Hello ,




I've built an rf receiver from this schematic :

i use 1n4148 diode instead of germanium diode and a 0-50 pf trim capacitor with 6 turns coil for receiving FM signal , but I do not hear anything !!!

please help me and Thanks.

 

...i use 1n4148 diode instead of germanium diode...

Doesn't that suggest a potential problem to you? It may not be your only problem, but a silicon diode requires roughly double the signal to work.

 

use a germanium 1n34 type diode, and try a different breadboard, those kind arent very good at vhf, too much capacitance between strips.

 

And that diode is for demodulating an AM signal NOT a quadrature detector, Foster-Seeley discriminator or ratio detector as required to demodulate frequency modulation. So it appears among other sited flaws that the detector should be added to the list.

 

an am detector can be used for detecting fm, you tune to one side of center, called a slope detector.

 

Actually ... Under the right conditions, what looks like a simple diode AM detector can function as a slope demodulator.

http://en.wikipedia.org/wiki/Slope_detection#Slope_detection

The problem here is the diode; it must be the one in the schematic. An OA91 is a Germanium diode with just 3 pF junction capacitance. This can function as an FM demod with enough signal strength. Construction technique is important at FM frequencies, and a protoboard probably is killing sensitivity with its stray capacitances all over the place. Use perf-board and keep all lead lengths as short as possible.

ak

 

Hey! This circuit is actually a simple AM Demodulator. Even2demod anAm signal U would need a strong Am signal. The OA79 or 1N34 Germanium diodes are very,very imp here. besides you have done away with this imp comp&inserted a 1n4148(Si).how can you expect this basic circuit 2 do demod of Fm signals even if you get ur maths right on the tuned circuit.NO WAY! Demodulating an Fm Signal is a very,very different ball game al2gether. 1St get the original circuit2do what it was supposedly designed2do. Then only try2aim4a Fm detector. 1more thing! Irrespective of what others may advice u will have a tough time getting even a feeble squeal from your local Fm radio station. This is because u would have 2have a rather "Hot" assured Fm signal in order2get what is commonly claimed as slope tuning. Tried it? Aw stay off... inspite of doing all of the above the results will not even be as clear as the audio coming out of ur aforementioned basic crystal set

 

I found the original circuit, and it is in fact intended as an AM band receiver. But there are many examples of 1-transistor FM receivers, including one that is a direct conversion colpitts oscillator.

I hope that last part isn't too far off-topic.

ak

 

an am detector can be used for detecting fm, you tune to one side of center, called a slope detector.

More commonly known as a SLOP detector.

 

Hey! This circuit is actually a simple AM Demodulator. Even2demod anAm signal U would need a strong Am signal. The OA79 or 1N34 Germanium diodes are very,very imp here. besides you have done away with this imp comp&inserted a 1n4148(Si).how can you expect this basic circuit 2 do demod of Fm signals even if you get ur maths right on the tuned circuit.NO WAY! Demodulating an Fm Signal is a very,very different ball game al2gether. 1St get the original circuit2do what it was supposedly designed2do. Then only try2aim4a Fm detector. 1more thing! Irrespective of what others may advice u will have a tough time getting even a feeble squeal from your local Fm radio station. This is because u would have 2have a rather "Hot" assured Fm signal in order2get what is commonly claimed as slope tuning. Tried it? Aw stay off... inspite of doing all of the above the results will not even be as clear as the audio coming out of ur aforementioned basic crystal set

Thanks I followed the following rule to adjust the received frequency :

I am a beginner in electronic please tell me if can i receive any radio frequency with this circuit and rule , because i want to receive many private radio station , For the germanium diode I did not find it in my old rf modules :

Also i want to build a simple fm receiver for my fm transmitter to make a walkie talkie.

 

Hello,

Perhaps the following online book may give you some ideas of receiver techniques:
http://www.mikroe.com/old/books/rrbook/rrbook.htm

This is one of the links of the analog radio page of the EDUCYPEDIA:
http://educypedia.karadimov.info/electronics/radiotuning.htm

Bertus

 

The RF module you show has plenty of amplification before the detector. The homebrew version is basically a crystal radio with an audio amplifier. The RF coming in from the antenna has a voltage rise from the resonant tuning circuit, but no actual amplification before your detector diode. 1N4148 will not work. A germanium, possibly a shottkey diode or a home made zinc oxide/lead diode will likely be required.

In any case, with this circuit you will need a strong antenna, and probably a strong local signal to hear anything. If you have a signal generator, you can inject a test signal of varying strength and frequency to test and adjust your home made radio. It will help.