A device that provides test signals from the device under test (DUT) to a measuring instrument.

Abstract

To provide a probe head for measuring both an AC electric signal and a DC electric signal of high-power electronic signal from an inverter or from power electronics similar to an inverter.SOLUTION: There is disclosed a device for providing a test signal from a test target device (DUT) to a measurement apparatus. The device includes a probe head 110 formed to receive an electric signal from the DUT. The probe head 110 has an electrooptical modulator. The device also includes a control box 120 with a light source. The light source is formed so that the electrooptical modulator will be provided with an input light signal and the electrooptical modulator is formed to provide an output light signal on the basis of an electric signal from the DUT. A control box 120 also includes an optical bias control circuit. A bias control signal alone is provided to the electrooptical modulator.SELECTED DRAWING: Figure 1

Description

Background Art

[0001] High-power electronics are ubiquitous in today's society and their applicability is increasing in various attempts such as in automotive vehicles and trains. For example, these power electronics often include an inverter that converts a direct current (DC) signal to an alternating current (AC) signal or an AC signal to a DC signal.

[0002] Often, it is desired to measure both AC electrical signals and DC electrical signals from an inverter or similar power electronics. However, the direct measurement of high-power electrical signals is not practical for known oscilloscopes or similar measurement devices that are electrically connected to ground.

Summary of the Invention

[0003] Therefore, what is needed is a device that overcomes at least the drawbacks of the known measurement devices discussed above.

[0004] Exemplary embodiments are best understood by reading the following detailed description in conjunction with the accompanying drawings. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions can be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and useful, like reference numerals refer to like elements.

Brief Description of the Drawings

[0005] [Figure 1] FIG. 1 is a simplified schematic block diagram of a system for measuring the voltage level of a high-power electrical signal from a DUT according to an exemplary embodiment. [Figure 2] FIG. 2 is a simplified schematic block diagram of a probe head and a control box according to an exemplary embodiment. [Figure 3A] FIG. 3 is a top view of a Mach-Zehnder modulator (MZM) according to an exemplary embodiment. [Figure 3B] It is the transfer function of the transmission of the MZM with respect to the applied voltage according to an exemplary embodiment. [Figure 4A] It is a simplified schematic block diagram of a bias control circuit connected to a Mach-Zehnder modulator (MZM) according to an exemplary embodiment. [Figure 4B] It is a flowchart of a process for setting a bias input to the MZM from a bias control circuit to maintain the MZM at an orthogonal point according to an exemplary embodiment. [Figure 5] It is a simplified schematic block diagram of a probe head and a control box according to another exemplary embodiment.

Embodiments for Carrying out the Invention

[0006] In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments that disclose specific details are set forth in order to provide a thorough understanding of one embodiment according to the present teachings. However, it will be apparent to those skilled in the art who can utilize the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein also remain within the scope of the appended claims. Moreover, descriptions of known devices and methods may be omitted so as not to obscure the description of the exemplary embodiments. Such methods and devices are clearly within the scope of the present teachings.

[0007] The terms used herein are for the purpose of describing only specific embodiments and are not intended to be limiting. The defined terms are such as would be generally understood and accepted in the technical meaning and scientific meaning of the defined terms, in addition to the technical field of the present teachings.

[0008] Unless otherwise specified, when we say that a first element (e.g., an optical waveguide or power line) is connected to a second element (e.g., an optical-electrical (O / E) modulator or an E / O modulator, respectively), this includes cases where one or more intermediate elements or intervening devices can be used to connect these two elements to each other. On the other hand, when we say that a first element is directly connected to a second element, this includes only cases where these two elements are connected to each other without any intermediate or intervening devices. Similarly, when we say that a signal is coupled to an element, this includes cases where one or more intermediate elements can be used to couple the signal to the element. On the other hand, when we say that a signal is directly coupled to an element, this includes only cases where the signal is directly coupled to the element without any intermediate or intervening devices.

[0009] As used herein and in the appended claims, “a” and “the” or “the” refer to both singular and plural objects unless the context clearly indicates otherwise. For example, “a device” includes one device and multiple devices.

[0010] As used herein and in the appended claims, the terms “substantial” or “substantially” mean, in addition to their ordinary meaning, within an acceptable limit or degree. As used herein and in the appended claims, the terms “approximately” mean, in addition to their ordinary meaning, within an acceptable limit or amount for a person skilled in the art. For example, “approximately the same” means that a person skilled in the art would consider the items being compared to be the same.

[0011] Various embodiments of a device that provides a test signal from a device under test (DUT) to a measuring instrument are described herein.

[0012] According to a typical embodiment, a device that provides a test signal from a device under test (DUT) to a measuring instrument includes a probe head configured to receive an electrical signal from the DUT. The probe head includes an electro-optic modulator. The device also includes a control box that includes a light source configured to provide an input optical signal to the electro-optic modulator. The electro-optic modulator is configured to provide an output optical signal based on the electrical signal from the DUT. The device also includes an optical bias control circuit. The bias control signal is provided to the electro-optic modulator.

[0013] According to another representative embodiment, a device that provides a test signal from a device under test (DUT) to a measuring instrument comprises a probe head configured to receive an electrical signal from the DUT, the probe head comprising an electro-optic modulator. The device also comprises a light source configured to provide an input optical signal to the electro-optic modulator, the electro-optic modulator configured to provide an output optical signal based on the electrical signal from the DUT, and a control box comprising an optical bias control circuit. Only the bias control signal is provided to the electro-optic modulator.

[0014] In particular, as can be seen from the examination in Figure 2, according to this instruction, the probe head transmits only optical signals to the control box and receives optical signals from the control box. Therefore, the probe heads of various representative embodiments are galvanically isolated from the control box and measuring instruments.

[0015] Figure 1 is a simplified schematic block diagram of a system 100 for measuring the voltage level of a high-power electrical signal from a DUT according to a typical embodiment.

[0016] System 100 includes a probe head 110 equipped with a sensor tip 112 and a probe 114. System 100 also includes a control box 120, which is connected to the probe head 110 at one end and to a probe interface connector 130 at the other end. The probe interface connector 130 is connected to an oscilloscope 140 that measures signals transferred from a device under test (DUT) (not shown in Figure 1).

[0017] In a typical embodiment, the probe interface connector 130 and the oscilloscope 140 are known devices and will not be described in much detail herein. The probe interface connector 130 transmits signals received from the control box 120 and provides power to the control box 120 during its operation.

