The simplest FM transmitter and receiver module

[Eshal]

I used a different LC calculator website:
88MHz, 0.1uH. Ct= 32.7pf.
100MHz, 0.1uH. Ct= 25.3pf.
108MHz, 0.1uH. Ct= 21.7pF.

Yes, I also tried.

You forgot that C7, stray capacitance of the wiring and the capacitance of the transistor reduces the required capacitance from the trimmer capacitor.

Right sir. Trimmer capacitance is reduced.

The RF oscillator has the output from its emitter which is a low impedance (emitter-follower). When I tried the output from its collector then it stopped oscillating.

It means, it is a common collector configuration and not a common base configuration. What would be the output impedance of the RF Oscillator?

R8 biases the RF amplifier transistor with base current (177uA) so it operates in class-A.

How can you say it operates in class A?

 

[zsolt1]

Hi,

a lot of complication for the simplest Fm transmitter and receiver .... hehe

 

[Audioguru]

It means, it is a common collector configuration and not a common base configuration. What would be the output impedance of the RF Oscillator?

The RF oscillator is common base but its output is from the emitter which is common collector.

How can you say it operates in class A?

The 47k base resistor has 9V - 0.7V= 8.3V across it then the base current is 8.3V/47k= 0.177mA. Its hFE is typically 200 so its collector current averages 35.4mA.
The signal at the 75 ohm antenna swings down 3.5V so the collector current increases 3.5V/75 ohms= 46.7mA to 82.1mA and the collector current decreases 46.7mA to zero mA for a moment. Then the RF amplifier is mostly class-A but operates in class-C for only a moment each cycle.

 

[Eshal]

The RF oscillator is common base but its output is from the emitter which is common collector.

Yes, that is what I am seeing too.

The 47k base resistor has 9V - 0.7V= 8.3V across it then the base current is 8.3V/47k= 0.177mA. Its hFE is typically 200 so its collector current averages 35.4mA.
The signal at the 75 ohm antenna swings down 3.5V so the collector current increases 3.5V/75 ohms= 46.7mA to 82.1mA and the collector current decreases 46.7mA to zero mA for a moment. Then the RF amplifier is mostly class-A but operates in class-C for only a moment each cycle.

Oh... nice analyzing. You know sir, this help me in increasing knowledge.

And sir, how to calculate the output impedance of the RF Oscillator? Because it has C7, C12 and L1 at its output. So how to find output impedance if we have these reactive components? How to add them in parallel or series sir?

I appreciate your knowledge sir.

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Hi,

a lot of complication for the simplest Fm transmitter and receiver .... hehe

Why are you laughing? :-(

 

[Audioguru]

And sir, how to calculate the output impedance of the RF Oscillator? Because it has C7, C12 and L1 at its output. So how to find output impedance if we have these reactive components? How to add them in parallel or series sir?(

C6 and L1 are resonating which is a high impedance. The collector of the transistor is also a high impedance.
C7 provides positive feedback but its reactance is 34 ohms at 100MHz.
Without the load of C12 and the base-emitter junction of the RF amplifier, a simulation shows that the output impedance at the emitter of the oscillator is 47 ohms.

 

[Eshal]

Yes, my simulation shows 47 ohms too.. I simulated it sir.
It is regarded as the total output impedance of the RF Oscillator?

 

[Audioguru]

Yes, my simulation shows 47 ohms too.. I simulated it sir.
It is regarded as the total output impedance of the RF Oscillator?

Yes, the output impedance of the oscillator is 47 ohms.

A strange thing happened. When I disconnected the RF amplifier from C12 then the output level of the oscillator dropped. I expected it to rise.

 

[Eshal]

A strange thing happened. When I disconnected the RF amplifier from C12 then the output level of the oscillator dropped. I expected it to rise.

It means the output impedance of the RF oscillator is reduced. That's why level is dropped. Is this the case sir?

Share your experience and problems with me too and I want to help you in learning to you as well as it helps me.

Thank you sir.. :wink:

 

[Audioguru]

In my simulation I expected to see a low distortion sinewave at the collector of the oscillator transistor but its bottom was clipped with some phase shift. I increased the value of R6 so that the oscillator transistor is not conducting so much current but it made little change.

