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How to tune FM receiver with O-scope?

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rehan

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simple fm receiver tuned to one frequency

Hi,

Need a procedure for tunning fm receiver with O-scope.
:?:
 

flatulent

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adjust fm receiver

Do you mean adjusting the internal circuitryafter constructing the receiver, or taking an already functioning receiver and listening to a signal off the air?

In the second case, you should look for clipping of the audio or demodulated signal and set the frequency tuning at the midpoint between the two clippling limits.

In the first case, for adjusting the IF amplifier tuned circuits you should know ahead of time which frequency each should be tuned to. Place your signal generator at that frequency and adjust the tuning for maximum output. After all are tuned make a graph of amplitude vs frequency to verify that the passband is the proper frequency range and the out of band frequencies are attenuated. Then put your signal generator at the center of the IF band and adjust the discriminator for zero DC volts out. Then FM modulate the signal generator and fine tune the discriminator for a good sine wave.

If you do not have a signal generator, find an unmodulated signal and tune around it for the IF alignment. This could even generated by you as a harmoinc of a crystal oscillator. Then tune in a modulated signal and adjust the discriminator for equal positive and negative amplitudes
 

rehan

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fm receiver 10.7 filter faulty

I have to tune receiver for radio control aircraft , freq: 72mhzFM.

You mean.

1. I am using RF ransimtter as signal source " should i disconnet its modulation input signal.

2. Then after demodulation i should tune for max output or after IF stage i should do that .

3. the signal i am receiving is PWM so i canot adjust sine wave.

4. How to detect if ceramic filter is working alright because many times after crash of model the filter is faulty and it gives gittering signal output.

Thanks.

Rehan
 

flatulent

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how to tune a radio with a signal generator

Here is one way to do it.

Use your transmitter as the source. Put a 50 ohm resistor across the antenna output instead of the antenna so that the signal level will be decreased.

Turn the transmitter on and then go far away with the receiver and oscilloscope. look at the PWM output. Look for noise level on the pulses. Adjust the receiver tuning for the least noise.

In your situation there may be other problems relating to the vibration by the motor or the transmitter not working well.

One way to test for defective parts is to gently tap them and watch the receiver output.

Overall, your case seems to be a problem of receiver sensitivity reduction. The best thing you can do is use a real signal generator to see if the receiver sensitivity has been reduced from the advertised value. Another fool proof thing which may be cheaper than reconstructing your airplane is to send the receiver and transmitter back to the manufacturer for repair.
 

FANT

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how to tune a receiver with a signal generator

You can try to use a broken and fixed trnsmitter as generator ( if it is your own ) disconnecting the power amplifiers and leaving only the oscillator and separator ( driver ).
Put everything in a shielded case and use a 100 ohm potentiometer ( I know, it is not the professional way ) in between the output of the driver and the output of the box.
Install a connector ( fair is a PL, good a BNC ) and connct about 20 cm of wire.
To see the demodulated signal, with the oscilloscope try to record the level of the noise in different points ( supposed all the time the receivers are the same ) both in DC ( to view the polarization points ) and AC ( to know the level of the signals ).
A photocopy of the PCB, with marked some reference points usually helps a lot.
When you have a receiver to test simply open the file with the photo (s) taken from the oscilloscope and the levels recorded and compare !

Hope to be helpful.

Mandi
 

E-design

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using am signal generator to test fm receiver

rehan said:
I have to tune receiver for radio control aircraft , freq: 72mhzFM.

2. Then after demodulation i should tune for max output or after IF stage i should do that .

3. the signal i am receiving is PWM so i canot adjust sine wave.

4. How to detect if ceramic filter is working alright because many times after crash of model the filter is faulty and it gives gittering signal output.

Rehan
I used to repair some of these devices while still active in this hobby. What I found was that merely tuning for the highest IF level was not the right way. The range of the radio often will be reduced due to noise etc. I managed to get hold of some service manuals from suppliers, and found it the best to follow their adjustment procedures. You might find that they will provide you with alignment/testing procedures if you ask. Maybe they will charge a small fee for photocopies only.

E
 

artem

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Tuning procedure is dependent on your receiver and can vary .
If you use heterodine(sorry for bad english ) you should find how many intermediate frequencies does it have . Have a look to schema (if you have one ) .
Try to tune first lwoest frequency by suppling the signal from oscillator .
For FM modulation if you do not have signal input there will be noice on the audio output (after detector )
When no modulated signal will come noice will be disappeared .

The lowest standard intemediate freq in your rec can be 10.7 MHz (or 465 kHz),
availability of second intermediate is dependent on receiver's quility . Good receivers have second intermediate freqs .
Then you can test receiver on working frequency (72 Mhz) . To save your time you shuld start from lowest inter freq to test its badn and demodulator (detector ).


Find the lowest one
 

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