Electronic – the difference between first order, second order and third order phase locked loops


What does PLL order represent?
What are the disadvantages in order 1 & 2 PLL comprared to order 3?
How to choose the pll type for an application like QPSK demodulator?

Best Answer

It seems to me the accepted answer (by Sparky256) views the PLL simply as a filter and completely ignores its actual purpose, which is being a control system, controlling the phase of a signal. The order of a control system signifies its number of internal states. In a system that has a single input, the states beyond the first state (order) are equivalent to derivatives of the controlled variable.

Specifically, in a PLL, the controlled variable is normally the phase of the signal. The PLL attempts to produce a phase lock. So, the first order is for the phase variable/state, the second state is a derivative of the first state - which is frequency, and so on.

For a simple frequency synthesizer a first order PLL might suffice, but with a QPSK demodulator a first order PLL would probably be lacking as any carrier frequency offset between the modulator and the demodulator will always produce a constant phase lag, which can only be removed by a second order PLL. A phase lag means the I and Q channels cannot be fixed (they constantly "move"). Therefore, a QPSK demodulator should have a PLL with at least 2 states (i.e. 2nd order or higher).

Also, contrary to some notion prevalent in comments and answers here, a higher order doesn't make a system slower, nor does it make it faster. Response time is determined by all of the system parameters, mainly by the value of its coefficients (or the position of its poles and zeros, in filter design jargon).