From the recording I can tell that everything above 1500 Hz or so is significantly dampened, which explains the muffled sound. I had a hunch that the BFO and crystal filter passband might not be properly aligned. It seems like that could be correct!
http://skywired.net/blog/2017/04/replacing-the-bitx-40-bfo/
I also found an explanation of how the BFO and filter are "factory aligned" from a BITX20 group.io post:
"11.059mhz is the nominal parallel resonant frequency of the crystals
The series resonant frequency (as used in the crystal ladder filter) is a few khz lower.
The old Bitx40 made use of this, the BFO oscillator used the 12mhz crystal in parallel resonant mode,
and happened to wind up at just the right frequency down from the 12mhz IF filter.
All those crystals were first sorted to be within 100hz or so of each other before stuffing."
https://groups.io/g/BITX20/topic/69443005
So the difference between the series and parallel resonance is cleverly used to get the offset from the BFO and the filter center frequency. Too bad that this apparently didn't work that well...
The PCB has an unpopulated place for a trimmer capacitor. I could add a cap there, or I could just go ahead and replace the BFO with one of the Si5351 outputs, which I wanted to do anyway (to get away from the 7.2 MHz spur and 41m broadcast band interference by switching between low side and high side injection).
Rasmus SA5RJS /
npub133…hfazx
2025-01-09 09:37:10
in reply to nevent1q…6upx
Author Public Key
npub133kx3vtsccvrn4uxjn85wacm45uqglre8qz7z7tr86p6lhy0whsqmhfazxPublished at
2025-01-09 09:37:10Event JSON
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"content": "From the recording I can tell that everything above 1500 Hz or so is significantly dampened, which explains the muffled sound. I had a hunch that the BFO and crystal filter passband might not be properly aligned. It seems like that could be correct!\n\nhttp://skywired.net/blog/2017/04/replacing-the-bitx-40-bfo/\n\nI also found an explanation of how the BFO and filter are \"factory aligned\" from a BITX20 group.io post:\n\n\"11.059mhz is the nominal parallel resonant frequency of the crystals\nThe series resonant frequency (as used in the crystal ladder filter) is a few khz lower.\n\nThe old Bitx40 made use of this, the BFO oscillator used the 12mhz crystal in parallel resonant mode,\nand happened to wind up at just the right frequency down from the 12mhz IF filter.\nAll those crystals were first sorted to be within 100hz or so of each other before stuffing.\"\n\nhttps://groups.io/g/BITX20/topic/69443005\n\nSo the difference between the series and parallel resonance is cleverly used to get the offset from the BFO and the filter center frequency. Too bad that this apparently didn't work that well...\n\nThe PCB has an unpopulated place for a trimmer capacitor. I could add a cap there, or I could just go ahead and replace the BFO with one of the Si5351 outputs, which I wanted to do anyway (to get away from the 7.2 MHz spur and 41m broadcast band interference by switching between low side and high side injection).",
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