3.1 PSK31 Requirements - The use of a PC sound card to modulate the audio of a SSB rig has seen explosive growth in the last few years. In particular, the PSK31 (Phase Shift Keyed, 31 Baud) has changed the nature of communications over ham radio and attracted thousands of hams to use this as their main mode of operation. PSK31 is extremely efficient of operating bandwidth use, typically requiring about 60 Hz. Signal separation of 100 Hz without interference is common practice. The nature of the PSK31 modulation system is such that it is effectively a two-tone modulation system. Because of this, good linearity must be provided throughout the audio path to prevent unwanted side bands from being created by the system.

3.1.1 Bandwidth - To realize this low bandwidth and close signal separation, some care must be taken in adjustment of the audio drive of the transmitter from the PC, and particular attention must be paid to minimizing stray signal pickup from sources such as the RF field of the transmitter, AC power lines and other sources. It is the job of the interface system to provide clean signals to the transmitter in this potentially hostile environment.

The basic concept is very straight forward. During transmit, audio output from the PC sound card is provided at the microphone (or accessory port) of the SSB rig. The SSB rig is keyed using the PTT input to the rig. Data input from the operator is provided by the keyboard to the sound card software. The sound card software converts it to the modulation format and uses the sound card to create the audio output for the SSB rig.

During receive, the audio from the SSB rig is connected to the audio input of the PC sound card, and the sound card software decodes the audio signal and provides text output on the screen.

3.1.2 Software Functions - The software running on the PC provides the functions of:

3.1.3 SSB Rig Audio Levels - are consistent with driving speakers or headsets. These signals have an extremely wide dynamic range: from a few millivolts to around 5 V p-p. The PC Sound Card accepts levels up to about 5v p-p. In general, the audio levels from the rig to the sound card are compatible. If signals approach the maximum range of the sound card input, a warning message from the software will be given for the operator to reduce the audio level. The receive audio level can be controlled by RF gain of the receiver, input gain of the sound card, or the RX level control potentiometer in the interface. In practice, all of these controls are used during setup and operation of the system.

3.1.4 PC Sound Card Audio Levels - The audio levels created from the sound card are in the range of 0.1 to 5 v p-p. Typical signal level is around 1V p-p. The SSB rig, however, expects signals that are compatible with microphones, i.e. in the tens of millivolts range. The interface must provide some level reduction to bring the signals to the right scale. The actual level required is a function of the microphone gain setting of the rig, the audio level of the PC soundcard and the TX level control potentiometer in the interface. All of these controls are set during the system setup, covered in chapter 7.

3.1.5 Noise & Feedback - One problem with lower level input is that any signal picked up on the transmit audio wiring can approach the expected signal level, degrading S/N and putting unwanted signals on the transmitted audio. If the noise source is the RF created by the transmitter, feedback will occur with broad band noise resulting on the transmitted audio. There are several lines of defense that can be taken to ensure clean, quiet operation:

3.2 Description of the Interface - The schematic diagram for the PCB is shown in figure 3.1 below. Sound card or PC functions are shown on the left side of the drawing. SSB rig functions are shown on the right side of the drawing. Signal flow is indicated by arrows to show the flow from the source to the destination.

3.2.1 Receive Audio Path - At the top of the page the flow of audio from the rig to the sound card is shown. There are two possible sources for this audio: a source terminated in an audio connector such as a 3.5mm monoaural source (speaker out, headset out or line out). The other possible source is from an accessory connector which contains both output audio, input audio, and PTT functions. The interface is designed to operate with either method of connection. If the 3.5mm audio connector in J4 is used, switches internal to the connector are used to disconnect the audio from the RJ45 connector (J5). This prevents two sources from the rig being connected together. During the time that the plug is inserted in the jack, J4, the cable can be momentarily shorted, hence resistor R8 is used to provide a load during this short time interval.

The audio from one of these two sources is sent to the primary of transformer X2. The output of the transformer feeds potentiometer R6 which is used to control the signal output level sent on to the PC sound card. The output connector is a 3.5mm stereo jack since the sound card expects stereo input. Only the TIP (left channel) connection is used. The connection to the sound card should be a shielded stereo cable.

3.2.2 Transmit Audio Path - In the center of the page, the audio path from the sound card to the rig is shown. The sound card output is provided in the form of a shielded stereo audio cable. The TIP (left hand channel) connection of 3.5mm audio jack J3 carries this signal. The other channel is not connected. As will be seen in chapter on system setup, there is an advantage to only using one of the two channels.

The audio goes to voltage divider R1/R2. This provides a signal level reduction of about 82:1. Signal levels of around 1 volts become around 10 millivolt after this divider. There are some rigs whose accessory ports require substantially higher drive, in the range of 100 millivolt. For these rigs, an extra set of pads in position R10 is provided to place a 1k ohm resistor in parallel with R1. This changes the signal level reduction to about 10:1.

Capacitor C3 is provided across the primary side of transformer X1. This acts as a filter for any RF present on the audio input line. The output of the X1 feeds a potentiometer (R5) to allow for final adjustment of signal level going to the SSB rig. The wiper of the potentiometer is fed through capacitor C1 to provide DC blocking of the signal sent to the SSB rig.

