Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Need Help With FM Modulator Simulation (Proteus)

Status
Not open for further replies.

blcksea

New Member
Hello,
I'm trying to simulate an fm transmitter circuit before building it but getting stupid results with Proteus. There is not an fm modulated signal at the output. Please help me find what is wrong with the simulation.

Here is the fm transmitter circuit:
FM Transmitter, #6, up to 1/2 mile range

And the .DSN file(Proteus-Isis) is in the attachment.
 

Attachments

  • fm transmitter.zip
    14.3 KB · Views: 2,413

blcksea

New Member
Thanks for the response.

I'm a beginner with such type of projects. My aim is simply to transfer human voice to a remote(less than 1 km) receiver as a starting point. So neither the circuit must be very complex nor it must be a bug type like this one. The reason of choosing this one was just making the simulation easier :)

Can you please suggest me an FM transmitter circuit for both simulating and applying with real components?

[Sorry for the title. "Modulator" must be "trasmitter"]
 

audioguru

Well-Known Member
Most Helpful Member
The carrier frequency is 100MHz.
The max deviation that makes FM is only 75kHz.

The amount of deviation of the carrier frequency is so small that you will not see it on a simulation nor on an oscilloscope. You will just see the carrier frequency that will appear to be at a single frequency.
 

blcksea

New Member
You will just see the carrier frequency that will appear to be at a single frequency.
I can not even see such a signal. I see irrelevant signals. Here they are:

9294-scopex.jpg


-First graphic shows the 1kHz sinusoid signal (instead of human voice)
-Second one is output of the preamplifier
-Third one is output of the colpitts osc (antenna)
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
The preamp is designed for a "mike" that has a signal level of only 5mV RMS. But you are blasting it with a signal level of 2V p-p which is 0.707V RMS which is 141 times too high. It kills the oscillations in the colpitts oscillator.

You cannot connect your 'scope to the antenna because the capacitance of the 'scope cable will kill the oscillations or drastically change the frequency. The 'scope can have its own "receiving" antenna that is 1m or 2m away from the transmitter.
 

blcksea

New Member
Oh, you're right! I've decreased the amplitude of input signal and there is a reasonable output now. Thanks for the solution. All your explanation is clear enough but I wonder why 2V p-p is high for the current amplifier. The factor that decides the input signals' amplitude is the DC supply, isn't it. Since the DC supply is 12V, shouldn't it amplify the signal with a maximum amplitude -approximately- 12V p-p?

Meanwhile I've found the fm transmitter that you developed in the forum and decided to build it. I'll use 7805 instead of LM2931A. Wish me good luck :)
Thanks in advance.
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
Oh, you're right! I've decreased the amplitude of input signal and there is a reasonable output now. Thanks for the solution. All your explanation is clear enough but I wonder why 2V p-p is high for the current amplifier. The factor that decides the input signals' amplitude is the DC supply, isn't it. Since the DC supply is 12V, shouldn't it amplify the signal with a maximum amplitude -approximately- 12V p-p?

No, the supply voltage doesn't affect the gain - it will simply clip against the supply rails.

Meanwhile I've found the fm transmitter that you developed in the forum and decided to build it. I'll use 7805 instead of LM2931A. Wish me good luck :)
Thanks in advance.

Don't do that, the 7805 isn't an LDO regulator.
 

blcksea

New Member
Don't do that, the 7805 isn't an LDO regulator.
OK but I couldn't found the LM2931A. Can any LDO regulator with 5V output do the job? Can you please suggest me another LDO regulator which will do the same job with the LM2931A? If there is not, I'll order it from out of town.
I have bought all the components except this regulator but there are two more problems:
-I couldn't find a 30K resistor; so I'm planning to use 3x10K resistors in series. Do you have a better idea?
-30pF cap is the another one. Is 33pF OK?
 

audioguru

Well-Known Member
Most Helpful Member
Every semiconductor manufacturer has low dropout 5V regultors. I used the LM2931A-5.0 because Digikey had thousands at a good price. I bought about 20 of them.

A 30k resistor is a common 5% value. It is available everywhere (except where you live).
A 33pF cap is almost the same as a 30pF one.
 

blcksea

New Member
Hello again.
Fortunately, I've ordered a LM2931 from an online shopping website. So the only problem seems to be the 30K resistor. 30K is not common here so I found 33K instead. Is it OK?
Meanwhile, the pcb which I designed using Ares is below. I've put components close to each other as much as possible. Is this a problem since the circuit is using RF?

 

audioguru

Well-Known Member
Most Helpful Member
30k is a common 5% resistor value. The preamp transistor will be almost cutoff if it is changed to 33k then the sound will be severely distorted.

The collector resistor could be changed to 8.2k so the 33k resistor can be used. Then the gain is a little less.

On my circuit I spaced the coils up about 0.2" above the circuit board.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top