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FM TRANSMITTER WITHOUT A TRIMMER CAPACITOR

Metor

New Member
The trimmer capacitor probably sets the output radio frequency. My FM transmitter uses its 5pF-35pF trimmer capacitor C6 to adjust the radio frequency anywhere on the 88MHz to 108MHz FM broadcast band. Your trimmer goes as high as 100pF to go down to the 76MHz low frequency limit of the Japanese FM broadcast band. My trimmer C13 peaks the RF strength at the radio frequency you are using.

Why didn't you post the schematic of the circuit you are building? Here is mine that is built on stripboard and has a 5V low dropout voltage regulator so that the radio frequency does not change as the 9V battery voltage runs down:
I tried to simulate the circuit in Multisim. Here are the result. The second photo is that of the signal analyzer. The one on the left is that from the microphone but that on the right just shows a straight line after a second transit. Please help me. Thanks
 

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audioguru

Well-Known Member
Most Helpful Member
Your 'scope tries to show an audio signal. But the output of an FM transmitter is not an audio signal, instead it is a radio signal at a MUCH higher frequency.
You have a very high audio frequency of 10kHz which is boosted by the pre-emphasis made with C4. Then your XLV2 audio signal level must be about only 1mV.

You are using the FM transmitter circuit I designed 14 years ago but I used a 9V battery for its design. You are using 12V which causes a much higher output power which is illegal without a transmitter licence. Also the circuit must be changed for using 12V so that the output does not produce distortion harmonics that interfere with important police, ambulance and fire dept. communications. If the RF cops catch you then you will be in big trouble.

Another problem you show is that your simulation is missing an important 75 ohms whip antenna.
Here is a simulation of the radio frequency from the radio parts of my transmitter:
 

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audioguru

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Most Helpful Member
I found the FM transmitter module only at Chinese sites and most say it is not available.
Some of the writing on the datasheet are in Chinese.
 

DerStrom8

Super Moderator
Most Helpful Member

Metor

New Member
Your 'scope tries to show an audio signal. But the output of an FM transmitter is not an audio signal, instead it is a radio signal at a MUCH higher frequency.
You have a very high audio frequency of 10kHz which is boosted by the pre-emphasis made with C4. Then your XLV2 audio signal level must be about only 1mV.

You are using the FM transmitter circuit I designed 14 years ago but I used a 9V battery for its design. You are using 12V which causes a much higher output power which is illegal without a transmitter licence. Also the circuit must be changed for using 12V so that the output does not produce distortion harmonics that interfere with important police, ambulance and fire dept. communications. If the RF cops catch you then you will be in big trouble.

Another problem you show is that your simulation is missing an important 75 ohms whip antenna.
Here is a simulation of the radio frequency from the radio parts of my transmitter:
Dear Sir. I have effected the changes of the battery but not getting the breakthrough. I'm missing something. What could it be
 

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Metor

New Member
Dear Sir. I have effected the changes of the battery but not getting the breakthrough. I'm missing something. What could it be
I'm using multisim 12.0 for the simulation. Will be glad if your design are in this or circuit wizard. Thanks in anticipation
 

audioguru

Well-Known Member
Most Helpful Member
Your left scope photo shows severely distorted rectified music or voices audio. The right photo shows 9 something (9V?).
The output of the transmitter has no audio, instead it has around 100MHz that has its frequency varied a small amount by the audio.
Speed up the timebase setting of the oscilloscope to show the 100MHz, or add an FM receiver to show audio.

I have never used Multisim so maybe it does not show oscillation unless the oscillator is kicked to get it started. The actual circuit starts with noise but the simulation probably does not have noise. Therefor the output is at 9V.
 

Metor

New Member
I used a signal
Your left scope photo shows severely distorted rectified music or voices audio. The right photo shows 9 something (9V?).
The output of the transmitter has no audio, instead it has around 100MHz that has its frequency varied a small amount by the audio.
Speed up the timebase setting of the oscilloscope to show the 100MHz, or add an FM receiver to show audio.

I have never used Multisim so maybe it does not show oscillation unless the oscillator is kicked to get it started. The actual circuit starts with noise but the simulation probably does not have noise. Therefor the output is at 9V.
I used a signal analyzer for both inputs and output in multisim. I will try using an oscillator
 

audioguru

Well-Known Member
Most Helpful Member
The second transistor is an oscillator operating at about 100MHz.
Your oscilloscope is setup to see audio but the output of the radio transmitter has no audio, instead its output is a 100MHz radio signal.
 

Metor

New Member
Noted
The second transistor is an oscillator operating at about 100MHz.
Your oscilloscope is setup to see audio but the output of the radio transmitter has no audio, instead its output is a 100MHz radio signal.
Noted Sir. I will look for an oscillator that shows the FM signal and get back to you. I really appreciate your help. Thanks
 

audioguru

Well-Known Member
Most Helpful Member
Why are you looking for an oscillator? The circuit already has the second transistor as its oscillator. You need to setup the simulation to see the output of the circuit.
What frequency do you expect to see at the output of this FM transmitter?
 

Metor

New Member
Why are you looking for an oscillator? The circuit already has the second transistor as its oscillator. You need to setup the simulation to see the output of the circuit.
What frequency do you expect to see at the output of this FM transmitter?
I want to see a FM modulated signal at the output
 

audioguru

Well-Known Member
Most Helpful Member
How can you see FM modulation? When at 100% modulation the frequency of the 100MHz signal changes only 75kHz which is less than one thousandth.
 

JimB

Super Moderator
Most Helpful Member
How can you see FM modulation? When at 100% modulation the frequency of the 100MHz signal changes only 75kHz which is less than one thousandth.
As usual AG you are quite correct.
However, there is a way but using real world equipment rather that some software simulator thing.

By connecting a good oscilloscope with stable triggering to the IF amplifier of a suitable receiver, it is possible to see the broadening of the trace after several cycles of the RF waveform.
It can be done with the usual 10.7MHz IF, but a lower frequency IF is better.

My Modulation Meter which is basically a receiver, has an IF of 1.5MHz.
Using this it is very easy to see the effects of FM modulation on the carrier.

JimB
 

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