Homebrew sBitx on 64 bit and Subthreshold Conduction Resolved

KK4DAS sBitx on 64 bit build by W9JES

The homebrew sBitx is now on the 64 bit version of the OS and software. Thanks to W9JES, JJ and team for their work on this upgrade to the factory sBitx Raspberry Pi image and sofware distribution. JJ and team have made some significant upgrades to the core - most important bringing it to the lates stable 64 bit release of the OS. In addition JJ has developed a suite fo add on applications that provide additional functionality. When I first booted it up I was not able to decode FT8 sigals in teh native sBitx app. JJ worked with me and we found that since I did not have a RTC clock installed the new version of the app was not using the correct time - and for the WSJT protocol to work the time must be in sync wiht UTC. He made the fix so the software will fall back to NTP if no RTC is installed and I am back in business! Thanks JJ.

While I was testing the 64 bit build the rig begain to exhibit another RFI gremlin - the Tx/Rx relay would begin to click randomly - sometimes one click at a time and other times it became a real chatter - annoying enough that I temporarily disconnected the connection from the Pi to the relay so I could get on with testing. I thought it was RFI from the Pi -- maybe related to the Pi's wifi - it seemed to get better when I turned off wifi. My build of the sBitx puts the rig in to Tx by putting a signal on a GPIO line that goes through a dropping resisitor to the base of an NPN transistor that turns on the relay. I suspected RFI was getting into the GPIO line. I put the scope at the base on it and I could see the voltage spikes that were triggering the relay. I put a .1 cap to ground where the line connects to resistor. That didn't help. I put ferrite beads on the line. That didn't help. I switche from a single piece of stranded wire to shielded coax. That didn't help. So I did what I often do when stuck on a thorny circuit problem, I asked my friend Pete, N6QW. Once again the Wizard of Newbury Park diagnosed my problem fromm across the country.

Here is Pete's response in its entirety:

"It may be a case of subthreshold conduction (a leaky transistor), 

I would do the following. 

Use a 2.2K versus a 220 Ohm. Get some small ferrite beads and slip those over the base lead and change out the 2N3904 transistor to a TIP31C.

Your narrative indicates it is a recent problem and you didn’t see spiking on the GPIO. The signs suggest a leaky transistor (or not). But the above steps are positive and should be done as good practice.

Leaky transistors are not limited to digital electronics and in fact this problem is in the analog hardware."

I didn't have a TIP31C in the drawer and since it was working previously I replaced the no-brand 2N3904 with a new known-good one from my MOUSER hoard, I changed the resistor value per Pete's advice and left the ferrite beads on the signal line.

Bob is my uncle!  No more chattery relay.  Key point above - this is an analog circuit problem that likely had little or nothing to do with the digital circuit - subthreshold conduction just means the transistror turns on when it is not supposed to.

Thanks Pete!

Here is Pete's blog post explaining in detail:

July 10, 2024. Subthreshold Conduction (n6qw.blogspot.com)

Next steps - install the RTC module and the directional coupler for the SWR and power meter 73 from Great Falls

Dean

KK4DAs

Links: HF Signals sBitx W9JES 64 bit release W9JES sBitx Toolbox