I think I just answered my own question.
All the Xtal Oscillator frequencies are multiples of 50KHz, apart from
on 30m where it is about 24,501.8KHz. I guess TT introduced the 1.8KHz
offset so that they could offset the VFO by the same amount and avoid
some in-band spurii - possibly caused by the VFO's second harmonic on
this band. The unusual Xtal oscillator frequency would require a
different reference frequency in the PLL chip.
73,
Steve G3TXQ
Steve Hunt wrote:
I set up my Omni VI Xtal Oscillator frequencies yesterday using a TT
Service recommended method posted on the list a few months ago. It
involves monitoring the LD (Lock Detect) pin on the MC145107 PLL chip
using an oscilloscope. When the loop is in lock, the waveform is a
series of very narrow, negative-going, pulses; you adjust the oscillator
for lock, and then for minimum width of the pulses.
I noticed that the pulses occurred every 20uS on every band except 30m,
where it was every 30uS. Looking at a data sheet for the chip, the pulse
repetition rate is the same as the (programmable) internal chip
reference frequency.
My question is: why would TT choose to use a different reference
frequency for stabilising the 30m Xtal?
73,
Steve G3TXQ
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