Problems of DDS drive PLL

[saulbit]

HI Everyone.
Now I am designing a synthesizer from 800MHz to 1000MHz. I have chosen the VCO. But I have to face a Difficult question: The output frequency Resolution is 1Hz. I have get a DDS covering 20MHz~80MHz, with frequency resolution of 0.2Hz,
Can I use this DDS as the reference of a PLL, such as ADF4107. Then what about the spurs of DDS? Would they be amplified by 20*log(N)? or They would be filtered outside the PLL loop filter? 3ks.

Added after 3 minutes:

I have designed some PLLs, but their reference all are OCXOs and their frequency resolution is 10MHz. I don't know whether DDS could be the PLL reference. Would anybode tell me some recommadations?

 

[summersonic]

HI saulbit
I have some experience on DDS and PLL hybird frequency synthsizer.if you use the DDS output as ADF4107 reference input,the spurs of output spectrum will be amplified as 20*log(N),so the purity of output spectrum won't be well.
you can improve the spurs by using mixer-PLL,that architecture equalize reducing N,then reducing spurs.

 

[fenfei]

The spur will be amplified as 20*log(N), you should make sure that after apmlified ,the spur will meet the requirement. DDS output is too wide , the spur in the loop will be very bad.

 

[biff44]

The art of Synthesizer design involves using several noisy sources, in various ways, to achieve the good end-result you want.

Yes a pll will amplify the spurs of the DDS by 20 Log N, but ONLY inside of the pll control loop bandwidth. If you have a 1 KHz loop bandwidth, and you have a spur at 1 MHz from the carrier, it will be amplified by 20LogN, but reduced by maybe 60 to 100 dB in the loop filter's lowpass response! So the answer to your question is "maybe".

Mixing the DDS with the VCO, and then presenting it to the digital divider eliminates the 20Log N DDS spur amplification, but adds parts/complexity, as you do not want to lock up to the image frequency out of the mixer! May require pretuning the VCO with a DAC, or maybe a image reject mixer, since the DDS you are thinking about is low frequency.