please help me with an FM transmitter simulation

[Keith9105]

so i tried simulating this circuit on multisim.. uses an audio input i switched that with a function generator.. and the output is at the inductor .. i can only seem to get what im putting in at the input of the function generator.. no modulated or amplified signal.... im really bad at this software stuff.. any suggestions are appreciated.. (in the plainest english possible, im not good at this electronics lingo)

its not letting me upload the file i made on multisim .. so ill just take a screenshot instead.

anyway about the inductor.. its a 5 turn copper wire on a 3.5mm drill bit.. im not sure how to find the inductance so i just put an estimate.. :/

 

[jjw]

The drill bit is used for winding the coil for the right diameter, not as core material.
Your estimated inductance is about 2000 times too big.
Here is a calculator for single layer air cored coils: **broken link removed**

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[JimB]

Apart from the inductor value, I see the following problems with your computer game errr sorry simulator,

The inputs from the function generator connect to the 0v line from C5.

There are no "earth" connections for the main circut or the scope.

The value of C6 is too large.

The connection from the scope to L1 is shown going to the +ve supply rather than a tapping point on the coil and would never display anything anyway.

Also, what are you expecting to see from this simulator?
I have a feeling that at best all you will see is the RF waveform, you will not see the effects of frequency modulation which will probably not be accurately simulated.

JimB

 

[crutschow]

Apart from the inductor value, I see the following problems with your computer game errr sorry simulator,
.........................................
Also, what are you expecting to see from this simulator?
I have a feeling that at best all you will see is the RF waveform, you will not see the effects of frequency modulation which will probably not be accurately simulated.

You seem to have a low opinion of simulators. Why do you think it won't show the frequency modulation?

 

[JimB]

You seem to have a low opinion of simulators.

My dislike of simulators stems from:

I am a grumpy old man.
If you want to learn electronics, then do the electronics, feel the components getting hot, burn your fingers with the soldering iron, smell the solder fumes, release the magic smoke (see my sig-line).
There are somethings which simulators do not do very well, like stray capacitance and inductance. As I am an RF enthusiast, the strays are what make a circuit work or fail quite often.
Did I mention something about my being grumpy?

Why do you think it won't show the frequency modulation?

All the text books show am FM modulated waveform in a highly exagerated manner with obvious changes in frequency, may be as much as 2:1 .
The reality of an FM modulated signal at 100Mhz viewed on an oscilloscope (real or simulated) is that the frequency change is a very small percentage of the carrier frequency.
That small change can be seen on a well triggered scope as the slight broadening of the sinewave trace after several cycles.
If the simulator had a spectrum analyser feature, then a wide deviation FM signal would be easily recognised.

Also, in this case, I am not sure that the simulator will actually simulate the FM effects due to the changes in parasitic capacitances within the transistor.
I have seen it postulated (I think it may have been by MrAl here on ETO) that the FM is actually due to loading of the tuned circuit by the collector resistance of the transistor. I did start to rig up an experiment to try and prove or disprove it, but real work got in the way and I dropped the idea.

So, that is why I was rather dismissive of the simulator, for better or worse.

JimB

 

[.ALJ.jun]

how to calculate the bandpass filter?