Test and measurement instrument and method of operation

Abstract

To effectively detect transition of a PAMn signal.SOLUTION: The test and measurement instrument 100 includes an input 102 to receive an n-ary pulse amplitude modulation (PAMn) signal, an analog-to-digital converter (ADC) 106 to digitize the PAMn signal, a memory 108 to store the digitized PAMn signal as a waveform, and a trigger circuit 110 coupled to the ADC and the memory. In addition, a clock and data recovery (CDR) circuit 112 decodes bits from the PAMn signal. The transition detection logic 114 causes the trigger circuit 110 to generate a trigger signal when it detects a transition of a particular symbol based on the decoded bits.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE This disclosure relates to test and measurement instruments and methods of operation, and more particularly to triggering techniques for test and measurement instruments such as oscilloscopes. [Background technology]

[0002] Pulse amplitude modulation (PAM) signaling is becoming more common. Multilevel PAM3, PAM4, PAM8, and PAM16 signaling are all part of major standards. Visualizing or measuring the signal integrity of these signals is much more complex than traditional non-return-to-zero (NRZ) signaling. Traditional oscilloscope triggering biases the waveform display to a subset of the possible transitions. PAM4 has 12 distinct transitions. PAM8 has 56 distinct transitions. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2017-512017 [Patent Document 2] Japanese Patent Publication No. 2023-033236 [Non-patent literature]

[0004] [Non-Patent Document 1] "What is an oscilloscope trigger? Mechanism, levels, and how to use it," Tektronix, [online], [Retrieved July 29, 2025], Internet <https: / / www.tek.com / ja / documents / primer / triggering-fundamentals-pinpoint-triggering-and-event-search-mark-dpo7000-0> Summary of the Invention [Problem to be solved by the invention]

[0005] Traditional oscilloscope trigger systems (analog and digital) can trigger on some of these transition types using runt triggering, but not on most, and triggering on one transition at a time does not provide a good visual representation of signal quality.

[0006] Figure 4 shows an example eye diagram display 400 of a PAM4 signal. The eye diagram shown in Figure 4 shows 12 different transitions between four different signal amplitude levels of the PAM4 signal and the corresponding four symbols (S0, S1, S2, S3). The example signal shown in Figure 4 uses gray coding, so symbol S0 (or 0) corresponds to bit 00, symbol S1 (or 1) corresponds to bit 01, symbol S2 (or 2) corresponds to bit 11, and symbol S3 (or 3) corresponds to bit 10.

[0007] As mentioned above, conventional oscilloscope trigger systems (analog and digital) can trigger on some, but not most, of the 12 different transitions between symbols using runt triggering. Runt triggering involves setting two amplitude thresholds with different amplitudes and triggering on pulses that enter or exit these thresholds (see Non-Patent Document 1). Furthermore, triggering on a single transition at a time does not provide an adequate visual representation of signal quality (signal integrity). For example, in the PAM4 signal example shown in Figure 4, if a user always triggers on the 0 to 1 transition, they may not notice a serious signal quality problem on the 3 to 1 transition.

[0008] Users often trigger on waveforms and enable persistence on the display to show signal noise and jitter. This persistence slowly decreases the waveform's intensity, making the more frequent parts of the waveform appear brighter and the less frequent parts appear darker, representing the waveform's frequency. However, this persistence display may not be able to adequately display problematic transitions using the current trigger. If gray scaling is applied to the persistence display, this further emphasizes the most frequently triggered transitions and hides problematic transitions in other transitions. Users may try to avoid this by taking longer waveform acquisitions, but this does not solve the problem because a relatively small number of transition-type waveforms still dominate over the others, and there is a lot of variation between one acquisition and another.

[0009] Modern oscilloscopes also offer high-speed serial pattern triggers, which can trigger on specific transitions or combinations of transitions. However, this does not solve the problem. The persistence effect tends to emphasize only the most frequent waveform patterns, especially as the n value in PAMn increases, and not display the complete picture of all transitions.

