[time-nuts] Basic question regarding comparing two frequencies

[J. Forster]

What about using an HP 3586 B or C, locked to a local standard, and GPIB
interface and averaging the data? It goes to 0.1 Hz right out of the box
as I remember.

FWIW,

-John

=============


> Hi
>
> Rather than having the 940 in there, why not just build a (simple) direct
> conversion receiver?
>
> Feed something like the 3335 or 6061 into one port of a suitable mixer.
> Feed
> the band pass filtered signal from the antenna into another port. Run the
> IF
> output into a preamp / filter and then into the sound card.
>
> You'll get DSB down to the audio chain, but that can be fixed with more
> hardware. Often it's a non-issue. It all depends on what sort of signal
> you
> are after.
>
> Another idea:
>
> Butcher the sound card and feed it a synthesized clock that's locked to
> the
> z3816. One less step in the data reduction / one less thing to worry
> about.
> The sound card *might* even run off of one of the outputs the z3816
> already
> generates. You'd have an odd sample rate, but that's not a big deal.
>
> A comment:
>
> Cleaner is always going to be better on the RF generator that is your
> ultimate reference. Anything you can do to improve close in phase noise
> will
> likely help things out.
>
> Lots of possibilities.
>
> Bob
>
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Guy Lewis
> Sent: Monday, July 26, 2010 4:51 PM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Basic question regarding comparing two
> frequencies
>
>
>
> -----Original Message-----
>>> There is another way to compare two frequencies, relevant when  they
>>> are
>>> very close together...................
> -------------------------------------
> I am trying to measure the frequency of a distant on-air signal, with path
> fading, Doppler shift, and maybe even AM modulation and would appreciate
> comments that might improve accuracy to better than .01Hz. The idea is to
> measure the frequency of an audio beat between a disciplined synthesized
> generator and the on-air signal, the subtract out the difference. Here is
> what I am doing:
>
> Equipment:
> GPS Disciplined Oscillator (HP 3816A with antenna)
> Synthesized generator with .001Hz resolution (HP3335A locked to GPS 10 MHz
> reference)
> PC running Spectrum Lab sound card audio spectrum analyzer software
> Second locked synthesizer (Fluke 6061A) to determine Spectrum Lab
> frequency
> error
> AM receiver (TS940 for 30kHz to 30 MHz) and antenna covering unknown
> frequency to be measured
> Input signal combiner (Merrimac 50 ohm combiner) or leak into receiver
> across Ext Rx switch
>
> Setup:
> 1a. Disable TS940 transmit mode (power set to minimum, PTT disabled, don't
> touch SEND)
> Install power splitter at Rx input to mix unknown and synthesized
> generator
> signals
> --or:--
> 1b. (preferred alternative, to avoid accidently transmitting into the
> generator), leak generator signal into TS940 across Rx antenna switch at a
> higher level
> 2. Lock generator to external GPSDO. All OCXOs run full time
> 3. Connect audio out to PC running Spectrum Lab
> 4. Allow PC to warm up for at least 30 minutes and measure second locked
> synthesized generator near the expected unknown frequency to determine
> Spectrum Lab measurement error
>
> Measurement of unknown signal frequency:
> 1. Set Rx to approximate frequency of unknown signal, AM mode
> 2. Adjust generator to create a clean audio beat note (power, freq + 600
> Hz
> audio freq, narrow AM filter)
> 3. Be sure clockwise rotation of generator frequency knob increases audio
> beat note frequency. Tune generator to upper side of signal if necessary
> 4. Read peak audio frequency from Spectrum Lab display
> 5. Subtract audio frequency (Spectrum Lab reading -measured .046 Hz error)
> from generator dial reading for result.
>
> Example measuring WWV @ 10 MHz:
>
> Rx tuned to 10 MHz, AM mode, Narrow Filter
> Antenna signal mixed with -70 dbm (-30dbm if leaked across Rx switch)
> generator signal. Adjust level for cleanest audio tone.
> Generator frequency tuned to generate 600 Hz beat note reading in Spectrum
> Lab
> Generator frequency reads 10.000599954
> Audio frequency increases as generator frequency is increased
> Spectrum Lab reads audio frequency 600.00 Hz
> Spectrum Lab frequency readout error known to be .046 Hz high (actual
> audio
> frequency is 599.954Hz)
>
> Calculation:
> Unknown freq = Fgen-(Fspeclab-Fspeclaberr)
> WWV freq = 10,000,599.954Hz-(600-.046Hz) = 10,000,000.000Hz +/-.01Hz
>
> Any suggestions appreciated.
>
> Guy
> N2GL