[0018] According to this instruction, the signal received from the DUT is an electrical signal that may include AC and DC components. As described more fully herein, the probe head 110 is configured to receive the electrical signal from the DUT via the probe 114. The electrical signal from the DUT is received by the electro-optic (E / O) converter 116. The E / O converter 116 converts the electrical signal from the DUT into an optical signal, which is transmitted to the control box 120 via the optical fiber 118. The optical signal transmitted to the control box 120 by the probe head 110 is converted into an electrical signal in the O / E converter 122 for measurement by an oscilloscope.

[0019] The E / O converter 116 and O / E converter 122 are arranged symbolically as shown in Figure 1 for convenience. However, as will be described more fully in this specification, these converters can be arranged only in the control box 120, only in the probe head 110, or both. Furthermore, the E / O converter 116 and O / E converter 122 in Figure 1 symbolize the electrical / optical and optical / electrical conversions that occur according to this teaching. Thus, and as will become clear as this description continues, the probe head 110 galvanically isolates the oscilloscope 140 from the electrical signal received by the probe 114 and the sensor tip 112. The electrical signal received by the probe 114 can be a relatively high-voltage electrical signal having both AC and DC components. The sensor tip 112 may incorporate a voltage reduction network such as a resistor divider, but the need to preserve DC means that a galvanic connection still exists between the high-voltage DUT and the probe head 110. As stated above, high-voltage signals cannot be safely measured by an oscilloscope such as oscilloscope 140 which is electrically connected to ground. By this teaching, relatively high-voltage AC and DC signals are converted to optical signals before being transmitted to control box 120. As will be described more fully herein, control box 120 provides oscilloscope 140 with electrical signals having acceptablely low-voltage AC and DC components representing the high-voltage AC and DC components from the DUT. Finally, and as will also be described more fully below, according to various representative embodiments, the term DC includes both voltage signals without time-varying components, and voltage signals with time-varying components having a frequency smaller than the frequency response of the optical bias control circuit (not shown in Figure 1) or the frequency of the crossover filter (shown in Figure 5).

[0020] Figure 2 is a simplified schematic block diagram of the probe head 210 and control box 220 according to a typical embodiment. The probe head 210 and control box 220 are adapted to function within system 100 to enable measurement of both high-voltage AC signals and high-voltage DC signals from a DUT (not shown in Figure 2). Therefore, various details of the description of system 100 are common to the description of the probe head 210 and control box 220 and can be avoided in order to avoid obscuring the description of the typical embodiment in Figure 2.

[0021] Referring to Figure 2, the control box 220 includes a current source 202 that provides input to the laser 204. In particular, since a portion of the output laser power is input to the current source 202 via a feedback loop, the power of the signal output from the laser 204 remains substantially constant for reasons that will become apparent as this description continues.

[0022] The output from laser 204 is supplied to the first optical tap 206. The first optical tap 206 supplies a significant portion of the signal output power from the laser to the probe head, as described below. A small portion of the power output from laser 204 (exemplifiedly about 1% to about 5%) is supplied to the first O / E converter 208, which supplies the input power (P) to the Mach-Zehnder modulator (MZM) 234 in the probe head 210. in As will become apparent as this explanation continues, the MZM234 receives an input electrical signal from the DUT and outputs an optical signal to the control box. Thus, the MZM234 functions as an E / O converter, and therefore electrically isolates the oscilloscope 140 from the relatively high-voltage AC and DC signals from the DUT.

[0023] The first O / E converter 208 exemplifies a photodetector (not shown in Figure 2) connected to a trans-impedance amplifier (TIA) (not shown in Figure 2), which can be partially or completely integrated into the optical bias control circuit 211. The first O / E converter 208 receives P from the laser 204.in An electrical signal 209 proportional to is provided as the first input to the optical bias control circuit 211. In particular, P in If the input is substantially constant, the first optical tap 206 can be defaulted and a constant input can be provided to the optical bias control circuit 211.

[0024] An electrical signal 209 from the laser 204, provided as one input to the optical bias control circuit 211, assists in biasing the MZM234. As will become clear as this description continues, a first output signal 212 from the optical bias control circuit 211 is provided to the DAC 218. The first output signal is a digital electrical signal that is proportional to, but opposite in sign to, the bias voltage applied to the MZM234 of the probe head 210. After conversion to an analog signal, the first output signal 212 from the optical bias control circuit 211 is provided to the adder 216. In particular, the AC and DC signals from the third O / E converter 226 and the DAC 218 are added together by the adder 216. The adder 216 is not required to provide filtering. Rather, the inductor symbol of the adder 216 simply indicates the DC path, while the capacitor symbol of the adder 216 indicates the AC path. The output from the adder 216 is provided to an oscilloscope (not shown in Figure 2) via a suitable electrical connector 228.

[0025] The probe head 210 receives an input signal from the DUT via a connection section 230, which can be equipped with a sensor tip 112 and a probe 114 as shown in Figure 1. The input signal includes AC and DC components, and the DC component includes either a voltage signal without a time-varying component, or a voltage signal with a time-varying component having a frequency smaller than the frequency response of the optical bias control circuit 211, as described above. This will be described in more detail below.

[0026] The output from connection 230 is provided to switch 232. As will be described more fully below in relation to Figures 3A and 3B, switch 232 is required to decouple the DUT signal from the input (RF) port of the MZM234 and set the inferred DC voltage to "zero". This ensures proper operation of the probe head 210 in the presence of drift in the MZM234. When switch 232 is closed and the MZM234 is stable and biased to a quadrature state, the second input 236 to the MZM234, the first output signal 212, and the signal from DAC 218 all represent the DUT signal from DC to the bandwidth of the optical bias control circuit 211. However, if the MZM bias drift due to external factors becomes significant, this teaching will cause switch 232 to be opened for a short time so that the optical bias control circuit 211 and the first output signal 212 are reset to "zero". The frequency of this reset procedure depends on the characteristics of the MZM234, environmental changes, and the precision requirements of the device. Generally, the signal provided to the second input 236 differs from the first output signal 212 by an offset proportional to the second input 236 when the switch 232 is open. The MZM234 is always biased to an orthogonal state, as described below in Figure 3B.

