And even more filters…

People might think I am crazy building all these filters all the time. But I think it’s really fun and I learn a lot by doing it. Now I think as I wrote in the previous post figured out how to do the filtering at the station. These RX filters will sit in the RX-path of the transceivers and I might do so that they are switched in depending on which bands the other radios are on. For example if the run station is on 20m then the 20m filter will be switched in on the mult stations and if one mult station is on 15m then that filter is switched in on the other mult plus also the run station. That way I will have the least amount of insertion loss possible, since it’s no need to use filters on bands which does not have any radio transmitting on them. Not that the insertion loss is high, but every 10th of a dB counts, at least in my opinion.

I first tried to build the filter on a PCB that I milled but it was extremely hard to tune it and I could only blame it on coupling between the filter nodes and I did not get near the performance that I expected from simulations. The filters are 5-pole chebyshev notch filters. So to test my theory I built a box with chambers and added pole for pole into it and measured and already with three poles I beat the old result and a lot better insertion loss as well. When adding two more poles the result was close to the simulations and I am very pleased with the performance.

Now I need to figure out if I should be pleased with just three poles or if I should go for 5 poles, we will see. I think I will make a bunch of filters in a big box and make chambers for it, will be a lot of work but I think it will be a great result and totally worth it. But I will make another test first, and that is to use toroids on all five inductors and that way maybe the coupling will be so low that I don’t need to use chambers, it’s worth a try because it would make it A LOT easier to build them.

I also put a Webswitch which we got sponsored from Microbit 2.0 AB into a box, that will be used at the station to be able to switch gear on and off over ethernet. That way I can reboot stuff if needed and not needing to have power on to the openASC system the whole time etc.

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