[TenTec] Omni VI Xtal Oscillator PLL observation

[Steve Hunt <steve@karinya.net>]

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