[0027] When the switch conducts, the input signal from the DUT is provided as the first input 233 to the RF input of the MZM234. As recognized, the first input 233 is an analog electrical signal. The second input 236 serves as the bias input to the MZM234. The signal to the second input 236 is the analog electrical signal after the bias signal 240 from the optical bias control circuit 211 has been converted by the DAC 238. As shown in Figure 2, the bias signal 240 from the optical bias control circuit 211 to the probe head 210 is a digital optical signal, thereby ensuring electrical isolation of the oscilloscope 140 from the DUT.

[0028] During operation, the input optical signal 241 is supplied to the MZM234, and the output optical signal 242 is supplied from the MZM234. The output optical signal 242 is supplied to the control box 220 and input to the second optical tap 224. As will be described in more detail below, the output optical signal 242 to the third O / E converter 226 of the control box 220 includes an AC signal with a frequency greater than the frequency response of the optical bias control circuit 211. In contrast, the optical bias control circuit 211 does not respond to time-varying (AC) signals with a frequency greater than its frequency response. This is expected because the optical bias control circuit 211 cannot respond to frequencies beyond its bandwidth. Rather, the optical bias control circuit 211 responds to frequencies within its bandwidth, including DC. Thus, the optical bias control circuit 211 contains information about DC signals and signal frequencies within its bandwidth, while the optical bias control circuit 211 removes this content from the output optical signal 242 by applying a bias signal to the second input 236 to the MZM234. Therefore, only AC signals larger than the bandwidth of the optical bias control circuit 211 are supplied to the output optical signal 242 of the MZM234, and as a result, are routed to the control box 220 and the summing circuit 216.

[0029] DC signals and signals having frequencies within the bandwidth of the optical bias control circuit 211 are provided to the adder 216 by the first output signal 212 and DAC 218. In the adder 216, the DC signal from DAC 218 is added to the AC signal from a third O / E converter 226 that receives the output optical signal 242. The addition can be weighted to compensate for different gains or losses in the AC path leading to the output of the third O / E converter 226 and the DC path leading to the output of DAC 218. The added AC and DC signals reconstruct the first input 233 (RF input) to the MZM 234, but with a sufficiently low voltage that can be safely routed to the oscilloscope 140.

[0030] A small portion (exemplarily about 1% to about 5%) of the output optical signal power from MZM234 is provided to the second O / E converter 222. Similar to the first O / E converter 208, the second O / E converter 222 exemplarily includes a photodetector (not shown in FIG. 2) connected to a TIA (not shown in FIG. 2) and can be partially or fully incorporated within the optical bias control circuit 211. The second O / E converter 222 provides an electrical signal 223 proportional to the output power (P out ) from MZM234 to the optical bias control circuit 211. Thus, the electrical signal 223 serves as a second input to the optical bias control circuit. Thus, according to an exemplary embodiment, the electrical signal 209 proportional to the input power (P in ) to MZM234 serves as a first input to the optical bias control circuit 211, and the electrical signal 223 proportional to the output power (P out ) from MZM234 serves as a second input to the optical bias control circuit 211.

[0031] As described more fully below, according to an exemplary embodiment, the ratio (P out / P in ) of the output power of the output optical signal 242 from MZM234 to the input power of the input optical signal 241 provided to MZM234 is maintained at 0.5 after considering the losses in MZM234, the first optical tap 206 and the second optical tap 224, and the optical fiber and interconnect. This operates MZM234 at the quadrature point, as described more fully below in connection with FIGS. 3A and 3B. Thus, during operation, MZM234 is biased by the optical bias control circuit to operate at its quadrature point. When the switch 232 engages, depending on its magnitude, the DC component of the electrical signal from the DUT input to the RF port of MZM234 can change the bias of MZM234 and shift MZM234 from its quadrature point. This, in turn, changes the P out / P inThis will change the ratio from 0.5 (or 50% transmission after considering losses). As will be described more fully below in relation to Figure 4, the optical bias control circuit 211 changes the bias signal 240 provided to the second input 236 to the MZM234 to a lower or higher value, thereby changing the P of the MZM234. out / P in It is adapted to return it to 0.5. Therefore, the bias shift of the MZM234 caused by the DC component of the electrical signal from the DUT changes (increases or decreases) the bias of the MZM234, and P is adjusted back from the desired 0.5. out / P in This results in a change in the ratio of MZM234's P out / P in To return the ratio to 0.5, the bias signal 240 and the second input 236 are modified by the optical bias control circuit 211. This modification compensates for the shift of the MZM234 from the desired quadrature bias caused by the DC component of the first input 233. Thus, the second input 236 is proportional in magnitude to the DC contained in the first input 233, but in some cases with the opposite sign. This is the P of the MZM234. out / P in The ratio is returned to 0.5. As mentioned above, and as will be described more fully below, the first output signal 212 from the optical bias control circuit 211 is proportional to the bias applied to the MZM234, which may have the opposite sign. Thus, the first output signal 212 provided to the oscilloscope represents the DC component of the input signal from the DUT.

[0032] To put it another way, the bias control of the MZM234 operates in a closed control loop. Therefore, the application of a DC electrical signal from the DUT to the RF port of the MZM234 at the first input 233 can move the MZM234 away from its orthogonal point. This movement away from the orthogonal point causes a change in the value of the output optical signal 242 from the MZM234 and P from 0.5. out / P in This causes a corresponding change in the ratio of P. out / P inTo restore the ratio to 0.5, a second input 236, whose magnitude is proportional to the DC electrical signal for the first input 233 from the DUT but whose sign is opposite, must be applied to the bias input of the MZM234 to offset the shift from the orthogonal point. The optical bias control circuit 211 includes DC information for the first input 233 so that the bias signal 240 and the resulting bias signal for the second input 236 restore the proper orthogonal bias. Thus, the first output signal 212 from the optical bias control circuit 211 and the resulting electrical signal from the DAC218 provided to the oscilloscope 140 are proportional to the DC electrical signal from the DUT. Conveniently, this teaching allows the DC voltage from the relatively high voltage signal from the DUT to be measured by the oscilloscope 140.

[0033] Figure 3A is a top view of the MZM300 according to a typical embodiment. As described above, the MZM300 can be mounted within the probe heads 110 and 210 and functions as an E / O converter for the probe heads 110 and 210. Certain details and aspects of the MZM300 are common to, for example, the MZM234 described above and may not be repeated in order to avoid obscuring the description of this typical embodiment.

