Re: uBitx Unfiltered

Jerry Gaffke
 

Allison had some good pointers on some first easy fixes to the uBitx in post
     https://groups.io/g/BITX20/message/57649
First and foremost, either fix the on-board transmit LPF routing somehow
or add an external LPF between the uBitx and antenna appropriate to the band you are using.

Beyond that, she suggested replacing Q90 with a BFR106, reduce C81 to 470-1000pf and
replace Q911,912,92,93,96,97 with 2n2222's to get a more even power across the bands.
These mods are not of much use if you are just using it on the lower bands to avoid spurs.

 
But if the current power amp is coupling into the IF amps and creating extra spurs
and we need significant mods to get the uBitx power amp to give reasonably clean and flat power
across all of HF, then perhaps best for the power amp to be a separate board.

So clean up the local oscillators and signal levels into the mixers on the uBitx.
Then saw off (or disable) the back end of the uBitx where we have the power amp and LPF's and LPF relays.
Replace that back end with a separate board for the power amp and LPF"s.
Perhaps use kits that Kees has up at 
    http://www.qsl.net/k5bcq/Kits/Kits.html
Perhaps use Han's upcoming QSX 10W amp, which he is planning to sell separately:
    https://groups.io/g/BITX20/message/57420
    https://radioaficion.com/news/qsx-sdr-transceiver/
If required, add per-band filters between the exciter and the new power amp board.

Would be nice to have a simple analog SSB exciter and receiver that works well from 3-30 mhz.
The uBitx can fill that role.  The QSX may turn out to be a fine rig, but the uBitx can remain
the rig of choice for analog experimentation.  Not all of us want to play with DSP firmware.
 
Jerry, KE7ER


On Thu, Aug 30, 2018 at 10:32 AM, ajparent1/KB1GMX wrote:
I still believe "there is a pony out there" .....small handful of parts and a good dual relay switched LPF board.
73 Kees K5BCQ 

Isn't the rest of that is "I can smell it...."

Yes ferrite and a few more caps in the right places plus some top pour of copper
with stitching to tie it all down so ground (there is no such thing) has less
transverse current paths and lower surface resistance.  

Why say that...  Adding parallel caps is useless if the ground it goes to has
RF resistance (or inductance).  The DC lines start right next to the power amp
so ferrite and caps liberally spread about limits the distribution.  At one point the 
R18, R28, R38, and R48 were tried at 330 ohms with 9.6V Zener at each
corresponding cap to ground was added.  That stuffed the spurs (not the
primary ones) down by 6DB.   So yes there undesired RF on the switched
DC.

Allison

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