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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.