RadioBanter - View Single Post

May 20th 04, 02:12 PM

(Avery Fineman) wrote in message

Coming in at the end of this discussion, I'll have to question the
observation of the so-called "frequency jump." At 100 PPB we are
talking quite serious test equipment

Yes, but the equipment here is up to the task. For the 10.5 GHz case
I am using this source as a receiver LO. A second receiver is
available and both can receive a third source. Both receivers
heterodyne the signal down to audio frequencies where any frequency
steps can be easily detected by ear. The receiver with the M/ACOM LO
jumps (frequency steps) with respect to the third source while the
second receiver does not. A similar scheme was used at 2.5 GHz.

If you are locking the X-band source to a crystal reference, then
there is a great deal of this frequency-control subsystem which can
be a cause for the "jumps." That can be the sampler or prescaler,
the phase detector (assuming it is a form of PLL), the loop filter, and
even the power supply rail voltage (affecting the voltage control of the
presumed voltage-controlled frequency adjuster circuit). ANY of
those can be the culprit in small frequency "jumps."

And from qrk:

Crystals can also jump.

Yes, I have not yet narrowed down the search in the M/ACOM 10.5 GHz
source, but was wondering about the likelihood of the silver mica caps
being the culprit. It has a measured supply voltage sensitivity of
about 50kHz/V (post regulator). The plug-in (not soldered) crystal in
an oven would also seem to be a likely source, particularly as the
jumping seems to improve after warmup (but long after the oven reaches
temperature).

I'm going to question all those others' claims about "jumpy silver
mica capacitors" after about 54 years of having hands-on
experience in RF and pulse circuitry.... I've never had one either open
or shorted and never "jumpy" in value and that includes the full-on
military environment testing of temperature, altitude, shock,
vibration, etc. A very few were found not quite within the capacity
value tolerance and not a single one experienced any "jumping"
of value. I've not heard of any such stories from contemporaries
in the industry...

Well, Tom Bruhns is the first one I have run across who has also noted
this, so it can't be a very commonly experienced effect. But I am
pretty sure about the source of the jumping in the 2.5 GHz source.
Each silver mica capacitor replaced made it better (unless the PC
board just needed the thermal cycling resulting from a few extra
soldering operations).

Steve