sBitx Hardware Calibration - drexjj/sbitx GitHub Wiki

Background

There are three sBitx models, available in essentially two hardware configurations. The DE and V2 models feature a 40-watt amplifier circuit using IRFZ24N MOSFETs, while the V3 model features a 25-watt amplifier circuit using IRF510 MOSFETs. At one time, a conversion kit was available to upgrade the DE model by adding a new T/R circuit, replacing the predrivers, and bypassing the IRF510s. A conversion kit was also available for the V2 models which replaced the IRFZ24N's with IRF510 MOSFETs, safeguarded by TVS diodes. However, both the DE and V2 models were discontinued due to their high failure rates.

Power calibration for the DE version can be set with the V3 Powercal tool when using v3.0 software and later.

In the earlier revision of the V2, power calibration was done using a built-in software function called TXCAL. However, this procedure is not compatible with the V3 model or converted units using IRF510 MOSFETs because of the MOSFET and power handling changes.

As of v3.025 64 bit and later; all three models read and store the calibration data in a file named hw_settings.ini located in the home/pi/sbitx/data folder which is read by the sBitx at every startup of the app. Each band in the file contains values for the frequency range and a scale factor, which compensates for non-linearities in the RF amplifier chain. Essentially, the scale value determines the amount of power gain that the amplifier will output from the audio channel.

Here is an example of the 80M band configuration in the file.

[tx_band]
f_start=3500000
f_stop=4000000
scale=0.0042

All V3 units shipped from the factory contained a test report. This report contains values such as PTT idle current, bias current, and PEP voltages for each of the bands tested. I will cover this procedure at a later time.

DE & V3 Model

This procedure outlines the calibration process of the V3 model and converted models. While not a precise method, it will provide a good starting point. Achieving exact alignment requires additional diagnostic equipment and voltage/current measurements at specific points on the mainboard and using the two-tone method which is beyond the scope of this procedure.

What's Required?

  • sBitx transceiver
  • Internet connection to the transceiver
  • 50 Ohm Dummy Load capable of handling at least 50 watts
  • External wattmeter
  • External keyboard/mouse (optional)
  • V3 Powercal Editor app
cd ~/Downloads && wget https://github.com/drexjj/sbitx-ham-apps/raw/main/v3powercal -O v3powercal && chmod +x v3powercal && ./v3powercal

Procedure

Tip: Rename your hardware.ini file if you want to save your old settings.

  1. Connect the wattmeter and dummy load to the sBitx via the BNC jack in the following order: sBitx → wattmeter → dummy load.
  2. Power on the sBitx transceiver.
  3. Download and open the V3 Powercal Editor (see above).
  4. Press "Start sBitx" from V3 Powercal Editor and set the Frequency: 3535, Mode: USB, Drive: 100, AGC: OFF, TNPWR: 100, then press the TUNE button ON/OFF briefly (the TUNE button is found in the menu).
  5. Observe the power output on the wattmeter. If you missed the reading, activate the TUNE button again.
  6. In the V3 Powercal Editor, adjust the Scale field incrementally, either increasing or decreasing the value. After each adjustment, press "Save".
  7. Press "Close sBitx" and "Start sBitx" from V3 Powercal Editor, then test the output with the TUNE button again.
  8. Repeat steps 4 through 7 for each frequency listed in the chart below, and remember to press SAVE in the V3 Powercal Editor after each adjustment.

Note: Make small adjustments in the V3 Powercal app and repeat steps 4 and 7 until you achieve the desired output. Typically, increasing the scale factor raises the output power unless oversaturation occurs. The best approach is to gradually increase the scale factor until you reach the target power shown in the chart below. If you raise the scale factor too much, you risk overdriving the output, which can reduce MOSFET lifespan and cause spurious emissions. Decrease the scale factor until you notice a 1-watt drop in power, then make very small adjustments increasing the scale value, such as 0.0045 or 0.0046, to fine-tune output power.

FREQUENCY WATTS
3535 25
5330.5 25
7035 24
10135 24
14035 21
18068 18
21035 15
24895 12
28035 10

V2 Model

If you are using an unmodified V2, then please follow this procedure. DO NOT ATTEMPT THIS ON MODIFIED V2 HARDWARE OR V3 HARDWARE!

Procedure

  1. Connect the wattmeter and dummy load to the sBitx via the BNC jack in the following order: sBitx → wattmeter → dummy load.
  2. Power on the sBitx transceiver.
  3. Open the sBitx app.
  4. Enter \txcal in the text box, then press enter. Do not touch the radio while this is in progress.
  5. Wait until the CW tone stops (there should be one long tone per band).
  6. Close and restart the sBitx app, then test the readings on your wattmeter.