[time-nuts] Basic question regarding comparing two frequencies

Didier Juges didier at cox.net
Mon Jul 26 18:01:06 EDT 2010


I like the 3586 a lot, it's amazing what you can do with it. However, if you send the audio (beat note) to a computer or other instrument, keep in mind that the BFOs are not phase locked to the reference, they are just free standing crystal oscillators, and they may be off by a few Hz. If you want to use the beat note for high accuracy frequency measurement, it would be a good idea to phase lock the BFOs to the reference (at least the one you are going to use, you don't need to do both).

The carrier frequency measurement system is independant of the BFOs.

Didier KO4BB
------------------------ 
Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: "J. Forster" <jfor at quik.com>
Sender: time-nuts-bounces at febo.com
Date: Mon, 26 Jul 2010 14:23:02 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: jfor at quik.com, Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] Basic question regarding comparing two frequencies

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



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