Friday, 2 December 2011

Cavity Filter Episode 3

A quick update while I'm in the lab to do some preliminary tests. I just did the very first frequency sweep on an RF network analyser and I'm quite satisfied so far. Although the top plate, that holds the tuning screws, is not soldered in place yet it seems as if everything works.

I still wonder why I chose male SMA for the cavity filter connectors, since cables usually have male connectos too. Luckily I found one gender changer and I'm also abusing an SMA T-connector.

One of the centre resonator pins was a bit lopsided, which I corrected by simply bending it back into place. I noticed that this particular pin is longer than the natural resonance length of 71.3 mm (1013.3 MHz) while I managed to keep the other pins at 69.0 mm. I will have to shorten this pin otherwise it will be useless.

Here's a picture of the first sweep with a span from 0.3 MHz to 3000 MHz. Apparently the filter resonates at 1029.6 MHz without the top plate and tuning screws. I was expecting a slightly higher frequency like 1060 MHz or so but 1030 MHz is nice.

Centring on that peak and narrowing the span to 20 MHz gave the above transmission plot. This is still without the top plate and tuning screws in place. I'm impressed by the quality factor of almost 1000. Granted, the skirt to the right has a hump and the insertion loss is rotten but this made me curious to see how the filter would work out with the top plate in place.

This is the transmission plot with the top plate attached and held in place with duct tape. A first attempt at tuning confirmed that the lopsided centre resonator is too long and adjusting its tuning screw does not change the filter curve at all. The other screws work very well although I think the tuning process is a bit too sensitive. With one of the cavities not tuned this filter has a very nice insertion loss of -6.1 dBm and a Q of roughly 600.

Curious to see the rejection at 1024 MHz (the second local oscillator) and 1034.7 MHz (the in-band image frequency) I changed the span to 45 MHz and did a new, slower sweep. The result is shown in the above picture.

I found -54 dBm at 1024 MHz and about -66 dBm at 1034.7 MHz, which is a lot less than what I want it to be (-120 dBm). But given the fact that one of the cavities is off-resonance and that this first test was not intended to be very accurate, I am looking forward to doing the measurement again later.

I haven't decided yet whether I need to re-solder the resonator pin or simply shorten it by removing about 2 mm of copper, but maybe re-flowing the solder joint is the proper solution. I still need to write episode #2 as well, which will tell more about how I soldered the base plate to the cavities.

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