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The above circuit diagram is one of a “PICAXE RTTY QRP Beacon”. The micro-controller is a “18M2” PICAXE device from the online store “www.picaxe.com”, and further supplies can be found from their online attached store “tech supplies”.
The AD9850 DDS device was obtained from ebay, costing just around £2·50 each. The DDS syntheiser is programmed in this application in the parallel mode. Using the PICAXE device in this application was my first attempt to program a AD9850 DDS frequency synthesiser for Ham Raduio uses. Normally when a RTTY modem is made, the audio tones are feed to the microphone socket, and on HF using SSB (USB for data), and for VHF or UHF or even Microwave bands, the audio tones frequency modulate the carrier. However in this design shown in the above circuit diagram, the AD9850 DDS synthesiser generates the RTTY 170Hz shift FSK signal directly at the R.F. output transmitter frequency. Pin 16 (c.7) of the PICAXE 18M2 device, will relay out the TTL RTTY data stream of the transmitted message, to be used perhaps to check the RTTY data signal parameters, or even to supply the TTL RTTY signal to an audio tone modem.
The PICAXE program supplied as a downloadable link, generates the output transmitter frequency on 40meters, more precisely actually on centre carrier frequency of 7·060MHz. For the logic zero space tone the carrier output generated is 7·061275MHz, while for the logic one mark tone, the generated output frequency is 7·061445MHz. To alter the centre carrier frequency, use the “Analog Devices url link” to generate the 5 Byte numbers required to reposition each of bit logic tone carrier frequencies individually required, whatever the FSK frequency shift pattern. In addition, pin 15 (c.6) of the 18M2 device, will logic switch between each resending of the RTTY message. The idea is to alter the transmitter power by ½ and then by 2, hence by 3dB in power, or by 6dBV, swinging the “S meter” of the receiver between an “S point” reading, say between S5 and S6. This will in turn calibrate the “S Meter” on the monitoring radio receiver.
The PICAXE Basic language program is set for a bit rate of 50bits/sec or 50Baud. Transmitting with a bit rate of 45·45baud, the appropriate delay values or listed as commented statements. However, within the program code supplied example, the RTTY message is “RYRYRYRYRY DE GW4YCT QRP BEACON TX”, such that any Radio Ham operator wishing to replace this with their own message, the baud-dot values are programmed in HEX numbers, the values found from in this case the ARRL manual for “HF Digital”.