# Estimating Frequency in PLL from estimated phase

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#### chessmath2009

##### Newbie level 6 Hi
Assume I have a digital PLL that outputs phase estimate of the reference signal, however I am wondering can I estimate the frequency of reference signal according to the given phase estimate? I know frequency is derivative of phase respect to time but dont we need a operating or center frequency of VCO(NCO). Can anyone guide me to find an algorithm to estimate the reference frequency ?:

#### KlausST

##### Super Moderator
Staff member Hi,

F_out = f_in * n(m) * n(d)
Where n(m) is the integer multiplying factor and n(d) is the integer divider

The other way round: F_in = f_out * n(d) / n(m)

It is a fixed relationship given by n(m) and n(d)

Klaus

#### chessmath2009

##### Newbie level 6 I do not have any divider by N in my PLL and I would like to estimate the frequency of the signal based on estimated phase. I have only phase detector, loop filter and VCO (NCO).

#### Warpspeed If the two frequencies are far apart, phase information will be changing so fast, it may be unable to find lock.
The reason being it tries to pull the frequency one way over half a cycle, then in the opposite direction during the following half cycle.

The solution is a combined phase/frequency detector.
Here is a good read on phase detectors, scroll down to page 18 for phase/frequency detector.

#### KlausST

##### Super Moderator
Staff member Hi,

If you don't have a divider, the the output frequency will be the same as the input frequency.

...
But the output of the loop filter is the input to the VCO. So yes, the voltage relates to the output frequency.
But it may have a lot of errors: offset, gain, unlinearity...

Maybe you could show us your circuit, so we can detailed talk about it.

Klaus

#### zorro Hi,

The voltage at the input of the VCO (or the number at the input of the NCO), i.e. the value at the output of the filter loop, gives the indication of the frequency at which the oscilator is running.
If the PLL is locked, that is the frequency of the signal.

Z

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