Did a bit more work on the Omni VI today, and I'm pretty happy with the
results.
The line carrying the RF output from the Xtal Osc board to the Logic
board phase comparator also returns the DC control voltage to the Xtal
Osc varactor diode. So looking at this complex waveform with a DVM to
measure the loop control voltage was probably not very sensible :)
Looking at it with a 'scope makes it much easier to see what is going on.
I've now pretty much centred the DC component between Gnd and +5v , and
all the oscillators are now stable and under the control of the loop -
i.e. you can make a small adjustment to the inductor and the frequency
stays the same! The loop time constant is very slow - presumably so as
not to damage the Crystal Oscillator phase noise performance - so when
you tweak the inductor you have to allow half a second or so for
everything to re-stabilise.
Yes, in the process I managed to damage one of the inductor cores.
Fortunately I was able to remove the coil, get at the core from the
other end, and re-use it "upside down". You need a very steady hand to
remove the coils - they're only held to the PCB by the thin entammelled
wire used for the winding !!
I've asked Ten Tec Service if they have a recommended setting-up
procedure for the board. If not I'm pretty happy with it as it is.
73,
Steve G3TXQ
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