When I was first licensed transmitters and receivers used a variable capacitor with a shaft that came out to the front of the radio that had knob attached to it to change frequency. Most modern radios have synthesizers to determine the frequency and the variable capacitor had been replaced by a diode. The diode is a called a varicap diode, varactor diode or tuning diode. This diode is what allows the Phase Lock Loop synthesizer to change frequency.
A semiconductor diode is a piece of germanium or silicone with a very small amount of imperative inserted to form a molecular structure in which part has more free electrons called N material and an other part has a lack of free electrons called P material. The how and why this happens is beyond the scope of this post but to understand the varicap there needs to have a little understanding of the N and P reigns. Between the N and the P is a thin neutral reign which is called the junction. This junction prevents the flow of current until the voltage reaches the breakdown point which is about .2 volts for germanium and .6 volts for silicone when the diode is forward biased. If the diode is reverse biased then this gap becomes wider and will continue to become wider as the voltage is increased until the avalanche point is reached. Avalanche or zener voltage is that point where the reverse bias voltage breaks down the junction which usually results in destruction of the diode’s properties. As we saw in the previous post zener diodes are designed to operate in this zener reign.
A capacitor is simply two conductors separated by an insulator. The NP junction acts as the insulator between the N and the P reigns which acts as conductors and thus forms a capacitor. If reverse bias is applied the junction will become thicker causing the capacitance to decrease. If the capacitive reactance of the diode is in parallel or series with inductive reactance it will form a tuned circuit and by varying the reverse bias voltage the frequency of that tuned circuit can be changed.
A VFO (variable frequency oscillator) can be constructed using the varactor diode as the capacitor but it is called a VCO (voltage controlled oscillator) instead of a VFO. The VCO can be stabilized to near crystal controlled stability. This is done by using a crystal controlled oscillator for a reference frequency and dividing the frequencies of both oscillators down digitally and feed the two new frequencies into a Phase Lock Loop (PLL) circuit with the output of the PLL going back to the VCO frequency controlling voltage.
A Phase Lock Loop (PLL) is a circuit that compares two frequencies and produces a DC voltage plus or minus when the two frequencies are not the same and a zero voltage when they are the same. If one of the oscillators (usually the crystal oscillator) has a fixed number of times it is divided and the other oscillator’s dividing circuit can be changed in the number of times it is divided the VCO will have to change frequency to make the two frequencies going into the PLL equal (6 MHz/60,000 = 100 Hz, 10MHz/100,000=100Hz, 10.001MHz/100,010=100Hz). Most modern frequency synthesizers use the PLL process just described to produce the desired frequency for the transmitter and the proper frequency for the local oscillator in the receiver.
If you are not acquainted with how a Phase Lock Loop Synthesizer works this description was probably a bit confusing. That is because my purpose here was not to explain how a synthesizer works but how a varicap diode is commonly used in our equipment. A more complete description of how the synthesizer works will probably be a series in the future.