[0034] The MZM300 is an electro-optic modulator, such as a known lithium niobate (LiNbO3) modulator. The MZM300 has an optical input 301 and an optical output 302. The RF input 303 is connected to an RF electrode 304, which is coupled to the lithium niobate material of the MZM300. Similarly, the bias input 305 is connected to a bias electrode 306, which is coupled to the lithium niobate material of the MZM300. As is known, lithium niobate is an electro-optic material. Therefore, the application of an RF signal (including a DC signal as per this teaching) and a bias signal results in a refractive index (n r This results in a change. As recognized, the application of voltages to RF input 303 and bias input 305 causes the MZM300 to function as an intensity modulator.

[0035] However, in addition to being an electro-optical material, lithium niobate is also a piezoelectric and pyroelectric material. Therefore, mechanical forces, acoustic waves, and temperature changes induce voltages within the lithium niobate material. Furthermore, a dielectric material (e.g., SiO2) is often placed between the RF electrode 304 and the lithium niobate, and between the bias electrode 306 and the lithium niobate. This dielectric material can act like a parasitic capacitor of the dielectric, causing charge accumulation and the resulting electrostatic field. Regardless of their source, these voltages induced by external factors in the MZM300 can affect the refractive index (n) of lithium niobate. r Undesirable changes in ) ultimately lead to undesirable modulation sources by the MZM300. Ultimately, these various factors can cause undesirable drift at the bias point of the MZM300. The two arms that make up the Mach-Zehnder interferometer amplify the effects described.

[0036] Figure 3B shows the transfer function 310 (sometimes called the transmission curve) of the transmission versus applied voltage of an MZM according to a typical embodiment. As will be recognized by those skilled in the art, transfer functions are often used to describe the function of an interferometer.

[0037] As described above, according to a typical embodiment, the MZM234 operates at its MZM orthogonal bias point 311, where the applied voltage results in 50% transmission. As recognized, when the MZM234 operates at the MZM orthogonal bias point 311, P out / P inThis is equal to 0.5 after considering losses in the MZM, coupler, and other components. However, the application of a DC signal from the DUT as the first input 233 at the RF input of the MZM234 can result in a shift in the bias point of the MZM on the transfer function 310. For example, based on the DC signal from the DUT, the bias point can be shifted to point 312 or point 314 on the transfer function 310. The shift can be compensated by applying a voltage to the bias input 305 of the bias electrode 306. The RF electrode 304 and the bias electrode 306 are typically subjected to different applied voltages (V π Because of this, the voltage applied to the bias electrode 306 that compensates for the shift is proportional to, rather than equal to, the DC value contained in the first input 233 to the MZM234. However, after calibration, the change in bias voltage is a measure of the DC signal from the DUT, excluding bias changes due to external factors for clarity at this point.

[0038] When the bias point increases or decreases from the MZM orthogonal bias point 311, the transmission of MZM234 changes accordingly. This change in transmission is greater than or less than 0.5 P after accounting for losses. out / P in This is particularly noticeable in the changes. Therefore, and in accordance with the typical embodiment described above, the second input 236 from the optical bias control circuit 211 changes in proportion to the DC signal from the DUT provided to the first input 233 of the MZM234. Thus, the first output signal 212 from the optical bias control circuit 211 is proportional to the bias input signal at the second input 236 to the MZM234, except for the offset estimated using switch 232 as described below (and above).

[0039] As mentioned above, the bias point drift of MZM234,300 may be induced by external factors such as temperature and mechanical force. As mentioned above, the switch 232 provided on the probe head 210 allows the first output signal 212 to be reset when the DUT test is not in progress. Therefore, engaging and disengaging the switch 232 prevents electrical signals from the DUT from being applied to MZM234. In this state, as usual, P out / P in It is maintained at 0.5, and the first output signal 212 is set to "zero" because there is no DC in the RF port of the MZM234. After switch 232 is re-engaged, the DC component of the first input 233 can be accurately estimated even if the bias point has drifted from the MZM orthogonal bias point 311 due to external factors. In all cases, the bias signal 240, and therefore the second input 236 provided to the bias input of the MZM234, after considering losses, gives the power ratio P out / P in Maintaining this value at the desired 0.5, the MZM234 operates at the MZM orthogonal bias point 311.

[0040] Resetting the first output signal 212 to zero is recommended before each calibrated measurement. One method used to determine when a reset is needed, though not exhaustive, is to characterize the drift of the MZM234 over time and temperature. These results can then be used to estimate the time before a reset. To put it another way, by determining how long it takes for the MZM234 to drift outside a specified range, the reset can be set to occur before that determined duration has elapsed.

[0041] Figure 4A is a simplified schematic block diagram of an optical bias control circuit 411 connected to a Mach-Zehnder modulator (MZM) 434 according to a typical embodiment. Many details and aspects of the various components of the typical embodiment described in relation to Figures 1 to 3B are common to those of the optical bias control circuit 411 of the typical embodiment described here, and can be omitted to avoid obscuring the description of the typical embodiment described here.

[0042] Referring to Figure 4A, the output of the laser (not shown in Figure 4) is supplied to the first optical tap 406. The first optical tap 406 receives a significant portion of the signal output power from the laser as input power (P in ) is provided to the MZM434 which is disposed inside the probe head (not shown in Figure 4). in A small portion of this (exemplarily about 1% to 5%) is supplied to the first photodetector 430, which in turn supplies its output to the first TIA 440. Here again, P in If is substantially constant, the first optical tap 406 is optional, P in A constant input representing this can be provided to the optical bias control circuit 411.

[0043] As suggested above, the first photodetector 430 and the first TIA 440 can provide one of the O / E converters used in connection with this teaching (e.g., the first O / E converter 208 shown in Figure 2).

[0044] The output of the first TIA440 is digitally converted in the first ADC441 and provided to a processor 446 or a controller 444 which may include a microprocessor and memory 447. As stated, the controller 444 is tangible and non-temporary and comprises a processor 446 representing one or more processors. As used herein, the term “non-temporary” shall be interpreted not as a permanent characteristic of a state, but as a characteristic of a state that persists over a period of time. The processor 446 may also be (including) a microprocessor, microcomputer, processor chip, controller, microcontroller, digital signal processor (DSP), state machine, or programmable logic device. The processor 446 may also be (including) a programmable logic device (PDL) including a programmable gate array (PGA), such as a field-programmable gate array (FPGA), or another type of circuit including discrete gates and / or transistor logic. The processor 446 may be a central processing unit (CPU), a graphics processing unit (GPU), or both. Furthermore, the processor 446 may include multiple processors, parallel processors, or both. Multiple processors may be included in or combined within a single device or multiple devices. The term “non-transient” specifically negates transient characteristics such as carrier waves or signals, or other forms of characteristics that exist only temporarily at any given time and place. As will be described more fully below in relation to Figure 4B, the processor 446 for the controller 444 is configured to execute software instructions stored in memory 447 to perform the functions described in the various embodiments herein. The processor 446 may be a general-purpose processor or may be part of an application-specific integrated circuit (ASIC).

