[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop
magnus at rubidium.dyndns.org
Sun Oct 12 19:48:41 EDT 2014
> 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.
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