[time-nuts] Basic question regarding comparing two frequencies

[Bob Camp]

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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