Hi Since (in a sense) it’s a single frequency SDR, it very much looks at the fundamental sine wave component.
Bob > On Jun 14, 2016, at 8:53 PM, John Swenson <johnswens...@comcast.net> wrote: > > Got it, I missed the 27 MHz low pass filter with 60 db attenuation. So the > ADC really is mostly seeing a sine wave. > > I guess it's back to the drawing board and doing this with the filter and the > ADCs. > > Thanks for setting me straight on this. > > John S. > > > On 6/14/2016 3:01 PM, Chris Caudle wrote: >> On Tue, June 14, 2016 2:35 am, John Swenson wrote: >>> The idea here is around a 80MHz sample clock with a >>> maximum input/ref signal of around 25MHz. >> >> Without some pretty steep low pass filtering that will violate the Nyquist >> criterion (for 80MHz sample clock the input must be strictly limited to >> less than 40MHz). You can't even get the first odd harmonic in of a 25MHz >> square wave input. >> >>> This is based on the TimePod with ADCs, which is >>> supposed to work with square waves. >> >> The ADC's would have a low pass filter in front. Think of it in terms of >> the Shannon information capacity, the amount of information conveyed is >> determined by the bandwidth and the signal to noise ratio. The bandwidth >> is determined by the sample rate, the signal to noise ratio by the number >> of (effective) bits of the ADC. >> I forget which ADC someone mentioned recently as being in the TimePod. >> Isn't it a 16 bit converter? So that is getting around 96dB integrated >> signal to noise ratio per converter, and you are starting with 6dB. >> >>> When you feed a square wave into this you have several samples at say >>> 50, then it jumps to 50,000 stays there for several samples, then jumps >>> down to 50 again. >> >> The key thing you are missing which happens with a multi-bit ADC is that >> the signal has a finite rise time, so it doesn't "jump" to 50,000, it has >> a transition region where you get several samples of different values. >> Those samples fit an infinite number of possible signals, but only one >> signal which is limited to the Nyquist criterion bandwidth. Using those >> samples and the knowledge of the system bandwidth you can interpolate >> where the zero crossing must have been. >> >> With a single bit quantizer (and no feedback to shape the noise), you get >> very little information about the signal values in the transition region. >> >>> This still seems like a binary sample. The difference >>> is that every now and then the sample hits during a ramptime of the >>> square wave and will give some intermediate value, >> >> No, every time you will sample during the transition, because the "square" >> wave still has a finite rise time, and if you have properly bandwidth >> limited the signal as required by the Nyquist sampling criterion (input >> signal must be less than half the frequency of the sampling clock) then >> you know what the upper limit on the rise time is. >> > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
