Ian Jackson wrote:
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.
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