[time-nuts] Designing and building an OCXO and GPSDO

[Bruce Griffiths]

Attached schematic is for a variant of the Wenzel circuit using a JFET 
high input impedance buffer to drive a cascaded pair of CB stages.

L104 is approximately resonant with C111 at 3MHz so that either an RF 
choke and series resistor or a relative;y low value resistor (10K) can 
be used to set the JFET gate voltage.
The impedance of C104 at 3MHz is about 530 ohms so a 1mA crystal current 
will result in about 530 mV rms across C104.
An additional inductor can be used in series with the crystal to 
resonate out the input capacitance of the oscillator stage if required, 
this will allow a greater tuning range to be achieved.

Inductor L104  needs to have a reasonably high Q ( > 100 @ 3MHz) to 
avoid degrading the loaded  Q significantly.

The drain current of JFET Q103 is determined by the collector current of 
Q104 this minimises the effect of JFET parameter variations from part to 
part on its drain current.

The RF voltage at the source of Q103 can be amplified (a widebandwidth 
opamp could be used) and used to drive a detector diode for AGC purposes.
Alternatively a common base stage connected to the other end of the 
secondary of T101 could be used an RF current source to drive a full 
wave diode detector with a low load resistance.
This will largely eliminate the effect of the detector diode forward 
voltage drops on the detected output. This detector dc load current can 
than be compared with a reference current and the dc collector current 
of Q101 adjusted to regulate the crystal current.

Bruce
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