[R-390] xtal wave spikes

Mon, 25 Nov 2002 10:01:46 -0800

Dan,

The presence of irregularities in a waveform is not immediate evidence of a
failing crystal. Either they don't work or they begin to lose output and
ultimately don't work. Unless the oscillator is tuned, at the output, for
the crystal frequency, and unless the oscillator is biased explicitly for
that particular crystal and mode of operation, there is very great
likelihood that some harmonic energy is being generated by the oscillator.
Also, some oscillator circuits are better at harmonic generation than
others. You would need a spectrum analyzer or a VHF/UHF receiver to
investigate this, and the fact that some of your crystals do not exhibit the
spike on a scope doesn't mean that significant energy isn't being produced
at *their* harmonics. The fact that this energy is being injected into a
mixer mitigates the presence of any harmonics, as the mixer itself is a
harmonic generator. They are sometimes referred to as a comb generator in
the older literature. In other words, don't worry. Is there sufficient
amplitude? Is it grossly distorted?

This begins to get off-topic, but there is ample and long standing evidence
that a square wave with very fast edges is the best waveform to drive a
mixer. Such a waveform has quite high energy in the harmonics. It is the
relatively slow transition in the interval between either fully saturated ON
or fully cut OFF that promotes susceptibility to intermod. Thus a fast
square wave minimizes the susceptible time.

Bob

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Dan Merz
Sent: Monday, November 25, 2002 9:22 AM
To: R-390 List
Subject: [R-390] xtal wave spikes

Hi,  in looking at the output from the front end high freq. oscillator
(40 to 60 Mhz xtals) on the Mackay 3010,  I noted double peaks on some
of the waveforms,  though the nominal frequency was ok.  My first
reaction was that this could be symptomatic of a failing crystal though
the output works ok and the right signals are received in the radio on
the band in use.   The spike occurs on the downside of the wave,  kind
of a minor glitch as the wave is decreasing.  Does anyone know what this
means or what is causing it?  Most of the xtals don't exhibit this
characteristic, but a couple are dead.  I'm going to replace the dead
ones - MH Electronics says they will provide replacements for $15 ea if
I'll send them a good crystal as example of spec's.  Is this a
reasonable price compared to other possible providers?  thanks,  Dan.

_______________________________________________
R-390 mailing list
[email protected]
http://mailman.qth.net/mailman/listinfo/r-390