Amateur Radio Transceiver

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AN-557 APPLICATION NOTE

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An Experimenter’s Project for Incorporating the AD9850 Complete-DDS Device as a Digital LO Function in an Amateur Radio Transceiver*
PIC “N” MIX DIGITAL INJECTION SYSTEM
By Peter Rhodes, BSc, G3XJP (email pirrhodes@aol.com) PART 1 OF 5
Thisconstruction project brings together a number of themes which I have been kicking around for some time. But first, why PIC “N” MIX? TWO ESSENTIAL TERMS PIC—A range of microcontrollers produced by Arizona Microchip Inc. In this application, the PIC16C84. DDS—Direct Digital Synthesis. The technique of digitally generating the output frequency directly (as opposed to typically mixing the output of aVFO with a crystal oscillator—or employing phase-locked loop techniques). In this application the Analog Devices AD9850 “complete DDS synthesizer” chip is used. IN BRIEF . . . PIC “N” MIX provides PIC controlled direct generation of the required injection frequencies into the signal frequency mixer in your transceiver. PIC “N” MIX also in the sense that you can pick and choose which functionalelements you build; and in the sense that there are by design a number of different mechanical configurations to best suit your circumstances. You are also presented with the radical choice of using the software I have designed—or writing your own. The PIC microcontroller (and about 400 hours of software development) provides control and operational flexibility while the DDS chip is used tosynthesize the RF output giving stability and low-phase noise. CONVERGING THEMES Discounting the value of your time, I would argue that for years it has been viable to build multiband HF transceivers which outperform their commercial counterparts at any point on the price versus performance graph—from the cheap and cheerful through to the truly exotic. Except, that is, for one critical element—theinjection oscillator. I have been building VFOs for years that for all practical purposes didn’t drift. Almost all were based on the Vackar running somewhere between 5 MHz–10 MHz. Besides some time consuming temperature compensation, I never gave them a second thought. But they need about eight x’tals, a mixer and switched bandpass filters before they can feed both the signal frequency mixer—and afrequency counter which gives a natural display of exactly not quite the frequency you are on! It can all be made to work, but only at substantial cost in time, money and space. And the only incremental feature easily obtained is IRT. Then in February 1996, Technical Topics reported the results of some phase noise measurements made by Colin Horrabin, G3SBI and Jack Hardcastle, G3JIR on a stable Vackar as“rather disappointing.” This set me thinking. Most of us ignore oscillator phase noise because we can’t measure it. Myself included. Does it really matter in practice? The ARRL handbook has an excellent section on the subject which concludes “. . . far-out phase noise can significantly reduce the dynamic range of a receiver. Farout phase noise performance has effects just as critical as blockingdynamic range and two-tone dynamic range performance of receivers.” Yes, but does it really matter in practice? I mean, am I truly going to fail to copy real signals on a significant number of occasions because of poor phase noise performance?

*This five-part article is reprinted in its entirety by permission of RadCom Magazine, a ham radio magazine publication in the U.K.(websitewww.rsgb.com), and the author. All international copyrights are reserved.

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I determined to find out by adopting the simple expedient of fitting a changeover switch between my traditional VFO and a phase-quiet alternative of the same power output. Then, under a variety of practical conditions, could I tell the difference? The problem, of course, was to find this alternative without...