# HMC833 frequency lock problem when switching between divide by N - fundamental - doubler

Question asked by Feruz on Mar 15, 2017
Latest reply on Mar 21, 2017 by Feruz

Custom board (not eval)

HMC Mode SPI

Fractional Mode
10 MHz ref

HMC833 Init params:
0x06, 0x20, 0x0B, 0x4A    //6
0x07, 0x00, 0x22, 0xCD    //7
0x08, 0xC1, 0xBE, 0xFF    //8
0x09, 0x15, 0x3F, 0xFF    //9
0x0A, 0x00, 0x20, 0x46    //10
0x0B, 0x07, 0xC0, 0x61    //11
0x05, 0x00, 0x16, 0x28    //5.5
0x05, 0x00, 0x60, 0xA0    //5.4
0x05, 0x00, 0x00, 0x00    //5 write end

Frequency set sequence (as in p1.2.2.1 datasheet):

1) Int part
2) vco 5.2
3) vco 5.3
4) vco 5 write end (zeros)
5) fract part

I am sweeping freq upward from 25 to 6000 MHz with 1 MHz step and downward (6000 - 25 with 1 MHz step)

All frequencyes locking good except switching between divide_by_N - Fundamental - Doubler modes.

For example when i switching from 3000 to 3001 i got (see pic 1)
When switching in other direction (from 3001 to 3000) i got (see pic 2)

Similar situation when i switching between 1500 and 1501 MHz

Other switches (1501 to 1502, 3001 to 3002, 1500 to 1499, 3000 to 2999) are correct

Also said that LD is always "1" - i.e. always locked (LD_SDO configured as LD while not SPI, performing ID read after power up with correct result)

Also my config params for different freqs:

Fo = 1500MHz   k = 2 doubler=off   Nint = 300   Nfrac = 0   deltaF = 0
Fo = 1501MHz   k = 1 doubler=off   Nint = 150   Nfrac = 1677722   deltaF = 0,238418579101563

Fo = 3000MHz   k = 1 doubler=off  Nint = 300   Nfrac = 0   deltaF = 0
Fo = 3001MHz   k = 1 doubler=on   Nint = 150   Nfrac = 838861   deltaF = 0,238418579101563

Question is: how to switch correct between modes and lock correct frequency?