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.

Thursday, 1 December 2011

Cavity Filter Episode 2

I was unable to gain access to a drill press so I had to use a cheap hand-drill to drill the holes in the tubes. I hated every minute of it! Our workshop has a drill press, unfortunately I didn't get access to it.

After I finished soldering the individual cavities together it was time to prepare the connectors and hairpin couplers. For that purpose I used a few pieces of 0.141" hard-line and extracted the centre conductor. The PTFE dielectric was cut into short pieces and provided spacers to keep the hairpin couplers from touching the cavity walls.

I used a hot air gun to solder the tubes together and got very nice joints using this method.

Instead of drilling holes in the end plate I notched the bottom of the tubes, bent the hairpins through the notch and soldered them before soldering the base plate in place.

The thick copper bars have almost been a show-stopper for me since it was impossible to drill holes for the hair pin couplers. Luckily, Sam recommended me to not drill holes at all and rather notch the tube and bend the hair pin through the notch. This worked very well.

The tubes were then placed on top of the bottom plate. I placed a ring of solder wire on the inside of each cavity while and simply fed solder to the outside joints as the bottom plate was hot enough to melt it. Added lots of flux and the result was excellent.

The second resonator from the right sticks too far into the cavity and will probably render it useless. I'll have to see how it affects the amplitude response.

If any of the resonators is lopsided, it can easily be bent back into the centre by sliding a drill bit into it and gently pulling it to where you want it to be.

I think I forgot to mention that I threaded the holes for the tuning screws. That way I will only need one nut to fixate them once the cavity is tuned. The screws I used are M4 x 40 but I think 25 mm  ones would have done the trick too.

The annoying part began when I wanted to attach the top plate next evening.