[0045] Memory 447 is a tangible, non-transient, computer-readable medium for storing instructions, which, when executed by processor 446, cause processor 446 to determine a specific bias level for MZM 434 to maintain its operation at orthogonal points, as discussed above. As used herein, the term “non-transient” is to be interpreted not as a permanent characteristic of a state, but as a characteristic of a state that persists over a period of time. The term “non-transient” specifically negates transient characteristics such as carrier waves or signals, or other forms of characteristics that exist only temporarily at any given time and place. The memories described herein are manufactured products and / or mechanical components. The memories described herein are computer-readable medium from which data and executable instructions can be read by a computer. The memories described herein may be random access memory (RAM), read-only memory (ROM), flash memory, electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), registers, hard disks, removable disks, tapes, compact disk read-only memory (CD-ROM), digital multipurpose disks (DVDs), floppy disks, Blu-ray disks, or any other form of storage medium known in the art. The memories may be volatile or non-volatile, secure and / or encrypted, or non-secure and / or unencrypted.

[0046] Memory 447 may include main memory or static memory, or both, and the memories may communicate with each other via a bus (not shown). Memory 447 as described herein is a tangible storage medium that can store data and executable instructions, and is non-temporary for the duration that instructions are stored within it. Among other instructions, memory, when executed by the processor, (P out / P inThe instructions are stored to cause the processor to determine the bias input from the optical bias control circuit 411 to the MZM434 in order to keep the MZM434 at the orthogonal point, such that the ratio is maintained at 0.5.

[0047] P out A small portion of this (exemplifiedly about 1% to 5%) is supplied to the second photodetector 450, which supplies its output to the second TIA 452. As suggested above, the second photodetector 450 and the second TIA 452 can provide one of the O / E converters used in connection with this teaching (e.g., the first O / E converter 208 shown in Figure 2). The output of the second TIA 452 is digitally converted in the second ADC 454 and supplied to the controller 444.

[0048] The controller 444 determines and outputs a bias signal 470, which is provided as the DC bias input 482 of the MZM434. Thus, as with the bias signal 240 described in relation to the typical embodiment in Figure 2, the bias signal 470 from the optical bias control circuit 411 provides a second input signal to the MZM434 and serves as a bias input to the MZM434.

[0049] Beneficially, as will be described more fully below with respect to Figure 4B, the optical bias control circuit 411 is the P of the MZM434. out / P in The bias signal 470 is adapted to change to a lower or higher DC value to return it to 0.5. Thus, bias drift of the MZM434 caused by the DC component of the electrical signal from the DUT is compensated by the bias signal 470 applied to the DC bias input 482. out / P inTo return the ratio to 0.5, the bias signal 470 is modified by the optical bias control circuit 411 by an amount proportional to the DC voltage contained in the first input 233 in Figure 2. As described above with reference to Figure 2, the first output signal 212 from the optical bias control circuit 211 is proportional to the bias applied to the MZM234. Thus, the first output signal 212 provided to the oscilloscope 140 represents the DC component of the input signal from the DUT.

[0050] The output optical signal 442 is supplied from the MZM434. The output optical signal 442 is incident on the second optical tap 424. The second optical tap 424 receives a significant portion of the signal output power from the laser from the output power (P) from the MZM434. out ) is provided to the O / E converter (not shown in Figure 4) as such. Exemplarily, this output of the second optical tap 424 is the third O / E converter 226 described in relation to the typical embodiment in Figure 2. As described above, the output optical signal 442 passes only AC signals with frequencies greater than the frequency response of the optical bias control circuit 411 to the O / E converter and finally to the oscilloscope. Again, this is expected because the optical bias control circuit 411 cannot observe signals outside its bandwidth. In contrast, frequencies within the bandwidth of the optical bias control circuit 411 are detected and a correction to the DC bias input 482 is applied through the DAC 472.

[0051] Figure 4B is a flowchart of process 460 for setting the bias input to the MZM from the optical bias control circuit in order to maintain the MZM at an orthogonal point, according to a typical embodiment. As mentioned above, process 460 is provided as instructions stored in memory. When these instructions are executed by processor 446, (P out / P in To maintain the MZM434 at an orthogonal point such that ) is maintained at 0.5, the processor 446 is caused to iteratively determine the bias input to the MZM434 from the optical bias control circuit 411 with respect to the DC bias input 482.

[0052] Many details and aspects of the various components of the representative embodiments described in relation to Figures 1 to 4A are common to those of the process 460 of the representative embodiment described here, and can be omitted to avoid obscuring the description of the representative embodiment. Furthermore, although the process 460 is mainly described in relation to maintaining the MZM434 at a desired orthogonal point, various aspects and details of the process 460 are applicable to the probe head 210 and control box 220 described according to the representative embodiment of Figure 2, and to the probe head and control box according to the representative embodiment of Figure 5.

[0053] In 461, controller 444 receives P from ADC441 and ADC462, respectively. out and P in The input signal representing this is measured.

[0054] Further loss corrections are applied in 462. These corrections may include considering the ratio of the first optical tap 406 to the second optical tap 424, the loss in MZM434, the O / E conversion coefficients of the first photodetector 430 to the second photodetector 450, and the gains of the first TIA440 to the second TIA452.

[0055] At 463, P out / P in The ratio of P is determined. out / P in Once the ratio is known, the specific choice of how to proceed depends on the slope of the MZM orthogonal bias point 311 and the specific gain of the signal path from the first and second photodetectors 430 and 450 to the controller 444. In the following exemplary example, it is assumed that the slope is as shown for the MZM orthogonal bias point 311 and that the signal path gain is non-inverting.

