Here's data showing Doppler (and other effects) on WWV as received in
Dayton, OH over several days. I took this by reading an HP 3586C
frequency counter output via GPIB -- which seems to be a good technique
for long-term HF frequency gathering. You need to figure out a way to
remove outliers and signal loss periods from any averaging, but the 0.01
dB amplitude readout gives you a tool to help do that.
http://www.febo.com/pages/hf_stability/
John
----
Didier Juges wrote:
The only way to have that kind of meaningful accuracy with an on-air signal
outside of ground wave range (a.k.a FMT) is to average over a long time (days)
to average out the shift due to variations in propagation. The altitude of the
layer reflecting the signals changes over time, so the distance the signal has
to travel changes too, causing a Doppler shift. Measuring WWV at 15MHz over a
24 hour period shows about 1Hz pp variation (that's what I found the last time
I did with my Thunderbolt locked HP3586). If you make a short term measurement
(a few minutes) you may be off by 1/2Hz easily regardless of the accuracy of
your equipment.
You may well be able to measure the frequency of the incoming signal to
0.001Hz, but it will be sheer luck if it is the same frequency they are
transmitting.
Didier KO4BB
------------------------
Sent from my BlackBerry Wireless thingy while I do other things...
-----Original Message-----
From: "Guy Lewis" <g...@coho.net>
Sender: time-nuts-boun...@febo.com
Date: Mon, 26 Jul 2010 13:50:49
To: 'Discussion of precise time and frequency measurement'<time-nuts@febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts@febo.com>
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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