Posted on Dec 30, 2012

This counter uses a four-digit display, but with a flip of the range switch, it can display frequencies from 1 to 40 MHz, with a resolution of 100Hz. The MM74C926 CMOS IC contains a four-digit decimal counter that can latch a given count and then use this information to drive a 7 -segment, common-cathode multiplexed (MUX) display. The block diagram and schematic show the operation of the counter. Crystal-controlled timer U1 through U3 produces a 5-Hz square wave used for timing the frequency count. Yl is a TV color-burst crystal operating in a reliable circuit that controls the oscillator frequency.

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U1 acts as the oscillator and also divides the fundamental operating frequency of 3.579545 MHz to produce a square-wave output of 60Hz. U2 divides the 60-Hz output of U1 by six. In turn, the 10-Hz output of U2 is divided by two in U3, a dual flip-flop, to produce the 5-Hz pulse. A quad, two-input, Schmitt-triggered NAND U4 is used for gating the rf-signal input and for generating the counter control pulses-Section U4D is unused. The 5-Hz output pulse of U3 is applied to the rf-signal input gate at U4A pin 2. When the timer output signal is low, the gate is closed.

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