and measurement system according to Example 9, wherein the test and measurement system includes a single test and measurement device having first and second test ports each coupled to a corresponding one of the pair of OEMs.

[0063] Example 12 is the test and measurement system according to example 9, in which the OEM comprises a photodiode.

[0064] Example 13 is the test and measurement system according to Example 9, wherein each of the optical waveguides comprises an optical fiber cable.

[0065] Example 14 is the test and measurement system according to Example 9, wherein each of the EOMs includes a Mach-Zehnder modulator.

[0066] Example 15 is the test and measurement system according to any of the preceding Examples 8 to 14, further comprising: a pair of coaxial cables coupled to each of the DUTs; a first coaxial cable of the pair of coaxial cables is coupled to a corresponding RF input port of the RF switch in one of the pair of remote heads; A second coaxial cable in the pair of coaxial cables is coupled to a corresponding RF input port of the RF switch in the other remote head in the pair of remote heads.

[0067] Example 16 is a method comprising: providing a plurality of electrical test signals to the remote head; selecting one electrical test signal from the plurality of electrical test signals to be provided to a test port of a test and measurement instrument; converting the selected electrical test signals to corresponding optical test signals at the remote head; transmitting the optical test signal via an optical waveguide to a test port of a test and measurement instrument; converting the optical test signal into an electrical test signal at the test port of the test and measurement instrument and providing the electrical test signal to a corresponding test channel of the test and measurement instrument; It is equipped with.

[0068] Example 17 is a method according to Example 16, wherein the process of selecting one electrical test signal from the plurality of electrical test signals to be supplied to a test port of the test and measurement instrument includes selecting one electrical test signal from the plurality of electrical test signals via an RF switch disposed in the remote head.

[0069] Example 18 is a method according to any of the preceding examples, wherein converting the selected electrical test signals into corresponding optical test signals includes modulating an optical carrier signal with the electrical test signals.

[0070] Example 19 is a method according to any of the preceding examples, wherein modulating the optical carrier signal with the electrical test signal includes providing the electrical test signal to a Mach-Zehnder modulator (MZM).

[0071] Example 20 is a method according to any of the preceding examples, wherein the process of selecting one electrical test signal from the plurality of electrical test signals to be supplied to a test port of the test and measurement device is performed by selecting the corresponding optical test signal transmitted through the optical waveguide using an optical switch.

[0072] Example 21 is a remote head of a test and measurement system, a plurality of electrical inputs each configured to be coupled to one or more devices under test (DUTs) for receiving electrical test signals from the one or more DUTs; a plurality of electro-optical modulators (EOMs), each coupled to one of the plurality of electrical inputs and each configured to convert the received electrical test signal into an optical test signal; an optical selection switch having a plurality of optical inputs each coupled to a corresponding one of the optical test signals from one of the plurality of EOMs, and selecting one of the plurality of optical inputs to output an output test signal; It is equipped with.

[0073] Example 22 is a remote head according to Example 21, in which the plurality of EOMs and the optical selection switch are integrated into a monolithic photonic integrated circuit.

[0074] Example 23 is a remote head according to any of the preceding Examples 21 to 22, further comprising a second optical selection switch having a plurality of second optical input sections, each of which is coupled to a test signal from one or more DUTs, and the second optical selection switch is configured to select one of the plurality of second optical input sections to output a second output test signal.

[0075] A twenty-fourth embodiment is the remote head of the twenty-third embodiment, wherein the output test signal and the second output test signal are differential signals.

[0076] Example 25 is the remote head according to any of the preceding Examples 21 to 24, further comprising a low frequency (LF) / DC correction loop.

[0077] Example 26 is the remote head according to Example 25, wherein the LF / DC correction loop a multiplexer configured to select one electrical input from the plurality of electrical inputs; a comparator configured to compare a signal at a selected one of the plurality of electrical inputs with an electrical signal converted from the output test signal; signal modifier and It has.

[0078] Example 27 is a remote head according to Example 26, wherein the signal modifying circuit is configured to control a bias input of the EOM.

