[time-nuts] DMTD Ideas

[Lux, Jim (337C)]

On 10/1/09 7:24 PM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:

> Brian,
> 
> Some quick comments...
> 
>> The Op Amps at Point #2 would be a something like LT1000s or so and they
>> b
> 
> Since getting rid of essentially all of the 20 MHz sum signal can be
> done using trivial passive networks close to the mixer, you can
> concentrate on the beat note or 100 Hz difference frequency. Notice that
> the beat note output level actually depends on the loading network.
> Essentially the trick is to let the 20 MHz see a short (cap) while the
> beat note sees high impedance. The loss in level is essentially due to
> the traditional 50 Ohm impedance, which doesn't match the mixers
> effective output impedence and besides, for that frequency we don't
> really care about reflections as lumped parameter models may be used and
> we count highest voltage and not highest power.
> 
> So, let's say you end up with 50 V/s and you want the counter to see say
> 5 V/us (really 5 MV/s or 5 MVHz) to avoid jitter-trigger then the
> slew-rate gain you need becomes 100.000.

What if you're going to run the beat note into an A/D (e.g. High performance
audio interfac) and sample it, then do the sinewave fit to calculate where
the zero crossing would have been?  Just a good low 1/f noise opamp to get
the level up where the A/D performance is good?