Re: Crystals - a little info

Bry Carling <af4k@...>

The oscillator will stop long before those kinds of VXO values are achieved. You will be lucky to pull them 1 kHz and still have a decent signal.

I always look for BJ stations around 7121.3 and have had luck so far.

Bry AF4K

From: 4sqrp@... <4sqrp@...> on behalf of n5ib@... [4sqrp] <4sqrp@...>
Sent: Sunday, January 22, 2017 2:14 PM
To: 4sqrp@...
Subject: [4sqrp] Crystals - a little info

As we've found, most of the crystals supplied with the Bayou Jumper, though nominally 7122, oscillate a few hundred Hz low. A number of folks have used small series capacitors to pull the frequency upwards somewhat.

A little info and tutorial...

A crystal in the 7 MHz range can be reasonably represented by a series RLC circuit, with a bit of capacitance shunting the whole affair. The principal quantities we are concerned with are Rs (series equivalent resistance), Lm (motional inductance), Cm (motional capacitance) and Co, the shunt, or holder, capacitance.Typical values for crystals with acceptable Q might be:
   Rs = 10 ohms
   Lm = 25 millihenries (yes, milli, not micro)
   Cm = 0.02 pF (yes, much less than a pF)
   Co = 5 pF

Think about what happens if you add some capacitance in series. The result is to reduce the net capacitance. But the amount will be small, since any amount you can solder in will be at least a couple of pF or so, much bigger than Cm. Lets use those values above and do a little math for a "back of the envelope" estimate:

Ignoring Rs and Co for the time being, 22 mH and 0.017 pF resonate at  7117.623 kHz

Now add in series about 4 pF (about the bottom end of a 4-50 pF trimmer)
the 0,02 pF capacitance becomes 0.01990 pF
and the new resonance would be theoretically 7135.487 kHz  --  18 kHz higher

    probably wouldn't happen, the crystal would quit

How about the 50 pF upper end of the trimmer

     now you'd get 0.1999 pF and 71199.049 kHz

How much you can actually achieve of those quick estimates depends a lot on the crystal itself - cut, mounting, quality, etc. The HC-49S types likely won't move as much as the HC-49U, and a real FT-243 would be quite different.

If you can obtain realistic values for Rs, Lm, Cm, and Co,  you could use tools like LTSpice to try and predict what you'd need to move a crystal, bearing in mind that what you achieve in practice will almost certainly fall short.


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