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AMATEUR RADIO BY GARRY CRATT, VK2YBX The Easytune FSK indicator for HF transmissions This project was born out of the frustration of trying to tune various HF data transmission modes. Initially, the problem was trying to correctly tune an HF receiver to allow the use of an automatic CW decoder. Our problem was that we could hear these signals but couldn't decode them. RTTY? Obviously, a switched multitone box was out of the question. As most RTTY stations feed AFSK tones directly to the transmitter from a modem, decoding the two tones, the mark and the space, had to be achieved simultaneously. This is also the case with fax and packet radio transmissions. While old timers will no doubt be shaking th eir heads in dismay, it is a sad fact of life that not all amateurs are skilled in the reception of CW. Knowing this, various manufacturers produce equipment that is able to deco de machine generated CW signals. The device we were using to decode CW signals called for an audio input of 800Hz plus or minus 80Hz. It incorporated both an active filter and a PLL filt er, driving a CPU and associated software, which in turn produced decoded text on a 2-line liquid crystal display. As any newcomer will The next consideration was a visual indicator. We needed a circuit that would activate a LED when the correct tone or tones were received. A quick check in the data book reveal ed that an LM567 would do the trick nicely. We had seen circuits for tone decoders using the 567 before but most gave few details. Fig. l(a) shows the circuit we built, which worked perfectly first go. The 567 is purpose designed for this task. Fig.2 shows the internal details of the 567 tone decoder. It contains a highly stable phase locked loop with synchronous lock detection and an output driver. The centre frequency, bandwidth and acquisition time are all determined by external components. Basically, there are only three components that determine the operating characteristics of the device. Rl and Cl determine the operating frequency of the internal PLL. C2 determines both the speed and bandwidth of the device. As can be seen from Fig.3, the 567 has a maximum detection bandwidth of 14%. Using this figure, the value of C2 can be determined from the graph. In our case, we wanted maximum bandwidth detection, so C2 is lµF. We found that any value greater testify, accurately tuning the receiver to produce an audio output of 800Hz without any audible or visual reference takes some finite time, enough to miss parts of the transmission. What was needed was some kind of indicator. Initially, we pondered the use of a fixed 800Hz oscillator driving a loudspeaker, and housed in a small plastic box. It should be simple enough to hear the beat difference in output fre quency and adjust the receiver accordingly. This would have been fine if CW was all we wanted to listen to . But what about facsimile, packet and 120!1 r------ -+- - -+-- - ~~Mh-0 +1 2-15V + 100 16VW+ LE01 201 8.2V J .,. AUDIO 0. 47 INPUT o---1 3 IC1 567 .,. VA1 22k + 1+ + 1+ Fig. l(a): this single tone version of Fig.1 can be used for tuning into CW transmissions where connection to an automatic Morse decoder is required. 78 SILICON CHIP IC tone decoder 120\! 100 PARTS LIST + LED1 16VWJ 1 PC board, code SC06104911, 77 x 50mm 2 22kQ trimpots, (VR1, VR2) 2 1kQ 0.25W resistors IC1 567 Semiconductors 2 NE567 tone decoders (IC1, IC2) 2 red LEDs 2 8.2V zener diodes (ZD1, ZD2) 120!! .,. VR1 22k ~t 1 AUDIO ,.____ INPUT ~ 100 + Capacitors 2 100µF 16VW electrolytic 2 2.2µF 16VW electrolytic 2 1µF 16VW electrolytic 2 .068µF metallised polyester (5mm lead pitch) 2 .01 µF monolithic or metallised polyester (5mm lead pitch) ZD2 16VWJ: 8.2V IC2 567 FSK TUNING INDICATOR Fig.l (b): this is the version of the Easytune Indicator circuit for FSK detection. The two 567 tone decoders are set up to indicate the two different FSK tones. than lµF caused the unit to be too slow in driving the output. By op erating the chip in the "high input level" mode (ie, more than 200mV of input sign al), th e bandwidth chan ges caused by input signal variation are eliminated; however, the chip then becomes sensitive to sub-harmon ics, as th e input stage w ill be limiting. By operating the chip in the "low input level " mo de , best n oise reduction an d out-of-band signal rejection is achieved, so we determined that feed ing the input from a low level source would be the best option. C3 sets th e b nd edge of the inter- PHASE DETECTOR I R2 C2 LOO P LO W- PA SS FILTER _ 3.9k R1 CURRENT CONTROLLED OSCILLATOR Other data modes Having sucessfully overcome th e CW tuning problem, we began to think of the other interesting HF data +V INPUTD---"f-.._--l V1 n al low pass filter which effectively attenuates frequen cies outside the detection band to eliminate spurious outputs. Th e device is fe d via a 9 volt zener regulated supply for temperature stability. This also ensures that the maximum DC voltage of 10 volts is never exceeded . The output, pin 8, is the coll ector of an internal trans istor which saturates w h en an in-band signal is received, an d can sink 100mA if necessary, although not in our application. We used the outpu t to drive a LED, which is our visual indicator. ~ :.! 100kt----'t---t--- I --+---< ;;, +---+---+---+----l u +V R3 QUADRATURE PHASE 1 -......-----1 DETECTOR RL 0 ~ ~ 5:,: 10k t---t---r"'~r"'~t---t---t-----i ~ Vrel 1k .___..___..___..___..___..___.._____, 0 C3 10 12 14 BANDWIDTH (%lo) 0/P FILTER! as Fig.2 : inside the 567 tone decoder IC, made Signetics, National Semiconductor and a number of other companies. It contains a phase lock loop and a lock detector (pin 8). Fig.3: the 567 has a m aximum detection bandwidth of 1 4 %. Using this figure, the value of C2 can be determined from the graph. In our case, we wanted maximum bandwidth detection , so C2 is lµ F. APRIL 1991 79 f[ C ,,J r.. ,J J l~ r 300 I 1070 1170 1270 F1S le F1M 300 387 2025 2125 2225 F2S le F2M FREQUENCY (Hz) 3300 ,) J 1200 MARK 2200 SPACE 3300 FREQUENCY (Hz) Fig.4: this diagram shows commonly used tone pairs for FSK transmissions and their respective standards. modes. HF packet at 300 baud, for instance, uses two tones, 1070Hz and 1270 Hz, which comply to the Bell 103/113 standard. At VHF, the Bell 202 standard predominates 1200 baud operation and uses 1200Hz and 2200Hz. So we proceeded to build another tone decoder, feeding both units with common audio and power. Fig.1 (b) shows the circuit details. Construction A small PC board has been designed for the FSK Tuning Indicator shown in Figl(b) and this can also be used for the circuit shown in Fig. l(a). Fig;5 shows the wiring layout on the PC board. Install all the parts as shown, taking care to ensure that all . '(}~ The PC board can be mounted inside the equipment or installed in a separate case. Power can come from a 9V battery, via an on/off switch. polarised components are correctly oriented. By pre-adjusting one decoder to each tone, it became a simple matter to tune the HF receiver, so that both LEDs were illuminated when the correct tones w ere being received. Fig.4 shows commonly used tone pairs and their respective standards. Alignment is easy. Conn ect a suitable audio frequency counter to pin 5 of the 567 and , ensuring that there is no audio input connected to the circuit, adjust VR1 until the desired frequency (the same frequency we wish to detect) is displayed. No doubt the same technique can be used for other tone pairs, used for packet, RTTY and Fax. The unit could be built into a receiver or enclosed in a plastic box and fed from the headphone socket of the receiver. Our unit needed lO0mV to op erate correctly and this level is easily achieved through the headphone socket. In some receivers, it may be possible to us e the external record socket, which provides a fixed level of audio, regardless of the volume control setting. This was the case on our Yaesu FRG7700 lab receiver. The unit could be powered from a 9 volt battery but as the current drain is around 20mA, an on/off switch is recommended. This is no inconvenience, as once the signal is tuned, only periodic ch ecks are required to monitor receiver drift. References Signetics Linear LSI Data and Applications Manual, 1985; CQ Magazine, January 1991; The ARRL Handbook. SC 2.2ue uF17 0 ~ LE °'u u• C . AUDIO INPUT +12· 15V + GNO + Fig.5: this is the PC hoard wiring diagram for the FSK versions of the Easytune Indicator. A single tone version can be built by leaving out all the components associated with IC2. 80 SILICON CHIP Fig.6: this is the full size artwork for the PC board.