Notebook PSK - SSTV Interface

This notebook was specifically developed order to provide information and education.

The information within is intended to provide useful circuits and theory to non-technical individuals. The content of this publication is not intended to be a complete works of circuits and techniques.

Although all designs and circuits have been tested, the author (Tony Gargasz, KB8WOW) assumes no responsibility for safety, fitness, or use of any circuit or information contained in this publication. THE DESIGN AND INFORMATION IN THIS NOTEBOOK IS COPYRIGHTED and is intended for individual personal use. Reproduction of this information may be obtained via Email from the author.

There has been allot of renewed interest in the HF digital modes of operation. One of them being PSK31. PSK refers to Phase Shift Keying, and the result is a 31 Hertz wide signal that rivals CW (Morse code) for its' characteristic ability to be heard when no other means of communication is possible.

The circuit described herein is actually a universal interface. If you need to connect the audio output from your rig to a computer and transmit signals from a computer, this is the ultimate interface.

You used to have to be near genius to construct a complex digital circuit and operate it. There is a newer and easier way of receiving and transmitting these digital modes. The latest wave is to have a personal computer run appropriate software and use a sound card to interface the radio to accomplish the desired mode of digital communications.

This notebook provides construction details for an interface from a personal computer sound card to a radio transceiver. This interface is primarily targeted toward PSK and SSTV (Slow Scan Television), however, it can be universally used for any computer to transceiver application that uses the computer sound card.

DESIGN CONSIDERATIONS AND DETAILS

When I first became interested in SSTV, I had severe problems with what everyone on the air was describing as RF in my audio. Some careful grounding techniques were applied that did indeed clear up RFI problems in general around my consumer electronics. However, on air reports persisted that I still had a problem. After much research and fact finding I found that the computer was inducing unwanted components into my transmissions.

This design is the result. It can't fix stray RF, but it does provide ultimate isolation between the rig and the computer. It also accommodates all types of rigs, regardless of the PTT requirements. Although the circuit appears simple, there have been many hours of design devoted to it. It's the "bullet proof" isolation solution.

Some newer sound cards only provide monaural microphone inputs. These cards typically use the left stereo input channel for the microphone. The 1/8" microphone connector on the sound card is a 3 conductor type. That is, it has a tip, center ring, and ring. It is specifically designed to accept a computer microphone. That's all fine and dandy except that it's not wired as you would expect. The tip supplies a voltage source for an electric mic element. The center ring is the real audio input, and the ring acts as the audio return and voltage ground. This design presented will work whether you use the microphone or line input of your sound card.

Sound cards, in general, use integrated circuits for outputs. These outputs (left and right channels) should never be connected in parallel. Severe damage , or at the very least, severe distortion will occur if they are. The design takes this in to consideration also.

As a general operating note, pay special attention to your software mixer. Most of them have special enhancement features turned on by default. They may be called "Spatial Effects", "3D", "Stereo Wide", or something of the sort. Turn all special effects off. They will only interfere with your ability to communicate. You need your signals (input and output) from your computer to be as pure as possible.

Pay careful attention to the construction details for this project (listed on a later page). Use a metal enclosure for the project. The construction details describe the essence of the isolation that the circuit provides. If it's not evident, the enclosure is connected to your radio ground as a spur of a "star grounded" system. Your computer is treated as a shielded extension. The circuit provides total and absolute isolation.

Many software programs offer some sort of automatic transmitter keying option. Many software programs give options that allow you to use DTR (Data Terminal Ready) or RTS (Ready To Send) signals from your computer in order to key your transmitter. This design allows you to switch between the two in order to facilitate any software requirements.

CIRCUIT DESCRIPTION

When receiving a signal from the transceiver , the speaker output is isolated by transformer T1. R1 provides a convenient way of making fine adjustments to the received signal. The adjustable signal from R1 is fed to the left input of the computer's line or microphone input jack on the sound card.

When the computer sends a signal to the DTR or RTS line of the selected Serial port, it goes high which in turn passes through switch S1 (assumed to be in the correct position) and through current limiting resistor R4 then turns on transistor Q1. The transistor pulls in relay RY1 which switches the microphone signal from the mic to the interface. The relay also switches the PTT line to ground and keys up the transmitter. Diode D1 protects the transistor from damaging transients when the relay is turned on and off.

The signal from the computer's sound card right line output is isolated by transformer T2. The isolated signal is passed through a 10:1 voltage divider, R2 & R3. The signal passes through a contact of RY1 and is fed to the transceiver's microphone or line input.