[time-nuts] DIY TimePod

Bob Camp kb8tq at n1k.org
Tue Jun 14 21:28:19 EDT 2016


Since (in a sense) it’s a single frequency SDR, it very much looks at the fundamental sine wave


> On Jun 14, 2016, at 8:53 PM, John Swenson <johnswenson1 at 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.
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