Re: Crystal CW filter with Version 4 uBITX boards? #ubitx

Bill Cromwell

Hi Jim,

My first question is in line with Alison's reply. Why not use 12 MHz xtals and keep it consistent? I have dome something similarin a couple of radios. But I got xtals to put the narrower filter on the same nominal frequency as the wider existing filter. Works a charm.

I am using narrow filters on the audio with my uBitx. The NEScaf works best for me. I really don't to make a $2500 radio out of it.


Bill KU8H

On 07/06/2018 01:47 PM, Jim Sheldon wrote:
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

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
bark less - wag more

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