Oscillator loading question

Hello,

My next question is:

When designing a buffer for the output of an colpitts oscillator, if you make the coupling capacitance not a significant component of the input impedance of the buffer, by choosing a capacitance of 1uF, using a 7MHz oscillator and the resistive component is 10Kohms. Would the oscillator frequency be affected by either the capacitive or resistive component of Zin of the buffer, or perhaps by both? It theoretically still meets the criterion for oscillation.

The site at the following link provides some info about the circuit: http://colpittsoscmxr.net63.net/

George

Note:
After first posting this question I modified the circuit so the common collector buffer is always connected to the oscillator output and the audio output amp vs the power output amp (to antenna) are switched in and out. I thought this would make the frequency stop shifting between Rx and Tx. I didn't help. I don't know what else could be the problem. Any suggestions?

The coupling capacitor should be as small as possible - 50pF? For the ultimate isolation the common base amplifier is much better. The problem with the common emitter is that its input impedance is Hfe X the output load, so while its a high value, it will change as the load at its emitter changes. Are you sure that your layout and screening is OK and the power lines are properly decoupled to earth?
Frank

IF you actually make the measurement....you need a surprising amount of load isolationn to not pull the oscillator frequency. In most applications, where the VCO is used in a "steady state" environment you do not care, as a slight pulling of the oscillator frequency can either be tolerated or compensated for in a PLL. But if you have a changing load (say switching between 2 antennas) and you need an accurate phase, you might need as much as 60 dB of load isolation.

Thank you for the replys fellows. I am not sure if my layout and decoupling is sufficient. I will try to make some improvements with those. As for the dB isolation. If I understand that correctly as an impedance ratio, there is more than adaquate (>60dB) isolation. I was thinking of trying a common base stage after the common collector. I have to wait until I get the parts. If I get it to work I will post it on above mentioned website. Thanks again.

George