Hi i just come out with this basic AM demodulator circuit with a low pass filter at fc=15khz and high pass at 159hz. It is for demodulating a 27mhz AM with 10khz information signal.

I know i am supposed to use LC filters which are more efficient for high frequency signals but i wanted to test this out using lab components which has very few inductors.

I am a beginner in this RF thing. So any comments would be very much appreciated.

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You would normally use a germanium diode for a demodulator, rather than a silicon one - simply due to the much lower voltage loss across them.

AM radio also doesn't have anything like the bandwidth you're hoping for, which is why it sounds so poor. If you're hoping to get 15KHz bandwidth from an AM radio you're going to be interfering over a good part of the spectrum!.

As a matter of interest, 15KHz is about the bandwidth from a VHF FM radio!.

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This is a very low efficiency detector because of the signal loss over the diode. You can do a lot better by using 2 matched diodes as shown. This nearly eliminates the voltage drop over the detector diode. An additional inductor can be placed in series with D2 to reduce RF losses to GND. This 2 diode arrangement will also produce lower distortion.

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super_detector1.gif (10.9 KB, 333 views)

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My signal is not audio. It will be digital from the pc's RS232 port

 

That's even worse! - how fast are you hoping to use?, and what sort of modulator/demodulator (MODEM) are you planning to use?.

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oh sorry for the lack of knowledge. How do i determine the bandwidth from the rs232 port? is it from the baud rate?

I planned to modulate it using an AM transistor modulator. Demodulate using diode detectors.

 

Perhaps you've noticed that telephone modems don't work like that?, that could be because there are good reasons for it!. A simple MODEM technique is to transmit two tones, one for 'One' and another for 'Zero', you can detect these at the receiver with a 567 PLL.

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