wrote:
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.

I understand now. I knew that adding capacitance tended to slow xtals down.
I was unfamiliar with how an inductor would act in series. I knew that this
circuit was trying to pull the crystal higher than spec so I, quite wrongly,
assumed that the inductor would have the opposite effect of capacitance.
And it seems that it does when used in parallel and that seems sensible to
my feeble mind. But in series, it has the opposite effect. What still
throws me is that raising series capacitance doesn't have the opposite
effect of adding parallel capacitance.


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.

Hmm....food for thought. :-)

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.

Doodling with reactance formulas, it appears that 20uH (coincidence?) would
offset 100pF of capacitance fairly well by having a an opposing reactance
(well resistance at this point) of about 450 Ohms at 3581kHz, the same as
100pF. I'll try putting the coil I made in parallel and see what happens.
Hopefully it won't short the oscillator and kill my 15 year old NE602, I
only have two spares.