[0010] Embodiments of the presently disclosed technology address these and other shortcomings of conventional oscilloscope triggering techniques. [Means for solving the problem]

[0011] Embodiments of the disclosed technology provide test and measurement instruments with the ability to trigger based on the "density" of any particular symbol transition type, i.e., the frequency of occurrence of a particular symbol transition type or combination of particular symbol transition types, present in a PAMn input signal, greatly enhancing the usefulness of test and measurement instruments for analyzing and troubleshooting modern communications systems utilizing multi-level PAM standards such as PAM4, PAM8, and PAM16. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a functional block diagram of a test and measurement instrument according to an embodiment of the disclosed technique. [Figure 2] FIG. 2 is a functional block diagram of a test and measurement instrument according to an embodiment of the disclosed technique. [Figure 3] FIG. 3 is a functional block diagram of a test and measurement instrument in accordance with an embodiment of the disclosed technique. [Figure 4] Figure 4 shows an example of an eye diagram display for a PAM4 signal. DETAILED DESCRIPTION OF THE INVENTION

[0013] The first problem to solve is detecting the specific transition. This can be done using a traditional analog or digital trigger system that uses multiple thresholds / edge detectors and timers. Depending on the signaling baud rate relative to the oscilloscope's sample rate, a digital trigger may require interpolation of the data before the trigger system. If the system under test has significant channel loss, it may be necessary to equalize the signal before the trigger machine. While this is a possible implementation, it is very complex and therefore not practical.

[0014] Another method for detecting transitions is to use dedicated transceiver circuitry to receive and decode serial data in parallel with waveform data acquisition of the analog signal. This circuitry is currently commonly used in oscilloscopes to trigger on high-speed serial data, but is currently only used for NRZ signaling. To detect transitions, this circuitry must be extended to PAM signaling in an ASIC or FPGA. Custom circuitry after the transceiver would be used to detect specific transitions.

[0015] FIG. 1 illustrates a functional block diagram of a test and measurement instrument 100, such as an oscilloscope, in accordance with some embodiments of the present disclosure. The test and measurement instrument 100 includes an input 102 for receiving a PAMn signal, where n is 3 or greater, such as PAM3, PAM4, or PAM8. The received analog PAMn signal is transmitted from the input 102 to an analog front-end circuit, which performs signal conditioning on the received input signal. The front-end circuit may include a preamplifier 104. The preamplifier 104 amplifies or attenuates the input signal and outputs the signal to both an analog-to-digital converter (ADC) 106 and a PAMn clock and data recovery (CDR) circuit 112. The ADC 106 digitizes the input signal and passes the digitized signal to both a memory 108 and a trigger circuit 110. The memory 108 is configured to store at least a portion of the digitized PAMn signal as a waveform. In parallel, PAMn CDR circuitry 112 operates to decode bits from the analog PAMn signal according to known CDR techniques. In some embodiments, PAMn CDR circuitry 112 may be implemented, for example, in an FPGA or ASIC. The decoded bits are passed to transition detection logic 114. Transition detection logic 114 uses the decoded bits from CDR 112 and compares them to the decoded bits of the previous clock cycle to detect any possible PAM transitions. For example, in the example PAM4 signal described above and shown in FIG. 4, if CDR circuitry 112 decodes bits 01 for the current clock cycle and the decoded bits of the previous clock cycle were 10, transition detection logic 114 will detect a transition from symbol 3 to symbol 1, which can also be represented as 3->1.

[0016] The transition detection logic 114 communicates with the trigger circuit 110 and configures the trigger circuit 110 to generate a trigger signal in response to detecting a particular signal transition. This trigger signal triggers the test and measurement instrument to acquire the waveform when the particular transition is detected. The particular transition (e.g., from symbol 3 to symbol 1) is user-configurable. In some embodiments, the trigger circuit 110 is a digital trigger circuit, while in other embodiments, an analog trigger circuit may be used.

[0017] Another technique for detecting transitions, according to some embodiments of the present disclosure, is illustrated in the test and measurement instrument 200 shown in FIG. 2. The test and measurement instrument 200 uses clock and data recovery circuitry based on sampled data rather than an analog input signal. This approach is similar to that used in the test and measurement instrument 100 shown in FIG. 1, but does not rely on existing analog CDR technology implemented in an ASIC or FPGA. This digital CDR relies on sampled data and allows for triggering on newer standards that may not be supported by existing IP.

