Just to set the record straight....

Someone suggested that you need to 'neutralize' the parallel resonance so
the series resonance can be tuned toward it. This is completely wrong! The
series resonance is, for practical purposes, invariant. The motional
parameters (L and C) of the series resonance are such high reactances (small
capacitance; high inductance) that external components have only a tiny
influence on the series resonance.

The series resonant frequency is the lower of the two crystal 'resonances'.
The parallel resonance is above it. When you make a VCXO with any
substantial tuneability, you're probably operating the crystal at its
parallel resonance. This leads to the common observation that you can 'pull'
a crystal up in frequency more than you you can pull it down. You can only
pull the parallel resonance to approach the series resonant frequency, but
you can't pass it because the crystal is effectively a short-circuit at that
frequency.

Also for the record, the crystal's quartz only has one fundamental and
significant natural resonance - the series resonance. The so-called
'parallel resonance' is actually a controlled spurious resonance caused by
the holder capacitance. At frequencies above series resonanve, the crystal's
RLC equivalent looks inductive, and at some frequency the holder capacitance
will resonate that net inductance.

Joe
W3JDR


"Ian Jackson" wrote in message
...
In message , Anthony Fremont
writes
Ian Jackson wrote:


As I said, sorry for the ramble.
Ian.

OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You
should set your clock to way in the future and repost that message so it
sticks around for a while. ;-)

So to make a long story short I need to put an inductor of roughly 400uH
across the crystal to cancel the 5pF of C2. Wow that's a ton of
inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do
it.
I'll let you know how that works out. I found another crystal,
unfortunately it's identical and possibly from the same batch. I haven't
tried it yet, but I'm not expecting any miracles. I'm tired of burning my
fingers unsoldering parts, so I'm goint to tinker on the breadboard with
another 602 set up just for the oscillator testing with capacitor changes.
I will apply the new coil to the soldered up version though.

The receiver hears, as we just had a storm earlier and I could hear
lightning crashes in the distance. In my narrow tuning range, I can hear
what is likely the carrier of a broadcaster too, or maybe my TV. Later
tonight when the band opens up some more, I should hear something from
W1AW
hopefully.

Thanks again



As you say, at around 3.5MHz, you will need a fairly large inductor to
resonate with 5pF. An alternative might be to make a bridge circuit, where
you actually use another (5pF) capacitor to balance out the unwanted 5pF.
I used to use an extremely simple balancing circuit to make accurate
measurements of the resonant frequencies and ESRs (equivalent series
resistance) of VHF crystals, and it should be possible to use something
similar in an oscillator. However, maybe someone out there can advise on a
tried-and-tested circuit which will definitely work. [There's no point in
re-inventing the wheel!]
Ian.
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