EMC issue with RF Amplifier

[ahsoopk]

Dear all,

I have designed a PA that feeds into a Filter bank, specific filter is selected by micro-controller. filter bank is in its own cavity and the PA and filter bank goes into a final box i.e. share the cavity. when filter is and PA are out of box and connect in open space they behave as they should but when put back into the box and are not in open space at some frequencies PA backs off by 3/4dBm.

Relays also start buzzing in filter bank when a specific filter is selected in the bank. One more thing filters are tuned in open space and then put into the cavity and they are lumped element filters mounted on the same PCB and get selected by the relays. frequency of operation is 30-512MHz.

if you guys have any ideas please through at me and if you think need anymore information please let me know.

thanks in advance

 

[reidintransit]

Depending on the proximity of the box to the filter elements you may be detuning the filters.

 

[D.A.(Tony)Stewart]

reactive elements like relays would have to be detuned with a small parallel cap below RF band, although any transients or arcs must also be suppressed or kept outside. Bad choice.

Filters could be be tuned in situ with wood or plastic tuning blades thru an access hole in the shield cover or with a variable cap mounted in wall. etc. or else have a shield soldered on board outside cavity.
Custom tin plated brass shields can be low cost and fabricated by board shops if given photo tools, which have dotted lines on one side for folders. and solid lines on both sides for cuts.

 

[jiripolivka]

There are two issues if I understand.
One is that filters become mistuned hen relocated in metal enclosures. Above contributors explained why, and how to readjust them once in the final installation.
The other is "buzzing relays". What does this mean? Do you really hear the relays buzzing? Who designed the driving circuits? Any knowledge of how to make reliably working relays?
When filters are used at UHF and higher, filter banks should use switches (relays) with terminations, to keep filters filtering well.
Connecting a RF power amplifier to a filter bank is nothing simple, the amplifier output should be matched over the wide band to keep filter response as designed.
Certain experience is needed, and the hands-on training will gradually show you how to optimize your design.

 

[D.A.(Tony)Stewart]

Speakers often buzz from Cell phones when they poll the server for closest tower reception.

It has to do with the AM or pulsed microwave energy gets rectified by nonlinear magnetics and causes radial forces making the coils move. In the case of speaker coil touching the magnets. In the case of relays buzzing the contacts from solenoid effects...

This is why I suggested a small RF cap across the coil