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Design >> RF Design >> Need help in Signal leakage
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Message started by RAM1984 on Jun 24^th, 2014, 4:39am

Title: Need help in Signal leakage
Post by RAM1984 on Jun 24^th, 2014, 4:39am

i am working on the up converter design, it converts in 100 MHz signal into 6 GHz RF Signal with double stage conversion.

As PCB level the board is working fine and meeting all technical requirement. LO1 and LO2 leakage is less than -35 dBc.

But if i am putting PCB into metalic enclosure , the LO1 and LO2 leakage is increasing app -20 dBc. In main path i have used enough BP filter to reject the LO leakages. But no effect in the output stage. I feel the unwanted signal is not traveling through the main path.

we have included the RF absorver in the top cover to reduce the unwanted signal, but we are geeting only 5dB improvment.

Please guide us how to reduce the signal leakage.

Title: Re: Need help in Signal leakage
Post by loose-electron on Jun 24^th, 2014, 3:04pm

What radiated into the lab is now inside a box and coupling into everything inside the box.

Simple answer: stage by stage isolation, often done with small metal covers soldered on the PCB over each section.

Make sure everything else is clean and quiet before you start adding shield cans over each section.

Title: Re: Need help in Signal leakage
Post by RAM1984 on Jun 24^th, 2014, 8:07pm

Hi loose-electron,

Thanks for your reply.

we have already done the shielding of each stage using metalic wall separation.

for more info : it is a 4-layer card, we have used L1 - RF ; L2 - RF Gnd ; L3- Control & power line and L4- DC Ground.

One more observation : In ground of PCB also i am getting my LO 1 and LO2 signal as -40 dBc down.

Title: Re: Need help in Signal leakage
Post by loose-electron on Jun 26^th, 2014, 5:43pm

coupling paths thru grounds and power supplies are common - these can be split and filtered

differential signal paths between boxes, with strict common mode and differential mode limited BW interconnects.

there's a bit of an art to the process of isolation, but it can be done.

Sometimes one LC section or one ferrite is not going to get it done.

Just sharing a common ground between stages may be enough to mess things up, when the PSRR of the ground pumping in one box does not get filtered out to the next box due to the shared ground.

Just some thoughts.