# FM receiver ?

Discussion in 'Wireless & RF Design' started by Mathematics!, May 17, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Now that I have a good understanding of how to make AM and FM transmitter. Ossilations coplitts ,...etc for the carrier waves ,...

I am know wanting to know how to go the other way around.
That is the reciever end.

What I don't get is once the wave hits the antenna of the reciever.
How do I make the tuning circuit is it just the same as the transmitter LC circuit.

So I guess what I want to know is how to make these stages tuner circuit , demodulator circuit , amplifier circuit for the reciever.

Does anybody have a simple scheme for FM recevier.
Some thing that can plug into an audio port of a computer or if not something that outputs it to a 8ohm speaker.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
It is easier to make an AM receiver rather a FM one.

However, you can find many schematics in the internet with the help of a search engine like google.

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I am more curious about the demodulation circuit for FM.

AM is really just using a diode to cut the bottom part of the wave out.

Also how about the tunning circuit part for a reciever would this be just the same as how you do it for a transmitter?

Jan 28, 2005
9,030
214
5. ### DonQ Active Member

May 6, 2009
320
11
And this (or some other form of "demodulation") is generally done after things like "local oscillator", "Mixing", "IF Bandpass", and a few other details.

What would probably work better for this range of subjects is to do a search for an already written tutorial, rather than to ask for a personal tutorial on such a broad topic.

Apr 5, 2008
15,799
2,386
7. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Your statement -
- isn't accurate. The non-linear conduction characteristic of the diode means that the modulation voltage is left on the following capacitor, while the carrier is "lost".

You could learn a lot by looking in a book.

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
A common way of teaching AM is your statement, but it is a way of getting a point across, and isn't really correct.

When you look at an AM signal there are sidebands. If you modulate a carrier wave with 1Khz there will be two sidebands, one 1Khz above the carrier, one 1Khz below the carrier. Either one will work for decoding back to audio, but it is where upper sideband modulation and lower sideband modulation come from. You can even eliminate the carrier, and just transmit the modulation, which really cuts down on the power sent on the transmitter. All of these are used by CBs in the USA.

The diode beats the carrier and sideband together. Beating in this case refers to heterodyne, same thing. Heterodyne the carrier and the sideband and out pops the 1Khz waveform.

The reason books teach AM crystal radios is the real theory is rather advanced, and the other concept gets you on the road. It is more of an analogy that a real theory of operation.

FM decoders are a bit harder to work out. Take two tank circuits, tuned closely to each other, each has a diode rectifier. One rectifier creates a positive voltage, the other a negative, and the outputs are summed. So as the FM freq swings between the two tanks you get a equivalent swing on the output. This circuit is called an FM discriminator, and was used for old style television signals on satellite receivers (or other types of signals, such as telephone), and was called base band. Base band goes from DC to 8.5Mhz, occasionally higher. Audio is a piece of cake compared to these applications, but the fundamentals are still there.

There is another technique used to discriminate FM, that is almost never used because it is so finicky and unreliable. Take the following tank circuit curve, with a diode...

It is theoretically possible to receive an FM signal with a AM receiver, but not practical.

• ###### temp5.GIF
File size:
2.2 KB
Views:
115
Last edited: May 24, 2009
9. ### Tesla23 Active Member

May 10, 2009
323
67
I used to think the same until I came upon the FM crystal set. A friend used them as student's practicals and they worked a treat. I queried the distortion aspect and he said that tuning highest output also seemed to give an undistorted signal, but he wasn't sure why. When you think about it this is correct as the maximum slope occurs when the second derivative is zero, this also minimises the distortion.

I'm sure if you google it you can find FM crystal sets, and it is possible for them to work well.

10. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I know they existed, but you won't find a commercial radio using them. I doubt they would work in my city, the dial is just too crowded, and that bell curve has two sides.

Radio Shack had a kit that used regenerative FM tuning. The sucker worked very well, but killed any other radio tuned to the same station. Used only one or two transistors, so it was very simple. Wish I could find that schematic again someday.

Apr 5, 2008
15,799
2,386
12. ### Tesla23 Active Member

May 10, 2009
323
67
I agree - this is just a beginner's kit.

Yes, I was skeptical, but the kit they used here in Sydney gave quite good performance (for a crystal set) and the dial here is modestly crowded. It was so successful that they built them year after year and the kids loved them.

Aug 21, 2008
2,768
669
14. ### radiohead Active Member

May 28, 2009
474
31
google, "Radio Shack Special" The schematic is simple and it works.