Hi,

I'm trying to design a demodulator for a radio. The AM frequency that I want to tune into is approximately 1.2MHz. The thing i'm confused about is how to choose the values for my RC low-pass filter. How do I find out what's the frequency of the actual useful signal and the frequency of the carrier? I'm planning on using the BC108 BJT (see the attached schematic), but my reason for that is because I'm most familiar with it, which I doubt is a very good technical reason. Any help there would be appreciated. Also, are there any other factors that I have to take into consideration?

Thanks a lot.

 

The "classical" AM demodulator is a germanium diode and a small capacitor. On a superhet receiver with an IF of 455 KHz, a 1N34 and 150 pF work just fine. If you're going to run the RF straight into the demodulator, use a smaller cap.

Since you are the designer, you are the one to decide on the carrier freq. Being able to tune the front end might be helpful. The demodualtor couldn't care less about the signal freq.

 

Thanks for the reply.

I have seen the circuits with the diode, but read somewhere that apparently, putting the BJT in helps to produce a full output current drive and also helsp to buffer the input signal from loading the next circuit... or does that not matter?

What did you mean by me deciding on the carrier frequency? I thought that the station transmits its stuff multiplied by a carrier signal, and in order to demodulate it, i need to have a low pass to filter out the carrier signal?

 

Here's the second hit on google - http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part9/page2.html. It should answer all you questions.

You still make the decision as to the frequency to tune to.

 

ah okay.. that makes sense now..
Thanks a lot