question about buffering on an RF input from antenna to mixer
I have a question about buffering on the input of a radio receiver. The way I am building it, there is an antenna and a resonant tuning circuit applied to the input of an active mixer. The active mixer is built around a 2N2222 transister, which has an hie of , I believe, less that 1Kohm. It also has a 10K ohm resister in series with the base (and a coupling capacitor). I was wondering if, since the impedance of the antenna is 50 ohms, would I need a buffer amp between the tuner and the mixer? The circuit can be seen at http://colpittsoscmxr.net63.net/
George
By the way I wanted to thank Brian and Walter from the other posting for their help with a previous question, but my internet connection wouldn't let me add any more replies. Or else there is a limit now, on replies. I'm not sure.
Yes. One advantage is that your LO will not be radiated by the antenna and disrupt reception of others.
Thank you. I will do that. I was also wondering, could lack of a buffer on it's input cause the oscillator to not work?
Every cheap transistor radio uses this configuration, i.e. no RF transistor. The secret is to match the input impedance of the base of the transistor to the high impedance of the tuned circuit. Also the aerial, this has typically about 20 turns or so on a coil (the free end of the coil is the earth connection) loosely coupled to the tuned circuit. For the main tuned circuit, use a 9:1 tapping point for the base, i.e. 200 turns, tapped at 20 turns from the earthy end. A big piece of ferrite will help to reduce the number of turns and increase the Q of the circuit (only if the ferrite is of the correct grade for the frequency). As for your circuit with a 10K in series with the base, this resistor must be bypassed or there will be a huge loss in signal.
Frank