# FM Transmitter Simulation Problem in Proteus Software

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#### Bong Tong

##### New Member
Hi, everyone. I really need some help from the experts to solve simulation problem of FM transmitter in Proteus software. I can't get desired output waveform through the constructed circuit. Here I attached my transmitter.DSN file. May I know the what's the problem? Does anyone have the sample of the circuit with well function simulation?

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#### audioguru

##### Well-Known Member
It is obvious that the negative terminal of the battery is not connected to the transmitter circuit, instead it is connected to C6 that is in series. C6 should be in parallel with the battery and the negative battery terminal should connect to the circuit.
Also the value of R1 is too high and the electret mic has its polarity backwards.
Also the value of C1 is too low to pass audio frequencies below about 500Hz.

#### Bong Tong

##### New Member
Is this ok for the circuit? I am using the online EDA tools (https://easyeda.com) but getting only a flat output waveform at antenna part.

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#### audioguru

##### Well-Known Member
The circuit should work now but the value of C1 is still so low that it will pass only high pitched voices from little kids.

#### Bong Tong

##### New Member
How about a value of 47mF capacitance?

#### audioguru

##### Well-Known Member
A 47mF capacitor would be huge because it would have 47 thousand micro-Farads. Maybe you mean 47μF?
The -3dB cutoff frequency is easily calculated. The electret mic is about 3.3k ohms and parallel to the 10k resistor that powers its Jfet inside produces 2.5k that is in series with the input impedance of the preamp transistor that is about 5k ohms. Then the total resistance is about 7.5k ohms and the frequency is 1 divided by (2 x pi x RC)= about 0.5Hz which is an earthquake frequency. A 330nF (0.33uF) film capacitor would produce a reasonable cutoff frequency of 65Hz.

#### Bong Tong

##### New Member
I replaced the microphone with a function generator for simulation purpose. However, it generates the only flat line of the output signal at antenna part except for the input sine wave.

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#### JimB

##### Super Moderator
What frequency do you expect the oscillator to produce?

How did you choose the values for L1 and C6 ?
The value of C6 is very high.

I am not familiar with simulators, but I believe that sometimes oscillators do not start in simulation. How you make them start I cannot remember, the problem has been reported here in the forum several times.

JimB

#### audioguru

##### Well-Known Member
It looks like your input audio signal level is 50 times too high at 500mV. A microphone level is only 10mV.
Your coil L1 is only 1μH but it should be about 100μH and the capacitor C6 value is much too high at 3.3nF when it should be about 27pF.
The circuit is extremely simple so the output will be a mixture of AM and FM. You will not see the FM modulation on the 'scope.

#### Bong Tong

##### New Member
Any suggestion of the design for the circuit to get FM modulation shown in the 'scope'? Is Proteus software suitable for simulation of FM modulation?

#### Bong Tong

##### New Member
The purpose of setting L1=1uH and C6=3.3nF is to get a carrier frequency of 87.5MHz in FM transmitter based on this formula :
f =1/2*√(L*C)

#### audioguru

##### Well-Known Member
Sorry, I was wrong. The coil should be 100nH and the capacitor should be 27pF. Your formula is missing "pi".

You cannot simply plug any numbers into the frequency formula, you need a coil that is actually a coil (1uH is a short piece of wire) and a suitable capacitor like 100nH and 27pF.
An FM radio station operates at around 100MHz. But its maximum frequency deviation (caused by very loud modulation) is only 150kHz or +/-75kHz. So the frequency changes only 0.15% that is so small that it cannot be seen on a 'scope.

#### JimB

##### Super Moderator
Actually a bit more than that.
The quick and dirty version of the LC resonance formula is
f = 160/√(L*C)
Where f is the frequency in MHz
L is the inductance in uH
C is the capacitance in pF

So the frequency changes only 0.15% that is so small that it cannot be seen on a 'scope.
I agree.
You will never see FM waveforms, as shown in the average textbook, in any practical radio system. The deviation is far too low.

JimB

#### audioguru

##### Well-Known Member
If the 3.3nF capacitor in your LC tuned circuit has very low inductance and resistance then its high value causes it to be a dead short circuit (o.49 ohms) at around 100MHz and the transistor cannot drive it. My 27pF capacitor is 59 ohms.

#### Bong Tong

##### New Member
Thank you for the suggestion. So, what should be the waveform for the output?Does it same as the input waveform?

#### JimB

##### Super Moderator
So, what should be the waveform for the output?
It should be a sine wave at whatever frequency L1 and C6 are resonant.

JimB

#### audioguru

##### Well-Known Member
The output is probably not a sinewave because the oscillator transistor is not properly biased. Some transistors will be saturated and some will be cutoff during parts of the waveform. The high impedance LC circuit is overloaded by the low antenna impedance of about 75 ohms. An added RF transistor at the output of the oscillator will allow the output to be a good sinewave like this:

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#### Bong Tong

##### New Member
Here is my result: Is that relevant?

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#### audioguru

##### Well-Known Member
Instead of calculating the values for audio capacitors you are simply guessing (wrongly).
Instead of calculating values for resistors to bias the audio transistor you are simply guessing (wrongly).
Your RF signal looks good but it has no 75 ohm antenna.

Why is the audio input a very high signal of 1V peak instead of 10mV peak from the microphone?
Why does your audio preamp transistor produce a voltage loss instead of voltage gain?
Why is the value of C3 so small that it passes only high pitched squeaks?

Here is yours and here is mine:

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