4 Band, Band Pass Filters

Double Tuned Bandpass filters

Being invasion day here in Australia with not much doing, between watching the cricket, sleeping and doing not a lot more I managed to build and test the bandpass filter board.

Overall, well its MEH. The filters are nothing to rave home about, they will work for now, but I do think i am going to redesign the board using a different topology and get much better attenuation in the stop band.

I ran all the filters though the bode plotter and remembered to screen grab 3 of them which i will now present here for laughter and ridicule.

40m bandpass filter is a 2 humped camel. The width between the peaks is fine, but i will have to play with the coupling cap value to see if i can take some of that hollow out.

20m bandpass filter looks ok, its just to narrow.

The 15m band filter is also to narrow and the high side attenuation is kind of poor at best.

The 80m filter was also MEH but i forgot to screen grab it, It was double humped, but only -3db down in the middle and the high side attenuation was also poor.
Anyway, these will allow me to get building and testing more parts of the radio and think about changing the topology and laying out a new board and having it made. I have 2 other board sitting there waiting to be sent to JLCpcb, the 3rd iteration of the audio board and the first of a final PA board. More on those to come.

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Radio Bebek LP and BP Filters

Ok, this has been a fun section to build. I spent the better part of an hour scratching my head over this and wondering why it was not working. Well, this is how I approached this section of the build.

First thing i did was rummage around in my parts bins for canned inductors. I do not have a lot of these things, but i do have a few and those are of a few different types. From what I understood from the few bits in information i have been able to glean from the internet, they used 10.7Mhz IF cans. This then narrows it down to 3 or 4 actual IF Transformers.

Of the ones i have in my parts trays, only one looked like it had suitable values for a 7mhz filter, with windings that measured 120nH and 2.2uH. That was going to get me ballpark and allow me to simulate the required capacitor values.

LTSpice Simulation of Bebek Bandpass Filter

The fun begins when you go to buzz out the actual pcb, because the input series cap is not on the board, but its on the schematic, first issue that did my head in. The output series cap is on the board and on the schematic. Simulating without the input cap seemed to make no difference at all. So i just left it out rather than cutting traces and soldering it to the bottom side of the board. This is not the first time in this build that i have found such discrepancies either. In fact, it would be a miracle indeed if someone could stuff the board with parts and it just worked.

LTSpice Simulation of Bebek Lowpass Filter

Not feeling very confident at this point, I thought it would be best to simulate the lowpass filter just to make sure it was ballpark and surprise it was actually quite ok. Perfect in fact.

Radio Bebek filters built and ready for testing

Ok now this is where the fun began. First thing i do when building something is buzz it out to make sure there are no shorts. Well the low value windings on the canned inductors are only 100nh. They look like a short to ground at DC. So sticking my continuity meter on both the antenna pins caused it to go beep and I spent the next 30mins trying to track down the mystery short to ground, before i worked out what was happening

Testing the Bebek filters

So finally I wired up some power and injected a signal and started to trace it though the board all the way to the input port of the mixer footprint. Initially there was nothing, so i probed around and found RF at a mystery pad showing a 6.8uh inductor, which is in series with the 7pf coupling cap. After some head scratching, I think that this meant to be a capacitor so that you can series 2 caps together to get a lower value. I dropped a wire link in there and things finally worked.

Signal scoped at the RF Post of the First Mixer

0.02V signal injected at the antenna port, through the lowpass filter, through the bandpass filter into the RF Preamp and we have 50mV. I do not think the losses in the filters are all that high.

So that end this nightmare 🙂 Its all worked so far, just some IF amps, IF filter and 2nd mixer to go to have the receiver working. Still plenty that can go wrong from here, but its getting there.

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