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

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Aug 12 23:16:48 EDT 2008


Hi Bruce,
 
do you have any phase noise/stability measurements for these  circuits?
 
thanks,
Said
 
 
In a message dated 8/12/2008 18:53:38 Pacific Daylight Time,  
bruce.griffiths at xtra.co.nz writes:

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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   Hi Bruce,



   do you have any phase noise/stability measurements for these circuits?



   thanks,
   Said



   In a message dated 8/12/2008 18:53:38 Pacific Daylight Time,
   bruce.griffiths at xtra.co.nz writes:

     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
     [cid:X.MA1.1218597403 at aol.com]
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     time-nuts mailing list -- time-nuts at febo.com
     To unsubscribe, go to
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     and follow the instructions there.
     __________________________________________________________________

   Looking for a car that's sporty, fun and fits in your budget? [1]Read
   reviews on AOL Autos.

References

   1. http://autos.aol.com/cars-BMW-128-2008/expert-review?ncid=aolaut00050000000017
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