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PLL Synthesized radio

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yoosefheidari

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Hi
I want to know how a PLL Synthesized radio work.(in a simple word)
I have many question and I want ask them in several posts.
my first question is how PLL Synthesized radio tuner work?for example in a mechanical tuning radio there are two resonant circuit including inductor and capacitor, one of them for RF and another for Osillator and by tuning them radio receive a station.but what about PLL radio?
 

Basically the tuning is the same. Instead of using a capacitor to tune the LC oscillator circuit, a varactor or varicap diode is used. It is a special diode designed to change it's capacitance when REVERSE biased according to the voltage applied across it. Being reverse biased it doesn't draw any current except for leakage but it allows the LC resonant frequency to be tuned by a voltage rather than mechanical capacitance.

The clever part is how it decides on the voltage to produce a particular frequency. It is done with digital divider circuits, the division ratio being adjusted to select the frequency you want. The voltage comes from a phase comparator (with suitable filter to smooth out the digital 1s and 0s) which is fed from two sources, one is a fixed reference frequency, the other is the frequency from the LC oscillator circuit having passed through the divider.

Example, if the reference (which itself is probably divided down from a higher frequency crystal oscillator) is 10KHz and the frequency you want is 5MHz. Selecting a division ratio of 500 will divide 5MHz down to 10KHz and the comparator will see the same frequency at both inputs. It will produce a voltage that says "I'm on frequency". Now change the division ratio to 501 and the divided frequency will now be too low compared to 10KHz, it will be 9.98KHz so the comparator will produce a new voltage to tune the circuit so it sees 10KHz again. In doing that, the LC circuit is now tuned to 5.01MHz.

There are many divider circuits, including ones that flip the division ratio periodically to produce fractional ratios but the same underlying principle apples that the frequency input to the PLL will be tuned so that input frequency = (reference frequency * division ratio) by the voltage produced by the comparator.

Where there are two tuned circuits, such as the input of a superhet receiver, the same voltage used to change the oscillator can be be used to tune the signal frequency stages because they track each other. Being slightly different in frequency, there may be some amplification or offset of voltage to align them.

Brian.
 
betwixt said:
Basically the tuning is the same. Instead of using a capacitor to tune the LC oscillator circuit, a varactor or varicap diode is used. It is a special diode designed to change it's capacitance when REVERSE biased according to the voltage applied across it. Being reverse biased it doesn't draw any current except for leakage but it allows the LC resonant frequency to be tuned by a voltage rather than mechanical capacitance.

The clever part is how it decides on the voltage to produce a particular frequency. It is done with digital divider circuits, the division ratio being adjusted to select the frequency you want. The voltage comes from a phase comparator (with suitable filter to smooth out the digital 1s and 0s) which is fed from two sources, one is a fixed reference frequency, the other is the frequency from the LC oscillator circuit having passed through the divider.

Example, if the reference (which itself is probably divided down from a higher frequency crystal oscillator) is 10KHz and the frequency you want is 5MHz. Selecting a division ratio of 500 will divide 5MHz down to 10KHz and the comparator will see the same frequency at both inputs. It will produce a voltage that says "I'm on frequency". Now change the division ratio to 501 and the divided frequency will now be too low compared to 10KHz, it will be 9.98KHz so the comparator will produce a new voltage to tune the circuit so it sees 10KHz again. In doing that, the LC circuit is now tuned to 5.01MHz.

There are many divider circuits, including ones that flip the division ratio periodically to produce fractional ratios but the same underlying principle apples that the frequency input to the PLL will be tuned so that input frequency = (reference frequency * division ratio) by the voltage produced by the comparator.

Where there are two tuned circuits, such as the input of a superhet receiver, the same voltage used to change the oscillator can be be used to tune the signal frequency stages because they track each other. Being slightly different in frequency, there may be some amplification or offset of voltage to align them.

Brian.
Click to expand...

thank you
I build a radio with CXA1619BS and use air core inductor+gang capacitor for RF and Oscillator(FM)for resonant circuit.
i want to replace gang capacitor with varactor and use atmega micro controller + frequency divider for tune both RF and Oscillator
So how should i put varactor in circuit?and how convert frecuency produced by atmega to voltage for apply to varactor?
can you help me by schematic?
 

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