Test and measurement instrument including asynchronous time-interleaved digitizer using harmonic mixing

A test and measurement instrument including a splitter configured to split an input signal having a particular bandwidth into a plurality of split signals, each split signal including substantially the entire bandwidth of the input signal; a plurality of harmonic mixers, each harmonic mixer configured to mix an associated split signal of the plurality of split signals with an associated harmonic signal to generate an associated mixed signal; and a plurality of digitizers, each digitizer configured to digitize a mixed signal of an associated harmonic mixer of the plurality of harmonic mixers. A first-order harmonic of at least one harmonic signal associated with the harmonic mixers is different from an effective sample rate of at least one of the digitizers.

BACKGROUND

This invention relates to test and measurement instruments and, more particularly, to test and measurement instruments including one or more asynchronous time-interleaved digitizers, which use harmonic mixing for reducing noise.

Useable bandwidths of test and measurement instruments, such as digital oscilloscopes, can be limited by an analog to digital converter (ADC) used to digitize input signals. The useable bandwidth of an ADC can be limited to the lesser of the analog bandwidth or one half of a maximum sample rate of the ADC. Various techniques have been developed to digitize higher bandwidth signals with existing ADCs.

For example, synchronous time-interleaving can be used to achieve an effective higher sample rate. Multiple ADCs can sample an input signal offset in time within a single sample period. The digitized outputs can be combined together for an effectively multiplied sample rate. However, if the analog bandwidth of the ADCs become the limiting factor, a high bandwidth front end, such as a multi-way interleaved track and hold amplifier is needed to achieve a higher bandwidth.

Conventional track and hold amplifier-based time-interleaved systems cause the track and hold amplifier to be clocked at a sample rate similar to or slower than the ADC channel bandwidth so that the ADC will have sufficient time to settle to the held value. The ADC is synchronously clocked to the track and hold amplifier to digitally capture each held value. Such a limitation on the track and hold amplifier in turn limits the ADC sample rate. Moreover, to satisfy the Nyquist sampling theorem, the ADC sample rate is lowered to less than twice the bandwidth of the ADC channel. As a result, many time-interleaved ADC channels are needed to achieve the desired performance.

As the number of ADC channels increases, the overall cost and complexity of the system also increases. For instance, the front end chip must now drive more ADC channels, including additional ADC circuitry, clocking circuitry, or the like, to get the overall net sample rate up to a suitable value. The size and complexity of the chip also results in longer communication paths, and therefore, an increase in parasitic capacitance, electromagnetic noise, design difficulties, and so forth.

In another technique, sub-bands of an input signal can be downconverted to a frequency range that can be passed through a lower sample rate ADC. In other words, the wide input bandwidth can be split into multiple lower-bandwidth ADC channels. After digitization, the sub-bands can be digitally upconverted to the respective original frequency ranges and combined into a representation of the input signal. One significant disadvantage of this technique is the inherent noise penalty when digitizing an arbitrary input signal whose frequency content may be routed to only one ADC channel. The recombined output will contain signal energy from only one ADC, but noise energy from all ADCs, thereby degrading the Signal-to-Noise Ration (SNR).

Accordingly, a need remains for improved devices and methods for digitizing any frequency input signal by all ADC channels in an asynchronous time-interleaved architecture, thereby avoiding the noise penalty.

DETAILED DESCRIPTION

This disclosure describes embodiments of an ADC system for a test and measurement instrument using harmonic mixing.

FIG. 1is a block diagram of an ADC system for a test and measurement instrument using harmonic mixing according to an embodiment of the invention. In this embodiment, the instrument includes a splitter10configured to split an input signal12having a particular frequency spectrum into multiple split signals14and16, each split signal including substantially the entire spectrum of the input signal12. A splitter10can be any variety of circuitry that can split the input signal12into multiple signals. For example, the splitter10can be a resistive divider. Thus, substantially all frequency components of the input signal12can be present in each split signal14and16. However, depending on the number of paths, harmonic signals used, or the like, the frequency responses for various split signals of a splitter10can be different.

The split signals14and16are inputs to harmonic mixers18and24, respectively. Harmonic mixer18is configured to mix the split signal14with a harmonic signal20to generate a mixed signal22. Similarly, harmonic mixer24is configured to mix the split signal16with a harmonic signal26to generate a mixed signal28.

