[time-nuts] Synths for FMT

[Bob Camp]

Hi

Some basic approaches to the LO issue:

1) Use a radio that is "fully synthesized" (R-1051 etc) off the GPSDO
2) Calibrate the radio with some form of synthesizer
3) Do both (trust but verify :)

A DDS running off the GPSDO is one way to do the calibration. A good old 
100KHz / harmonics of a square wave type calibrator is another way to do it. In
this case the 100 KHz (or what ever) would come off of the GPSDO. A PIC
can be used as the divider and set to an arbitrary “magic number” that does
what you need. 

There is a “sweet spot” in most setups in terms of beat not frequency. Lower
is better when you get into post processing (unless you play with some DSP 
code). Lower ultimately gets into issues with audio rolloff and noise issues. 

Lots of fun

Bob

> On May 14, 2015, at 2:44 PM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> I'm thinking about sticking my toes in the water with the ARRL Frequency
> Measuring Test. http://www.arrl.org/frequency-measuring-test
> 
> Frequencies used on the air, are 14121 khz, 7055 khz, 3598 khz.
> 
> A current idea, is to use my Z3801A's 10MHz output to generate a LO that
> would be used with a mixer to convert the FMT's down to audio frequencies,
> to be recorded by a PC's sound card and then post-processed.
> 
> Would feed the output from the mixer into one channel of the sound card.
> Would feed 10MHz/10000 = 1kHz square wave (derived from Z3801A) into other
> channel of sound card as a "Calibration" audio signal.
> 
> LO Synthesis ideas:
> 
> For 3598 kHz, make a 3600 kHz LO this way: Triple 10MHz to 30 MHz, then
> triple again to 90MHz, then divide by 25 to get to 3.6 MHz LO. This will
> result in an audio frequency circa 2kHz.
> 
> For 7055 kHz, make 7058.8(...) kHz LO this way: Triple 10MHz to 30MHz. Then
> double and double again to 120MHz. Then divide by 17 to get to 7.0588MHz
> LO. This will result in an audio frequency circa 3.8kHz.
> 
> Don't have any good ideas for 14121 kHz.
> 
> Any thoughts?
> 
> Tim N3QE
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