Has anyone successfully added a narrow crystal filter to the uBITX version 4 boards using Ashhar's included header connections?
So far I've not been able to get a signal through filters at 4.6, 6.2 or 8 MHz. The filters use closely matched crystals, They are Chebychev filters and have been designed with "ELSIE" for a 200 ohme input/output impedance with 4:1 transformers to match them to Ashhar's specified 50 ohms. Software has also been modified to allow moving the BFO and IF frequencies to match the filter's frequencies. Can't seem to get enough signal through them to be useful though the software DOES put the IF where it should be. I can hear a signal generator on whatever frequency the uBITX is tuned to, but it takes better than 10 microvolts input at the antenna for what I'd consider to be a usable CW signal in the headphones.
I built the filter using a 4 crystal - 200 ohm 8 MHz circuit with crystals and other components purchased from INRAD (Vibroplex now owns them) but either I haven't figured out the proper 4:1 transformers or there's just too much loss through the filter. I have tried it with 4 and 6 MHz filters I made from scratch with only slightly better results (6 microvolts at the antenna for a usable signal)
Sure would be nice if someone was able to get something working to the point it was usable and repeatable, even on perf-board.
The idea of switching the IF frequency to switch filters appears to be sound - just need to find the magic crystal/capacitor/transformer combination and for that I need someone with better knowledge of crystal filters and better test equipment that I have.
Jim Sheldon, W0EB