In message .com,
writes
On Mar 13, 12:02 pm, "Anthony Fremont" wrote:
Hello all,

I was playing around and saw that my junk box had all the parts so I started
tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg
The problem is (well I think it's a problem) is that I'm all the way down to
a 10pF cap for the crystal trimmer and the highest frequency I can get out
of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm
thinking that the 20uH inductor is supposed to pull the colorburst crystal
high in frequency

The inductor will pull the crystal down in frequency, as others have
suggested.

In fact it is extremely difficult to pull a crystal's series resonance
up in frequency more than a few Hz. This is because the crystal's
parallel resonance is just above its series resonance. If you put a
capacitor in series with the crystal the series resonant frequency
goes up...BUT...if you approach the parallel resonant frequency you
can no longer get a low impedance resonance condition since the
crystal's parallel resonance makes the crystal look like an open
circuit, regardless of what you put in series with it.

If you want to understand this better, try to find a reference with a
good discussion of the equivalent circuit of the quartz crystal
resonator. the only one I know of at present is Kenneth K. Clarke and
Donald T. Hess, Communication Circuits: Analysis and Design, Addison-
Wesley Publishing Co., 1971. It may be a bit hard to find outside a
good university library.

As Ian Jackson suggested a parallel inductor might work....this
modifies the parallel resonance. Ian..is there a schematic available
for that ? I'd be interested in what actually worked.

Steve


I'll try and draw what I have in mind, and post it in
alt.binaries.schematics.electronic. However, in the meantime, let me try
and explain. The explanation may not be absolutely 100% complete, or
even 100% correct, but it may help in moving a crystal more HF than it
wants to go. Sorry that it's a bit rambling!

In Anthony's circuit (http://newenglandqrp.org/files/w1aw-receiver.jpg),
the crystal will probably be functioning as a series-tuned circuit.

As Steve has stated, a crystal suddenly goes into parallel resonance
just HF of its series resonance. This limits how far the series
resonance can be pulled HF by the addition of a series trimmer
capacitor. However, if this parallel resonance can be removed (or moved
further HF), it should be possible to move the crystal further HF. The
technique described certainly does work with VHF overtone crystals
(between 50 and 200MHz), but should also work with HF crystals working
on their fundamental frequencies.

A crystal is a mechanical device, but can be represented as being a
series-tuned L-C circuit. (Call these L1 and C1.) Also, across the two
is a parallel C (C2). Forget about losses (represented by a resistor).
[Note: L1 and C1 are not actual electrical components, and only appear
to have these values at or near to the L1-C1 resonant frequency.
However, C2 essentially is a physical electrical capacitor consisting of
the plating on each face of the crystal, with the crystal as the
dielectric between.]

L1 is very large (possibly 1H or more, depending on the frequency). C1
is very small (say only a few pF or even a fraction of a pF - again
depending on the crystal frequency). [So adding a relatively large
series trimmer capacitor has very little effect on the frequency.] C2 is
typically around 5pF, regardless of frequency.

Imagine doing a test where you look at the resonant frequency of a
crystal, using a signal generator. This feeds an RF signal through a
crystal, into a 50 ohm load. You measure throughput of the crystal by
measuring the voltage across the load.

Swing the sig gen frequency slowly from LF to HF, through the resonant
frequency of L1-C1. [Let's forget about C2 for the moment.] Below the
resonant frequency of L1-C1, the L1-C1 circuit acts like a small
capacitor, so there is very little throughput. Above the series resonant
frequency of L1-C1, the L1-C1 circuit acts like a large inductor, so
again there is very little throughput. However, when you hit the series
resonance of L1 and C1 (F1), reactance of L1 and C1 cancel. The crystal
acts like a short-circuit (or nearly so) and there is a large
throughput. Because the L-C ratio is very high, the resonance peak is
very sharp.

The effect of C2 across the L1-C1 circuit is to produce a second
(parallel) resonant circuit. VERY slightly HF of the L1-C1 resonance, C2
resonates with effective inductance of the L1-C1 circuit. This produces
a parallel resonant circuit (F2). Another way of looking at it is that
L1 resonates with the series combination of C1 and C2 (so F2 must be
higher than F1). The parallel resonance is, of course, a high impedance,
where there is almost no throughput through the crystal.

As a result of this double resonance, the crystal acts as a series-tuned
circuit at F1 (one you want), and a parallel-tuned circuit at F2. The
transition between the two is very sudden. The frequency response peak
of the throughput is very lopsided, and gets chopped off suddenly on the
HF side.

The difference between F1 and F2 is very small (a few Hz to a few kHz,
depending on the frequency and type of the crystal). If F1 is lower than
you want, and you add an external series trimmer capacitor to try and
pull the crystal L1-C1 series resonance HF, you effectively hit a brick
wall with the parallel resonance at F2. The parallel resonance will
block any throughput at (or near) this frequency.

A possible solution is to neutralize C2.
[Note: Neutralization is a technique sometimes required when using VHF
crystals, as C2 may be large enough to allow the oscillator to free-run,
instead of being locked to the frequency of L1-C1. However, it may also
be used with advantage, as described below.]
You can neutralize C2 by adding an inductor across the crystal (ie in
parallel with C2). The value required is that which parallel-resonates
with C2 at the crystal frequency. In effect, C2 no longer exists. With
C2 neutralized, there is no longer a sudden transition from the wanted
series resonance F1 to the unwanted parallel resonance F2. The peak the
response curve of the throughput of the crystal (at F1) is now nice and
symmetrical, without the sudden cutoff at F2. In practice, the actual F1
peak will probably be somewhat more HF than before, and the crystal
should be more pullable with a series capacitor.

Finally, if you reduce the value of the inductor so that its resonance
with C2 is somewhat higher than the crystal frequency, this tends to
pull the F1 resonance peak even higher in frequency. However, if you
overdo this, the oscillation will probably unlock from the crystal, and
start to free-run.

As I said, sorry for the ramble.
Ian.
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