This computer sound card interface was constructed and refined for use as a flexible interface between the computer and the TS430 HF transceiver at my previous residence in Melbourne, Victoria from 2005 - 2007

Presented here is a transceiver to computer sound card interface complete with automated transmit key function. A sound card interface is simply the audio coupling of a computer soundcard and a transceiver to allow various computer applications that send and receive SSTV, RTTY, PSK31 and other similar modes based on soundcard generated signals. 

The interface is designed to be left inline with the ability to easily be switched between phone and data operations. This version is my first attempt to interface a computer soundcard with my Kenwood TS-430 transceiver and it has proven to be very successful and easy to operate.

While designed specifically to be connected to a Kenwood TS-430 transceiver the design should suit many other Kenwood radios with little if any modification and should be in principle adaptable to many other radio types.   

Design

The design is based on the variety of similar circuit that can be found on the internet to produce a model that is suited ideally to my needs.

One of the main aims of this circuit was to develop a simple method of switching too and from normal voice communication and a range of digital modes at the flick of a switch. 

The circuit design is however fundamentally about transferring a clean and undistorted signal between computer and radio. Preventing unwanted signals, noise and even damaging voltages passing between the computer and the moderately powerful HF transceiver can be quite difficult. At 100 Watts the Kenwood TS-430 may introduce RF feedback into sensitive audio circuits within the radio via connecting leads, other attached equipment and the computer may find a path to introduce noise into the sensitive HF receiver.

Audio isolation transformers are placed in both, transmit and receive audio circuits to eliminate the possibility of ground loops. All cabling between equipment is shielded audio cable and small value capacitors are place around the circuit to suppress any stray RF that may have been picked up. An Optical coupling IC package is used to isolation the transmit PTT (Push To Talk) keying circuit and the DB9 comms port.

The both transformers are common 3k to 3k ohm impedance (1:1) audio transformers complete with a simple resistor and trim pot attenuation networks to reduce and allow adjust of the signal levels. The amplitude of soundcard signal to transceiver microphone input has been shown to require an attenuation of around 33dB and the transceiver speaker output to soundcard AUX input requires an attenuation of around 25dB.

I have chosen to arrange the circuit so that the attenuation is achieved after the signal has passed through the transformers, with the view that the transformers may be susceptible to coupling with strong unwanted external signals. This would leave the ratio of desired signal to unwanted induced signal at its greatest in this configuration.

The transmit key function requires that the RTS (Request To Send) Pin 7 and Ground Pin 5 from the computer com port be relayed via a 4N25, or 4N32 Optocoupler chip to the transceiver PPT pins for additional isolation. Also included is LED transmit key indicator.    

Construction

The unit’s components have been laid out on a section of versa strip board and assembled in a sealed polycarbonate box, size 115 x 90 x 55 (Dick Smith Electronics) H2863) which is a nice quality box. The positioning of the components is not critical, but as always neatness counts and makes it easy to keep track of where you during assembly. Internal signal wire pairs are twisted to further reduce unwanted signal pick up.

Short screened leads connect the computer to the interface and the location of the device in relation to other station equipment like the ATU, power supply etc. are all crucial to mitigating stray signal pick up.

The computer's CRT screen is a bad source of radio hash and while I have operated these modes with a CRT screen its replacement with a new LCD display removed near all the noise from the shack.

 Schematic of the interface between my computer sound card and TS-430S HF transceiver with computer communications port (9 pin port).

Operating

Once the interface is inline the first thing to do is set the trim pots to a useful level with the mic gain and the AF gain in the nominal position so that when switching from phone to data the levels will be about right.

The mic gain will still need to be adjusted to achieve the final transmitter output power level. This is critical for if the transmitter is over driving it will result in a lot of adjacent channel interference and possibly damage to the transmitter.

For SSTV, the generated signal is a frequency modulated tone causing a normal SSB transmitter to work hard, exceeding its nominal duty cycle. Therefore it is recommended that the transmit power be wound back to around twenty watts for the average 100W HF SSB transceiver or to about the AM rating of the transceiver. Be conservative!

With everything adjusted, the interface can be left switched to phone for normal voice operations where the microphone controls the PTT function or switched into the data mode where the monitor speaker is switched out (No noise) and the computer software operates the PTT function. Even in the phone mode the soundcard is monitoring the audio, which can be useful.

This simple but effective means of interfacing radio and computer and the related software may open up a whole new radio communication experience if you have not already tried it! 

Related sound card interface site. See:

http://www.qsl.net/wm2u/interface.html

General sound card information and standards. See:  

http://en.wikipedia.org/wiki/Sound_card

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Page last revised 13 October, 2024