I haven't found the way to set the prescaler of AD9520-5 to 1. In datasheet says that prescaler in fixed division can be set to 1, 2 or 3. But also below the "VCO/VCXO Feedback Divider N: P, A, B, R" says that P can be 2, 4, 8, 16 or 32. I have a 100 MHz VCXO connected to CLK input and I tried to start phase detector working at this frequency (reference clock is the same 100 MHz from external generator). If I set STATUS pin control = 4 or 1 (prescaler output or N divider output) I see divided by 2 frequency. If I set STATUS pin control = 2 (R divider output) I see 100 MHz. Bits 0-2 of 016 register = 0. Bit 0 of 1E1 register is 1 (bypass the VCO divider). Where's true?

Thanks.

• Hi vdpavel,

Did you also set R0x16 bit  = 1? This sets the B-counter to a divide by 1 state and allows the prescaler value to determine the entire N divide.

• Dear VDPavel,

First, the evaluation software is a handy tool for configuring the part:

You're using the -5, which is the same as the other versions, other than the internal VCO is disconnected. Therefore, you can select any of the dash versions while running the eval software.

I've also written a user's guide to assist you with running the software:

http://www.analog.com/static/imported-files/user_guides/UG-076.pdf

To answer your question, there are two modes for the feedback divider: dual modulus (which is where the P=2,4,8,16,32 comes from) and fixed divider (/1,/2, /3).

My colleague, Neilw is correct. To do a /1, select fixed divide (/1), leave the B counter =3, and bypass the B counter.

-Paul Kern

• Yes, I do the B counter >= 3 and bypass the B counter. Register I write to AD9520 0x016 = 0b00001000. But it still divides by 2. It's behave very strange. If I set prescaler = 0 (divde by 1) it divides by 2. If I set prescaler = 1 (divide by 2) it divides by 4. I can't understand why it is happening in this strange manner. A counter is always = 0.

• I've found the problem! Clock for PLL was supplied from zero delay block. But the clock from first divider was divided by 2. Now I bypassed divider0 and everything OK. I did it! Thanks for your reply!

Pavel

• Hi Vdpavel,

Glad to hear. Please let us know if we can be of further assistance.

-Paul Kern