The sBitx is a 40 watts, high performance SDR that you can easily build, it has all popular digital modes like FT8 integrated. This is a blog that gathers all the help, tips, modifications and information about this little radio.
PA sequencing to avoid blow outs
PA sequencing to avoid blow outs
There have been many reports of the PA blowing up. The PA transistor is a MOSFET that can blow up due to high input voltages that can lead to the input gates blowing up or drawing too much current and blowing up the source-drain channel.
What is going on?
We applied an oscilloscope probe to the TX_LINE. The TX_LINE goes high under raspberry Pi control during transmission. We applied the second probe to a dummy load connected to the antenna socket. What we found is below.

As you can see, there is an enormously high power spike of 220V peak to peak (more than 100 watts of power). Even more telling is that the gate RF t has 20 volts of RF. The maximum rating for the gate is +/-20V as per the datasheet some of the spikes are hitting that as well. See below

Why is this happening?
The power amplifier is permanently connected between the exciter portion and the harmonic filters. On receive, the power to the drivers and the bias is switched off and the relay K9 with U3 bypass the PA to directly the signal to the rest of the radio.
When the transmission ends, the TX_LINE goes low and the power to the PA is switched off, simultaneously, the relay K9 and the analog switch U3 are released, allowing the RF to flow back from the PA output to the input. The power amplifier takes a few hundred microseconds to power down as the bypass capacitors still hold charge. This sets up the oscillations that we are seeing. To confirm this, I desoldered C14 and the spike went away, (it also renders the receiver dead) by breaking the feedback loop.
The spike of the oscillations remains on for as long as the bypass capacitors of the PA hold charge and they quickly discharge, killing the oscillations. Not quickly enough to do damage.
What is to be done?
The powering up and down of the PA has to be sequenced. When we switch to transmit, we must first switch off the relay and U3, give it a millisecond and then power up the PA. When transiting to receive from transmit, we have to follow the reverse sequence. First power down the PA, wait for the caps to discharge and then switch the relay K9 and the switch at U3 to bypass the PA.
How can it be done?
The PA power is turned on and off in the existing sBitx when TX_LINE goes high at the gate of Q10. Now, instead of the TX_LINE, we take another GPIO line (we can call it TX_POWER) to separately control the powering up and down of the PA. This makes the spike go away.
Four things to do:
- We pull in the new code from github. This software mod does not impact the unmodded sbitx but it can potentially blow up the modified sBitx if this mod is not in place.
Power on the sBitx, open the terminal with Ctrl-Alt-T and type the following:cd sbitx git pull ./build sbitx
- We cut the track that brings the TX_LINE to the gate of the Q10 with a sharp knife.
- We add a wire from the Gate of Q10 to be taken to pin 36 of the Raspberry Pi’s connector on the digital board.
Gate of Q10 isolated from TX_LINE and connected to TX_POWER - We solder the wire coming from the Q10 gate to pin 36 of the Raspberry Pi’s connector on the digital board.
Wire soldered to Pin 36 of RPI to connect to Q10 gate (TX_POWER)
Result
You can see that the spike is gone. The two pictures are taken with different timing bases to be sure.


sBitx Basic Kit Assembly Notes
sBitx Basic Kit Assembly Notes
How to set the bias
How to set the bias
Siegfried Jackstien, DG9BFC
- Set both the POTS to minimum current
- Select Mode as USB. Reduce the MIC to zero.
- Press the PTT.
- Without speaking into the microphone, carefully increase the DRIVE_BIAS POT until the current increases by 200 mA. Note the current.
- Keeping the PTT on the microphone pressed, now increase the PA_BIAS POT until the current increases by another 200 mA.
- Release the PTT.
Eliminating Periodic Noise in the Speaker (Developer Edition)
Eliminating Periodic Noise in the Speaker (Developer Edition)
Ashhar Farhan, VU2ESE
The LM380 audio amplifier is picking up the Wifi RF and it results in a noise that repeats every 8 seconds of so as a series of quick pulses.
The noise is not present if you use earphones as they bypass the LM380 audio amplifier.
The current solution is to just turn off the WiFi and use Ethernet instead for Internet.
Corrupted sBitx – How to reset the settings.ini
Corrupted sBitx – How to reset the settings.ini
Anthony Good
Before you replace files, try deleting (or renaming) your settings file, /home/pi/sbitx/data/user_
cd /home/pi mv ./sbitx ./sbitx.old git clone https://github.com/afarhan/sbitx.git cd ./sbitx ./build sbitx ./sbitx
Upgrading to switching regulator (Development Edition)
Upgrading to switching regulator (Development Edition)
From Anthony Good, K3NG, Anil Rayporula, VU2DXA
The very early sBitx (Developer Edition) were shipped with a linear regulator, the LM338 to drop the voltage down from 13.8V power supply to the 5V needed by the Raspberry Pi and other digital circuitry. This often led to heating issues. An upgrade kit was shipped to these users. These are the instructions to install it properly.


