Kenwood TS-50 radio - laptop interface with internal USB soundcard and VOX circuit.

A new radio-to-computer interface was needed for portable and camping type radio operations to support a range of digital modes, specifically modes like Winlink and JS8Call.

The available Kenwood TS-50 transceiver is well-suited for portable operation, especially when used with the AT-50 automatic antenna tuner. However, it lacks a VOX (voice-operated transmit) function. While there are various ways to activate the PTT (push-to-talk) for digital modes, some—such as Winlink—require either an advanced radio interface or rely on VOX to key the transmitter. Due to limitations in both the TS-50 and certain software, the interface must include a built-in VOX circuit.

The interface design discussed below has been successfully used with digital modes such as Winlink, JS8Call, WSPR, and Olivia, and should be compatible with most similar modes.

After several unsuccessful attempts to build a VOX circuit using substitute transistors from various internet-sourced designs, I decided to approach the problem from first principles. This led to the design, modelling, testing, and implementation of a custom VOX circuit. Powered by 5V DC from a USB soundcard dongle, the circuit uses the audio signal to trigger a 4N25 opto-coupler, which in turn keys the transmitter.

To better understand the input signal, I measured the soundcard’s output audio using an oscilloscope and found it to be around 100 mV peak-to-peak. After revisiting transistor biasing theory, I modelled a simple Class A amplifier in LTSpice as a starting point. Since linearity is not critical in this application, I modified the design to prioritise gain by reducing resistor R5 to 390 ohms and adding C4, a 47 µF electrolytic capacitor, as a bypass. This significantly increased the amplifier's gain and ensured reliable VOX triggering. 

Fig 1 LTSpice model of the VOX and PTT switching circuit. The 4N25 optical coupler provides electrical isolation from the radio.

At the collector of Q1 (a 2N2222 transistor), the signal appears across resistor R4 (3.3 kΩ), producing an output voltage of approximately 3.0 V peak (based on a 50 mV input multiplied by the transistor’s gain, or hFE). The signal then passes through capacitor C2, which blocks the DC component, resulting in an AC signal of about 2.0 V peak-to-peak.

This amplified AC signal is then rectified by diodes D1 and D2, which allow only the positive half-cycles to pass. Capacitor C3 (1.0 µF) charges rapidly to the peak of the rectified signal, minus the diode forward voltage drop (approximately 0.7 V), reaching around 800 mV.

When the audio signal stops, C3 discharges through resistor R7 (100 kΩ) to ground. As a result, the LED remains illuminated briefly—about 30 milliseconds—after the signal ends. The voltage across C3 is applied to the base of Q2 (another 2N2222 transistor) via resistor R6 (2.2 kΩ). Q2 requires a base-emitter voltage (Vbe) of about 0.7 V to turn on. As long as the voltage on C3 exceeds this threshold, Q2 conducts, completing the path to ground for the LED circuit.

Current then flows from the 5 V supply through the LED and current-limiting resistor R8 (390 Ω), lighting the LED. The current is limited to approximately:
(5 V – 2 V LED drop) / 390 Ω ≈ 31 mA, which is within the safe operating range for most standard LEDs.

The 4N25 opto-coupler, which is used to key the transmitter, is connected in parallel with the LED and uses the same value current-limiting resistor.

LTSpice modelling was used to simulate and analyze the circuit, allowing measurements to be taken at key points. One of the most important parameters was VOX switching time. To achieve the desired fast response, the values of C3 and R7 were adjusted experimentally based on simulation results.

Fig 2 LTSpice modelling VOX fast switching time.

Real-world measurements were taken from the breadboard prototype using an oscilloscope and compared with results from LTSpice simulations. As expected, the measurements were similar but not identical, which led to some minor adjustments in component values.

One of the challenges in this project was the lack of clearly defined performance targets—such as the required VOX switching time for various digital modes. As a result, several assumptions had to be made, including the idea that faster switching would generally be better.

Photo 1 Breadboard testing of the VOX circuit.

The final circuit design was established from the VOX circuit experimentation and from previous experience with a number of much earlier radio interface projects, such as the Computer Sound-card Interface for a TS-430S project.

A key objective of the design was to electrically isolate the computer from the radio. This isolation helps prevent RF power from interfering with the computer and also reduces computer-generated noise that could degrade radio reception. PTT switching is handled via optical isolation using a 4N25 opto-coupler, while both audio input and output are transformer-isolated, effectively eliminating unintended signal paths.

The interface is built around an unmodified CM108 USB soundcard dongle, as shown in the schematic below. It incorporates audio isolation and adjustable attenuation to ensure clean and reliable transmit and receive signals.

R10 is a 5k logarithmic potentiometer mounted on the front panel, providing precise control over transmitter audio gain.
R11 is a 5k logarithmic trimmer potentiometer mounted on the circuit board, used for general gain setting. Final fine-tuning can then be performed using the radio’s audio gain control.

Resistor R13 (2.2 kΩ) is connected between Ring 1 (bias voltage) and Ground (Ring 2 or Sleeve) to simulate the presence of a microphone. Without this resistor, the CM108 soundcard will not process incoming audio.

The USB cable is directly soldered to the contact tabs of the CM108 USB soundcard dongle, and the USB 5 V supply is used to power the VOX circuit.

Fig 3  Final interface with internal USB soundcard circuit design.

The USB cable connecting the computer to the interface unit is soldered directly to the USB soundcard dongle contact per the USB standard cable colour code and description below.

Photo 2 CM108 USB soundcard dongle.

USB standard cable colour code and description.

All connections between the Kenwood TS-50 and the interface unit are made via the microphone jack. While this pinout is standard across many Kenwood models, it's important to note that pin 6 (audio out) is not present on all standard 8-pin configurations, it is however available for the TS-50 radio.

	Pin	Description
	1	Microphone Audio
	2	PTT
	3	Channel Up
	4	Channel Down
	5	+8 VDC
	6	RX Audio (Not all Kenwood models)
	7	Microphone Audio Ground
	8	Chassis Ground

Fig 4  Kenwood TS-50 microphone pin outs.

The components are mounted on a section of Veroboard (strip-board) and housed in a sealed polycarbonate enclosure measuring 115 x 90 x 55 mm. While the exact positioning of components is not critical, maintaining a neat and organized layout greatly helps during assembly and troubleshooting.

  Photo 3 Radio to laptop interface project under construction.

 Photo 4 Completed radio to laptop interface project.

Operational

Microphone gain should be carefully adjusted using the front-panel gain control to achieve the desired transmitter output power. This is critical—if the transmitter is overdriven, it can cause significant adjacent channel interference and may even damage the transmitter.

Most digital modes produce a continuous, 100% duty cycle signal composed of modulated tones. This places a heavy load on a standard SSB transmitter, often exceeding its intended duty cycle. For this reason, it’s recommended to reduce transmit power to around 20 watts for a typical 100-watt HF SSB transceiver—or slightly below the transceiver’s AM rating.

It's also important to disable the VOX feature when it's not needed, as any audio generated by the computer can unintentionally key the transmitter.

LTSpice model file: radio_soundcard_interface_20250415_fl01.asc

USB soundcard circuit design: radio_soundcard_interface_20250415_fl02.pdf

CM108 soundcard details: radio_soundcard_interface_20250415_CM108_v1.6.pdf