Don Pearce wrote:

On Mon, 01 May 2006 16:26:57 GMT, "Michael A. Terrell"
wrote:

In our analog receivers the DC offset was measured, and a 18 bit ADC
was used to trim the offset to keep it to about a millivolt from
wherever it was set. Some external decoding required a fixed DC offset,
and this was available from the front panel. We also had a 63 dB range
in the output level control, digital with accurate .1 dB steps. This
was on the Microdyne 700 and 1620/1670 series telemetry receiving
equipment.

Mine was the error amplifier in an ultra low noise RF synthesiser. The
low frequency drift that resulted from the air currents resulted in
phase noise from the oscillator in the .01 to 1Hz range. It was really
hard to fault find.


I had to clean up an older design RF synthesizer because of multiple
engineering changes and parts going obsolete. The unit was inside a
double shield, so it was quite temperature stable, but the uncased
ceramic disk capacitor they originally used as standoffs was no longer
available. They switched to a good quality microwave ceramic capacitor,
and soldered them next to the large vias that were originally used to
solder the caps to the board. This caused the phase noise to go through
the roof. The fix was simple. Solder the holes closed, then solder the
cap where it was in the original design. The PLL was broken into three
band segments to reduce noise, and forward biased diodes were used to
short out part of the inductor to raise the frequency. The rest of the
phase noise was from the two RF output amps ringing due to a minor
change in the layout. I was working to use a Maxim IC to replace the
two amps when I was laid off in Sept. 2001. I can't remember the number
right now, but the two outputs were 180 out of phase to reduce the
current load variations. I also made some changes to the reference
frequency traps to clean up the DC loop control voltage.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida