Homebrew sBitx Walkthrough

 

Here is a video walkthrough and quick homebrew sBitx update.  I've intergrated the PA into the rig and calibrated the Tx levels.  I am very pleased to get 20 clean watts CW out on all bands from 80-10.  All modes are working well.  Final calibration meant deriving all my own band settings and updating the sBitx hardware configuration file hw_settings.ini.  Since my analog build is unique I had to start from scratch with the configuration.  I tested transmit on each band through the LPF and check the signal quality and strengthh on the scope.

When I first installed the PA it worked great on 80 and 40 but on the higher bands the relays were chattering and buzzing.   I added .1uF caps to the DC lines and that fixed the problem on all bands except 10 meters.  Farhan suggested adding bypass caps to the GPIO lines that drive the transistors that control the relays.  As N2CQR says - "Bob was my uncle!"  That fixed it.  

The radio sounds great and I have been getting good audio reports.   Had a QSO yesterday with PV8AL,Helio in Brazil on 17 meters SSB.  I got a 5:7 signal report plus good audio.  If you've ever worked Helio you know you can sometimes here a rooster crowing in the background.  Not so this time.

Its scary - I'm running out of bugs and problems.  I better be careful what I wish for.

73,

Dean

KK4DAS

Homebrew sBitx 20 Watt Power Amplifier

 

20 RD15HVF1 HF Power Amplifier

Between Field Day and other volunteer activities this past weekend I managed to complete a build of a 20 watt RD15 amplifier for the hombrew sBitx transceiver.  I spent the last week or two studying these amps and looking at various implementations. I had previously built EI9GQ, Eamon Skeleton's amp from his book  "Building A Transceiver which I documented earlier.   That build was copied directly from his schematic and I didn't give much thought to the design.  

Push-pull MOSFET RF amplifiers pretty much all share a common architecture.  The transitors are configured as common source amplifiers and biased for class AB.  The differences are in how the input an output transformers are configured and how the bias circiut is designed. Compared the the EI9GQ amplifier I simplified the input and output transformers and tripled the value of the feedback resistor to get more gain.  I  also learned that adding a cap across the drain side of the output transformer will flattend the gain curve and increase gain at higher frequencies. Using cut and try I ended up at 330 pF which results in flat gain from 3 MHz to 21MHz and about 3db more gain at 28 MHz.  This was a very satisfying build experience. Essentially everything worked as expected almost right off. 

You may download the schematic here.

73 from Great Falls

Dean

KK4DAS