HMC832A Direct FM Modulation?

Question

I would like to know if it would be possible to get FM by digitally modulate the HMC832A PLL? 
 I've learned that HMC832A can be used in fast hopping application, which infer that it can be directly modulated. This is the main reason why I chose this chip for FM application. 
 I've programmed the PLL to tune at a frequency, wait until it got a lock, then programmed Reg0Ah[11] = 1 (disable VCO auto calibration). 
 With 20 kHz rate, I programmed Reg04h (with sine shaped values) and expected to get an FM output. However, the output signal did not resemble an FM signal. The sidebands were too wide and too high, much like a wideband noise. I got the same result when I successively programmed Reg04h with a constant value. 
 I tried to adjust various parameters such as charge pump current, offset, seeds, etc. to no avail. 
 I've read Is it possible to Frequency modulate the internal VCO of the HMC829 PLL? Is it also applicable to HMC832A? 
 I attached the output spectrum. Green trace is unmodulated signal, red and blue are modulated signal with different DSM seed.

ADIApproved · Accepted Answer

Try with autoseed off (Reg 0x06[8]=0) and turn off N-divider init (Reg 0x08[8]=1). This will prevent the DSM from initializing the seed and N divider on every Reg 0x04 write. Keep in mind constant SPI activity will degrade phase noise a bit. You can check if this is a problem by writing the same value to Reg 0x02.