An RJ-45 jack is provided as the method of connecting to the SSB rig. This connector is set up to allow feeding the microphone jack of the rig, or an accessory connector, if available. Signals on this connector include the audio received from the SSB rig, the audio sent to the SSB rig and the PTT control of the rig. Notice that the return lines (normally chassis ground of the rig) are not tied together in the interface. They are only tied together at the connector to the rig itself.  Pinouts used on the connector are chosen to be compatible with the twisted pair sets on a standard CAT5 patch cable.

3.2.3 PTT - The Push To Talk (PTT) rig function is used to turn on the transmitter when sending audio to the rig through the interface. The PTT Control circuit is described below.

3.2.3.1 PTT Control - The PTT (Push to Talk) control circuit is shown at the bottom of the diagram. The interface is set up to activate PTT under the control of a serial port of the PC. Historically, two different signals have been used to perform this function for ham radio sound card applications: RTS (request to send) and DTR (data terminal ready). One or both of these signals have been used by different software written for these applications. Today, most software has been rewritten to activate BOTH of these lines to maintain compatibility. Hence there are two diodes D1 and D2 which perform a logical OR of these two signals so that either one will activate the rig. The output of the diodes is used to feed an opto-isolator (U1) which in turn is used to activate the PTT line. The connection to the interface is via a 2.5mm three conductor jack, J1. An external cable which connects the DB9 type connector of the PC to the 2.5mm plug must be provided. The construction of this cable is described in section 5.2.1.

3.2.3.2 PTT Configuration -1 With Relay - Two configurations of PTT drive are provided with the interface. The first configuration, designated -1 drives a reed relay in the interface whose output contacts drive the PTT line. A capacitor, C2, is provided across these contacts in case they feed an inductive source such as a relay coil. The -1 configuration is used for older, tube type rigs which have relay coils connected to the PTT line. This configuration requires a power source from 12V to 13.8 volt DC to power the reed relay coil. This is connected to the board via DC power connector J6, mounted external to the board on the back of the enclosure. There is a short "captive cable" between J6 and the PCB. In this configuration, components R7 and D4 are NOT stuffed on the board, nor are jumpers A-A, B-B or D-D used.

3.2.3.3 PTT Configuration -2 Without Relay - The -2 configuration is in fact the "normal" configuration for most modern transceivers. It expects the PTT to be connected to solid state, low current requirement, non-inductive loads. It is driven directly by the output of the opto-isolator. It does not require an external power source for the interface. The wiring of the LED which indicates PTT activation is moved to be in series with the serial port control signals arriving via D1 or D2. The output of U1 is moved to connect to the PTT lines of the SSB rig. "Just in case" there is a diode placed across the output of U1 to protect it if an inductive load is ever connected to it. In the -2 configuration, several components are NOT stuffed. The components NOT used are J6, K1, R3 D3 and C2. Jumpers A-A, B-B and D-D ARE installed for this configuration.

Some laptop computers have lower than standard RS 232 voltage levels on the RTS and DTR lines. The current derived from these signals may not be high enough to properly light the LED or activate the opto-isolator circuit. A set of pads has been provided at location R9 to allow a 2.2k ohm resistor to be inserted to parallel R7. This reduces the series resistance and increases the current drive. See the discussion on PTT test in section 7.4.2.6 to determine if you need to add this component.

3.2.3.4 PTT Indicator - LED1 is a low current, red LED used to indicate that PTT has been activated. In the -1 configuration, it means that the output PTT control lines have been activated. In the -2 configuration, the LED means that either RTS or DTR on the serial port has been activated.

3.2.3.5 PTT Mode Control Switch - Switch SW1 provides control over the PTT lines of the rig. If the switch is in the UP position, automatic PTT has been enabled. Any time either RTS or DTR on the serial port is active, the PTT output will be activated (low impedance). If the switch is in the CENTER position, the PTT lines can not be activated. If the switch is in the DOWN position, the PTT lines are forced to the active state, and the rig is keyed. This latter mode is useful for testing, and for those installations which do not have a serial port, or VOX available to key the rig. It can be keyed manually by activating the switch.

Check the manual for your rig. Often, it will tell you what the current will be drawn when the PTT input is activate. If not, you can measure it using the following procedure.

Find the pin out configuration of the microphone connector on your rig from the rig manual. There will be a PTT pin, and a return pin for the PTT. Sometimes the PTT return and the microphone audio return are on the same pin. Using the VOLTAGE scale on a multi-meter set at 50v or higher, measure the voltage from the return pin to the PTT pin. Note the voltage and polarity. Typical measurements would show something on the order of + 5 to + 14 volts on the PTT pin as referenced to the return pin.

Next measure the current between these same two pins using the CURRENT scale of the multi-meter. Set the range for around 1 amp. When you do this, you will be keying the rig, so before making the measurement, set the rig control to USB and connect it to a dummy load. Momentarily connect the meter across the PTT and return pins. You should see activation of the rig, and see some current on the meter. You may see current in the range of a few ma. to a few hundred ma. If the current is really low, you may have to reduce the scale on the multi-meter to get a good reading.

If the current measures around 30 ma. or less, then it is most likely a solid state connection, or else a low current reed relay. If the measurement is more than 30 ma., you probably have a larger relay coil being activated. Use the -1 option if you have a current above 30 ma., or voltage measurements above 15 volts. Use the -2 option if the current is less than 30 ma. and the voltage is less than 15 volts.

During construction, follow the procedures for the appropriate configuration.