As used herein, a harmonic mixer is a device configured to mix a signal with multiple harmonics. Although multiplication and/or mixing has been described in connection with harmonic mixing, as will be described in further detail below, a device that has the effect of multiplying a signal with multiple harmonics can be used as a harmonic mixer.

In some embodiments, the multiple harmonics can include a zero-order harmonic, or a DC component. For example, in some embodiments, the harmonic signal20can be a signal represented by equation (1):
1+2 cos(2πF1t)  (1)

Here F1represents the first-order harmonic and t represents time. Thus, a signal having the form of equation (1) has harmonics at DC and at frequency F1.

Harmonic signal26can be a signal represented by equation (2)
1−2 cos(2πF1t)  (2)

Similar to harmonic signal20, harmonic signal26has harmonics at DC and frequency F1. However, the first-order harmonic at frequency F1is out of phase by 180 degrees relative to the similar first-order harmonic in harmonic signal20.

A digitizer30is configured to digitize mixed signal22. Similarly, a digitizer32is configured to digitize mixed signal28. The digitizers30and32can be any variety of digitizer. Although not illustrated, each digitizer30and32can have a preamplifier, filter, attenuator, and other analog circuitry as needed. Thus, the mixed signal22input to the digitizer30, for example, can be amplified, attenuated, or otherwise filtered before digitization.

The digitizers30and32are configured to operate at an effective sample rate. In some embodiments, the digitizer30can include a single analog to digital converter (ADC). However, in other embodiments, the digitizer30can include multiple interleaved ADCs operating at lower sample rates to achieve a higher effective sample rate.

A first-order harmonic of at least one of the harmonic signals20and26is different from an effective sample rate of at least one of the digitizers30and32. For example, the first-order harmonic F1of the harmonic signal20could be 34 GHz. A sample rate of the digitizer30could be 50 GS/s. Thus, the first-order harmonic F1is different from the effective sample rate.

In some embodiments, the first-order harmonic of a harmonic signal need not be an integer multiple or sub-multiple of the effective sample rate of the at least one of the digitizers. In other words, in some embodiments, the first-order harmonic of a harmonic signal associated with the harmonic mixers is not an integer multiple or sub-multiple of the effective sample rate of the at least one of the digitizers.

In some embodiments, the first-order harmonic of a harmonic signal can be between the effective sample rate of the at least one of the digitizers and one half of the effective sample rate of the at least one of the digitizers. In particular, as will be described in further detail below, such a frequency allows higher frequency components above and/or below the first-order harmonic to be mixed down in frequency to be below one half of the sample rate of the digitizer30. Thus, such frequency components can be digitized effectively by the digitizer30.

It should be understood that all bands of the input signal12go through all paths. In other words, when more than one channel is combined for processing a single input signal12, each channel or path receives substantially the entire bandwidth of the input signal12. As the input signal12is transmitted through all of the digitizers, the signal to noise ratio is significantly improved.

A filter36can be configured to filter the digitized mixed signal34from digitizer30. Similarly, a filter42can be configured to filter the mixed signal40from digitizer32. Harmonic mixers46and52are configured to mix the filtered mixed signals38and44with harmonic signals48and54, respectively. In some embodiments, the harmonic signals48and54can be substantially similar in frequency and phase to the corresponding harmonic signals20and26. While the harmonic signals20and26are analog signals, and the harmonic signals48and54are digital signals, the scaling factors for these harmonic signals can be the same or similar to each other. The output signals50and56are referred to as remixed signals50and56. A combiner58is configured to combine the remixed signals50and56into a reconstructed input signal60. In some embodiments, the combiner58can implement more than mere addition of signals. For example, averaging, filtering, scaling, or the like can be implemented in the combiner58.

The filters36and42, the harmonic mixers46and52, harmonic signals48and54, the combiner58, and other associated elements can be implemented digitally. For example, a digital signal processor (DSP), microprocessor, programmable logic device, general purpose processor, or other processing system with appropriate peripheral devices as desired can be used to implement the functionality of the processing of the digitized signals. Any variation between complete integration to fully discrete components can be used to implement the functionality.

