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frequency sweep problem of HMC769

Hi, all

I'm trying to use HMC769 as a FMCW signal generator. After 2 days' groping for the SPI interface, I have finally made HMC769 output fixed frequency in integer or fractional modes.

I followed the "PLLs WITH INTEGRATED VCO - MICROWAVE APPLICATIONS PRODUCT & OPERATING GUIDE" (page 20) exactly, but can not get frequency sweep.

The start frequency is 8.96 GHz, and stop frequency 10.24 GHz, although a little beyond the specification, but it works in fixed frequency mode. The reference is 10 MHz, and R is 1. After some computations, the register sequence to be written is:

(1) Reg 06 = 0x1F1F             <the PLL in fractional mode>

(2) Reg 0B = 0x1E071

(3) Reg 09 = 0x3264

(4) Reg 03 = 0x1C0

(5) Reg 04 = 0x0

(6) <Press the external Trigger button>

(7) Reg 0A = 0x68DC

(8) Reg 0C = 0x200

(9) Reg 0D = 0x0

(10) Reg 06 =0x1FFF           <the PLL in auto 2-way sweep mode>

(11) <Press the external Trigger button>

After Step(6), the PLL is locked and oupouts 8.96GHz correctly. But after Step (11), the lock is lost, and VCO is free running and outputting its highest frequency.

I do not know what is wrong with the above register sequence. I have tried to change the order, no luck.

BTW, I did not set RFDiv2Sel (Reg 08 bit[17]), because it seems HMC769 automatically set it already. If I set RFDiv2Sel, then the PLL does not work. This is contrary to the manual.

Thanks for any advice.

PS: I am from China. If you do not know how to write my name in Chinese, you can call me "Qian" here.

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