Cascading these gives a divide by ten with BCD output of the counter state. It’s a dual decade divider, each side has a divide by two, and a divide by five stage. I decided on the 74hct390 chip which I have used before. Internet searching did not turn up many choices for decade frequency dividers. Later on, this proved to be the most troublesome part of the project. To get line level output I decided to buffer the ‘294 output with an LM358 op amp connected as a voltage follower. The MAX294 has an uncommitted on board op amp but it is intended as a pre or post processing filter and has weak output specs. When the signal generator display shows 25 megahertz, the output would be 25 kilohertz. If I add one additional decade to the ‘294 chip’s clock divide by 100 requirement, the total division would be 1000. That sixth requirement means decimal dividers throughout. Direct frequency readout from the signal generator display.Be as portable as the signal generator, meaning battery operation in an Altoids tin.Line level output, one volt peak to peak into 600 ohms.My new Si5351 RF generator could be a crystal controlled clock source and all I would only need the divider and filter circuits to add audio capability. Keith generates his clock with a 555 timer and then feeds a divide by 128 chip to develop the base output frequency. The ‘294 is fed with a square wave at the desired frequency and also a clock signal 100 times the desired frequency. The heart of Keith’s circuit is a MAX294 filter chip, a sophisticated 8th order switched capacitor filter capable of rounding off a square wave into a pretty good sine wave. January is the annual Do It Yourself issue and there was an intriguing article by Keith Kunde, K8KK titled “A Low Distortion Digital Audio Oscillator”. I had just finished building a three port RF signal generator when the January 2018 QST arrived in the mail.
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