[time-nuts] Racal-Dana 1992 switches

[Jeff Mock]

How do you pick the optimal difference frequency?  I see that 1kHz has a 
nice numerical property where you can read the frequency directly off 
the counter, you just need to mentally prepend the first 4 digits. With 
computers it's not that important, the difference can easily be a 
strange number if it optimizes performance. I'm wondering what 
difference frequency optimizes the performance of the mixer thing or if 
it really matters?

Do you worry about the phase-noise contribution of the 10.001MHz source? 
As I do the math, it seems that the phase noise of the mixing signal is 
subtracted out after the mixing, so it shouldn't mater that the 
10.001Mhz source comes from a frac-N synthesizer and has a few random spurs.

You say this isn't state of the art.  Why not?  Can't you run the timing 
collection for longer runs and get higher resolution results?

jeff


Pete wrote:
> This topic has been addressed earlier; though with some
> debate. I have proposed a simple heterodyne scheme for
> beating 2 stable sources against each other & observing
> a 1KHz difference frequency to resolve 1uHz deltas.
> This is NOT a "state-of-the-art" scheme, but it will
> provide better than 1E-12 resolution in less than 10s.
> 
> This scheme does require some non-standard items.
> 
> 1. You need a stable synthesizer with external clock
>     capability to yield 10.001 Mhz, phase locked to 
>     one of your sources. The HP3336C or a PTS040
>     work fine.
> 
> 2. You need a 1KHz zero crossing detector to drive
>     your counter input with low jitter. The ZCD requires
>     2 opamps & a few passive parts, including 2
>     inductors you'll need to wind by hand.
> 
> 3. You need a level 7 double balanced mixer to
>     heterodyne the second source & the 10.001Mhz
>     signal. Mixers optimized as phase detectors, like
>     the  mini-circuits SYPD-1 work well for this.
>     You also need a diplexer on the mixer output
>     to separate the 1KHz beat signal from the other
>     mixer products. The diplexer is 6 passive parts.
> 
> The results are stable & provide counter readings of
> 9 significant digits down to 1uHz with the leading 4
> digits of frequency assumed from the mixing process.
> The counter gate time setting provides useful & often
> necessary averaging of the readings; so a variable
> gate time counter is handy. I've used a H-P 5335A,
> & don't know much about the Racal 199x series,
> but I suspect they would do just fine.
> 
> Pete Rawson
> 
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