LMX2595: deterioration in phase noise

Hello

I'm using an lmx2595 in my circuit and have made a evaluating module using the EVM as a reference. Some changes have also been made to simplify the module, and these changes have caused deterioration in phase noise in the 7.5-8 GHz band at around 100kHz offset, as is plotted in the following figure. In other bands no deterioration appears. 

The reference signal of 100MHz is given by R&S SMA 100B, and the chip is programmed using a usb2any. Doubler is turned on to set fpd as 200MHz, and the out put is 7.6 GHz, for example. I suspect that something is wrong with my schemit or PCB design. 

The schemit is the one plotted below. 

Please give some advice about how to solve this problem or any potential cause of the deterioration.

Thank you!

Qiuyi Zhang

Kailyn Chen：

Hello ,if the design is referred to the EVM,  the schematic should be no problem. 

Do you used TICSpro to configure the registers? 

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Qiuyi Zhang：

yes, the registers are configured using TICSpro software and USB2ANY.

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Kailyn Chen：

Hello， the  deterioration should come from external noise, for example, power supply, USB2ANy which can sometimes carry noise over from the host PC.

Or carrier , if convenience, you could change another spectrum analyzer to test again. 

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Qiuyi Zhang：

Hello. Removing the usb2any really works. Thank you very much! By the way, what is the reason? I'm pretty curious about the resource of spurs, because I did not see any potential noise source of 7.5-8 GHz. I also tried removing connection of usb2any connector one by one, and finally found that when the CSB pin is disconnected, the spur would disappear. Is anything special in the CSB pin? I bought an official version EVM. When the official EVM is connected to the USB2ANY, the spur just did not appear. Are there any key rules I should refer to?

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Kailyn Chen：

Hi, I have found that you have resolved your questions by E2E. Our foreign engineer has replied to you about the CSB effect to the spur.

I am very glad you have resolved your questions, so I close this post temporary， any other questions, please contact me again.

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Qiuyi Zhang：

Thank you also Kailyn for your advice! Your team have provided high quality suggestion and instructions and solved my problems well.

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Kailyn Chen：

Thank you very much. If you have any other question about other questions in future,  please don't be hesitate to contact us again. 

We would try our best to resolve your questions, if not, we could ask our senior engineer for help.