But a logic analyzer is not a scope. It is pretty rare that I need this sort of buffer for an analog waveform. It is a trade off between features you actually need and ones that you will use once in a blue moon.
In fact, I don't feel the need for a 4 input scope anymore. But what I do want is a scope that can work with a logic analyzer. It has been awhile, but I seem to recall analog scopes that have a trigger output on the back which can be used to trigger other devices.
My customer buys PC attached devices. He is very happy with their Intronix logic analyzer. But they have a PC attached scope that he was having trouble figuring out the triggering. I might ask to borrow the scope and see if I can make it work for me.
What can you do with a 4 channel scope that you can't do with a 2 channel one by moving the probe, when testing a 4 channel AWG? This looks to me like the perfect example of why you don't need 4 channels.
I can work my way through a list of 265 distinct serial commands, testing each one and its arguments, and watch what happens to all 4 waveform generators after each command, without mating/unmating cables about a thousand times.
Hmm.. Just out of curiosity, do you implement a real-time digital inverse-sinc filter on the arbitrary waveform samples?
I'm currently looking at combining a DDS (external professional audio type DAC) with high precision digital multipliers.. fortunately only sine waves.
Best regards, Spehro Pefhany
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No. The waveforms come out of RAM, get multiplied twice (one user gain, one cal), get added to three times (user DC offset, cal offset, channel summing) and go to the dacs. We limit the user's frequency request to half Nyquist (32 MHz for a 128 MHz clock) so sinc rolloff isn't bad, about 1 dB. We use a fairly soft (7-pole transitional Gaussian with a notch) lowpass filter after the dacs to get decent time-domain steps, so that keeps us away from Nyquist. We're overall down about 2 dB at 32 MHZ.
We did implement linear interpolation (using dual-port waveform rams) against the lower-order phase accumulator bits. That does good/interesting stuff, especially at low frequencies. Steps become linear ramps!
If you're just doing sines, you might just tweak the downstream gain (or dac reference!) as a function of frequency, rather than doing a tedious reverse-sinc digital filter. Assuming that phase don't matter much. One could also do the corrections in the analog filter after the dac, or maybe in some opamp stage. Sinx/x is only about 3 dB at Nyquist.
Incidentally, the Nuhertz filter design software is dynamite. It designed a perfect LC filter, using finite-Q standard-value parts. I didn't think that was possible.
Rolling your own DDS is fascinating. You can scribble the block diagram on a whiteboard and walk through cases and *see* all the math happening... Nyquist, aliasing, negative frequencies, sinc bouncing-balls, all that.
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