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| Arbitrary Waveform Generator with fast edges in ARB mode |
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| gf:
--- Quote from: _pat_ on January 20, 2023, 01:19:43 pm ---Though I also think we might be slightly cross threads here, so for the avoidance of doubt, I was referring to edge jitter on *square* edges, not zero-crossing jitter on sinusoids. I would be interested to know the source of your claim that even units such as the 335xx runs its DAC(s) at fixed sample clocks - because Keysight claim that with Trueform they can get edge jitter down to 1ps. I don't see how that is in any way possible for *square* edges unless you are clocking your DAC at 1THz (which they don't claim). --- End quote --- It is possible if you soften the edges of the square wave, so that the highest frequency component in the signal is below fs/2. Of course it is no longer a "square wave" then, but a square wave with zero rise/fall time does not exist anyway in practice. [ With a sufficiently large phase accumulator and sample memory, even DDS can be pretty low jitter, if you don't violate the Nyquist sampling theorem when the advancing phase accumulator of the DDS re-samples the data in the sample memory. Just make sure that the data you load into the sample memory are sufficiently band-limited for the desired phase accumulator step size, which is a function of the desired waveform repeat rate. ] EDIT: Always keep in mind, that these AWGs are still a generators for an analog signal, and they are based on Nyquist-Shannon reconstruction of a band-limited sampled signal. They are not dedicated pulse generators. |
| joeqsmith:
--- Quote from: _pat_ on January 20, 2023, 01:19:43 pm ---Joe, Sounds like an interesting setup you have there too :) If it's of interest you might want to have a look at how Motorola (back in the day) implemented the angle clock in the (e)TPU in devices such as the MPC555 / MPC5554. An interesting approach that avoids the need for division to get from time ticks to angle ticks :) --- End quote --- We were using the 68332 for the TPU function prior to the Power PC. This is going back 25-30 years. Still have an ICE for it sitting around for when I open my home museum. I use my setup to test prototype firmware for the ECM. It's easier to put it through the paces on my desk than fitting it to the bike and riding it, plus the advantage of being able to simulate cases that you may not want to while driving. |
| nctnico:
--- Quote from: gf on January 20, 2023, 01:29:29 pm ---EDIT: Always keep in mind, that these AWGs are still a generators for an analog signal, and they are based on Nyquist-Shannon reconstruction of a band-limited sampled signal. They are not dedicated pulse generators. --- End quote --- Except for the TTi TGF4000 unit I mentioned. That is also sold as a pulse generator! |
| switchabl:
--- Quote from: _pat_ on January 20, 2023, 01:19:43 pm ---I have a propensity for throwing Occam's Razor out the window, LOL - so yes perhaps I am over thinking it. Though I also think we might be slightly cross threads here, so for the avoidance of doubt, I was referring to edge jitter on *square* edges, not zero-crossing jitter on sinusoids. I would be interested to know the source of your claim that even units such as the 335xx runs its DAC(s) at fixed sample clocks - because Keysight claim that with Trueform they can get edge jitter down to 1ps. I don't see how that is in any way possible for *square* edges unless you are clocking your DAC at 1THz (which they don't claim). --- End quote --- In the "Trueform" whitepaper (https://www.mouser.com/pdfdocs/to5991-0852EN.pdf) they talk about having a "virtual variable clock". I don't know if they ever explicitly state that this is used for square/pulse mode as well as ARB mode but it is the obvious thing to do: As gf said, the trick you are missing is that the generator can simply treat the edges as analog waveforms with limited bandwidth and rise-time. At the risk of oversimplifying: the DAC will not output an edge as 0, 0, 1, 1 but instead something like 0, 0, 0.5, 1, 1 or 0, 0, 0.2456, 1, 1 which gives you a high resolution for sub-sample phase adjustment. In reality, things are a little more complicated because the coefficients need to be chosen in such a way that the reconstruction filter will turn them into a smooth edge with correct rise time (and no overshoot) but I hope you get the idea. Mathematically speaking, this is really just a special case of "Trueform" resampling with a different interpolation filter. In theory you could more or less re-use the ARB filter pipeline with different coefficients (in practice you would want an optimized implementation that gives you better resolution and features like modulation). |
| nctnico:
IIRC the Keysight generators allow you to choose the type of filter so you can make the trade-off between bandwidth and overshoot yourself. |
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