# [time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

Simon Marsh subscriptions at burble.com
Sat Oct 11 22:31:46 UTC 2014

```I (mostly) understand this when considering an analogue mixer, but I'm
lost on whether there are any similar effects going on with a digital
signal ?

TBH, I'm not really sure 'mixing' is the right phrase in the digital
case, and my apologies if I got that wrong.

What's actually going on is sampling one (digital) signal at a rate
close to the signal frequency. This gives a vernier effect and the
result is a purely digital set of pulses at the beat frequency, aligned
to when the signal and sample clock are in phase. It does not have a
high frequency component to filter out.

Cheers

Simon

On 11/10/2014 21:11, Bob Camp wrote:
> Hi
>
> Your glitches are (in part) coming from the 20 MHz (10 + 10) component on the mixed signal. Since they have no direct relation to the beat note, filtering them after limiting is not a simple task. It is far easier to keep filter the signal pre-limit than to do so post limit.
>
> The other component of the glitches is related to the limiting process. The paper by Collins is a good one to read for information on gain, bandwidth and the limiting process. Again, there is very little you can do “post limit” to sort things out.  None of the zero crossings you are getting may be “correct”. It’s not simply a process of picking one out of the group.
>
> ——————
>
> Some math:
>
> You have two 10 MHz signals and a (say) 10 Hz beat note. You are looking for 1x10^-13. You get 1x10^-6 from the downconversion. You need to get 1x10^-7 out of the beat note.
>
> Put another way, 1x10^-13 at 10 MHz is 1x10^-5 Hz.
>
> If your beat note is 3 V p-p, it will cover 6V every 1/10 second. It’s about 1.2X faster than a triangle wave as it zero crosses (memory may be failing me here), so that makes it equal to a 7.2V triangle excursion.
>
> 1x10^-6 of 7.2V is 7.2 microvolts.
>
> That’s how accurate your limiter / filter combination needs to be, pre-limiting.
>
> It can be in a fairly narrow bandwidth, so it’s not quite as daunting as a radio front end.
>
> Since you have a very large signal, and very small noise, the normal “dithering will help me” effect of the noise can not be counted on.
>
> The thing you *want* to come up with is essentially a random signal (ADEV), so massive filtering will not do the trick either.
>
> Bob
>
> On Oct 11, 2014, at 3:33 PM, Robert Darby <bobdarby at triad.rr.com> wrote:
>

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