[time-nuts] FMT

John Ackermann N8UR jra at febo.com
Fri Dec 29 15:10:31 EST 2006


Mike Suhar said the following on 12/29/2006 02:32 PM:

> I used an HP 3586C Frequency Selective Voltmeter with the tracking generator
> looped back to the RF input via a step attenuator.  The audio output went to
> the PC.  An ICOM IC-745 was tuned to the 160M signal to hear the test
> announcements throughout the test period.  The antenna on the HP was a SWL
> slopper antenna that favors the lower frequencies.  The ICOM was on a
> 40-meter dipole. 

I too use the 3586C but there is one thing to be aware of: while the
main frequency synthesis system can be locked to an external reference,
the BFO signal comes from an independent oscillator that is not
disciplined.

The last IF is at 15.625 kHz and the BFO frequency (generated on board
A22) is (for the "C" version) is either 17.475 kHz (USB) or 13.775 kHz
(LSB).  The BFO is derived from a crystal -- 1.7475 MHz for USB and
1.3775 MHz for LSB; the crystal frequency is divided by 100.
Interestingly, it appears that the BFO signal is injected into the
product detector as a square wave; unless I missed it, there's no filter
following the divider.

After a bit of warm-up, the BFO is very stable (dividing by 100
certainly helps reduce any drift) but the nominal 1850 Hz tone generated
by a signal on the tuned frequency is likely to be off by a fraction of
a Hertz.

One of my plans is to build a BFO synthesizer to replace the crystals
and thereby eliminate any frequency offset.  Having done that I should
be able to directly determine frequency by measuring the output tone.

John




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