Wednesday, 11 July 2018

450 MHz CDMA duplexer tear down and analysis


450 MHz CDMA duplexer tear down and analysis- draft

Introduction


Why?

I am interested in how modern duplexers work. The club purchased a new 70cm duplexer, only 50 mm tall and not obvious how it worked, but they didn't want me opening it for a look!

I purchased a CDMA duplexer from Russia on the 450 MHz band on eBay. It was similar to the 70 cm one. I could get some idea how the UHF one works and some(?!) chance of re-tuning it for either a 70 cm DVB-T TV filter, 7 MHz bandpass, or as a 70 cm narrow pass band repeater duplexer (or both, as there are three chains of seven cavities in the device.

Unfortunately, I did not take photos of the duplexer's response before I opened it. However, it was a 6 MHz pass band, with steep skirts, and low pass in the 450 MHz band. I will do it when I put the top back on, but have probably disturbed the tuning. CDMA signals are 1.23 MHz wide, so it is unclear why the pass band is 6 MHz.

The outside, with a zillion screws out. The left three connectors are all SMA, the right are 7/16 DIN and N adaptors that I added.

The gizzards!

Click photo to see captions larger!!


It is a complex beast, requiring a very detailed examination to see all its features. Pore over the photo to see.

?? = I think!

The duplexer has three chains of cavity filters, RX (top), RX monitor (middle) and TX (bottom). RX has its own antenna. TX and RX monitor share an antenna, but are on different frequencies. Tx input is to left. The resonators use big capacitive hats to electrically shorten the resonator to one rack height, otherwise four rack high. The resonator adjustment screws are the larger screws.

The filters are pass band and low pass. The low pass comes from the capacitive tuning into the resonator??

The filter chains are pass-band, iris-coupled (port-tuned) filters that have a sharp cut off. The iris-coupling screws are the smaller screws. In addition to iris-coupling, both inductive (on lid) and capacitive inter-cavity coupling are used. I don't know why, presumably to get a sharper response (or impedence matching??). There are no notch filters, as are used in DVB-T filters, to get a very sharp cut at the edges of the pass band.

The input/output are either a separate smaller resonator with a gamma match?? coupling (left), or a conventional gamma match?? coupling direct to the main resonator (right).










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