[time-nuts] DMTD - analog multiplier vs. diode mixer ?

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jan 5 22:42:01 UTC 2016


On 01/05/2016 10:37 PM, Poul-Henning Kamp wrote:
> --------
> In message <553575724.582265.1452024437677.JavaMail.yahoo at mail.yahoo.com>, Bruce Griffiths writes:
>> The noise of such Gilbert cell based analog multipliers far exceeds that of the traditional mixer.
> Yes, but does that really matter in this case ?
> The interesting output will be coming out of a LPF so
> most of the noise will die there?

You still raise the noise-level which is in the pass-band of those 
filters. This will be true both for white noise and flicker noise.
The white noise will be particularly annoying as it then converts to 
jitter through the slew-rate limitation as you go into the 
trigger-circuit. To reduce this effect, we amplify up the signal in 
steps, with higher and higher bandwidth to balance noise contribution 
with slew-rate incrementation. Using noisy mixers rather than quieter 
mixers makes this more worthwhile. The diode mixers needed does not have 
to be very rare, just look at the 2N2222A based mixer out of NIST, 
actually being a Harris chip with four transistors and a pair of off the 
shelf transformers.

Yes, I've played with this, ran into the issues.
Tried to build a DTMD, but didn't manage to handle some problems before 
it got side-tracked.

As always, choosing the trigg-point to have the highest slew-rate have 
always been key to reducing timing jitter.k Turns out that most counters 
isn't optimized for this property.


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