fm modulation with dds and pll help needed

[leonardo0]

Hi to forum

I want to make fm modulation to a 1.7 to 1.8 ghz pll . For this purpose i used adf4106 pll with umc vco . I though to drive the reference with dds signal at 40 mhz . My question is how to make fm modulation for fm bradcast signal . I can fm modulate the dds reference so this multiplied produces the needed 1.8 ghz fm modulation .
By running adisim i show i must use 1 mhz loop badwidth so gain in linear from 0 to 56 khz .
The problem is phase which has 80 degrees deviation at 56 khz from 0 degrees at 1 khz .
Will this confuse stereo emmisions since the pilot at 19 khz will be in different phase than the 38 khz subcarrier ?. Finally rds will have problems if i phase lock it to 19 khz ?.
Can someone enlighten me ?

 

[biff44]

There are two ways to FM modulate a phase locked oscillator:

1) Inside of the control loop (like you are doing)

2) Outside of the control loop (using a very large digital divider N and a narrowband loop)

If you choose #1, then you have to live with the distortions that the PLL will provide. It helps if you understand classical control theory, when I tell you that you will want to place an "zero" somewhere around 56 KHz to compensate for the phase lag, and another pole at a higher frequency to close out the loop gain, and then another zero to help keep the loop stable when it crosses the open loop unity gain frequency.

I would also watch out for time lag in your digital divider in these high frequency loops, it will make the loop more unstable.

 

[leonardo0]

In case i use the dds with upconversion to 600 mhz and mix the two signals together . With The subtraction i drive the pll signal input . So eg. Fvco =1700 mhz , fpll(inp)=1700-500=1200 mhz .
In that case if i fm modulate the dds wil i have problem with distortion ?.
Outside the loop bw how much will the spurious of the dds be attenuated ?.