General > General Technical Chat
Review: Hantek DDS 3X25. Anyone own one?
marmad:
--- Quote from: zibadun on December 18, 2012, 02:05:58 am ---isn't this a transition problem? like a DDS settle time which equals some N number of samples. I've seen this in a digital mixer in another device. every time you retune the mixer there was all kinds of garbage injected to the signal,but it lastsed only a few clock cycles. It may even be specified in the datasheet.. In some cases you may be able to account for that transition in whatever is on the "receiving" end of the sweep.. At least that is what I'm hoping to do in my project...
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I don't think so. I think it's a limitation of the hardware design or bad firmware implementation. If you use DDSDownload to change frequencies on the fly, you get large glitches in the output - where the waveform vanishes then reappears while the new sample memory is being loaded. If you use DDSSetFrequency to change frequencies on the fly, you don't get the big glitches - but the firmware just appears to change the clock frequency as soon as it receives the command (instead of waiting for the zero-crossing / beginning of sample memory) because the frequency 'shift' does not appear to be regular. If you look carefully at sweeps, you see the 'shift' happen at different points on the waveform. I don't have another sweep generator to test against (yet) but I think a properly designed and programmed one would be able to make frequency changes that would be smooth (or at least with the exact same shift at every changeover).
torch:
--- Quote from: marmad on December 18, 2012, 03:00:50 am --- - but the firmware just appears to change the clock frequency as soon as it receives the command (instead of waiting for the zero-crossing / beginning of sample memory) because the frequency 'shift' does not appear to be regular. If you look carefully at sweeps, you see the 'shift' happen at different points on the waveform. I don't have another sweep generator to test against (yet) but I think a properly designed and programmed one would be able to make frequency changes that would be smooth (or at least with the exact same shift at every changeover).
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So the randomness is introduced by the timing of the instruction from the computer. I wonder if there could be a hardware work-around? Either through the microprocessor control interface or perhaps some sort of USB buffer controlled by the sync output of the Hantek.
Maybe something even simpler: connect the sync out to the trigger in, send the first waveform and turn on "ext trig mode". Would the Hantek then wait for it's own signal falling signal before starting the next frequency change?
torch:
So much for the simple cable between sync and trigger -- it only works randomly.
zibadun:
got something semi-useful out of the generator in the 1-30mhz sweep. attached is a couple of filter responses (aka bode plots) made with the help of hantek... (don't laugh :-DD)
I also noticed the DDSDownload function will randomly fail to change the frequency and needs to be retried.
PuterGeek:
--- Quote from: Mechatrommer on December 17, 2012, 12:32:17 am ---
--- Quote from: PuterGeek on December 17, 2012, 12:10:31 am ---What would be a good cutoff frequency do you think I should use?
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100 or 200MHz?
--- Quote from: PuterGeek on December 17, 2012, 12:10:31 am ---Maybe I'll whip up a schematic to share in case someone wants to try my idea.
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yes please do. i will be interested. ;)
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It is going to require a Bessel filter and will need to be more than 5 poles to get good results. I will need to run a simulation to verify some things and may need to probe around with my scope too.
Not sure I will have this done before New Year because of the holidays but I am working on it.
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