[0018] FIG. 2 shows a functional block diagram of a test and measurement instrument 200, such as an oscilloscope, according to some embodiments of the present disclosure. The test and measurement instrument 200 includes an input section 102 and a front-end circuit. The front-end circuit may include a preamplifier 104, an ADC 106, a memory 108, and a trigger circuit 110, which are substantially similar to the corresponding blocks in the test and measurement instrument 100 of FIG. 1. However, as shown in FIG. 2, in the test and measurement instrument 200, the digitized PAMn signal is output from the ADC 106 to the memory 108, the trigger circuit 110, and a digital PAMn CDR circuit 212. Therefore, in the test and measurement instrument 200, the CDR is based on sampled data rather than a copy of the analog signal sent to a separate receiver circuit. Optionally, the digitized PAMn signal may pass through an interpolation block 202 and a continuous-time linear equalizer (CTLE) block 204 prior to the digital CDR. The interpolation block 202 and CTLE 204 may be required depending on the rate and channel type of the input signal. Similar to the analog PAMn CDR 112 of Figure 1, the digital PAMn CDR 212 decodes bits from the digitized PAMn signal and passes the decoded bits to the transition detection logic 114. The transition detection logic 114 compares these with the decoded bits from the previous clock cycle and communicates with the trigger circuit 110 to cause the trigger circuit 110 to trigger the test and measurement instrument 200 on a particular symbol transition.

[0019] According to some embodiments of the present disclosure, once each distinct transition is detected, a randomized or round robin trigger can be used to display the persistence effect with an equal overlay of all transitions. This can also be done with gray scaling. This round robin or random transition selection function can be implemented in the trigger circuitry 110.

[0020] In analytical applications, ideally, the number of acquired waveforms would be equal for each transition type. However, for most patterns, perfectly equal distribution is not possible. One solution, according to some embodiments of the present disclosure, is to provide a counter for each transition type, and increment the count value of each of these counters by one each time the corresponding specific transition is detected, as shown in the example of FIG.

[0021] 3 is a functional block diagram of a test and measurement instrument 300 including counters for tracking the number of each detected transition type, in accordance with some embodiments of the present disclosure. As shown in FIG. 3, the test and measurement instrument 300 includes an input section 102 and front-end circuitry, including a preamplifier 104, an ADC 106, a memory 108, a trigger circuit 110, an analog PAMn CDR circuit 112, and a transition detection logic circuit 114, each of which is substantially similar to the identically numbered blocks in the test and measurement instrument 100 of FIG. 1. The test and measurement instrument 300 also includes a symbol transition counting circuit, which may include, for example, a transition memory controller 302, m transition counters 304a, 304b, ..., 304m, m transition count threshold blocks 306a, 306b, ..., 306m, a logic gate 310, and a peak tracking block 308.

[0022] The transition memory controller 302 is coupled to the transition detection logic 114. Each of the transition counters 304 is coupled to the transition memory controller 302 and the transition detection logic 114. Each of the transition count threshold blocks 306 is coupled to a corresponding one of the transition counters 304.

[0023] The total number m of transition counters 304 and transition count threshold blocks 306 is equal to the number of distinct symbol transitions in the PAMn signal. For example, for a PAM4 signal, m is equal to 12 because there are 12 distinct symbol transitions. For a PAM8 signal, m is equal to 56. Thus, each transition counter 304 and transition count threshold block 306 is associated with a corresponding m distinct symbol transition type. The symbol transition count circuit is configured to increment the count value of the associated transition counter by one each time a particular symbol transition is detected. This count value may be compared to a configurable threshold value in the transition count threshold block 306. The logic output of this comparison process is input to a logic gate 310. The logic gate 310 combines the outputs of all m transition count threshold blocks 306 and sends a signal to the trigger circuit 110 based on the combined output, for example, if the outputs of all m transition count threshold blocks 306 match.

[0024] The peak tracking block 308 is coupled to each of the transition counters 304a, 304b, ..., 304m and each of the count threshold blocks 306a, 306b, ..., 306m. The peak tracking block 308 is configured to track the maximum count (peak count) value of all m possible transitions and can be further configured to require that, for the transition with the maximum count value, the count values ​​of all other transitions meet a certain percentage of the maximum count value. So, for example, if the 0->2 transition is the most dominant in a PAMn signal and has 1000 edge occurrences, all other transitions must meet some user-configurable percentage of the 1000 edge occurrences. According to some embodiments, the peak tracking block 308 can automatically set the count thresholds of these other transitions to a specific percentage of the count value (i.e., peak count value) of the densest transition. The peak tracking block 308 can be useful for PAMn signals where it is not possible to determine in advance which transitions are most dense.