Some form of synchronization of the harmonic signals20,26,48, and54is used. For example, the harmonics of the harmonic signals20and26can be locked to a clock related to the digitizers30and32. In another example, the harmonic signal can be digitized. Thus, the first-order harmonic would be available to synchronize the harmonic signals48and54. In another example, out-of-band tones can be added to one or more of the mixed signals22and28. Using a first-order harmonic of 34 GHz, 19.125 GHz and 21.25 GHz tones, or 9/16 and 10/16 of 34 GHz, can be added to the mixed signal22. Since these tones are outside of a bandwidth of the filtering eventually established by filter36, i.e., approximately 18 GHz depending on the transition band, the tones can have a substantially negligible effect on the reconstructed signal60. However, as the tones can be less than a Nyquist frequency, i.e. less than 25 GHz for a 50 GS/s sample rate, the tones can be acquired by using the digitized mixed signal34before filtering. Regardless of the technique used, a phase and frequency relationship between the harmonic signals20and26and the digital harmonic signals48and54can be maintained.

FIGS. 2-8illustrate examples of spectral components of various signals in the ADC system for the test and measurement instrument ofFIG. 1. Referring toFIGS. 1 and 2, spectrum100can be a spectrum of the input signal12and hence, the split signal14. Using the above example of the harmonic signal defined in equation (1), a DC component of the split signal14is passed, as represented by spectrum100. However, the spectrum100in the input signal12is also mixed with the first-order harmonic at frequency F1. The resulting spectrum102is the product of such mixing. Thus, the mixed signal22includes components of spectrum100and spectrum102. Here, and in other figures, the spectral components are illustrated as separate and overlapping however, the actual spectrum would be the combination of the spectra100and102.

Referring toFIGS. 1 and 3, spectrum110similarly represents components of the mixed signal28due to the mixing of input signal12with the DC harmonic of the harmonic signal26. However, in contrast toFIG. 2, the spectrum112has a 180 degree phase difference relative to the spectrum102ofFIG. 2. As described above, the first-order harmonic of the harmonic signal26is phase shifted by 180 degrees from the first-order harmonic of the harmonic signal20. This 180 degree phase shift in the harmonic signal26induces a 180 degree phase shift in the spectrum112. The 180 degree phase difference is illustrated as a dashed line.

FIGS. 4 and 5represent the spectrums of the filtered mixed signals38and44. In some embodiments, the filtering can be a function of inherent filtering of the corresponding digitizers30and32, the filters36and42, or the like. Although filtering is illustrated inFIG. 1as occurring after the digitizers36and42, filtering can be performed in other locations. For example, some filtering can occur prior to digitization. The mixed signals22and28could be filtered with a low pass filter having a cutoff frequency near one half of the effective sample rate of the digitizers30and32. The filtering of filters36and42can add to such inherent and/or induced filtering.

In some embodiments, the net filtering of the mixed signals22and28can result in a frequency response that is substantially complementary about one half of a frequency of the first-order harmonic of the harmonic signals20and26. That is, the frequency response at a given offset higher than frequency F1/2 and the frequency response at a given offset lower than frequency F1/2 can add to one. Although one has been used as an example, other values can be used as desired, such as for scaling of signals. Furthermore, the above example is described as an ideal case. That is, the implemented filtering can have different response to account for non-ideal components, calibration, or the like.

In a particular example of the frequency response, using the 34 GHz F1described above, frequency F1/2 can be 17 GHz. From DC to 16 GHz the frequency response can be one. From 16 to 18 GHz, the frequency response can linearly change from one to zero, passing through ½ at 17 GHz.

The resulting spectral components inFIG. 4, representing the filtered mixed signal38include a lower frequency portion of spectrum100, illustrated by spectrum120, and a lower frequency portion of spectrum102, illustrated by spectrum122. Note that due to the mixing, spectrum122includes frequency components of a higher sub-band of spectrum100, albeit reversed in frequency. Similarly, the spectral components130and132ofFIG. 5correspond to the lower frequency components of spectra110and112ofFIG. 3. The 180 degree phase relationship of spectrum112is preserved in spectrum132.

Accordingly, through the harmonic mixing, two sub-bands of an input signal12have been digitized even though the span of the sub-bands would have exceeded a Nyquist bandwidth associated with the digitizers30and32. In this embodiment, each mixed signal, whether analog, digital, filtered, or the like, includes components of each sub-band of the input signal12. That is, in this example, each signal from the mixed signals22and28to the filtered digitized mixed signal38and44includes both a low frequency sub-band and a high frequency sub-band of spectrum100.

