23 July 2011

The withering filters - adding a blocking capacitor when space is tight

Adding the DC blocking capacitors to protect the ceramic filters in your  rig may sometimes prove a bit difficult with the newer, extra small SMT components. Sometimes, there just isn't enough space or PCB trace length to accommodate the new part. In such cases, alternative methods might help, like the one in the photo (click to enlarge).
Here, the capacitor has been added only at the output pin of the filter (top right in the photo), because the output side of the filter is the most vulnerable (owing to the thinner ceramic resonator there). Due to the very small dimensions of the cicuit (the distance between the input and output pins of the filter in the photo is 8.3mm) there wasn't enough PCB trace length to comfortably add the capacitor by cutting the trace. So the solder was sucked out of the pin's plated-through hole, which left the pin standing in the center, unconnected. Then, the size 0603 capacitor was soldered at an angle, bridging the distance between the top of the filter's pin and the PCB trace. Problem solved, without PCB cutting (which may also sometimes prove dangerous to nearby SMT components)...Be sure to check whether the filter's pin actually stops making contact to the plated-rhrough hole, after you suck out the solder!

(The photo is from a President brand marine portable VHF tranceiver, that had gone completely "deaf" due to filter deterioration - after reviving the filter and adding the cap, it's back singing and dancing. The manufacturer [what a surprise!!] had omitted the DC blocking capacitors between the IF IC and the 2nd IF ceramic filter.)