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

Magnus Danielson magnus at rubidium.dyndns.org
Sun Oct 12 19:48:41 EDT 2014


Robert,

> Bob Camp mention Collins low jitter hard limiters but I suspect that's
> much more of an issue on the very shallow slopes you see on 5 or 10 Hz
> mixer outputs.  The LTC6957 is probably overkill on 10 MHz inputs but I
> believe they're a tad better than a 74AC gate, but then again maybe not
> all that much better.  Lot more expensive.  Bob C discussed sine to
> square conversion in a recent post (IIRC) perhaps in connection with 5V
> to 3.3V conversion, and for a low cost solution the 74AC gate looks
> pretty good and they're easy to dead bug.

Sine-to-square conversion and Collins low jitter hard limiters is 
related to slew-rate limited resolution.

If you have 1E-12 resolution, and see about 20 steps, then over a period 
of 20 ps the sampler is unable to make a stable sampling. For a 74AC 
that is not very surprising. You might benefit from some slew-rate 
improvement on the input. Maybe look at the TADD-2 input for inspiration.

The slew-rate adaptation aims to reduce the slew-rate as being the major 
impact on noise, but the inputs inherent noise then needs to be handled. 
This is both the samplers/DFF jitter and the other signals jitter.

Cheers,
Magnus



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