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Timebase accuracy, SFDR and RMS noise for USB scopes with waveform generators


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Hi, I'm looking for a USB scope/waveform generator with good timebase accuracy (e.g. <2ppm), SFDR (>70dB) and low RMS noise (-45dB). Digilent appear to have some options that might work but I can't seem to find these specifications in the documentation. Can anyone help?

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A problem with Digilent's sampler/waveform generator products, either in the instrument or FPGA development platform line is the lack of an external clock input.

For their SYZYGY products the lack of an external clock input is a mind-boggling blunder for most serious work.

For low end instruments like the Analog Discovery line I suppose that a fixed time-base clock of unknown accuracy and stability is good enough for some applications. I'm not sure that the market that these product are geared toward even merit a high performance timebase option at a surcharge. I haven't tried working out the ramifications. No doubt the current timebase is adequate for the rest of the hardware.

The specifications that you are looking for are not unreasonable for low to moderate instrumentation ( excluding the 2 ppm bit ).

I'm not sure what you mean by SFDR and RMS noise as referenced to a clock source. Frequency accuracy, stability, phase noise, jitter etc. are more common specifications for clock modules. I'm reasonably certain that all of Digilent's AD line use a common clock module. Beyond the time-base specifications certainly ADC and DAC specifications are something to consider.

Perhaps you could use the datasheets for Eclypse-Z7 clock module and ZMOD ADC and DAC devices as a guide if Digilent doesn't want to specify these things for it's instrument product line. This would likely be a reasonable guesstimate.

The nice things about the ZMODs and, by inference the more expensive AD instruments, is that they are geared toward low end "scope/waveform generation" applications, and at least initially had great documentation. The worst limiting thing about the Digilent products that use them, or versions of them, is the lack of an external clock reference.

There's not a lot of selection out there for making your own ADC/DAC FPGA based instrument. I've used the Terasic DCC. It has clock inputs to accomodate any time-base requirement. The ADC and DAC Fs is higher than the ZMODs. Unfortunately, the converters connect to wide-band transformers so DC and low frequency performance isn't so good. This depends on your application of course. The DCC and the Cyclone V GX Starter board make for a nice alternate to the ZMODs and available SYZYGY FPGA base-boards, if you don't need the convenient features that the ZMODs supply.

The problem with ADC/DAC applications is that there just isn't a small set of criteria sufficient to know if the hardware is up to the task required by the measurement. Making general purpose ADC hardware is almost guaranteed to eliminate a range of potential applications. Edited by zygot
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