[time-nuts] Designing and building an OCXO and GPSDO
bruce.griffiths at xtra.co.nz
Tue Aug 12 21:51:45 EDT 2008
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.
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 44691 bytes
Desc: not available
Url : http://www.febo.com/pipermail/time-nuts/attachments/20080813/70229078/attachment-0001.gif
More information about the time-nuts