Increase Input Frequency circuit

Thread Starter

MrsssSu

Joined Sep 28, 2021
251
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Referring to website link at here , this circuit tries to replicate the waveform of input voltage and make it to a high frequency, I try to simulate the circuit in LT Spice but the output is weird. May I know the solution to this? Below is my attached LT Spice for your convenience.

Thank you for reading and have a nice day:)
 

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crutschow

Joined Mar 14, 2008
30,788
The antenna has an impedance, which should be added to the simulation.
But simulating oscillating circuits can be difficult with a simulator, since stray impedances are often a significant part of the real circuit.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
251
The antenna has an impedance, which should be added to the simulation.
But simulating oscillating circuits can be difficult with a simulator, since stray impedances are often a significant part of the real circuit.
Hi, thanks for replying. Should the voltage source that i attached to the audio in in schematic be replaced by a current source ? Does that mean that I have to build the circuit in real life to test it because simulator cannot do it? :)
 

Audioguru again

Joined Oct 21, 2019
5,164
The circuit from the students at Circuits Today in India is horrible and it does not work. Maybe their old circuits were not tested?
If it did work then it would sound awful on an FM radio because the audio transistor produces severe distortion and the circuit is missing the pre-emphasis (treble boost) that all FM radio stations have. All FM radios have de-emphasis.

My FM transmitter oscillator works when feeding an RF buffer (it isolates capacitance at the antenna from the oscillator) and my transmitter sounds perfect.
 

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Thread Starter

MrsssSu

Joined Sep 28, 2021
251
The circuit from the students at Circuits Today in India is horrible and it does not work. Maybe their old circuits were not tested?
If it did work then it would sound awful on an FM radio because the audio transistor produces severe distortion and the circuit is missing the pre-emphasis (treble boost) that all FM radio stations have. All FM radios have de-emphasis.

My FM transmitter oscillator works when feeding an RF buffer (it isolates capacitance at the antenna from the oscillator) and my transmitter sounds perfect.
Hi, thanks for the diagram. Is the green node (input voltage) and RF buffer is the output voltage?
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
251
The circuit from the students at Circuits Today in India is horrible and it does not work. Maybe their old circuits were not tested?
If it did work then it would sound awful on an FM radio because the audio transistor produces severe distortion and the circuit is missing the pre-emphasis (treble boost) that all FM radio stations have. All FM radios have de-emphasis.

My FM transmitter oscillator works when feeding an RF buffer (it isolates capacitance at the antenna from the oscillator) and my transmitter sounds perfect.
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Hi, your circuit does not seem to increase the input frequency. For FM transmitter, I believe we need to increase the input frequency so that its high enough to be transmitted :)
 

Audioguru again

Joined Oct 21, 2019
5,164
Why is your "audio" extremely high level short duration pulses instead of a low level sinewave? Their level exceeds the maximum allowed 5V reverse-bias EB voltage of the transistor.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
251
Why is your "audio" extremely high level short duration pulses instead of a low level sinewave? Their level exceeds the maximum allowed 5V reverse-bias EB voltage of the transistor.
Hi, yes you are right. But, the problem is it does not increase the input voltage frequency :)
 

MrChips

Joined Oct 2, 2009
27,117
You are not increasing the frequency in a radio transmitter.
The transmitter sends out an RF carrier signal generated by an RF oscillator.
You are modulating the high frequency carrier using either AM or FM.
 

Audioguru again

Joined Oct 21, 2019
5,164
I have a microphone preamp circuit with pre-emphasis that feeds the capacitor C3 in my 100MHz oscillator.
Why are you still feeding extremely high level short duration pulses as "audio" instead of lower level audio sinewaves?

Your oscillator is defective and does not oscillate. It is supposed to oscillate at around 100MHz and its frequency is the same as FM radio stations, then it does not need its frequency to be increased.
 

Thread Starter

MrsssSu

Joined Sep 28, 2021
251
I have a microphone preamp circuit with pre-emphasis that feeds the capacitor C3 in my 100MHz oscillator.
Why are you still feeding extremely high level short duration pulses as "audio" instead of lower level audio sinewaves?

Your oscillator is defective and does not oscillate. It is supposed to oscillate at around 100MHz and its frequency is the same as FM radio stations, then it does not need its frequency to be increased.
Hi sir, is your circuit a RF oscillator which increases the input frequency while maintaining the same waveform shape?
 

MrChips

Joined Oct 2, 2009
27,117
Hi sir, is your circuit a RF oscillator which increases the input frequency while maintaining the same waveform shape?
No. You cannot alter the frequency and maintain the same waveform shape.
A monotonic oscillator by definition has to be sinusoidal.
You are ignoring the point I have already made. RF transmission is usually AM of FM of RF.
 

Audioguru again

Joined Oct 21, 2019
5,164
My RF oscillator produces a sinewave at around 100MHz.
The audio at the base of the oscillator transistor increases and decreases its conduction which increases and decreases its capacitance that increases and decreases its frequency (FM is frequency modulation) about 75kHz up and 75kHz down for loudest sounds. The transmission is also AM that an FM receiver ignores.

Your audio transistor should be like this:
 

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Audioguru again

Joined Oct 21, 2019
5,164
On the other forum I notice that your 100uH is 1000 times more than the 0.1uH in my FM transmitter. Therefore LTspice thinks the RF frequency is way too low. Remove C7.
Please use a low level audio sinewave as the audio input.
Please understand what is "FM modulation".
 
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