[0056] If the ratio is determined to be less than 0.5, then in 464, the bias input signal from DAC472 to the DC bias input 482 of MZM434 will have an increment V equal to an integer multiple of the minimum resolution of DAC472.step It is increased by only that much. Using the minimum resolution of the DAC472 provides sufficiently precise control of the vial voltage relative to the DC bias input 482, but larger integer multiples can provide a faster response.

[0057] Next, the increased bias voltage is supplied to the low-pass filter 469 to limit the maximum frequency response of the optical bias control circuit described above and below, and at 480, the output of the DAC 472 is updated with the new bias voltage applied to the DC bias input 482 of the MZM 434.

[0058] At 481, the bias voltage from DAC472 applied to the DC bias input 482 of MZM434 at 480 is maintained for a duration of ΔT. Here, 1 / ΔT is equal to the system sampling rate, which is the time interval between each execution of 461. By the known Nyquist-Shannon sampling theorem, the sampling rate is at least twice the desired frequency response of the optical bias control circuit 411. The delay ΔT, along with the cumulative delay of all preceding steps, defines the bandwidth of the controller.

[0059] Due to the delay ΔT at 481, process 460 repeats the start at 461.

[0060] Alternatively, P at 463 out / P in If the ratio is determined to be greater than 0.5, then at 465, the bias input signal for the DC bias input 482 of the MZM434 is equal to an integer multiple of the minimum resolution of the DAC472. step It will decrease by only that much.

[0061] Next, the reduced bias voltage is supplied to the low-pass filter 469 to limit the maximum frequency response of the optical bias control circuit described above, and at 480, the bias voltage is updated to a new bias voltage applied from DAC 472 to the DC bias input 482 of MZM 434.

[0062] At 481, the bias voltage applied to the DC bias input 482 of the MZM434 is maintained for a duration of ΔT.

[0063] Due to the delay ΔT at 481, process 460 repeats the start at 461.

[0064] Finally, at 463 P out / P in If the ratio is determined to be 0.5 in or within a small percentage (relative to the least significant bit of DAC472), then in 466, the bias input signal from DAC472 to the DC bias input 482 of MZM434 is maintained at its current level.

[0065] Figure 5 is a simplified schematic block diagram of a probe head 519 and control box 520 according to another typical embodiment.

[0066] The probe head 510 and control box 520 are adapted to function within the system 100 to enable measurement of both high-voltage AC signals and high-voltage DC signals from the DUT (not shown in Figure 5). Therefore, various details and aspects of the description of the system 100, probe head 210, control box 220, optical bias control circuit 411, and process 460 described above in relation to the typical embodiments in Figures 1 to 4B may be common with the description of the probe head 510 and control box 520, but may be avoided in order to avoid obscuring the description of the typical embodiment in Figure 5.

[0067] Referring to Figure 5, the control box 520 includes a current source 502 that provides input to the laser 504. In particular, since a portion of the output laser power is input to the current source via a feedback loop, the power of the signal output from the laser 504 remains substantially constant for reasons that will become apparent as this description continues.

[0068] The output from the laser 504 is supplied to a first optical tap 506 located within the probe head 510. The first optical tap 506 supplies a significant portion of the signal output power from the laser to the MZM 534, as described below. A small portion of the power output from the laser 504 (exemplifiedly about 1% to about 5%) is supplied to a first O / E converter 508 located within the probe head 510. This small portion of the optical power from the first optical tap 506 supplies the input power (P) to the MZM 534 within the probe head 510. in As will become apparent as this explanation continues, the MZM534 receives an input electrical signal from the DUT and outputs an optical signal to the control box 520. Thus, the MZM534 functions as an E / O converter, and therefore electrically isolates the oscilloscope from the relatively high-voltage AC and DC signals from the DUT.

[0069] The first O / E converter 508 exemplifies a photodetector (not shown in Figure 5) connected to a transimpedance amplifier (TIA) (not shown in Figure 5), which can be partially or completely integrated into the optical bias control circuit 511 as discussed above. The first O / E converter 508 receives P from the laser 504. in An electrical signal 509 proportional to this is provided as a first input to the optical bias control circuit 511. As described above, the electrical signal 509 from the laser 504 is provided as one input to the optical bias control circuit 511 to assist in biasing the MZM 534.

[0070] During operation, the input optical signal 541 is supplied to the MZM534, and the output optical signal 542 is supplied from the MZM534. The output optical signal 542 is input to the second optical tap 524. The output optical signal 542 to the third O / E converter 526 of the control box 520 includes an AC signal having a frequency greater than the frequency response of the optical bias control circuit 511. A small portion of the output optical signal power (exemplarily about 1% to about 5%) is supplied to the second O / E converter 522. Like the first O / E converter 508, the second O / E converter 522 exemplary comprises a photodetector (not shown in Figure 5) connected to a TIA (not shown in Figure 5) and can be partially or completely integrated into the optical bias control circuit 511. The second O / E converter 522 receives the output power (P) from the MZM534. out An electrical signal 523 is provided that is proportional to the input power (P) to the MZM534. The electrical signal 523 serves as a second input to the optical bias control circuit 511. Therefore, according to a typical embodiment, the input power (P) to the MZM534 is provided. in The electrical signal 509, which is proportional to the output power (P) from the MZM534, serves as the first input to the optical bias control circuit 511. out The electrical signal 523, which is proportional to (P), serves as a second input to the optical bias control circuit 511. According to a typical embodiment, as will be described in more detail below, the ratio of the output power of the output optical signal 542 from the MZM534 to the input power of the input optical signal 541 provided to the MZM534 (P out / P in This value is maintained at 0.5 after considering the losses in the MZM534, the first optical tap 506 and the second optical tap 524, as well as the optical fiber and interconnect. This causes the MZM234 to operate at the orthogonal point, as described above in relation to Figures 3A and 3B.

[0071] In contrast, as described more fully above, the optical bias control circuit 511 does not respond to time-varying (AC) signals with frequencies greater than its frequency response. Similarly, the optical bias control circuit 511 cannot respond to frequencies beyond its bandwidth. It responds to frequencies within its bandwidth. Therefore, the bias controller incorporates information about the MZM534 bias drift and compensates for the bias drift by applying a bias signal to the second input 536 to the MZM534. The crossover filter 540 separates the frequencies from the DUT into low-band (DC) and high-band (AC) frequencies, and since the crossover frequency is somewhere between 1Hz and 10kHz, only the high-band frequencies are applied to the MZM534. Only these high-band frequencies are modulated on the output optical signal 542 of the MZM534 output and consequently routed to the control box 520 and the summing circuit 516.

