[time-nuts] DIY TimePod

Bob Camp kb8tq at n1k.org
Tue Jun 14 07:00:15 EDT 2016


The input to the ADC is lowpass filtered. If you use a square wave it “sees” a more or less sine wave. If you are running
a very low frequency square wave, you can use an external lowpass filter. Since one channel is a reference, you may only 
need one external lowpass filter. 


> On Jun 14, 2016, at 3:35 AM, John Swenson <johnswenson1 at comcast.net> wrote:
> The idea here is around a 80MHz sample clock with a maximum input/ref signal of around 25MHz. This is based on the TimePod with ADCs, which is supposed to work with square waves. Its is using a 77MHz clock if I remember correctly, so somewhere in the neighborhood of 12-13 ns per sample.
> 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. 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, is this enough of a difference to invalidate the concept of a binary sample?
> Or is the difference that the ADC won't stay at 50, but will be bouncing around say between 45 and 55 when the square wave is low and this noise makes it work? If that is the case then wouldn't a longer measurement time do essentially the same thing with slight variations of timing of the edge due to the noise in the binary sampler?
> John S.
> On 6/13/2016 8:17 PM, Chris Caudle wrote:
>> On Mon, June 13, 2016 9:38 pm, Bruce Griffiths wrote:
>>> If the quantisation noise is random and spread uniformly over the Nyquist
>>> bandwidth (~40MHz??) then the noise floor is about -82dBc/Hz.
>> How do you spread the quantization noise randomly with a one bit
>> quantizer?  I'm mostly familiar with single bit quanitizers in the context
>> of audio range delta-sigma converters where the quantizer is in a feedback
>> loop to move most of the noise to a higher frequency range. That also
>> requires a clock much higher than the minimum nyquist requirement.
>> Maybe I need to see a block diagram of what is being described.  Where is
>> the clock for the ECL flip-flop generated?  I don't recall seeing a
>> description of what the effective sample rate will be, or the highest
>> clock signal accepted for analysis.
>>> With a high resolution RF ADC internal noise is usually sufficient
>>> (>= 1 lsb)) to ensure this.
>> Can't really dither to >= 1lsb when lsb=msb (single bit quantizer).
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