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fm transmitter simulation

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rahul.sadtm

New Member
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?????

fmtx.JPG

(this is the waveform that i am geting)



View attachment fmtx2.asc

(this is my circuit)



fm_reqd.gif

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


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

ericgibbs

Well-Known Member
Most Helpful Member
hi,
What was wrong with simulation I posted for this project a few days ago.:confused:

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.???
 
Last edited:

BrownOut

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

audioguru

Well-Known Member
Most Helpful Member
Why have two threads for the same simple circuit?
Maybe the moderator can merge them together.
 

ericgibbs

Well-Known Member
Most Helpful Member
I think this schematic won't work. Q2 is loading the oscillator and will prevent oscillations.

hi,
This was posted on the other thread.:)
 

Attachments

  • fmtx1a.asc
    3 KB · Views: 273

BrownOut

Banned
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.
 

Attachments

  • FMmodulator.JPG
    FMmodulator.JPG
    208.5 KB · Views: 668

audioguru

Well-Known Member
Most Helpful Member
Your sim shows AM (amplitude modulation). You cannot see the FM (frequency modulation) because it is so small.
An FM radio ignors the AM.
 

Sceadwian

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

audioguru

Well-Known Member
Most Helpful Member
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.
 

BrownOut

Banned
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.
 

rahul.sadtm

New Member
please tell me am i right or wrong!!!

hi...

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

imgeting.JPG

imgeting1.JPG

imgeting3.JPG

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

imgeting2.JPG

at_high_freq.JPG

(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???
 

BrownOut

Banned
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.
 

JimB

Super Moderator
Most Helpful Member
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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audioguru

Well-Known Member
Most Helpful Member
You should not blast an extremely high modulating signal (4V p-p) into the oscillator. Try 20mV peak (40mV p-p) because the input is from a microphone.
 

BrownOut

Banned
You won't see much modulation at 20mV. My sim uses a 2V signal, but it doesn't show the over modulation. I think you're problem is a simulation setup one, and not a circuit error.
 

audioguru

Well-Known Member
Most Helpful Member
The peak loudest deviation for a 100MHz FM signal is only 75kHz, not 75MHz!
You will not see such a small deviation on a 'scope nor on a sim.
 

BrownOut

Banned
Where do you see 75MHZ deviation? Not in any of my simulations you don't. The O/P asked about a demonstration of FM modulation, he didn't say anything about listening. In fact, that's irrelevant, since you can't hear a 1MHZ signal anyway. The levels and frequencies are chosen to demonstrate the process of frequency modulation, as per the O/P's request. I haven't seen any voice FM transmitter that connects a microphone directly to the modulator's input.
 

audioguru

Well-Known Member
Most Helpful Member
A transistor Colpitts oscillator used as an FM broadcast band transmitter uses a very low modulating signal, not anywhere near 4V p-p.

Most simple FM transmitters use a preamp transistor so that they can be used as a "bug" and detect voices anywhere in a room.

The first link in Google (there are probably many more) shows this FM transmitter that connects the low level output of an electret mic directly to the modulation input of the oscillator transistor:
 

Attachments

  • simple FM transmitter again.PNG
    simple FM transmitter again.PNG
    8.1 KB · Views: 201
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