[0072] According to the typical embodiment described above, the probe head 510 receives an input signal from the DUT via a connection part 530 which can be equipped with a sensor tip 112 and a probe 114 as shown in Figure 1. The input signal includes AC and DC components, and the DC component includes either a voltage signal without a time-varying component, or a voltage signal with a time-varying component having a frequency smaller than the crossover frequency of the crossover filter 540, as described above. This will be described in more detail below.

[0073] As described above, the input signal from the DUT is split into a low-frequency band and a high-frequency band by the crossover filter 540. The low-frequency band includes the DC signal as taught in this instruction, and the crossover filter has a crossover frequency of 1 Hz to 10 kHz. The remainder of the input signal from the DUT includes the signal in the high-frequency band and is transmitted through the first input 533, which is the RF input to the MZM534.

[0074] In contrast, the low-frequency band containing the DC signal defined above is provided to the ADC 546 as an analog signal as shown in the figure by the crossover filter 540. The DC signal included in the low-frequency band is then provided as a digital optical signal to the DAC 518 and summer circuit 516 of the control box 520. Thus, the ADC 546 and DAC 518 are part of the low-frequency optical link, and possibly part of the digital optical link. The output from the summer circuit 516 is provided to an oscilloscope (not shown in Figure 5) via a suitable electrical connector 528.

[0075] Since the weighted sum of the low-frequency and high-frequency bands determined in the summer circuit 516 must yield a scaled version of the original signal from the DUT, the transfer function describing the crossover filter must ultimately be 1. To put it another way, in order to ensure proper signal reconstruction, there must be absolutely no loss of information or frequency contained in the original signal from the DUT.

[0076] As mentioned above, the low-frequency band is transmitted through a low-frequency optical link including the ADC546 and DAC518. This link implementation is free from DC drift because the link does not include the MZM534.

[0077] While the crossover filter 540 ensures that DC is never present at the input of the MZM 534, bias point drift can still occur due to environmental influences, such as temperature fluctuations. However, the optical bias control circuit 511 keeps the MZM in an orthogonal state at all times, thus ensuring proper transmission in the high-frequency band.

[0078] Since the low-frequency and high-frequency bands are transmitted through two separate optical links, which may have different gains or losses, the summing circuit 516 may require appropriate weighting of each signal so that when each signal is added together, the final transfer function equals 1. This may involve DC subtraction and addition. Determining the weighting described above is part of instrument calibration.

[0079] A person skilled in the art will be able to understand and implement other variations of the disclosed embodiments by examining the drawings, disclosures, and appended claims when carrying out the claimed invention. In the claims, the word "including" does not exclude other elements. If certain means are listed in multiple distinct dependent claims, it is not shown that those means cannot be combined and used advantageously.

[0080] While representative embodiments are disclosed herein, those skilled in the art will see that many variations are possible according to this teaching and fall within the scope of the appended claims. Therefore, the present invention is not limited to the scope of the appended claims. The claims as originally filed are as follows: Claim 1: A device that provides a test signal from a device under test (DUT) to a measuring instrument, A probe head (110) is configured to receive an electrical signal (209) from the DUT and is equipped with an electro-optic modulator. Control box (120) and Equipped with, The control box (120) comprises a light source configured to provide an input optical signal (241) to an electro-optic modulator configured to provide an output optical signal (242) based on the electrical signal (209) from the DUT, and an optical bias control circuit (211), wherein only the bias control signal is provided to the electro-optic modulator. Claim 2: The apparatus according to claim 1, wherein the optical bias control circuit (211) receives a portion of the output optical signal (242) from the electro-optic modulator and maintains the bias of the electro-optic modulator at a substantially orthogonal point (312). Claim 3: The apparatus according to claim 2, wherein the output optical signal (242) from the electro-optic modulator supplied to the optical bias control circuit (211) is first converted to the electrical signal (209). Claim 4: The apparatus according to claim 1, wherein the electro-optic modulator includes a Mach-Zehnder modulator (MZM(234)). Claim 5: The apparatus according to claim 1, wherein the input optical signal (241) has an input power level, the output optical signal (242) has an output power level, and the ratio of the input power level to the output power level is maintained at a substantially constant value. Claim 6: The apparatus according to claim 1, wherein the output optical signal (242) has an output power level, and the output power level is maintained at a substantially constant value. Claim 7: The apparatus according to claim 1, wherein the electrical signal (209) has an alternating current (AC) component and a direct current (DC) component. Claim 8: The apparatus according to claim 7, wherein the frequency response of the optical bias control circuit (211) is substantially DC. Claim 9: The apparatus according to claim 8, wherein the DC component of the bias is coupled with the AC component received from the electro-optic modulator to reconstruct the original electrical signal (209). Claim 10: A device that provides a test signal from a device under test (DUT) to a measuring instrument, A probe head (110) is configured to receive an electrical signal (209) from the DUT and is equipped with an electro-optic modulator. Control box (120) and Equipped with, The control box (120) comprises a light source configured to provide an input optical signal (241) to an electro-optic modulator configured to provide an output optical signal (242) based on an electrical signal (209) from the DUT, wherein the electrical signal (209) has a direct current (DC) component and an alternating current (AC) component; and an optical bias control circuit (211) configured to provide a bias control signal to the electro-optic modulator, wherein the bias control signal to the electro-optic modulator is proportional to the magnitude of the DC component. Claim 11: The apparatus according to claim 10, wherein the bias control signal for the electro-optic modulator includes the DC component of the electrical signal (209), and the signal received from the electro-optic modulator includes the AC component of the electrical signal (209). Claim 12: The apparatus according to claim 10, wherein the optical bias control circuit (211) receives a portion of the output optical signal (242) from the electro-optic modulator and maintains the bias of the electro-optic modulator at a substantially orthogonal point (312). Claim 13: The apparatus according to claim 12, wherein the output optical signal (242) from the electro-optic modulator supplied to the optical bias control circuit (211) is first converted to the electrical signal (209). Claim 14: The apparatus according to claim 10, wherein the electro-optic modulator includes a Mach-Zehnder modulator (MZM(234)). Claim 15: The apparatus according to claim 10, wherein the input optical signal (241) has an input power level, the output optical signal (242) has an output power level, and the ratio of the input power level to the output power level is maintained at a substantially constant value. Claim 16: A device that provides a test signal from a device under test (DUT) to a measuring instrument, A probe head (110) configured to receive an electrical signal (209) from the DUT, comprising an electro-optic modulator and a crossover filter (540) having a low-frequency output and a high-frequency output, the high-frequency output being supplied to the electro-optic modulator, A control box (120) equipped with a light source and Equipped with, The apparatus is configured such that the light source provides an input optical signal (241) to an electro-optic modulator configured to provide an output optical signal (242) based on the electrical signal (209) from the DUT, and the control box (120) is configured to receive a direct current (DC) component from the crossover filter (540) and an alternating current (AC) component from the electro-optic modulator. Claim 17: The apparatus according to claim 16, wherein the control box (120) further comprises an optical bias control circuit (211). Claim 18: The apparatus according to claim 17, wherein the optical bias control circuit (211) receives a portion of the output optical signal (242) from the electro-optic modulator and maintains the bias of the electro-optic modulator at a substantially orthogonal point (312). Claim 19: The apparatus according to claim 16, wherein the electro-optic modulator includes a Mach-Zehnder modulator (MZM(234)). Claim 20: The apparatus according to claim 16, wherein the input optical signal (241) has an input power level, the output optical signal (242) has an output power level, and the ratio of the input power level to the output power level is maintained at a substantially constant value. Claim 21: The apparatus according to claim 16, wherein the electrical signal (209) has an AC component and a DC component, and the probe head (110) further comprises a circuit adapted to prevent the DC component from being input to the electro-optic modulator and to provide the DC component to the control box (120). Claim 22: The apparatus according to claim 16, wherein the DC component received from the low-frequency output of the crossover filter (540) is coupled with the AC component received from the electro-optic modulator to reconstruct the electrical signal (209) from the DUT.

