Re: Tinny sounding electric miss on uBitx

ajparent1/KB1GMX
 

OK, we have a lot of misunderstanding here...  uBitx.

If the master osc is not set on frequency everything else is likely to be off frequency and work poorly.

The filter is ladder filter (not a Cohn) and like all ladderfilters due to the parallel caps are loaded to
the lower frequency.  so technically it is a filter of about 2.1Khz with a center frequency a of
11.9989mhz built with nominal 12mhz crystals.   From here out we call it a 12mhz filter as 
11.99989 is just longer! ;)

Also the default startup setting for the software uncalibrated is 11.995 (at least 1khz too low).

The IF filter 12mhz has a roughly 2.1khz pass band with the upper edge just below 12mhz
and the bottom ends around 11.997ish  Small errors in BFO placement (100hz) is enough to
impact audio quality both RX and TX.   FYI OSC-0 is the BFO.   In mine the magic number was
11.996750mhz.  NOTE: the BFO is not changed to change sideband.

ubitx changes sideband by flipping oscillator 1 between 33mhz and 57mhz with a small correction
for the 45mhz filter that is not at 45mhz but about 5-10khz lower.    If the calibration is off or not
done correctly these oscillators will be off for the 45mhz filter and sensitivity will suffer (if far enough off).

Now if the 25mhz crystal on the Raduino board is off and generally they will be its the master
oscillator for the whole game.  So the first thing is to get the "calibration" dialed in so when the
MPU/software commands xxx frequency you actually get it and the other two oscillators are also
where they should be.

Changing out the crystals in the filter can improve the filter but not likely solve the "tinny sound"
as that set will likely have a different offset for the BFO.   FYI changing the parallel capacitors 
will also move the filter center frequency and the bandwidth requiring BFO change.  
They are not SMT they are conventional leaded HC49U parts and easily desoldered without
art or rework station.

If the BFO is right the low end should cut off at about 200hz and the high end around 2300hz.
That will not be tinny.

For an FYI: I included a image of the filter pass band while testing a personal mod to get 
to a wider band width (2.7khz) for better fidelity.  The ripples on the top of the pass band
are about 1.5db and not at all bad.  Even widened up the upper edge is barely 12mhz
(11.999985ish mhz).  Best sideband setting is 11.996685mhz for that filter.

Allison

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