What does the ratio of two phase mean?

pll_check

Joined May 28, 2017
23
I am confused with ratio of two phases in dB mean? I am referring to PLL loop transfer function in which Y axis is ratio of Output Phase to Ref. Phase?

I am familiar with signal gain/loss in dB but I am unable to visualize the phase gain/loss in dB.

Papabravo

Joined Feb 24, 2006
17,562
You would have to be careful about divide by zero problems if you want to express phase ratios in dB. I'm not sure it makes much sense, so can you provide a reference that talks about it.

pll_check

Joined May 28, 2017
23
May be I am confused with two different parameters here.

When we plot PLL transfer function in terms of frequency and gain, what does those two parameters indicate?

pll_check

Joined May 28, 2017
23
I Think I got the answer.

PLL must track any deltas in input phase and change the output phase by same amount to maintain the input and output phase relation which is called Phase Locking
So gain = d(φo)/d(φi).

The PLL Transfer function talks about how fast the delta is changing in input phase.

If rate of change of output is same as rate of change of input, Gain=0dB.
If rate of change of output is slower than the rate of change of input, Gain < 0
If rate of change of input is faster than the rate of change of output, Gain > 0.

Ideally you want gain to be at 0dB for any fast or slow changes in the input phase, this does not happen. So if you look at any PLL transfer function, what it says is, PLL can track low frequency changes in frequency (Wander/Jitter/Phase Noise/Phase Variation/delta) but cannot track high frequency (jitter/phase variations/delta)

Last edited:

Papabravo

Joined Feb 24, 2006
17,562
May be I am confused with two different parameters here.

When we plot PLL transfer function in terms of frequency and gain, what does those two parameters indicate?
The frequency in question is the frequency of the VCO (Voltage Controlled Oscillator). It has a range over which it can be adjusted. It also has a free running or center frequency. The capture range is the range over which an input signal can be "captured" by the PLL, and the lock range is how much variation in frequency on the input can be tolerated before the loop loses lock. And as you have already pointed out the gain is related to how fast the loop responds to phase disturbances.

shteii01

Joined Feb 19, 2010
4,644
May be I am confused with two different parameters here.

When we plot PLL transfer function in terms of frequency and gain, what does those two parameters indicate?
We observe which frequencies pass and which frequencies are blocked.