PA hardware Calibration

PA Hardware Calibration

Background

Each zBitx leaves the factory with calibrated PA (power amplifier) values. They are stored in a file that the zBitx application reads when it starts. You may need to recalibrate if that file is corrupted or deleted, or if you want to fine-tune output power.

As on the sBitx, the values are stored in hw_settings.ini in the /home/pi/sbitx/data folder. Each band has a frequency range and a scale factor. The scale factor sets the gain of the mixer input to the RF amplifier chain. In other words, it controls how much of the audio signal is passed on to the amplifier, and so how much power the radio puts out.

Example entry for 80 m:

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

Scope: This procedure gives a good starting point, not a precision alignment. Exact alignment needs additional test equipment, voltage and current measurements at specific points on the mainboard, and the two-tone method, which are beyond the scope of this page.

---

What You Need

• zBitx transceiver
• 50 Ω dummy load rated for at least 10 W
• External wattmeter
• Access to the radio's desktop through browser VNC or network VNC (see VNC and Remote Desktop)
• The zBitx HW Calibrator app, available in the zBitx Toolbox

---

Before You Start: Back Up Your Settings

Make a copy of hw_settings.ini before you change anything, for example by renaming it to hw_settings.ini.bak. See Saving Your Configuration Files.

Important

The calibrator removes two settings from the file. When you save with the HW Calibrator, the following lines are removed from hw_settings.ini:

center_bin=xxx
ssb_val=x.x

Write down your current values before you start. When calibration is finished, open hw_settings.ini in a text editor and add both lines back by hand with your original values.

---

Procedure

1. Connect the wattmeter and dummy load to the zBitx BNC jack in this order: zBitx → wattmeter → dummy load

2. Power on the zBitx.

3. Open the zBitx HW Calibrator from the zBitx Toolbox.

4. If the radio is not already running, press Start zBitx in the HW Calibrator. Then set the radio to:

   Setting	Value
   Frequency	3535 (or the next frequency from the table below)
   Mode	USB
   Drive	100
   AGC	OFF
   TNDUR	3
   TNPWR	100

   Press TUNE.

5. Read the output power on the wattmeter. If you missed it, press TUNE again.

6. In the HW Calibrator, adjust the Scale value for that band in small steps, then press Save.

7. Press Close zBitx, then Start zBitx, and press TUNE again to check the new output.

8. Repeat steps 4–7 until the output matches the target in the table below.

9. Repeat for each frequency in the table. Press Save after every adjustment.

10. When all bands are done, restore the center_bin and ssb_val lines in hw_settings.ini as described above.

Tuning tips

• Raising the scale factor normally raises output power, until the amplifier saturates.
• Work up gradually toward the target power. Do not overdrive the amplifier. Too much drive shortens MOSFET life and causes spurious emissions.
• If you have gone too far, decrease the scale factor until output drops by about 1 W, then increase it again in very small steps (for example, 0.0045 → 0.0046) to fine-tune.

---

Typical Output Power (v2 transceiver)

Frequency (kHz)	Output (W)
3535	4.9
5330.5	4.1
7035	5.0
10135	4.0
14035	5.0
18068	4.2
21035	2.9
24895	3.8
28035	3.5