[0025] According to some embodiments, transition events may be stored in memory 108 adjacent to the sample acquired for the edge or the sample acquired before the edge. The transition event is stored only once, regardless of the number of samples at the edge. It is not critical that the transition event be stored using the exact sample; it is used only to track the number of transitions of each type. If the memory is a circular buffered memory, as waveform data is overwritten, each transition type in the overwritten sample decrements its associated counter by one. In this way, the symbol transition counting circuit reflects the number of transitions currently in memory. For higher-order PAM signaling, transition data can be encoded before being stored with the acquired samples, eliminating the need for PAM4 to store 12 bits in parallel with 8-bit or 16-bit data samples. This consumes an equivalent amount of memory bandwidth as the stored data can be encoded into 4 bits. This becomes even more important for PAM8 or PAM16.

[0026] If there are counters that reflect the number of transitions of each type, a counter-based trigger can be constructed. This trigger allows the user to configure the minimum number of each transition that must be present. For example, if the user specifies at least 3000 transitions for each transition type, a statistically significant jitter calculation can be made based on each transition type. The number of transitions required by the user will vary depending on the standard and what the user is trying to measure. Therefore, the number of transitions required is left to the user as a configurable setting.

[0027] Another possibility is that the user may want the distribution of different transition types to be roughly similar. This requires tracking which transition type has the most counts and the user to set a threshold for the counts of all other transition types as a percentage of the maximum count. This may be enabled by the peak tracking block 308.

[0028] 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.

[0029] 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.

[0030] "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.

[0031] 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, substrate integrated waveguide (SIW), glass / quartz substrate waveguide, or any other medium suitable for communicating electrical, optical, radio frequency (RF), infrared, acoustic, or other types of signals. Example

[0032] 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.

[0033] Example 1 is a test and measurement device, an input for receiving an n-ary pulse amplitude modulated (PAMn) signal, where n is 3 or greater; an analog-to-digital converter (ADC) coupled to the input for digitizing the PAMn signal; an analog-to-digital converter (ADC) coupled to the input for digitizing the PAMn signal; a memory coupled to the ADC and configured to store at least a portion of the digitized PAMn signal as a waveform; a trigger circuit coupled to the ADC and the memory and configured to generate a trigger signal to trigger a test and measurement instrument to acquire the waveform; a PAMn clock and data recovery (CDR) circuit configured to decode bits from the PAMn signal; transition detection logic coupled to the PAMn CDR circuit and the trigger circuit, the transition detection logic configured to detect symbol transitions based on the decoded bits and to cause the trigger circuit to generate the trigger signal in response to detecting a particular symbol transition; It is equipped with.

[0034] Example 2 is the test and measurement instrument of Example 1, further comprising an amplifier coupled to the input section for amplifying the PAMn signal, the PAMn CDR circuit being coupled to the amplifier and including an analog PAMn CDR circuit configured to decode bits from the amplified PAMn signal amplified from the amplifier.

[0035] Example 3 is the test and measurement instrument of Example 1, wherein the PAMn CDR circuit is coupled to the ADC, and the PAMn CDR circuit comprises a digital PAMn CDR circuit configured to decode bits from a digitized PAMn signal from the ADC.

[0036] Example 4 is the test and measurement instrument of Example 3, further comprising an interpolation block coupled between the ADC and the PAMn CDR circuit.

[0037] Example 5 is the test and measurement instrument of Example 3, further comprising a continuous time linear equalizer (CTLE) coupled between the ADC and the PAMn CDR circuit.

[0038] A sixth embodiment is the test and measurement instrument of the first embodiment, in which the PAMn CDR circuit is implemented in an FPGA or an ASIC.

[0039] Example 7 is the test and measurement instrument of Example 1, wherein the trigger circuit is further configured to generate a trigger signal for each of the m different symbol transitions, where m is equal to the number of different symbol transitions of the PAMn signal.

