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 = 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.