OL interference in my PCB design

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DCuadrado

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Hello everybody!

I have recently built a module which up-converts a baseband signal between 90 and 190 to an IF signal between 2263,5 and 2363,5 MHz. This module needs an OL signal of 2453,5 MHz @ 0dBm which passes through a buffer amplifier consisting of a band pass filter, an amplifier and an attenuator. The OL reaches the mixer with a level of +12 dBm.

Now that I have exposed the situation here is the problem: the OL tone is interfering every part of the PCB. When I connect the spectrum analyzer to either the output or the input of my circuit, there is a tone of 2453,5 MHz and a considerable level. In fact, if I desolder the AC coupling capacitors, so that there is no physical conexion in the microstrip circuit between the three ports, the interference is still in both the input and the output.

It seems that the OL is interfering all the circuit through the air.

Please, do you have any solution to avoid this interference? The box is already in an aluminium enclosure box.

Thank you very much in advance.
 


As you do not show the design of your converter module, it is difficult to offer you an advice.

With microwave mixers like yours, there is often a LO leakage problem. The LO itself is best to install in a metal box with a coaxial connector output to the mixer. The DC feed should be done with a good capacitive feedthrough. Then all inputs and outputs of the mixer should pass through good band-pass or low-pass filters which reject the LO signal and its harmonics from getting to important points.

Your test equipment should also be connected by good coaxial connectors, and possibly provided with additional filters. LO signal should only be allowed into mixer LO port, nowhere else.

A your design is a PCB, unmatched stripline sections tend to radiate signals around. In many cases it may be a good idea to replace the "live" RF and LO lines with sections of a semirigid coaxial cable which prevents such radiation.

RF design is often tricky if one needs to enclose a LO signal into a desired location only.
 

Thank you very much for your answer jiripolivka. I'll take all your advices into consideration for my design.

Please, I'd appreciate if you or somebody answered a couple of questions.
- What distance (clearance) should there be between the RF line and the ground plane (both on the top layer)?.
- Since I don't have a machine for metallizing the vias, does it matter if the vias in the ground plane are soldered with metal (tin)? I mean, there won't be air connection between the two planes.
 

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