AF4Z Multi-Modem Part II

Technical Description

By Jan A. Heise WA4VQD

In the first article about the AF4Z Multi-Modem, I discussed how it started as a club project. It generated a lot of RTTY interest, and a number of us have done quite well contesting and DXing with it. In this article I will review the technical aspects of the unit.

The AF4Z Multi-Modem was designed to provide excellent results when used in the 45 Baud RTTY mode. However, it can also be used in other areas of digital communications such as ASCII and CW. It also supports multiple Amtor modes. The unit has a built-in comparator demodulator which allows it to support WEFAX and NAVTEX. With software such as JVFAX or SSTV, it can be used for SSTV transmit and receive.

Circuit Description: Power Supply: The Multi-Modem is powered from a 9-14VDC source which is protected from a reverse voltage connection. Three voltages are used on the circuit board to power all the circuitry. The data filters are powered from the +12 volt and -10 volt supplies generated by an on-board DC-DC converter. The other supply is +5VDC which is regulated from the +12 volt source. The regulated +5 Volt is used to power the internal AFSK tone modulator. It is recommended that a separate power from your HF Radio supply be used to power the unit. A wall plug-in power supply works fine.

Demodulator: Audio tones are received from the radio at J3 and are amplified with a JFET input op-amp. J4 is connected to J3 to provide an audio source for an external speaker or other device. The amplified signal is then split into several paths. The audio goes to a zero-crossing comparator and will generate an output digital signal at the audio rate. This signal is used with HamComm and other software needing this type demod to determine the timing of the audio signal. HamComm derives the data for the software audio spectrum analyzer and tuning indicator from this signal. This signal is output to pin 6 of the RS232 cable connected to the computer.

The audio is also sent to two op-amps which form a cascade CW bandpass filter. The center frequency of this filter is adjustable from 500Hz to 1Khz. This filter is selected when in the CW position of the selector switch.

The third path of the audio is to the MARK and SPACE RTTY filters consisting of 6 Op-Amps. Three 2 pole cascaded filters are used in each channel. This approach provides less transition ringing and better filter performance.

Data Detector and Automatic Threshold Correction (ATC) Circuit: Audio from the filters is fed to the data detectors. There are two sets of detectors. One is designed as the data detector, and the other set is for peak detection and is used as part of the ATC detector. After being detected, the data is filtered by the data filter and amplified. At this point the tones have been filtered out passing only the data stream to one input of the data comparator. The signal from the ATC is connected to the other input of the data comparator. The comparator uses these two signals to determine the data. This circuit is designed to provide optimum performance during periods of fading. Even if one of the tones goes away, the comparator will still continue to demodulate the data. The comparator then sends the demodulated data to the computer using pin 5 of the RS232 cable. When using CW, the comparator threshold will adjust itself to decode the CW tones.

The Modulator: AFSK Modulator consists of a tone generator which is adjusted to the correct frequency during alignment. The tones are generated by using the FSK signal line from the computer which determines the tone to be transmitted. The AFSK tones are then lowpass filtered to prevent distortion. An adjustment is provided to control the amplitude of the signal. There is a jumper option to allow selection of the internal AFSK generator or computer generated tones. If the radio supports FSK, the FSK line from the computer is buffered and is available on an RCA jack.

Switches and Indicators: On the front panel are three LEDs (one for power and two for tuning indicators) and a four position mode switch (OFF, RTTY, RTTY Reverse, and CW). RTTY reverse is used to swap mark and space filters on receive only. This is useful for receiving an inverted signal. All cable connections needed are located on the rear panel. The unit is housed in a Silver and Black metal cabinet with silk-screened front and back panels.

Unlike some kits, this one comes complete including hardware, solder, cable and connectors. The only thing you will need to supply is the connector for your radio and the 12 volt source.

Modifications and Enhancements: Several people who have built these units have made modifications and enhancements to the units. I added scope tuning output by simply connecting two wires from the board through resistors to RCA connectors on the back panel. Others have built nice external LED tuning indicators using a simple driver circuit which Don, AF4Z, also designed. Several club members built the unit using the European low tone standard, and instructions for doing this are in the manual. The design of the unit is straight forward and easy to understand. This allows it to be easily modified to suit the needs of the user.