CB Radio FM Transmitter

881

11:35:48

This transmitter consists in a common emitter colpitts VCO and a Class E amplifier with 50 ohm matched resonant network. The capacitance of the LC tank is given by varactor diodes. Incresing the inverse voltage on each diode, the depletion zone will increase and the junction capacitance drops... and viceversa.
Biasing the varactors with a DC offset you can change the center frequency and select channels. The Class E amplifier is matched for 50 ohm loads, you can send the signal with 50 ohm coax cable directly to a matched antenna. If you turn on the transmitter without a 50 ohm load the mosfet will blow up.

Use a switch and a relay to turn off the 30V power supply when you are not transmitting like here.
http://everycircuit.com/circuit/5568943585230848

published 6 years ago

lmccoig

6 years ago

CB is AM. What is the FM part?

hurz

6 years ago

CB is AM mostly used by truckers and FM is also available but more for the cheap mass markt. Anyway, CB is still online? In some countries maybe...

lmccoig

6 years ago

United States truckers use AM Channel 19 mostly. Locals come in on Channel 2. Above Channel 30 or Channel 35 they may go SSB.

bryanthehacker

6 years ago

in Citizen Band many people use AM, but others use FM or SSB. Anyway, AM transmitters are easy to made. In this circuit, FM is done with varactors and a common emitter Colpitts oscillator

hurz

6 years ago

The statement "CB is AM" is just wrong

lmccoig

6 years ago

Seems UK may have the FM for CB. Some countries have UHF bands for CB. FM for CB frequencies in U.S.A. might win a $25,000 fine.

hurz

6 years ago

FM is independed of the carrier frequency. Its a modulation methode and works on any frequency. Even years ago there were users here on EC who takling bullshit about FM and AM and bands available/working dont trust them too much. I know CB from 27MHz what is that 11m? Starts in begining for sure with simple AM transceiver but later the mass was FM for little mobile tranveiver walkitalki. Trucker prefered AM, cuz its with a tuned Mic and over modulation its range was much better, and higher power for AM build in devices in their trucks was also allowed. Ch 19 yes i think that also here in central europe the same tradition.

Bushmills

6 years ago

You ought to separate technical from legal aspects. Consider, for example, that frequency modulating CB carrier frequencies will demonstrate the technical feasibility while not earning you any sanctions at all when using a dummy load rather than an aerial.

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