[0079] Example 28 is the remote head according to example 26, wherein the signal modifying circuit is configured to control an electrical signal converted from the output test signal.

[0080] Although the above-described versions of the presently disclosed subject matter have many advantages that have been described or that will be apparent to those skilled in the art, not all of these advantages or features are required in every version of the disclosed devices, systems, or methods.

[0081] Additionally, the description in this application refers to specific features. All features disclosed in this specification, including the claims, abstract, and drawings, and all steps in all disclosed methods or processes, may be combined in any combination, unless they are at least partially mutually exclusive. Each feature disclosed in this specification, including the claims, abstract, and drawings, may be replaced with an alternative feature serving the same, equivalent, or similar purpose, unless otherwise specified.

[0082] Furthermore, when this application refers to a method having two or more defined steps or processes, these defined steps or processes may be performed in any order or simultaneously, unless the circumstances do not preclude this possibility.

[0083] Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Claims

1. 1. A test and measurement system comprising: one or more remote heads each configured to be coupled to a corresponding device under test (DUT) and to receive electrical test signals from the DUT, each remote head having an electro-optical modulator (EOM) configured to convert the received electrical test signals into optical test signals; an optical interconnection circuit coupled to receive optical test signals from the EOMs of the one or more remote heads, the optical interconnection circuit responsive to a control signal to select one optical test signal from among the plurality of optical test signals, convert the selected optical test signal into an electrical test signal and provide it to a test port of a test and measurement instrument; A test and measurement system comprising:

2. The optical interconnection circuit, an optical switch including a plurality of optical input ports each coupled to receive the optical test signal from the EOM of one of the one or more remote heads, the optical switch configured to select the optical test signal on one of the plurality of optical input ports in response to the control signal and provide the optical test signal on an optical output port; an optical waveguide coupled to the optical output port of the optical switch; an optical-to-electrical modulator (OEM) coupled to the optical waveguide for receiving the selected optical test signal, converting it into the electrical test signal, and providing it to the test port of the test and measurement instrument; 10. The test and measurement system of claim 1, comprising:

3. 3. The test and measurement system of claim 2, wherein said optical waveguide comprises a fiber optic cable.

4. 3. The test and measurement system of claim 2, wherein the OEM comprises a photodiode.

5. 2. The test and measurement system of claim 1, wherein each of said EOMs comprises a Mach-Zehnder modulator.

6. 2. The test and measurement system of claim 1, further comprising a pair of coaxial cables coupled between each of said DUTs and a corresponding one of said one or more remote heads, said pair of coaxial cables providing a differential electrical test signal from said DUT to said corresponding remote head.

7. 7. The test and measurement system of claim 6, wherein each of the remote heads further comprises a differential input amplifier configured to amplify the electrical test signal from a corresponding DUT.

8. 1. A test and measurement system comprising: a pair of remote heads, each remote head being coupled to a corresponding device under test (DUT) and configured to receive one of the differential electrical test signals from the DUT; an RF switch having a plurality of RF input ports each coupled to receive a corresponding electrical test signal from one of the plurality of DUTs, and configured to select one RF input port from the plurality of RF input ports in response to a control signal and provide the electrical test signal of the selected RF input port to an RF output port; an electro-optical modulator (EOM) configured to convert the electrical test signal on the RF output port into an optical test signal; the pair of remote heads having an optical interconnection circuit coupled to receive the optical test signals from the EOMs in the pair of remote heads, and configured to convert each of the optical test signals into a corresponding electrical test signal and provide it to a test port of a test and measurement instrument; A test and measurement system comprising:

9. The optical interconnection circuit, a pair of optical waveguides each coupled to the EOM of one of the pair of remote heads; a pair of optical-to-electrical modulators (OEMs), each coupled to one of the pair of optical waveguides, for receiving the corresponding optical test signal, converting it into an electrical test signal, and providing it to the test port of the test and measurement instrument; 9. The test and measurement system of claim 8, comprising:

10. 10. The test and measurement system of claim 9, wherein the test and measurement system comprises separate first and second test and measurement instruments, and further comprises a synchronization circuit coupled to the first and second test and measurement instruments and configured to synchronize capture of electrical test signals received from the pair of OEMs at test ports of the separate first and second test and measurement instruments.

