Building and testing microwave duplex band pass filters

A project log for Cell Phone 4G LTE Repeater / Booster / Femtocell

An outside pole mounted aerial picks up 4G signals which are then filtered, amplified and re-transmitted through a second inside aerial.

capt-flatus-oflahertyCapt. Flatus O'Flaherty ☠ 03/25/2017 at 14:450 Comments

I decided to try and be a bit more logical in my approach to this project and start building the gadget from the beginning - the band pass filters. Why do we need them? They're to filter out all those FM radio stations etc and provide (hopefully) a nice clean break between the incoming signals and the outgoing ones, which operate on well defined frequency bands eg band 20 base stations transmit on frequencies between 791 and 821 MHz.Our cell phones, if they're actually using band 20, transmit on 832 to 862 MHz and we don't really want each of our amps trying to process both sets of frequencies at the same time .... And certainly not FM radio stations!

Ok, less of the theory and more of the mechanics. Looking at the actual filter units, the USD020 s from CTS, it looks like they've been attacked by some kind of cyber-kinetic squirrel:

And looking more closely we can see that they've actually been deliberately gouged out with a precision machine:I contacted CTS, worried that somebody had sabotaged my filters, but they reassured me that this was in fact fine tuning at the manufacturing plant - FANTASTIC!

So, check, re-check, quadruple check the soldering guide for these beasts and place them on their mini PCBs and into the toaster oven for breakfast:

The filters are heavy blocks of metals so my thermometer probe is going to lag behind the actual readings by about 30 seconds or so by my estimates. In any case, the maximum temperature was not exceeded and it all working out fine - thank God, as these components are not cheap!

Now for testing on the LimeSDR:

I paused the FTT waterfall halfway through and changed the filter connections from Rx to Tx and we can see nice clean separation between the two sets of frequencies - wonderful - those cyber-kinetic squirrels did a really good job! Now to solder up the rest of them and test them as well.