In particular, the sub-bands of the input signal12have been frequency shifted to be within the bandwidth of a baseband sub-band. In some embodiments, each sub-band of the input signal12can be frequency shifted to be within the bandwidth of the single sub-band. However, depending on the number of sub-bands, and the harmonic signals, each sub-band may not be present in each mixed signal.

FIGS. 6 and 7represent the spectra of the remixed signals50and56. Referring toFIGS. 1 and 6, the spectrum represents the remixed signal50. As described above the filtered digitized mixed signal38can be mixed in the harmonic mixer46with the harmonic signal48that is substantially similar in frequency and phase to the harmonic signal20. Accordingly, the spectra ofFIG. 4are mixed with a DC component and a first-order harmonic.

Spectra140and142represent the spectra from mixing the spectra120and122ofFIG. 4with the DC component. Spectrum144represents the result of mixing the spectrum120with the first-order harmonic. Spectra146and148represent the mixing of spectrum122ofFIG. 4with the first-order harmonic.

Similarly,FIG. 7represents the spectra of the remixed signal56. Spectra150and152represent the mixing of the DC component with the spectra ofFIG. 5. Spectrum154represents the mixing of the first-order harmonic of the harmonic signal54with the spectrum130ofFIG. 5. In particular, as the first-order harmonic of harmonic signal54has a relative 180 degree phase shift, the resulting spectrum154also has a 180 degree phase shift, represented by the dashed line.

Spectrum132ofFIG. 5is also mixed with the first-order harmonic of harmonic signal54; however, the spectrum132already had a 180 degree induced phase shift. Thus, the additional 180 degree phase shift results in an effective 0 degree phase shift, represented by the solid line of spectra156and158.

FIG. 8illustrates a spectrum160of the reconstructed input signal60ofFIG. 1. Spectra162and164represent the component sub-bands forming the spectrum160. Spectrum166represents an additional sideband from the mixing described with respect toFIGS. 6 and 7. In this embodiment, spectrum166can be filtered out; however, in other embodiments sub-bands can extend beyond the first-order harmonic frequency F1. In such an embodiment, spectrum166, being generated from a lower frequency sub-band, can be eliminated through destructive combination.

Due to the relative phasing of the components of the remixed signals50and56, sub-bands in their original frequency range combine constructively, while sub-bands outside of their original frequency range are phased to combine destructively. Referring toFIGS. 6-8, when combined, spectra140and150combine constructively, resulting in spectrum162. Spectra142and152combine destructively as the spectra are out of phase by 180 degrees. Thus, of the spectra within the baseband sub-band, the remaining sub-band is the original sub-band.

Similarly, for the sub-band from approximately F1/2 to F1, spectra146and156combine constructively into spectrum164, while spectra144and154combine destructively. Spectra148and158combine constructively into spectrum166; however, spectrum166can be filtered out as it is beyond the expected input frequency range which in this case is about less than frequency F1.

As illustrated by spectra162and164, a transition occurs around frequency F1/2. This transition is the result of the filtering described above in reference toFIGS. 4 and 5. In particular, the slopes of spectrum162and spectrum164are complementary. Thus, when the frequency components of the spectrums162and164are combined, the resulting portion of the spectrum160substantially matches the original frequency spectrum.

Accordingly, by mixing the input signal12with various harmonic signals, sub-bands of the input signal12can be passed through the lower bandwidth of a digitizer. Although the mixed signals included overlapping sub-bands, because of the phasing of the harmonic signals, the sub-bands combine constructively and destructively when combined as described above to create a substantially accurate representation of the input signal12.

FIGS. 9-12are block diagrams of examples of harmonic mixers ofFIG. 1. In some embodiments, a mixer can be used to mix the split signals14and16with the respective harmonic signals20and26. A mixer that can pass DC and baseband signals on all ports can be used as a harmonic mixer.

FIGS. 9A and 9Billustrate examples of a harmonic mixer, which can represent any one or more of the harmonic mixers18,24,46, and/or52discussed above.FIG. 9Aillustrates a 2-way time-interleaving switch.FIG. 9Billustrates an N-way time-interleaving switch.

In these embodiments, switches180and/or181are configured to receive an input signal182. When using the 2-way switch180, the input signal182is switched to outputs184and186in response to a control signal188. When using the N-way switch181, the input signal182is switched to the outputs184,186, on through to the Nth output187, in response to the control signal188. For example, the switch181can be a three-throw switch, a four-throw switch, etc., up to an N-throw switch, which causes the input signal182to spend 1/Nth of its time at each point or output. As further paths and sub-bands are added, the harmonics of the harmonic signals can be appropriately phased. In some embodiments, the relative phase shifts of the harmonic signals can be spaced in phase by time shifts of one period divided by the number of sub-bands.