11. 10. The test and measurement system of claim 9, wherein said test and measurement system comprises a single test and measurement instrument having first and second test ports each coupled to a corresponding one of said pair of OEMs.

12. 10. The test and measurement system of claim 9, wherein the OEM comprises a photodiode.

13. 10. The test and measurement system of claim 9, wherein each of said optical waveguides comprises a fiber optic cable.

14. 9. The test and measurement system of claim 8, wherein each of said EOMs comprises a Mach-Zehnder modulator.

15. a pair of coaxial cables coupled to each of the DUTs; a first coaxial cable of the pair of coaxial cables coupled to a corresponding RF input port of the RF switch in one of the pair of remote heads; 9. The test and measurement system of claim 8, wherein a second coaxial cable in said pair of coaxial cables is coupled to a corresponding RF input port of said RF switch in the other remote head in said pair of remote heads.

16. providing a plurality of electrical test signals to the remote head; selecting one electrical test signal from the plurality of electrical test signals to be provided to a test port of the test and measurement instrument; converting the selected electrical test signals into corresponding optical test signals at the remote head; transmitting the optical test signal via an optical waveguide to a test port of a test and measurement instrument; converting the optical test signal into an electrical test signal at the test port of the test and measurement instrument and providing the electrical test signal to a corresponding test channel of the test and measurement instrument; A method comprising:

17. 17. The method of claim 16, wherein selecting one electrical test signal from the plurality of electrical test signals to be provided to a test port of the test and measurement instrument comprises selecting the one electrical test signal from the plurality of electrical test signals via an RF switch located in the remote head.

18. 17. The method of claim 16, wherein converting selected electrical test signals into corresponding optical test signals comprises modulating an optical carrier signal with the electrical test signals.

19. 17. The method of claim 16, wherein modulating the optical carrier signal with the electrical test signal comprises providing the electrical test signal to a Mach-Zehnder Modulator (MZM).

20. 17. The method of claim 16, wherein selecting one of the plurality of electrical test signals to be supplied to a test port of the test and measurement instrument is performed by selecting the corresponding optical test signal transmitted through the optical waveguide with an optical switch.

21. 1. A remote head for a test and measurement system, comprising: a plurality of electrical inputs each configured to be coupled to one or more devices under test (DUTs) for receiving electrical test signals from the one or more DUTs; a plurality of electro-optical modulators (EOMs), each coupled to one of the plurality of electrical inputs and each configured to convert the received electrical test signal into an optical test signal; an optical selection switch having a plurality of optical inputs each coupled to a corresponding one of the optical test signals from one of the plurality of EOMs, and for outputting an output test signal by selecting one of the plurality of optical inputs; A remote head comprising:

22. 22. The remote head of claim 21, wherein the plurality of EOMs and the optical selection switch are integrated into a monolithic optical integrated circuit.

23. 22. The remote head of claim 21, further comprising a second optical selection switch having a plurality of second optical inputs, each of the plurality of second optical inputs coupled to a test signal from the one or more DUTs, the second optical selection switch configured to select one of the plurality of second optical inputs to output a second output test signal.

24. 24. The remote head of claim 23, wherein the output test signal and the second output test signal are differential signals relative to each other.

25. 25. The remote head of claim 21, according to any one of the preceding Examples 21 to 24, further comprising a low frequency / DC correction loop.

26. The low frequency / DC correction loop a multiplexer configured to select one electrical input from the plurality of electrical inputs; a comparator configured to compare a signal at a selected one of the plurality of electrical inputs with an electrical signal converted from the output test signal; Signal modification circuit and 26. The remote head of claim 25, comprising:

27. 27. The remote head of claim 26, wherein the signal modifying circuitry is configured to control a bias input of the EOM.

28. 27. The remote head of claim 26, wherein the signal modifying circuitry is configured to control an electrical signal converted from the output test signal.

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