Audio interface

Audio interface for HAM operators

Features

This device was designed to connect amateur radios to computers to operate digital modes.

• Isolation between the radio and the computer to avoid ground loops
• CW keying
• PTT keying
• DC isolation between radio input / output and GND
• PTT resistor for HT (Handheld Transceivers)
• Serial port for external devices (see below)

This device works well with the following programs and user cases.

• Direwolf or other software TNC for AX.25 and APRS operation
• FLDIGI for PSK and other digital modes on HF and VHF bands
• FT8 and its friends

Theory of operation

The schematic. Click on the image to download as PDF.

The devices is equipped with a DB-9F connector that connects to the serial port of your computer. An USB to serial converter can also be used. The DTR and RTS signals are used to key CW and PTT lines. These lines are optically isolated from the radio. There is a 3 pin header (J104) provided to connect to external serial devices such as a GPS module. LEDs are provided to monitor PTT and CW activity.

Audio signals are routed through audio transformers to provide galvanic isolation. DC blocking capacitors can be introduced to the audio paths both on the RX and TX loop at the radio (DCE) side. This is especially useful, if the device is used with HT (handheld transceiver) radio, which often uses balanced speaker output with DC offset. (Like the FT-4X.)

Ground concept

There are three GND connection in the board.

1. GNDA is the analogue GND from the audio interface of the computer
2. GND1 is the digital GND from the serial port of the computer
3. GND is the ground of the transceiver.

GNDA and GND1 can be shorted together using solderable jumpers JP101 and JP104 respectively. It is only advised to short grounds when no isolation is needed.

Mounting options

Isolation / bandwidth

1. Isolation
   • Mount T101, T102, U101, U102
   • Don't mount Q101, Q102
   • Leave open JP101, JP102, JP103, JP104
2. High bandwidth
   • Mount Q101, Q102
   • Don't mount T101, T102, U101, U102
   • Short JP101, JP102, JP103, JP104

Configuration

DC block radio speaker (For HT)

1. DC block
   • Open JP106, JP107
2. Direct connect
   • Close JP106, JP107

PTT and Mic on same line (For HT)

1. Mic and PTT on same line
   • JP105: 2-3
2. Mic and PTT on different line
   • JP105: 1-2
3. Mic and PTT on different line with DC block
   • JP105: No connection

Connections

Connection to the sound card (DTE)

Soldering pads.

Serial connector (J105)

This is a standard RS232/V.24 pinout using DB9F connector.

External serial device header (J104)

3 pin header. This might be wired to an external DB9M connector. You obviously loose control lines, but in most cases, they are not needed anyays.

Radio (DCE) (J106)

DB9F connector.

Adjustments

PTT resistor

Adjust R103 if PTT is not triggered with your HT.

LED current

Measure the current through R101 and R102. (Practically, by measuring the voltage on the resistor, and applying Ohm's law.)

Adjust R101/R102 if your current is over 7mA, or under 4mA.

Software configuration

PTT and CW lines

Set PTT output to RTS, and CW key to DTR.