As the pulses get shorter compared to the overall clock cycle, the harmonic content gets richer. For instance, for a two-way or a three-way switch, the zero-order harmonic (DC) and the first-order harmonic are used. For a four-way or five-way switch, the zero-order harmonic, the first-order harmonic, and a second-order harmonic can be used. For a six-way or seven-way switch, the zero-order harmonic, the first-order harmonic, a second-order harmonic, and a third-order harmonic can be used. As N increases, the pulses get narrower, thereby generating the richer harmonic content. The control signal188can be a signal having a fundamental frequency of the first-order harmonic, or other suitable harmonic frequency, described above.

All bands of the input signal182go through all paths, i.e., to each of the outputs paths (e.g.,184,186, through the Nth output187).

For example, referring to switch180, the control signal188can be a square wave with a fundamental frequency of 34 GHz. As a result of the switching, output184will receive the input signal182during one half-cycle of the control signal and will be approximately zero during the opposite half-cycle. In effect, the output184is the input signal182multiplied by a square wave oscillating between zero and one at 34 GHz. Such a square wave can be represented by equation (3).

Equation (3) is the Taylor series expansion of such a square wave. The DC and first two harmonics are listed. Here F1is 34 GHz. Although the magnitudes of the components are different, equations (1) and (3) include similar harmonics.

Output186is similar to output184; however, the time period over which the input signal182is routed to the output186is inverted relative to output184. The effect is again similar to multiplying the input signal182with a square wave defined by equation (4).

Similar to equation (3), equation (4) is similar to the harmonic signal described in equation (2) above. Thus, the multiplication effect of the switching of the switch180is substantially similar to the mixing of a split signal with the harmonic signal described above. In addition, in this example, the switch can act as both the splitter10and harmonic mixers18and24. However, in other embodiments, the switch180could be a single pole single throw switch and act as a single harmonic mixer.

Although the relative magnitudes of the DC component and the first-order harmonic are different, such imbalance can be corrected through a compensation filter in the appropriate path. For example, the sub-band described above between frequency F1/2 and frequency F1can have a different gain applied during recombination in the combiner58than a baseband sub-band.

In addition, equations (3) and (4) above also list third-order harmonics. In some embodiments, the third-order harmonics may be desired. However, if not, the effect of such harmonics can be compensated with appropriate filtering. For example, the input signal12can be filtered to remove frequency components above frequency F1. Thus, such frequency components would not be present to mix with a frequency at 3*F1. Moreover, filtering before a digitizer can remove any higher order frequency components that may otherwise affect the digitized signal due to aliasing.

In the event of interleaving errors due to analog mismatch, hardware adjustments can be made for mixing clock amplitude and phase. The adjustments can then be calibrated to minimize interleave mismatch spurs. Alternatively, or in addition to the above approach, hardware mismatches can be characterized, and a linear, time-varying correction filter can be used to cancel the interleave spurs. Further, in some cases, the switches might not always operate perfectly. For example, an errant switch might spend more time in one direction than the other, thereby causing a skewed duty cycle. The digital harmonic mixers46and52can be configured to compensate for phase or amplitude errors that may be present in the analog harmonic signals20and/or26by making subtle adjustments to the amplitude or phase of the digital harmonic signals48and/or54.

FIG. 10is an example of another harmonic mixer. A switching circuit200is configured to switch two input signals202and204alternatively to outputs208and210in response to the control signal206. The control signal206can again be a square wave or other similar signal to enable the switches of the switching circuit200to switch. During one half-cycle of the control signal206, input signal202is switched to output208while input signal204is switched to output210. During the other half-cycle, the input signal202is switched to output210while input signal204is switched to output208.

In some embodiments, the input signal204can be an inverted and scaled version of the input signal202. The result of such inputs and the switching described above is a rebalancing of the DC and other harmonics from the levels described above with respect to the switch180ofFIG. 9A. For example, input signal204can be a fractional inverted version of the inputs signal202. Instead of switching between 1 and 0 with the switch180ofFIG. 9A, the effective output of outputs208and210can be switching between 1 and (2−π)(2+π), for example. Thus, the amplitude and DC level can be adjusted as desired to create the desired balance between the harmonics.

