QRP Forum Speaker
Craig Johnson, AA0ZZ
"HF-VFO Using an AD9854 Direct Digital Synthesizer"
Craig will be discussing his latest
digital VFO project design, featuring the Analog Devices AD9854 Direct Digital
Synthesis device. The AD9854 DDS is a step beyond the widely-used AD9850
for certain applications in that it has two output signals with a 90 degree
phase difference between them (quadrature). This phase relationship
remains constant regardless of the frequency. When used with a direct
conversion phasing-type HF receiver (such as an R2Pro) or a single-sideband
transmitter, these two outputs make it easy to provide opposite sideband
suppression over a frequency range of 0 to 30 MHz without switching components
for the various frequency bands. Of course, the VFO can easily be used for a CW
transmitter as well.
AA0ZZ will discuss the principles of Direct Digital Synthesis, and he'll discuss how it is done in his current project, using two PIC microcontrollers to control the AD9854 operation. Pros and cons of using a DDS device vs other digital frequency synthesis techniques or analog techniques (such as Hartley or Colpitts oscillators) will be discussed.
When Craig and his colleague developed the code for the two PIC microcontrollers in this project, they did it with other experimenters in mind. They attempted to document the code, to make it as clear a possible by reading the comments in the source code. The source code will be made available for experimenters, to allow them to understand the current design and to modify it to suit their own needs.
Craig will present the VFO project as a work-in-progress. He will discuss a number of options for changing and extending design, to encourage experimenters to improve the current design and to find answers to the questions about how a DDS device performs in various situations.
About Craig Johnson, AA0ZZ ...
Craig lives in St. Paul Minnesota. He has a Bachelor's degree in Electrical Engineering as well as an MBA. He has worked his entire career for one company, Unisys, where he is involved with the design/development of large computers. After starting out in computer hardware, he moved to software development where he works on the operating system.
Craig is married, and has three children. His two daughters are students at Bethel College in St. Paul, and his son is a junior in high school.
Craig got his first ham license in 1964 at the age of 14. However, during his college years and following, he let his license lapse and concentrated on computers. In 1995, two if his children (age 14 and 10) expressed interest in amateur radio so Craig offered to help them get their licenses. All three went on to get their licenses together. Then Craig went on (in the next 5 weeks) to his Extra Class license. For several years after that, Craig led a team of Volunteer Examiners, helping hundreds of people in the St. Paul area get or upgrade their licenses. He still serves as a VEon occasion. He is an active member of the Minnesota QRP Society.
Craig enjoys operating QRP, DXing and contesting. However, he is happiest when he is tinkering, building, or experimenting with new designs and new circuits. Working with microcontrollers and experimenting with new digital components are his current interests.
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