PN5139

The output of the CA3179 is fed through the D1/Q1 circuit. Those components serve to boost the 1-V output of the CA3179 to a standard TTL level. Then, depending on the position of range switch S2b, the signal is passed directly to the 7216, or through the divide-by-four circuit built from the two D flip-flops in IC3.

The other half of the range switch S2a controls the voltage at pin 3 of the CA3179. When pin 3 is high, the signal applied to pin 9 is fed through an extra internal divide-by-four stage before it is amplified and output on pins 4 and 5. When pin 3 is low, the signal on pin 13 is simply processed for output without being divided internally.

A 3.90625-MHz crystal provides the time base; the crystal yields a fast gate time of 0.256 second.The displayed frequency equals the input frequency divided by 1000 in the fast mode. In slow mode, gate time is 2.56 seconds. The displayed frequency equals the input frequency divided by 100 in the slow mode.

Switch S4, gate time, performs two functions. First it selects the appropriate gate time according to which digit output of IC1 the range input is connected to. Another of the 7216's inputs is also contro0ed by S4: the dp select input. The decimal point of the digit output to which that pin is connected will be the one that lights up. The correct decimal point illuminates, according to the position of S4, to provide a reading in MHz.