What about the FM character of the VCO output in the case reference is FM modulated?

[saulbit]

I am confronted a difficult to make the RF signal FM modulated. I don't have good knowledge about the FM modulation. I learnt from something that if the reference is FM modulated, then the VCO output of the PLL would be also FM modulated. However, the loop bandwidith is about 100 kHz (I got this from the phase noise curve in the spectrum with phase noise measuring feature, the bandwidth is about 3dB minor than the phase noise hump, please tell me if I am wrong). When I use a reference as 100MHz, 2k deviation and 2k modulation rate, the 2250MHz VCO output shows FM character but with a distortion more than 8%, a deviation about 46kHz, just within the loopbandwidth; when it comes to 200 deviation and 2k modultion rate, the 2250MHz output would be with a deviation of 4.6kHz, a distortion less than 1%. Would someone bring me suggestions about the FM deviation and the loop bandwidth in what case could be fit? Thanks.

 

[biff44]

If you have a PLL that is controlling a VCO, you can impart FM modulation onto the VCO output in 1 of 2 ways:
1) as you stated, modulate the lower frequency reference. The VCO output will be FM modulate by the same deviation in ppm as you modulate the reference. You DO need to keep the PLL loop bandwidth many times higher than the modulation rate. Since you have a 100 khz loop bandwidth, modulating the reference with a 2 khz modulation rate is fine. And you probably want an old fashioned PLL chip (no fractional N).
2) Make a very narrow loop bandwidth, and apply an AC coupled signal directly to the tune line of the VCO. In your 2 khz modulation rate example, you would probably need < 100 Hz control loop bandwidth.