# 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.

Regards,

Parents
• Hi Marty,

So according to the design & analysis tool, HMC440 can achieve the goal we want.

However, our test result shows otherwise. We are using LT1028 for the filter, with the parameters the tool provided.

The phase noise is measured with R&S FSWP8, and I've plotted them on the tool's plot, as following.

The loop bandwidth is set to 9kHz (this bandwidth is optimised for a different reference).

The output PN roughly follows the scaled reference curve up to 800Hz, where the reference PN starts reducing.

It seems some other term is dominating the PN performance up to some 400kHz, where the measured PN match the tool's simulation again.

Could you please suggest any potential source of this noise term, thanks.

Best regards,

• Hi Marty,

So according to the design & analysis tool, HMC440 can achieve the goal we want.

However, our test result shows otherwise. We are using LT1028 for the filter, with the parameters the tool provided.

The phase noise is measured with R&S FSWP8, and I've plotted them on the tool's plot, as following.

The loop bandwidth is set to 9kHz (this bandwidth is optimised for a different reference).

The output PN roughly follows the scaled reference curve up to 800Hz, where the reference PN starts reducing.

It seems some other term is dominating the PN performance up to some 400kHz, where the measured PN match the tool's simulation again.

Could you please suggest any potential source of this noise term, thanks.

Best regards,