Re: uBitx Modulation only lows no highs in frequency response

Jerry Gaffke

You got it right.
Raising the BFO frequency brings it closer to signals coming from the filter, which lowers the audio frequencies..
We have a BFO below the 12mhz crystal filter passband  which mixes (at D5,T7) with signals in that passband creating audio.
Lets give it some numbers, the numbers for your particular rig might be a bit different.
I'm assuming we are in receive mode, transmit is the same math but in the opposite direction.

The BFO frequency in the stock firmware is at 11996500 hz.
The 12mhz crystal filter has a roughly 2000 hz 3dB passband, something like 11997000 to 11999000.
A signal coming in through the crystal filter on the bottom edge of the passband creates an audio tone of 11997000-11996500 = 500 hz.
A signal coming in through the crystal filter on the top edge of the passband creates an audio tone of 11999000-11996500 = 2500hz.

If we now raise the BFO frequency from 11996500 to 11996700 hz, 
the audio coming through would fall between 11997000-1196700 = 300 hz and 1199900-1199700 = 2300 hz.


On Sun, Jul 22, 2018 at 06:19 AM, RCC WB5YYM wrote:
With my radio/engineering skills very much lacking, I probably don't even need to be trying to contribute to this conversation, but here is what I ran into on my build. My tx audio was very distorted when I first tested. I then checked the transmit using the tones sent from WSJTx. I found that no tones were passing that were around 500hz or lower, but higher tones around 1600hz caused more power to be output from the radio. I adjusted the  BFO until I had maximum power out when sending a 1khz tone. This gave me power out in a range from 200hz to 1600hz. This seemed to clean up the audio on SSB. Apparently I have a very narrow filter, but I am getting good audio reports on SSB. The down side is that with it being so narrow, when operating FT8, I need to shift the VFO frequency to see all the transmissions. If I remember right, to shift the audio frequency down, I had to adjust the BFO frequency up, but please don't hold me to this statement . I did adjust about 100hz at a time, and checked the power out using different frequencies generated by WSJTx. I don't exactly know what the above means, but it did help me to have a good working radio. Hope this helps someone. 

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