PLL performance and phase noise between two structures

Is there a significant difference in phase noise between these structures :
1) design a L-band PLL and the multiply it up to ku-band or
2) design a ku-band pll directly?
I think that the first one will be better in phase noise ( practically not theoretical) . Isn't it?

The best phase noise ?. How to get.

It is depending on a few factors. You have to see what the VCO's performance is without PLL loop. Check datasheet. If this is better than you need you should design a PLL with a loop bandwidth as small as possible so your Phase noise of your PLL will not effect the normal VCO phase noise.

If your VCO's phase noise direct is not as good as needed you need to make a PLL with a loopband width that is large enough to lower your phase noise performance what you need.

What is your frequency stepsize needed. If you only need ONE fixed frequency you have it easy. simply use the highest possible compare frequency with the lowest divider valua. So 100 MHz compare and main divider is 200 making it 20 GHz VCO frequency. Now if you take a low noise XTAL of 135 dBc/Hz at 1KHz offset your final phasenoise on your VCO will be. 20 log (200) = 46 dB resulting in a phase noise of:
135 - 46 = 89 dBc/Hz (this is only true if your noise produced by the Phase comp. is mutch smaller than 135 dBc/Hz and this is normally the case)

This example is assuming that your loop band width is more than 2 KHz. Normally you take something between 20 and 50 KHz loop bandwidth.

regards,

Paul.

Generally, you can assume that you will get a 6 dB degradation in phase noise for each doubling. Also, how are you planning on multiplying this signal? If you plan on mixing it up, then you will get a contribution from the mixer and the other signal source.

One scheme that some 802.11 chip's have used is a LO at 1/3 the final output frequency. The output of the PLL goes into a doubler, and then the un-doubled version is mixed with the doubled output, and you take the sum frequency @ the output. You probably get worse noise floors, and spurious may be a problem, but it avoids load pull problems.

Dave