FIG. 11illustrates an alternative example of a harmonic mixer. The harmonic mixer170includes a splitter172, a mixer175, and a combiner177. The splitter172is configured to split an input signal171into signals173and174. Signal174is input to the combiner177. As signal174is not mixed with another signal, signal174can act as the DC component of a harmonic mixer described above.

Signal173is input to the mixer175. A signal176is mixed with the signal173. In some embodiments, signal176can be a single harmonic, such as the frequency F1described above. If additional harmonics are desired, additional mixers can be provided and the respective outputs combined in combiner177.

In another embodiment, the signal176can include multiple harmonics. As long as the bandwidth of the ports of the mixer175can accommodate the desired frequency ranges, a single mixer175can be used. However, since the DC component of the harmonic signals described above is passed to the combiner177by a different path, the ports of the mixer receiving signals173and176need not operate to DC. Accordingly, a wider variety of mixers may be used. Once the signals179and174are combined in the combiner177, the output signal178can be substantially similar to a mixed signal described above.

In some embodiments, the splitter172can, but need not split the input signal171symmetrically. For example, a side of the splitter that outputs signal174may have a bandwidth that is at or above the filtering cutoff frequency described above. A side of the splitter172that outputs signal173can have a frequency range centered on a harmonic of the signal176and a bandwidth of twice or greater of the filtering cutoff frequency described above. In other words, the frequency response of the splitter172need not be equal for each path and can be tailored as desired.

FIG. 12is another example of a harmonic mixer of the general topology ofFIG. 9A. In this embodiment, a harmonic signal224can be input to a diode ring220similar to a mixer through transformer225. The input signal222can be input to a tap of the transformer225. Accordingly, depending on the harmonic signal224, the input signal222can be switched between outputs226and228. For example, the harmonic signal224causes either the left diodes227to turn on when the bottom of the transformer is positive and the top is negative, or the right diodes229to turn on when the polarity of the transformer is reversed. In this manner, the input signal222is alternately routed to the output228and the output226. In some embodiments, an additional diode ring could be used to terminate the outputs and/or inject an inverted portion of a sub-band of the input signal222to achieve a higher gain, compensate for imbalanced harmonics, or the like, as in the topology ofFIG. 10.

In some embodiments, two paths and two overlapping sub-bands are implemented. However, as mentioned above, any number of paths and sub-bands can be used. In such embodiments, the number of harmonics used can be equal to one plus one half of a number of sub-bands, rounded down, where DC is included as a zero-order harmonic. For example, for three sub-bands, only two harmonics can be used. Using the above frequency ranges as an example, the first-order harmonic can frequency shift frequencies higher than frequency F1to the baseband sub-band. The first-order harmonics of the harmonic signals can be phased with 120 degree relative phase shifts.

Accordingly, when a sub-band is in the proper frequency range during combination in the combiner58, the sub-band spectra will have the same phase shift, such as a 0 degree relative phase shift. In contrast, the three components of a sub-band in the incorrect frequency range would offset in phase from one another by 120 degrees. The resulting spectra would destructively combine to eliminate the incorrect sub-band. As further paths and sub-bands are added, the harmonics of the harmonic signals can be appropriately phased. In some embodiments, the relative phase shifts of the harmonic signals can be spaced in phase by time shifts of one period divided by the number of sub-bands.

Although embodiments have been described above where digitized signals can be substantially immediately processed, such processing after digitization can be deferred as desired. For example, the digitized data from digitizers30and32can be stored in a memory for subsequent processing.

Moreover, although the digital filtering, mixing, and combining have been described as discrete operations, such operations can be combined, incorporated into other functions, or the like. In addition, as the above discussion assumed ideal components, additional compensation, can be introduced into such processing as appropriate to correct for non-ideal components. Furthermore, when processing the digitized signals, changing frequency ranges, mixing, and the like can result in a higher sample rate to represent such changes. The digitized signals can be upsampled, interpolated, or the like as appropriate.

As mentioned above, the digital harmonic mixers46and52can be configured to compensate for phase errors that may be present in the analog harmonic signals20and/or26by making subtle adjustments to the amplitude or phase of the digital harmonic signals48and/or54. Shifts in delays of various components over time or temperature may cause unacceptable amounts of phase shift. Delay shifts in the circuitry generating the analog harmonic signals, in the analog mixers, and/or in the analog-to-digital channel aperture would all contribute to a phase error between the analog mixers18and24and the digital mixers46and52, respectively.

