[time-nuts] FMT

[John Ackermann N8UR]

Didier Juges said the following on 11/10/2006 10:05 PM:
> Hi Mike,
> 
> Interesting, I was just doing pretty much that, except that I did not 
> think of using the tracking generator of the 3586A as a reference, I 
> used an 8657B synthesizer phase locked to the Thunderbolt GPSDO to 
> inject a reference signal 20Hz above the test signal (I used WWV at 5 
> MHz for test), so that I could display both signals at the same time in 
> the FFT using Spectrum Lab. Setting the two very close eliminates the 
> risk of selecting the wrong sideband and reduces the error caused by the 
> sound card sample rate being off.

I've used the 3586 tracking generator trick for the last couple of
years.  It makes the tuning process much, much simpler because there's
only one knob to twiddle.

I usually use the 400Hz bandwidth in the receiver, and a window running
a shallow FFT for quick response.  The tracking generator signal will
always show up at nominally 1850 Hz in the receiver passband (it may be
a Hz or more off because the BFO is generated from a separate crystal
and isn't locked to the reference).  It stays put as the unknown signal
moves with the tuning knob.  So, look around 1850 Hz on the display, and
tune so the unknown is maybe 50 Hz away from the tracking generator
signal.  Then take your measurement.  It's important to have a good
variable attenuator on the tracking generator line as you need to match
its level pretty closely to the unknown signal level in order to get
comparable amplitudes on the display.

John