Baofeng UV-3R in packet world using ArduinoTNC

Since it is so cheap and works reasonably well for 2m/70cm band in FM mode, I wanted to test also it's capabilities as FM modulator for AFSK digi modes and APRS.

First I needed a cable to connect to the radio. Fortunately that is relative easy and requires only four ring 3.5" jack and some standard components.

The cable pinout is easy:

Baofeng UV-3R TNC cable

To get to the packet world a TNC is needed. I was experimenting with different software solutions (soundmodem, agwpe), but this test was done with Arduino based TNC (because I had one laying around and it has USB to serial chip on board).

ArduinoTNC shield and cable

The design is based on KI4MCW's, but the software was updated to work well with latest BertOS (2.7.0) and to support KISS escapes and command structure.

Arduino TNC schematics (the filter cap on PTT line is missing here)

You can also download the complete source codes (complete with configured BertOS).

The TNC was connected to Xastir running on Mac OS X with the proper settings - Serial KISS TNC, 19200baud, /dev/cu.usbmodem24311 and "Transmit now" was triggered.

The radio switched to TX mode and... stayed transmitting. Oops. RF noise induced enough energy on the unshielded Arduino to trigger the PTT optocoupler and keep it triggered.

All that was needed to remedy this was to connect one SMD ceramic 100nF cap between PTT line and ground.

With that done, I triggered Transmit now again and measured what happened:

• Blue line (CH1) measures the PTT line on the PCB (pin 11/PORTB3)
• Red line is the audio output of the simple DAC, measured before the pot

With STE turned on in the menu, the radio showed about 230ms PTT to TX delay and 300ms TX tail as you can see on the attached pictures (watch the RF induced noise on the blue PTT line).

TxDelay with STE on

TxTail with STE on

The TX tail was pretty long so I turned the STE (Tail tone elimination) function off and tried again:

TxTail with STE off

With full power, STE turned off and resistance between the pot wiper and ground measured as 572 ohm the transmission was finally properly received by our local IGATE (9.2 km). So the UV-3R is indeed usable as an APRS transmitter.