# How do PLL work (for example 565 IC)

1. Feb 25, 2013

### dexterdev

Hi PF,
PLL was always a beast for me to tame. I lost a job bcoz of PLL question in interview.
I wanted to know what a PLL is, how it works and what is it for etc? At least can one suggest easy tutorials pdf or good links , books etc.

-Devanand T

2. Feb 26, 2013

### jim hardy

Here's where i learned. It's that book that Digoff posted a photo of a few days ago. You need it in your library.

I suggest breadboarding a PLL, i think LM567 is still around.
Fire up your oscillator and watch some capture transients on a dual trace scope. Use chop mode so you can see it synch. The darn things are so much fun they're really addictive.
I used them for ultrasonic communication over a PA system so as to not interfere with audio conversation...

old jim

3. Mar 1, 2013

### dexterdev

Thank you sir,.......... Let me go through it :)

4. Mar 2, 2013

### skeptic2

You understand how op amps work don't you? A PLL is similar to an op amp except that it works with frequencies instead of voltages.

For instance with an op amp you may have a reference voltage applied to the non-inverting input (+I) and an output voltage that is the gain of the op amp times the difference between +I and -I.

With PLLs it is similar. You apply a reference frequency at one input and you get a output voltage that is proportional to the phase difference between the two inputs. The output voltage can be used to control the frequency of a variable frequency oscillator whose output frequency is fed back to the other input of the PLL. Simple phase comparitors can be made from an XOR gate or an RS flip flop. By dividing down the frequency of the VFO, a PLL can be used to create frequencies higher than the reference frequency.

Last edited: Mar 2, 2013
5. Mar 2, 2013

6. Mar 2, 2013

### rbj

the one thing that needs to be mentioned is that the op-amp has this thing called a "differential amplifier" which, essentially, measures the difference between voltages. and then the op-amp circuit with negative feedback will do something to drive that voltage difference to zero.

this concept of using negative feedback to force to zero some "error signal", which is the difference between what you have and what you want (what your target is), that whole idea is essentially what control systems or control theory is all about. so really understanding a PLL requires understanding something about control theory and feedback loops.

a PLL must have a component called a phase discriminator that measures the difference between the phases of two waveforms of virtually the same frequency. and the PLL needs something called a voltage-controlled oscillator (or VCO) to convert a voltage into a frequency. then remember frequency is the derivative of the changing phase. that means there is an added integrator that goes into the control loop.

conceptually, especially of the waveforms are sinusoids, the phase discriminator need only be a simple multiplier with a low-pass filter. nowadays, people write software PLLs in a few different applications of DSP.