A question about HMC440 Loop Filter

Dear all,

I've been working on a HMC440 PLL with Wenzel 100MHz OCXO as reference, and a very narrow band (Ko = 120kHz/V) VCO runs at 1GHz.

The "Hittite Microwave PLL Design & Analysis Tool Version 1.1" shows the output phase noise is dominated by neither the reference nor the VCO, but the filter (as shown in the following figure, dark green line).

When I look into the "Closed loop Modulated Filter Noise" plot, it suggests the RL and R1 are the dominating sources.

Since the simulation result matches our test result pretty well, and because this -100dBc/Hz@10kHz is really not good as a 1GHz PLL, I just wondering is there way to improve the performance? Shall we change the R1/RL values, although 200 Ohm is suggested on the datasheet? Is there another type of loop filter which is better in doing this job?

Thanks a lot.



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