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LF: Xtal osc "delta emitter" tuning

To: [email protected]
Subject: LF: Xtal osc "delta emitter" tuning
From: "Vernall" <[email protected]>
Date: Mon, 7 Jun 2004 20:36:24 +1200
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Hi all,
I've been experimenting with high stability crystal oscillators for methods of fine tuning by external electronic means.  Varicap diodes are nonlinear for voltage and temperature, so I was wanting to use some other way.  Crystal oscillators usually have coarse adjustment with an NPO trimmer capacitor, so fine tuning would be acceptable even if it was less than 1 ppm in adjustment range (sufficient to correct for ageing over a fairly long period).
What I came up with is variation of emitter load resistance in a Colpitts crystal oscillator.  20% or so of change in emitter resistance gives smooth control of fractional ppm change in oscillator frequency.  I have been using a fixed resistor to the emitter that is ten times the installed emitter resistor, and then a pot (rheostat) to ground also of ten times the emitter resistor.  This arrangement gave 0.4 ppm range for fine tuning with the home brew 10 MHz oven oscillator being tested.  There are minor effects with thermal equilibrium when emitter current changes, so if a precise setting is needed then more than one tweaks is needed, but it is still quite a convenient way of carrying out fine tuning with a panel mounted control.  The lead can be bypassed at the oscillator so the control is "cold" at RF.  Also the control can be a floating with earth return to the oscillator circuit to minimise a ground loop influencing the oscillator.  Probably a wirewound is the best choice for the type of pot.
Some types of oven crystal oscillators on the surplus market do not cater for fine tuning by electronic means (only have screwdriver access to a trimmer capacitor).  If the oscillator housing can be opened up is is fairly easy to implement a "delta emitter" wire for fine tuning.  Sometimes it is frustrating to adjust the capacitor as the only means of making a precise setting, as there is inevitable backlash effects, whereas the "delta emitter" method gives smooth fractional ppm adjustment (with due regard for temporary thermal equilibrium). 
73, Bob ZL2CA 
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