Автор Тема: Direct Digital Synthesizer Board  (Прочитано 655 раз)


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Direct Digital Synthesizer Board
« : Ноябрь 09, 2017, 10:02:10 pm »
Removed and replaced the original CPU board that controlled the radio (including the front panel buttons, knobs, and display). Dual Flash programmed Freescale 9S12D64 chip(s) now runs the show (including Filter selection, DDS control, Display, buttons and knobs)

A Direct Digital Synthesizer replaced the original Heathkit designed PLL controlled analog synthesizer's. Feeding the DDS with a temperature controlled cystal oscilator eliminated the need for any PLL circuits. The idea is to use a predictible computer algorithm to generate a sine wave where the stability is now based on the stability of the osc. reference used.
The HW-5400 radio originally used 3 PLLs and VCOs to synthesize the LO reference sine wave on all bands. The circuitry was found on the CPU board, synthesizer board, and VCO boards. The IF signal was generated on the BFO board and was crystal controlled. The PLL arangement was fairly new technology for 1983 (for consumer use), but had a limiting slew rate and limited the frequency selection down to only 50Hz steps.
This project was to eliminate the PLL, VCO, BFO circuits while increasing the LO frequency resolution, increasing the slew rate, and keeping the frequency stability.