If the phase error is uncorrected, the mixing phase error will effect an equal phase error in the frequency components within the upper bands of the reconstructed waveform, leading to distortion in the step response of the system. Additionally, amplitude errors will result for frequency components within the cross-over band, as the unconverted and the twice-converted vectors representing the frequency component, as will be discussed in more detail below, will not be properly aligned when added together near the end of the reconstruction process.

Some embodiments of the test and measurement instrument contain a compensation oscillator300and a switch302as shown inFIG. 13. A compensation oscillation signal304from the compensation oscillator300can be switched into the input of an ATI digitizer, described above, via switch302. The compensation oscillator300can be used to determine the phase and amplitude errors, as discussed in more detail below, so the phase and amplitude errors can be removed.

The compensation oscillator300and switch302are included within an integrated circuit for the ATI digitizer so the compensation oscillator300adds little cost or power overhead to the system. Further, the compensation oscillator300is tunable over a frequency range wider than the integrated circuit process uncertainty of the center frequency, insuring that the system can find an appropriate tune voltage to place the compensation oscillator300frequency within the cross-over band.

Since the frequency of the compensation signal304from the compensation oscillator300is tuned to be within the cross-over band, the compensation signal304travels through an ADC channel of the ATI digitizer both at its original frequency and as a down-converted and subsequently digitally-up-converted frequency component. The phase of the original frequency component of the compensation signal304is not impacted by the phase error between the analog and digital harmonic mixing signals, but the phase of the twice-converted component is impacted.

A phase error value can be determined based on comparing the original frequency component of the compensation signal304that was not affected by the phase error and the twice-converted component which has been affected by the phase error traveling through one ADC channel of the ATI digitizer. Comparing these values provides the phase error value between the analog and digital mixers in that ADC channel. The phase error can then be used to adjust the mixing function of either the analog mixer18or the digital mixer46, if in the upper ADC channel. Adjusting the mixing function of one of the mixers18or46, or if in the lower ADC channel inFIG. 13, mixers24and52, allows for the phase error to be removed from the reconstructed waveforms. Alternatively, the phase error may be removed by changing the delay of digital filter36in the upper ADC channel or digital filter42in the lower ADC channel, as a phase shift in either input to the digital mixers46and52will effect a phase shift in the output.

Preferably, the compensation oscillator300compensation signal304is switched into the input via switch302immediately after an acquisition of a signal to be tested, rather than beforehand, as the measurement of the phase error can be applied to correct the mixing functions of the digital mixers46and52or the delays of digital filters36and42. The information is not needed until the ATI reconstruction of the signal occurs post-acquisition.

As seen inFIG. 14, a memory400may be provided between digitizer30and filter36in the upper ADC channel and a memory402between digitizer32and filter42in the lower ADC channel. An acquisition can be performed and the digitized mixed signal34or the digitized mixed signal40can be stored in memories400and402, respectively, before being sent to filters36and42, respectively.

After the digitized mixed signals34and40have been stored in memories400and402, respectively, switch302can be triggered to automatically provide the compensation signal304from the compensation oscillator300without a user input. For example, a digital signal processor (DSP), microprocessor, programmable logic device, general purpose processor, or other processing system with appropriate peripheral devices as desired can be used to automatically switch to the compensation signal304from the compensation oscillator300. The phase error can be determined as discussed above, and the mixing functions of the digital mixers46and52or the delays of digital filters36and42can be adjusted. Once the mixing functions or filter delays have been modified based on the phase error, then the digitized mixed signals34and40can be processed through the remaining portions of the ADC channels as discussed above with respect toFIG. 1.

Running the compensation after the acquisition minimizes the opportunity for a phase drift between the compensation and acquisition modes. A compensation run before a signal acquisition may be performed an arbitrary time before a signal acquisition, as there is no way of knowing how long an acquisition will be running waiting for a trigger event. However, if the system phase stability is sufficiently good, then the compensation process can be run before acquisition. Further, if a user decides a compensation is desired, the user may begin the compensation via a menu on the test and measurement instrument.

When the compensation oscillator300is enabled, the input signal acquisition is automatically switched off via switch302and replaced with the compensation signal304, allowing the compensation to run without requiring user interaction. Further, the compensation signal304may be switched on after a trigger event has been detected, without user input, using a processor, or the like, as discussed above. The compensation oscillator300can also automatically be switched on after every signal acquisition to provide the compensation signal304to determine a phase or amplitude error.

