1. BITX20@groups.io

Re: uBitx Modulation only lows no highs in frequency response

Bill Cromwell
 

Hi Jerry,

That is a pretty good description. I have my BFO set so that the incoming signals start rolling off at 300 Hz just as you have described. A few decades ago when we were looking at "compandered" SSB we were shown that the human voice has three main 'bands' of audio spectrum that are required for intelligibility. The lowest band is at 300 Hz. there is a mid band (I don't recall the frequency range) and high band around 1500 to 1800 Hz. Compandering used some Rube Goldberg approaches to squeeze those bands all together (with multiple filters and mixers) so as to reduce the RF bandwidth used to transmit it. There was also some kind of pilot tone to help sort it back out at the receiver. Did I mention Rube Goldberg? It kind of worked in the lab. But not in the wild.

So our SSB filter would like to bridge all of those bands without mashing or bending them. Even with the narrower passband of the stock filters in the uBitx we can get all of that fitted in if the BFO presents that lowest frequency band at about 300 Hz (as you have described). The high end will be close to 19 or 20 kHz. Mine is right around 1900 Hz. "Narrow SSB filters are designed and installed with a 1.8 kHz bandwidth. So the uBitx filters are at the narrow side of SSB. They may sound a little better for armchair chatting at 2400 to 3000 Hz. The bottom end still wants to be rolling off stuff starting around 300 Hz and the wider passband used to to increase the high end of the audio spectrum.

That will probably be very good for digital operation too. I am a CW guy and it does nothing to hurt CW either. I have several times outlined how to set the BFO this way by using the noise shadow displayed on all of the digi-mode screens and the audio frequencies shown on the scale below it. If yours is working well for you just go take a look - without changing the BFO. The noise starts to drop off about 300 Hz on one end (or wherever your is actually set) and the other end it starts dropping off well above 1800 Hz. With CW (or other narrow) filters that noise band is obviously much narrower and the width of that 'noise shadow' is much less - as expected.

73,

Bill KU8H

On 07/22/2018 10:29 AM, Jerry Gaffke via Groups.Io wrote:
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.

Jerry


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.

--
bark less - wag more
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