>
>
>Dave,
>
>Keep in mind that the Midland 13-509 was built for the Amateur Radio market,
>and the transmit crystals are only specified to maintain .001%, which is 10
>PPM. There is no temperature compensation in the crystal circuit, and you
>may make the drift worse by using an NPO capacitor. If you have the time
>and the test equipment to do it, you can determine a crude temperature
>compensation by finding out how much the TX crystal drifts for a given
>change in temperature, then using a capacitor whose TC has an equal but
>opposite effect. This capacitor will definitely not be an NPO type, which
>is stable over a wide temperature range. In fact, you want an "unstable"
>capacitor that exactly balances the crystal drift.
>
>Commercial radios of the same vintage often used bare crystals with a color
>dot on the side of the can, and you were instructed to install the
>appropriate color TC capacitor with that particular crystal. Not perfect,
>but adequate.
>
>You might also consider replacing the bare TX crystal oscillator with a
>small TCXO unit from any of several sources, including ICM.
>
>73, Eric Lemmon WB6FLY
>
>
One other thing you might try - tape a 100 ohm 1 watt resistor to the
side of the crystal and put 12 VDC across it. Not a pretty sight, and
Eric will probably really bad mouth this one, but it can move the
crystal temp into an area of its temperature range that stays more
stable frequency wise with external temperature changes.
I have had good luck and bad luck with this scheme. Sometimes it makes
the frequency stability better from a temperature change standpoint, and
sometimes makes it worse. It all depends on just how your crystal is cut.
73 - Jim W5ZIT
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