Digitizers30and32may suffer from phase drift between their respective sampling clocks, such that the unconverted signals passing through the analog mixers are not sampled at the same time. Also, digitizers30and32may themselves employ interleaving techniques, such as synchronous time interleaving, to achieve their effective sample rates. In that case, the interleaved acquisition pipes within digitizers30and32may similarly suffer from phase drift of their respective sample clocks. Compensation oscillator300can also be used to provide a compensation signal304through the ATI front-end to each ADC channel for the purpose of determining phase errors of acquisition pipes within and/or between the ADC channels. This can be accomplished by tuning the compensation oscillator300out of the cross-over band so that only one tone is output from the analog mixers18and24within the bandwidth of each ADC channel. Alternatively, if compensation oscillator300frequency is left within the cross-over band, a sine-fit algorithm used to measure the phases of each ADC pipe could be set to fit just the unconverted frequency component and not an image component, or vice versa.

The measured phase errors may be used to adjust the phase response of digital filters36and42to correct for the impact of the sampling time errors. Adjusting the delay of one digital filter with respect to the other digital filter may compensate for phase error between the digitizers30and32. If the digitizers30and32are internally interleaved, pipe-dependent phase shifts may be applied within each digital filter36and42to compensate phase errors within each digitizer30and32, respectively. Alternatively, the phase errors could be used to adjust the sample-clock timing of the acquisition pipes to minimize the error in subsequent acquisitions.

The compensation oscillator300can be built from a cross-coupled NPN differential pair amplifier, to generate negative resistance, and a shorted transmission-line stub, to set a nominal frequency. The compensation oscillator300is turned on and tuned by setting an emitter current in the differential pair amplifier. Once the current is high enough to provide sufficient transconductance to support oscillation, further increase in current serves to increase the devices' input capacitance, which in turn loads the transmission-lines and lowers the resonant frequency. That is, the tunable compensation oscillator300is tuned predominantly through varying an input capacitance of at least one bipolar junction transistor.

Use of the input capacitance tuning provides a relatively large and linear tune range compared to varactor tuning at these frequencies. The large tune range is helpful to overcome process modeling uncertainty and process variability. If the large tune range of the compensation oscillator300causes excessive frequency instability within the duration of the compensation acquisition, the acquired compensation record can be split into multiple shorter segments and analyzed for phase errors using separate sine-fits with potentially different frequencies in each of the segments. The measured phase error between the unconverted and twice-converted components in each segment represents the phase error between the analog and digital harmonic signals, and is independent of the exact frequency of the compensation signal used. Thus the results of the segment phase error measurements may be averaged to gain the same noise immunity as the single long record.

As mentioned briefly above, an amplitude error can also be determined using the compensation oscillator300. To determine the amplitude error, the input of the ATI digitizer may be swept with the compensation signal304over at least two frequencies symmetrically opposed within the cross-over band. When the input frequency is below the center of the cross-over band, the ratio of amplitudes of the digitized signal at the converted frequency and the input frequency will be the product of the conversion gain and the digitizer frequency response roll-off. When the input frequency is symmetrically above the center of the cross-over band, the ratio of amplitudes of the digitized signal at the converted frequency and the input frequency will be the ratio of the conversion gain and the digitizer frequency roll-off. The geometric mean of these two amplitude ratios then represents the conversion gain. The amplitude of the analog mixing functions20,26or the digital mixing functions48,54may then be adjusted to bring the conversion gain to the desired value, generally 1.0.

Another embodiment includes computer readable code embodied on a computer readable medium that when executed, causes the computer to perform any of the above-described operations. As used here, a computer is any device that can execute code. Microprocessors, programmable logic devices, multiprocessor systems, digital signal processors, personal computers, or the like are all examples of such a computer. In some embodiments, the computer readable medium can be a tangible computer readable medium that is configured to store the computer readable code in a non-transitory manner.

Although particular embodiments have been described, it will be appreciated that the principles of the invention are not limited to those embodiments. Variations and modifications may be made without departing from the principles of the invention as set forth in the following claims. For example, it is anticipated that a re-ordering of the digital filtering, mixing, and/or combining may allow for more efficient execution of the digital processing while still providing for reconstruction of a digital representation of the input signal.