Phase comparator gain calculation

Apr 14, 2005 4 Replies
J

Hi, I am trying to design a PLL using the 74HCT7046 chip. I am using the Phase frequency detector and feeding this into a single supply op-amp active PI filter. This has its non-inv input tied at vdd/2=2.5V whilst the amp is on +15V to give the VCO tuning voltage



+2-+15V for a 5MHz change in frequency.

Now what is the gain of the PD. I have reckoned it as 3*vdd/2pi, is this correct? Do I need to take into account that the non-inv input is at half VDD?



Has anyone got a schematic for a TTL PFD feeding a fast totem pole type discrete output at +15V, to up the voltage swing?



Thanks, John.



Vote
Twitter Facebook LinkedIn
A

Yes.

When the pull-up transistor is on, the phase detector output voltage w.r.t. op-amp virtual-earth is +2.5V When the pull-down transistor is on, it's -2.5V.

If you want to think of it as a voltage-output phase detector, the gain is

2.5/2pi

Alternatively, treat it as a current output: Let's say the resistor between the PD output and the summing node is R, then the gain is (2.5 / R) / 2pi. You now have a current-input active loop filter, of which R is not a part.

BTW You probably want to split R in two and put a small capacitor to ground from the middle. This takes the sharp edge off the pulses before they reach the op-amp.

Vote
Twitter Facebook LinkedIn
A

single

VDD?

voltage w.r.t.

gain is

between

/ 2pi.

a part.

to ground

they reach

I

No. The PFD gain is not affected by the op-amp power supply.

Vote
Twitter Facebook LinkedIn
J

Thanks for the reply Andrew, and sorry for being obtuse, but firstly I had the gain for the PFD wrong, it is VDD/4pi. Are you saying that now I have the split supply to the op-amp this is now doubled? ie 2*vdd/4pi.

Many thanks, John.

Vote
Twitter Facebook LinkedIn
J

The charge-pump pd's are generally loaded into a capacitor, and the chip output is usually considered to be a current source. So the transfer function from phase to pin voltage is of the form k/S, an integral.

John

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required