RF amp problems

[stoveitin]

Hello,
I haven't done much in RF design atall so thought would have a play to try and learn some basics.

I thought I would try a very simple way. Transmit a carrier frequency and encode information by basically turning it on and off.

I have made a small 9V transmitter basically a pierce oscillator running from a 16Mhz crystal.
Antenna just a wire coming from the CMOS inverter output about 9Volts.

This seems to work as I have a receiving tank circuit around 100micro henrys with 1pf and measuring this with my oscilloscope I can see the transmitted signal that varies in strength depending on distance its around 100 to 200 milli volts incoming.

Trouble is I have tried all sorts of transistor amplifiers and MOSFET amps and seem to struggle to be able to amplify this signal from the incoming tank circuit.

the frequency is to high for op amps to work with.

Anyone have any ideas on how RF receivers successfully amplify the small incoming voltages?

Cheers tom

 

[duke37]

Your input circuit must be tuned to the received frequency. It should have a larger capacitance, say 50pF and an inductance to suit. This will be a more realistic L/C ratio. Any loading by the scope probe will give severe detuning.

To drive a transistor, the drive must be taken from a tap on the circuit to match the impedance.

The transistor must have a high frequency response, many times the input frequency. You can get MAR-1 to MAR-6 which contain two transistors with a very high frequency response. Ordinary op-amps are just too slow.

Get a circuit diagram of an FM transistor radio and study the input circuitry.