# Phase Locked Loop Schematic Question

#### Tony Elliott

Joined May 8, 2015
158
pg: 8 fig: 13
http://www.ti.com/lit/an/snoa593b/snoa593b.pdf

I have been looking to build this circuit and on the schematic I looked at the optional (+N frequency to voltage divider), can anyone tell me what this is? And as it is optional what is its function as I want to use the circuit to convert frequency to voltage with a presition response time.

Anthony

#### Dodgydave

Joined Jun 22, 2012
11,235
Its optional extra, you can use a cd4017 decade counter or other ripple counter, there are other pll chips, CD4046, NE565.

#### dl324

Joined Mar 30, 2015
16,672
I have been looking to build this circuit and on the schematic I looked at the optional (+N frequency to voltage divider), can anyone tell me what this is?
If this is what you're referring to:

The optional counter is used to lock on a multiple of the input frequency.

#### Lestraveled

Joined May 19, 2014
1,946
You would put a frequency divider chip (binary counter) there, if you wanted to measure frequencies lower than what the LM331 can operate at.

#### ian field

Joined Oct 27, 2012
6,536
You would put a frequency divider chip (binary counter) there, if you wanted to measure frequencies lower than what the LM331 can operate at.
The 331 is equivalent to the Raytheon RC4151 - that may open up the search to a bigger resource of appnotes.

When a PLL is used for frequency conversion - a divider is used to put a divided down frequency on the phase comparator so the VCO has to go faster to keep up. That's the classic textbook example of using a divider to multiply frequency.

#### Lestraveled

Joined May 19, 2014
1,946
Remember, this is a PLL precision frequency to voltage converter. The "F input" is the frequency to be measured.

#### Tony Elliott

Joined May 8, 2015
158

#### Tony Elliott

Joined May 8, 2015
158
Thank you for these notes, just having a good read of the info.