[time-nuts] Basic question regarding comparing two frequencies

[J. Forster]

It's a remarkable, and largely unappreciated, instrument. I passed them up
for years, thinking they were only useful for analog multiplex telephony.
It was not until I bought one, almost by accident, at the tail end of a
flea and started to play with it, did its utility became apparent. Thje
ability to lock onto a received carrier and count it is a delight, IMO.

A note on the data you get out. If you digitally high pass filter it, you
should be able to get a measure of the path stability. I've done this with
both an HP 117A on WWVB and WWV but not yet with the 3586C. The day-to-day
variation is dramatic.

Best,

-John

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


> 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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