Hi Ed,
On 4/25/2012 9:41 AM, ed breya wrote:
Ed,
I downloaded that service manual - an interesting read. So much stuff
was well-figured out even back then.
Yes, the building blocks were all in place and are still used today.
But when you look at some of the implementation details you see a few
things that just have you scratching your head. My favorite is: How do
you power TTL chips when you only have 15 or 20 volts available?
Simple, just stack 3 or 4 chips in series and use capacitors to couple
the inputs and outputs. I would never have thought it would work, but I
guess it does.
You should look closely at section 4.4.7.1. regarding the presence of
the 2f (310 Hz) signal.
Yes, that's the normal method of operation. The presence of the 2nd
harmonic plus absense of fundamental equals a locked condition so stop
the sweep, turn on the green light and connect the output of the
Synchronous Detector (the Loop Error Signal) in series with the
Oscillator Frequency Control to close the frequency control loop. But,
as the OCXO drifts, it's the Loop Error Signal that nudges the OCXO back
on frequency. The 2nd harmonic is only the indicator.
Look at section 3.5.3. That's the only mention of the Sweep switch.
But when I traced it out I found that it would allow locking without any
2nd harmonic because it bypasses the 2nd harmonic circuit, disables the
sweep and connects the output of the Synchronous Detector in series with
the Oscillator Frequency Control. This is the identical connection that
normally happens when the 2nd harmonic is detected. The green light
doesn't turn on because that function is controlled by the 2nd harmonic
only. The result is a closed frequency control loop and, as long as
there's enough Loop Error Signal to maintain it, a locked condition
regardless of the presence or absence of the 2nd harmonic.
Ed
Ed Breya
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