[time-nuts] Xtal-like Colpitts Oscillator

[Bruce Griffiths]

Pete wrote:
> Jenny,
>
> In your schematic I'm assuming the uppermost
> transistor connection is the emitter. 
>
> Your bias scheme is unlikely to exhibit its "paper"
> impedance at 34MHz. Any stray capacitance(s)
> can easily result in AC paths to ground lower
> than the L & R impedances in your circuit.
>
> The bias inductor will have a sef-resonant
> frequency well below your planned operating
> frequency. This inductor is probably useless.
>
> You need to modify your circuit model by
> adding a bunch of stray elements before it
> will be apparent why it doesn't work.
>
> Regards,
> Pete Rawson
>
>   
If you use a VHF choke similar to those produced by Siemens Matsushita, 
a 220uH choke has a maximum impedance at about 34 MHz. Since its 
impedance drops steeply thereafter a somewhat lower value should be used 
to allow for manufacturing tolerances. If you are using non VHF chokes 
then the frequency at which the maximum impedance is achieved for a 
given inductance can be substantially lower. These VHF chokes are 
relatively large (~30mm long).

The substantial shunt capacitance of C6 may cause some difficulty in 
this circuit, tuning it out with a shunt inductor may help a little but 
the presence of R8 will make this lesseffective than it would otherwise be.

C1 and C2 seem a little high for 34MHz operation using a resonator with 
an effective ESR of around 200 ohms or so.

One of the Driscoll HF crystal oscillators may be more useful than a 
Copitts.
Depending on the required stability an RF transformer connected between 
the collector and tank of such an oscillator may be used to extract a 
low noise current signal to drive a common base buffer transistor.

Bruce