[R-390] xtal wave spikes

Mon, 25 Nov 2002 13:16:51 -0800

Bob,  the wave amplitude ranges from about 1 to 1.7 volts pp output from the
cathode follower that feeds to the single-balanced diode ring detector,  with a
couple of the xtals producing 0.8 volts pp.  I don't think there is  gross
distortion other than what I described but I'm just looking at it with a scope.
I'll order the dead crystals after trying a couple of different tubes in the
oscillator but I don't expect a miracle at this point with tube substitution.
Your comments about the square wave brought back some faint memories of fourier
series representation of square waves but my memory bank need refreshing about
that and what a mixer really does besides adding and subtracting  the
frequencies of waves put into it. I do remember that a square wave is generously
endowed with waves of many frequencies,   and hence its utility in testing.
thanks for the comments,  Dan

Bob Tetrault wrote:

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