RadioBanter - View Single Post

May 19th 04, 11:38 PM

In article ,
(Steve Kavanagh) writes:

Thanks for all your comments. Since speculation has started here is
what I know about the capacitors.

Those used in the 2.5 GHz source are surplus from a company that makes
high quality stuff. They were probably procured to a military or
space specification but I am not sure. The 10.5 GHz source was
manufactured by MA/COM about 20 years ago. All of them are the usual
deep maroon (is that the right word ?) to brown colour.

Keep in mind I am being pretty picky. I consider short term frequency
jumps of much over 100 Hz to be unsatisfactory - that is 10-40 parts
per billion depending on which source is considered. The largest
observed jumps are about ten times this. Of course, since these are
crystal oscillators, the corresponding capacitance jumps must be much
larger, since the crystal should dominate the oscillator stability. I
would not consider them "crappy", just not as good as one might be led
to expect. I have used capacitors from the same provenance as those
in the 2.5 GHz source in LC oscillators at a few MHz with no observed
problems. The smooth portion of the warm-up drift is reasonably
normal in both cases...only the jumpiness is unusual.

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 hook-ups and a number of
different techniques very much uncommon in home workshop
practice.

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." That would
include whatever it is you are using to heterodyne with the X-band
source to enable frequency measurement.

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'm talking primarily the
"dipped" coating DMs with some excursions into the molded
plastic case axial lead models. 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...and I HAVE heard lots of urban-myth stories on
other things within all of electronics.

Len Anderson
retired (from regular hours) electronic engineer person