I expected to see a low distortion sinewave at the collector of the RF amplifier transistor but its top was clipped with some phase shift. I increased the value of C5 to 100pF then the sinewave looked smooth.

With C5 at 100pF I increased the values of C5 and C6 to 33pF then the waveform at the antenna looked good with a higher amplitude.

 

[Eshal]

I see, it means we need amendment in the transmitter. But it can be done by an expert like you who can see and trace the fault.

But sir, your transmitter without these changes is working fine than other which I have seen on the internet and simulated them.

I expected to see a low distortion sinewave at the collector of the RF amplifier transistor but its top was clipped with some phase shift. I increased the value of C5 to 100pF then the sinewave looked smooth.

I can see and understand what we need to do further with the transmitter.
But what do you say about this change? What meanings do you extract from the above statement? I want to know your view.

Thank you.

 

[Audioguru]

The RF output signal of a radio or TV transmitter must be a pure sinewave to avoid harmonics causing interference to other communications.

 

[Audioguru]

Here is a review of my FM transmitter. Some remarks are wrong. It is called "Spot The Mistakes" on the website called www.talkingelectronics.com :

 

[zsolt1]

sorry if misunderstood, not laughing of you , i was laughing with you ... anyway to make sens to my my post here : Do you remember your AM transmitter you successfully built ?
What changed now in this case ?
what changed is the type of modulation . Just make a parallel between them (and between your circuits). In AM you where modulating the amplitude of a carrier frequency with the frequency to be transmitted. Now if you only change the modulator type you can modulate the carrier in frequency domain instead of amplitude , with the same project.

Ps: put beck that smiley again , i'm shore it's better

 

[Eshal]

@Aduioguru
Is it your transmitter's remarks?

@zsolt1

Just make a parallel between them (and between your circuits). In AM you where modulating the amplitude of a carrier frequency with the frequency to be transmitted. Now if you only change the modulator type you can modulate the carrier in frequency domain instead of amplitude , with the same project.

Sorry, what did you say? I didn't get your mean. Elaborate it please.

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Here is a diagram of common base Colpitts oscillator.


Here is a diagram of equivalent of common base Colpitts oscillator.


3 question regarding the equivalent circuit of the common base Colpitts oscillator.

1) Where does RtL and RtC come from?
2) Where did R1 and R2 go?
3) Where does CB go?

 

[Audioguru]

The "Spot The mistakes" remarks described my FM transmitter project but some of it is wrong.

The Colpitts oscillator in my FM transmitter and in most simple FM transmitters use stray wiring capacitance for C2 in the circuit you found.
The circuit you found uses C1 in series with C2 to tune the inductor but my circuit uses a trimmer capacitor parallel to the inductor.
The circuit you found biases the transistor from a voltage divider made with R1 and R2 but my FM transmitter has a voltage regulator, a transistor with a narrow range of hFE and only R1 to bias it.
The circuit you found has the output from the collector so the resistance RL, capacitance Rtc and inductance RtL of its load affects its Q and frequency. My circuit does not have those problems because the output is from the emitter.
CB in the circuit you found and in my circuit connects the base to ground at RF frequencies to make the base common.

 

[Eshal]

Yes sir, I know your oscillator has better result than the one I posted in post#74.

But I am still confuse where does RtL and RtC come from and where does R1 and R2 go in equivalent circuit?

Also, you have modified your oscillator according to the required output. Right?

 

[Audioguru]

I assume that RtL is the inductance of the wiring and Rtc is stray capacitance.
The equivalent circuit does not have a transistor that needs to have its base biased by R1 and R2, instead it has a source of gm.

 

[Eshal]

Yes, gm is the transconductance of the transistor, I think.

In DC equivalent, do we assume stray capacitance (RtC) or RtL?

 

[zsolt1]

hi,
just go back to those posts and you will figure out ...
do you understand the principle of modulation? I did not figure out exactly what you did not understand.

 

[Audioguru]

I think the text book where the equivalent circuit came from or your professor should explain about RtC and RtL.