Claims

1. A device that provides a test signal from a device under test (DUT) to a measuring instrument, A probe head (110) configured to receive an electrical signal from the DUT and equipped with an electro-optic modulator, the probe head (110) provides the electrical signal from the DUT to the electro-optic modulator via a switch (232), Control box (120) and Equipped with, The control box (120) controls the output power level (P) based on the electrical signal from the DUT. ＯＵＴ The electro-optic modulator is configured to provide an output optical signal (242) having an input power level (P ＩＮ The system comprises a light source configured to provide an input optical signal (241) having ), and an optical bias control circuit (211) configured to provide a bias control signal to the electro-optic modulator, The aforementioned input power level (P ＩＮ ) and the output power level (P ＯＵＴ ) ratio (P ＩＮ / P ＯＵＴ The device wherein the value is maintained at a substantially constant value and is reset when the optical bias control circuit opens the switch (232).

2. The apparatus according to claim 1, wherein the optical bias control circuit (211) receives a portion of the output optical signal (242) from the electro-optic modulator and maintains the bias of the electro-optic modulator at a substantially orthogonal point (312).

3. The apparatus according to claim 2, wherein the output optical signal (242) from the electro-optic modulator, which is supplied to the optical bias control circuit (211), is converted into a second electrical signal (223) and then supplied to the optical bias control circuit (211).

4. The apparatus according to any one of claims 1 to 3, wherein the electro-optic modulator includes a Mach-Zehnder modulator (MZM(234)).

5. The aforementioned input power level (P ＩＮ ) and the output power level (P ＯＵＴ The apparatus according to any one of claims 1 to 4, wherein the ratio to ) is substantially equal to 0.

5.

6. The apparatus according to any one of claims 1 to 5, wherein the output optical signal (242) has an output power level, and the output power level is maintained at a substantially constant value.

7. The apparatus according to any one of claims 1 to 6, wherein the electrical signal from the DUT has an alternating current (AC) component and a direct current (DC) component.

8. The apparatus according to claim 7, wherein the frequency response of the optical bias control circuit (211) is substantially DC.

9. The apparatus according to claim 8, wherein the DC component provided as a second bias control signal from the optical bias control circuit is coupled with the AC component received from the electro-optic modulator to reconstruct the original electrical signal from the DUT.

10. A device that provides a test signal from a device under test (DUT) to a measuring instrument, A probe head (110) configured to receive an electrical signal from the DUT and equipped with an electro-optic modulator, the probe head (110) provides the electrical signal from the DUT to the electro-optic modulator via a switch (232), Control box (120) and Equipped with, The control box (120) is configured to provide an input optical signal (241) having an input power level (P ＯＵＴ ), to an electro-optic modulator configured to provide an output optical signal (242) having an output power level (P ＩＮ ), wherein the electrical signal from the DUT has a direct current (DC) component and an alternating current (AC) component, and a optical bias control circuit (211) configured to provide a bias control signal to the electro-optic modulator, wherein the bias control signal for the electro-optic modulator is proportional to the magnitude of the DC component, and a ratio (P ＩＮ / P ＯＵＴ ) between the input power level (P ＩＮ ) and the output power level (P ＯＵＴ ) is maintained at a substantially constant value, and the optical bias control circuit (211) is reset by the optical bias control circuit (211) opening the switch (232).

11. The apparatus according to claim 10, wherein the bias control signal for the electro-optic modulator includes the DC component of the electrical signal from the DUT, and the signal received from the electro-optic modulator includes the AC component of the electrical signal from the DUT.

12. The apparatus according to claim 10 or 11, wherein the optical bias control circuit (211) receives a portion of the output optical signal (242) from the electro-optic modulator and maintains the bias of the electro-optic modulator at a substantially orthogonal point (312).

13. The apparatus according to claim 12, wherein the output optical signal (242) from the electro-optic modulator, which is supplied to the optical bias control circuit (211), is converted into a second electrical signal (223) and then supplied to the optical bias control circuit (211).

14. The apparatus according to any one of claims 10 to 13, wherein the electro-optic modulator includes a Mach-Zehnder modulator (MZM(234)).

15. The aforementioned input power level (P ＩＮ ) and the output power level (P ＯＵＴ ) ratio (P ＩＮ / P ＯＵＴ The apparatus according to any one of claims 10 to 14, wherein ) is substantially equal to 0.

5.

16. The apparatus according to any one of claims 10 to 15, wherein the electrical signal from the DUT has alternating current (AC) and direct current (DC) components, the frequency response of the optical bias control circuit (211) is substantially DC, and the DC component provided from the optical bias control circuit as a second bias control signal is coupled with the AC component received from the electro-optic modulator to reconstruct the original electrical signal from the DUT.

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