Wishing for a circuit diagram on VAR. FM transmitter

[adrian22]

Hello

Anyone has a FM transmitter which we can set the freq(pre-set resister) like the range of 88 ~ 1057 HZ?

Or any circuit diagram on this?

Regards
Adrian

 

[audioguru]

Your required "88 ~ 1057 HZ" frequency is a low pitched audio tone.
The FM broadcast band is from 88Mhz to 108MHz, a million times higher.
You can make an adjustable voltage with a pre-set resistor and apply the voltage to a vari-cap diode in the transmitter's oscillator. The voltage changes the capacitance of the vari-cap which changes the RF frequency.
Since the modulation in FM occurs by changing the RF frequency, a vari-cap diode can also be the modulator.
Here is an example of a fairly high power FM transmitter that uses a pot to adjust the RF frequency and the vari-cap for modulation:

 

[Someone Electro]

I didnt even notice that ne wrote Hz and not MHz.

Yea 1khz is way too low to be useful for radio.

You can trasmit at 1khz but you wod need an huge antena.

 

[ThermalRunaway]

I suppose that in theory, provided of course that you could build an aerial that was physically big enough (I realise this would not be practical) the frequencies that he originally mentioned would make for better transmissions. I mean, the transmissions themselves would be less susceptical to interference would they not?

I'm thinking that he meant to put Mhz on the end of his figures though.

Brian

 

[Someone Electro]

Yea gues he mised the M key

 

[mstechca]

Hello

Anyone has a FM transmitter which we can set the freq(pre-set resister) like the range of 88 ~ 1057 HZ?

Or any circuit diagram on this?

Regards
Adrian

It depends on how you want to set the frequency. The simplest way to do it is to take a simple transistor transmitter, and look at the tank circuit (where an inductor and capacitor are connected in parallel).

Replace the capacitor with a bunch of random capacitors in parallel, all having different values.

For each capacitor, stick a switch (or use jumper pins) in series with it.

Now to use the transmitter, one or more switches need to be on to set the frequency. The more switches there are on, the lower the frequency you transmit.

For higher precision, lower the inductor's value and/or lower each capacitor's value.

 

[adrian22]

oh... i am so sorry that i didn't state clearly it should be 88Mhz to 108MHz thx for all the correction. So are there any circuit no need to use a inductor?

Regards
Adrian

 

[audioguru]

MStechca,
Wouldn't a switch and wires to it add so much capacitance, maybe 10pF, to the tank that the frequency would be way too low?

Wouldn't the inductance of the wire from the tank to the switch be so high that it cancels the capacitive reactance of the capacitor that you are trying to add anyway?

The average frequency on the FM broadcast band is 98Mhz. We are talking about adding or subtracting a pico-Farad here to tune different stations.If you want fine-tuning then you must add or subtract tenths of a pico-Farad. A pico-Farad is a very small amount of capacitance, one-millionth of a micro-Farad, you know.

 

[demokesola]

Your required "88 ~ 1057 HZ" frequency is a low pitched audio tone.
The FM broadcast band is from 88Mhz to 108MHz, a million times higher.
You can make an adjustable voltage with a pre-set resistor and apply the voltage to a vari-cap diode in the transmitter's oscillator. The voltage changes the capacitance of the vari-cap which changes the RF frequency.
Since the modulation in FM occurs by changing the RF frequency, a vari-cap diode can also be the modulator.
Here is an example of a fairly high power FM transmitter that uses a pot to adjust the RF frequency and the vari-cap for modulation:

Hi I'm working on one FM transmiter now but i really like the idea of it being variable. What text do i refer as par values for the inductors L1, L2 L3 and L4?
Thanks

 

[audioguru]

Hi Demoke,
The text is here: