Cavity filter deconstruction

[SeanB]

I have a cavity filter from an old Lotto machine, rescued (temporarily) from the scrapheap. Must be a lot of these old Uthingo machines out there sitting in a storeroom somewhere at the shops they were at, they were never collected when the new operator took over and issued touch screen machines that use GSM data instead via a GPRS modem attached to the machines.

Interesting way to have a 413MHz receive and a 423MHz transmitter using the same antenna and separating the 2 radio paths enough so that they do not interfere with each other.

First is the unit, with the central N connector that was connected to the antenna on the roof.

Next is the bottom after removal, with the spacers used to separate the tuning screws and the silicone grommets used for compliant mounting of the bottom of the tuning rods. Not sure what plastic is used for the spacers, looks like acrylic.

A look up the bottom. A lot of screws to hold the bottom on, 32 screws just to take the cavities off. Copper coated steel ends with an aluminium cavity and copper insides with silver wire.

Matching inductors, otherwise a slightly bent wire adjusted on final alignment by bending it a little.

The unit bare, look at the different construction on the transmit side ( right 3 pipes) to handle the high power on transmit ( 5W from the amplifier specs) but good enough as a filter that the receiver can be active at the same time with no problems.

[Deckert]

Most interesting, particularly the amount of copper in the filter. "Thieves" should raid the scrapheap of these machines rather than pull out cables from the ground. I sometimes marvel at the amount of unrecycled junk we produce that still has substantial amounts of viable metal in them.

But I digress... This frequency-tuned plumbing has always made me nervous - reminds me of a receiver we built for a signal theory class while at varsity. Thing was built on single-sided copper board and ran at 1.2GHz. The receiver failed to demodulate the instructor's transmission until somebody inadvertently placed a half-empty coke can about half a meter away. Voila.. it demodulated. Take the can away, pfffft, no demod.

That's what propelled me into digital electronics and computer engineering. Today I'm happily manipulating 1's and 0's with C++. Analogue RF electronics scare me - the closest I get to analogue is the PSUs, mini-charging circuits and other basic analogue odds and ends I've built over the years.

--deckert