[0040] Example 8 is the test and measurement instrument of Example 7, wherein the trigger circuit is further configured to generate the trigger signal for each of the m different symbol transitions in a round-robin or randomized manner, wherein a number of the trigger signals generated is approximately equal to the number of the m different symbol transitions.

[0041] Example 9 is the test and measurement instrument of Example 1, further including a symbol transition counting circuit configured to track count values ​​that count detected symbol transitions, each of the count values ​​being associated with a corresponding one of m different symbol transitions of the PAMn signal.

[0042] A tenth embodiment is the test and measurement apparatus of the ninth embodiment, wherein the symbol transition counting circuit comprises: a transition memory controller coupled to the transition detection logic; m transition counters respectively coupled to the transition detection logic and the transition memory controller, each associated with a corresponding one of the m different symbol transitions of the PAMn signal; m transition count threshold blocks each coupled to a corresponding one of the m transition counters, each outputting a comparison result between the count value from each of the m transition counters and a count threshold; a logic gate configured to combine the outputs of the m transition count threshold blocks and to send a signal to the trigger circuit based on the combined output; It has.

[0043] Example 11 is the test and measurement instrument of Example 10, further comprising a peak tracking block configured to determine a peak count value among the count values ​​and to set the count threshold as a selectable percentage of the peak count value.

[0044] Example 12 is the test and measurement instrument of Example 9, wherein the symbol transition counting circuit is configured to increment the count value by one each time an associated symbol transition is detected.

[0045] Example 13 is the test and measurement instrument of Example 9, wherein the symbol transition counting circuit is configured to decrement the count value by one each time an associated symbol transition is overwritten when a portion of the PAMn signal is stored in the memory.

[0046] Example 14 is the test and measurement instrument of Example 9, wherein the trigger circuit is further configured to generate the trigger signal based on a selectable combination of the count values.

[0047] Example 15 is a method of operating a test and measurement instrument to trigger on a symbol transition of an n-ary pulse amplitude modulated (PAMn) signal, comprising: receiving a PAMn signal (n is 3 or more) at an input; digitizing the PAMn signal; storing at least a portion of the digitized PAMn signal in a memory; decoding the bits of the PAMn signal using a clock and data recovery (CDR) circuit to determine the current symbol and the previous symbol; detecting a symbol transition between the previous symbol and the current symbol; configuring a trigger circuit to generate a trigger signal in response to detecting a particular symbol transition; It is equipped with.

[0048] Example 16 is the method of example 15, further comprising amplifying the PAMn signal before digitizing the PAMn signal, and wherein decoding bits of the PAMn signal includes decoding bits of the amplified PAMn signal using an analog PAMn CDR circuit.

[0049] Example 17 is the method of example 15, further comprising storing data indicative of the particular symbol transition detected in the memory along with the digitized PAMn signal.

[0050] Example 18 is the method of example 15, further comprising configuring the trigger circuit to generate the trigger signal for each of m symbol transitions in a round-robin or randomized manner, where m is a number of distinct symbol transitions in the PAMn signal, and wherein the number of generated trigger signals is substantially equal to the number of distinct symbol transitions.

[0051] Example 19 is the method of example 15, further comprising incrementing or decrementing a count value of each of the m distinct symbol transitions of the PAMn signal to reflect the number of each of the m distinct symbol transitions present in the digitized PAMn signal stored in the memory.

[0052] Example 20 is the method of example 19, determining a peak count value among said count values ​​for each of said m different symbol transitions; setting a count threshold for said count value for each of said m different symbol transitions as a selectable percentage of said peak count value; configuring the trigger circuit to generate the trigger signal until all of the count values ​​for each of the m different symbol transitions satisfy the corresponding count threshold; It further comprises:

[0053] Additionally, the description of this application refers to specific features. It should be understood that the disclosure herein includes all possible combinations of these specific features. When a specific feature is disclosed in connection with a particular aspect or example, that feature can also be used in connection with other aspects and examples, to the extent possible.

[0054] 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 preclude this possibility.

