hi...

i have got a fm waveform but its amplitude is changing and its not exact fm waveform.


i am sending you the circuit and the waveform that i am geting from this circuit. I am also sending you the waveform that i need to bring at my output.

please tell me where i am doing wrong?????



(this is the waveform that i am geting)



fmtx2.asc

(this is my circuit)





(this is the waveform that i required at my output)


Please please please help me out........

 

hi,
What was wrong with simulation I posted for this project a few days ago.

I posted an image of the 3Hz modulation and an image of the 100MHz carrier.

I did explain in detail on the Chatline that you cannot see the 3KHz modulation on the 100MHz carrier on the same X time base.

On your sim, you have the V2 set to 'nothing' no modulation.!

Also you are showing an image of 'noise' ????

How do you expect to see the waveform you have drawn.?
The time scale is in nanosecs and the modulation you have added is in microsecs.???

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Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

I think this schematic won't work. Q2 is loading the oscillator and will prevent oscillations.

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You don't need a quadraphonic Blaupunkt -- you need a curve
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Why have two threads for the same simple circuit?
Maybe the moderator can merge them together.

__________________
Uncle $crooge

 

hi,
This was posted on the other thread.

Attached Files

fmtx1a.asc (3.0 KB, 20 views)

__________________
Eric " Good enough is Perfect "
I will NOT answer PM's requesting technical help, please use the Forum
PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/

 

Well, shut my mouth! You are the master of simulation.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Hey guys, I didn't read the former thread, so maybe someone already did this. But I used Eric's simulation, and increased the frequency of the modulating frequency. You can clearly see the modulation in this simulation.

Attached Thumbnails

 

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Your sim shows AM (amplitude modulation). You cannot see the FM (frequency modulation) because it is so small.
An FM radio ignors the AM.

__________________
Uncle $crooge

 

Actually you can see the FM if you look close enough.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

I can see the FM modulation, I can also see a gawdawful amount of distortion, what's the FFT of that output look like?

__________________

 

The distortion is caused by the output RF amplifier biased wrong and it is clipping. I sim'd it with a tuned circuit for its collector load and it has much less distortion.

__________________
Uncle $crooge

 

The distortion isn't really all that bad compared to some modulators I've seen. That's why lots of filtering is used in the downstream stages. Many modulators source distortion of the carrier signal.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

hi...

first of all thanks to Eric for helping me out. i did the simulation and i am showing the waveforms to you....







(these all images are when my modulating signal is 1KHz frequency)





(these are when my modulating signal is 1MHz)


According to these images i think i have got the fm waveforms please suggest me am i right or wrong???

 

The last one looks like you're on the right track. Using 1Mhz in not practical in a real tramsmitter, but for evaluation your modulator, it will be best for simulation. I think you need to change the time step to 1n in the last simulation. I was getting the same thing, and I think that's what I did to fix it.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

The main problem with trying to examine frequency modulated signals in the time domain (ie using an oscilloscope) is that the deviation (the change in frequency due to the modulation) is very small compared with the carrier frequency, in practical circuits.
The result is that usually the best you can see with an oscilloscope is a broadening of the trace at the right hand side of the display.
What you will not see is a trace like that in the first post of this thread.

JimB

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Experience is directly proportional to the value of the equipment ruined.