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: [email protected] [mailto:[email protected]] 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