[0055] Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be appreciated that various modifications can be made therein without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims. [Explanation of symbols]

[0056] 102 Input section 104 Preamp 106 Analog-to-Digital Converter 108 memory 110 Trigger Circuit 112 Clock and Data Recovery (CDR) Circuit 114 Transition Detection Logic Circuit 200 Test and Measurement Equipment 202 Interpolation Block 204 Continuous Time Linear Equalizer (CTLE) Block 212 Digital PAMn CDR circuit 300 Test and Measurement Equipment 302 Transition Memory Controller 304 m transition counters 306 m count threshold blocks 308 Peak Tracking Block 310 Logic Gates

Claims

1. 1. A test and measurement device comprising: an input for receiving an n-ary pulse amplitude modulated (PAMn) signal, where n is 3 or greater; an analog-to-digital converter (ADC) coupled to the input for digitizing the PAMn signal; a memory coupled to the ADC and configured to store at least a portion of the digitized PAMn signal as a waveform; a trigger circuit coupled to the ADC and the memory and configured to generate a trigger signal to trigger a test and measurement instrument to acquire the waveform; a PAMn clock and data recovery (CDR) circuit configured to decode bits from the PAMn signal; a transition detection logic circuit coupled to the PAMn CDR circuit and the trigger circuit, the transition detection logic circuit configured to detect symbol transitions based on the decoded bits and to cause the trigger circuit to generate the trigger signal in response to detecting a particular symbol transition; A test and measurement device comprising:

2. 2. The test and measurement instrument of claim 1, wherein the trigger circuitry is further configured to generate the trigger signal for each of the m distinct symbol transitions, where m is equal to the number of distinct symbol transitions in the PAMn signal.

3. 2. The test and measurement instrument of claim 1, further comprising: a symbol transition counting circuit configured to track count values ​​that count detected symbol transitions, each of the count values ​​being associated with a corresponding one of the m different symbol transitions of the PAMn signal.

4. The symbol transition counting circuit a transition memory controller coupled to the transition detection logic; m transition counters respectively coupled to the transition detection logic and the transition memory controller, each associated with a corresponding one of the m different symbol transitions of the PAMn signal; m transition count threshold blocks each coupled to a corresponding one of the m transition counters, each outputting a result of comparing the count value from each of the m transition counters with a count threshold; a logic gate configured to combine the outputs of the m transition count threshold blocks and send a signal to the trigger circuit based on the combined output; 4. The test and measurement instrument of claim 3, comprising:

5. 5. The test and measurement instrument of claim 4, further comprising a peak tracking block configured to determine a peak count value among the count values ​​from each of the m transition counters and to set the count threshold as a selectable percentage of the peak count value.

6. 4. The test and measurement instrument of claim 3, wherein the symbol transition counting circuit is configured to increment the count value by one each time an associated symbol transition is detected.

7. 4. The test and measurement instrument of claim 3, wherein the symbol transition counting circuit is configured to decrement the count value by one each time an associated symbol transition is overwritten when a portion of the PAMn signal is stored in the memory.

8. 4. The test and measurement instrument of claim 3, wherein the trigger circuitry is further configured to generate the trigger signal based on a selectable combination of the count values.

9. 1. A method of operating a test and measurement instrument to trigger on symbol transitions of an n-ary pulse amplitude modulated (PAMn) signal, comprising: receiving a PAMn signal (n being 3 or greater) at an input; digitizing the PAMn signal; storing at least a portion of the digitized PAMn signal in a memory; decoding the bits of said PAMn signal using a clock and data recovery (CDR) circuit to determine the current symbol and the previous symbol; detecting a symbol transition between the previous symbol and the current symbol; configuring a trigger circuit to generate a trigger signal in response to detecting a particular symbol transition; A method of operating a test and measurement instrument comprising:

10. 10. The method of claim 9, further comprising incrementing or decrementing a count value of each of the m distinct symbol transitions of the PAMn signal to reflect the number of each of the m distinct symbol transitions present in the digitized PAMn signal stored in the memory.

11. determining a peak count among said counts for each of said m different symbol transitions; setting a count threshold for the count value for each of the m different symbol transitions as a selectable percentage of the peak count value; configuring the trigger circuit to generate the trigger signal until all of the count values ​​for each of the m different symbol transitions satisfy the corresponding count threshold; 11. The method of operating a test and measurement instrument of claim 10, further comprising:

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