Questions about implementing FM transmitter on PCB

Some friends recommended me that FM Transmitter , and they told me that they implemented it on PCB and the TX Worked very good , but i have 2 Questions ?


1- In the Red Rectangular it's the RF Oscillator right, So What is the Function of the 1n Capacitor connected to the bet the base of the transistor and ground ?

2- In the Blue Rectangular it's the RF Power Amp , Is it better than the Tuned calss c Amp , this TX range is 200 ft , Is it easy in any TX to put this stage and it will act as a good power RF Amp , i want someone has practical expierence .

Re: About FM TX

Hi,

the red box is a common base colpitt oscillator and hence the 1nF capacitor acts as a path to ground for signal frequencies (100MHz). But for the voice signal which modulates the carrier frequency it will be a high impedance as the frequency is around 4 KHz, (which is 25000 less than carrier frequency).

The blue part not much of idea.

Prakash.

About FM TX

Hehhehh..

This circuit comes from maybe 35 years ago that is from my childhood. I used that circuit to transmit ( when I was at 11 years old ) AF signals to receive by a FM radio.. It's nostalgique..

But last "RF amplifier" which is not an real RF amplifier can produce some harmonics due to low bias condition( 2.5mA collector current ). If you put a variable current source instead of constant resistor devider circuit, you can adjust operating point of the transistor that will give you less harmonics.

If you would make a real FM transmitter, you should use varicap modulated oscillator circuit instead of AM-FM conversion techniques.Because that circuit doesn't modulate frequency directly, micrphone signals change only operating point of Collpits oscillator an then a frequency shift is achieved.

It's better the last RF amplifier is a class C because of efficiency considerations.Because circuit is battery powered and current consumption is important..

Re: About FM TX

BigBoss said:

micrphone signals change only operating point of Collpits oscillator an then a frequency shift is achieved.

..

Click to expand...

I don't really understand that part , can u explain how the operating point is changing and what is the result of that ( Is that change Capacitance ) ?

Re: About FM TX

musliman said:

BigBoss said:

micrphone signals change only operating point of Collpits oscillator an then a frequency shift is achieved.

..

Click to expand...

I don't really understand that part , can u explain how the operating point is changing and what is the result of that ( Is that change Capacitance ) ?

Click to expand...

When you apply a microphone signal, this will change collector current of the oscillator then this influence will give a FM result due to Ccb and feedback mechanism of the oscillator.Because frequency of oscillation is not only depending on tank circuit also feedback capacitances between C-B and C-E...

Re: About FM TX

Thanks BigBoss , I have final Question , What will happen if the mentioned 1nf Capacitor is removed from the ct ? Will the Ct work

Re: About FM TX

Hi,
If u remove the 1nf capacitor the circuit will became unstable. It should not work as desired. You can try this, it will not harm anything.

Regards
P.

Re: About FM TX

Hi, The 1nF can be removed is not needed. 100nF will already ground the Base of the 2N2222A. Better place the 1nF across the emitter resistor of the Osc. Increases gain and than you don't need the buffer. regards, Paul.

Re: About FM TX

tanx
I have a question on this circuit
how can i measure bandwidth ?(without spectrum analyzer and oscilloscope)
or how can i measure deviation frequency??
how can i increase bandwidth ?
tanx

About FM TX

plz help me

Re: About FM TX

Without a spectrum analyzer or oscilloscope you can't do very much.

Dave

About FM TX

tanx for reply
but how can i increase or decrease bandwidth?