Varactor Diode Modulator

[remon7]

Varactor Diode Modulator is a semi-conductor diode having a variable junction capacitance. The variation is achieved by applying a reverse bias. The junction capacitance of varactor diode varies linearly with applied voltage. The disadvantage of this varactor diode modulator is that is uses a two terminal device. Thus it is used mostly in automatic frequency control and remote tuning. How can I resolve frequency control?

 

[alec_t]

To control frequency you need either a controlling voltage or a controlling current, both of which involve two terminals/nodes. Why are two terminals a disadvantage?

 

[dr pepper]

You can usually ground one end of a varicap depending on the circuit, then a single signal can be used to apply bias.
Varicaps are not often linear, maybe over part of the operating range, but not overall.
I've used them in a few designs to pull a the freq of a crystal with a voltage.

 

[audioguru]

The capacitance of ANY semiconductor junction (diodes or transistors) changes when the reverse voltage changes.
Then a simple one-transistor FM transmitter can be made.

Here is an FM transmitter that uses one transistor for FM modulation and two paralleled transistors for tuning:

 

[unclejed613]

varactors are also used in PLL circuits to voltage control the oscillator.

i don't exactly know what you are asking about "resolving frequency control"..... do you mean to use one varactor for coarse frequency control, and a smaller one for fine tuning?

i've used 2N2222 transistors connected across the Local Oscillator cap in an old FM tuner to frequency modulate the LO, then use the LO as the basis for a low power FM station. one old Delco car radio i did this with didn't even need the 2N2222 installed, the AFC (automatic frequency control) varactors did the trick nicely when audio was inserted (and the rest of the AFC loop disconnected)

 

[remon7]

yeah"" one varactor for coarse frequency control, and a smaller one for fine tuning?""
Thanks unclejed613

 

[JimB]

Have look at the attached fragment from a rather old (1980s) VHF radiotelephone, a Pye MX294 to be exact.

There is a varicap diode (D1) for the modulation, a second varicap (D2) is part of the frequency synthesiser control.

There are also two switching diodes D3 and D4 which are used to switch the trimmer capacitors C8 and C11 to help give a rapid frequency change between transmit and receive.

JimB