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 .

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.

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..

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 ) ?

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 ) ?