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PROGRAMMABLE COUNTER A N D TX-OFFSET CIRCUITS
These circuits, consisting of IC5 to IC12, are basically a MODULO-N PROGRAMMABLE counter of IC5 to IC7 added with an EXTENDER consisting of a D-flip-flop of lClO and a logic circuit of IC8, 9, II and 12. I t belongs to the high-speed scaling method. Fig. 4 shows the operation of the circuits. The operation is simply described below. A division ratio is preset in the MC74416 of IC5 to IC7 with a BCD code. The division ratio preset lies between 400 and 799 in relation to digital indication (144.00 147.99). While, since the IF signal entering the MC 74416 is 5.37 9.36 MHz t o eliminate beat interference in reception, the division ratio must be 537 936 actually. For this purpose the gate, No, serves to raise the division ratio by 137. The gate circuit, U and D, shifts frequency by f600 kHz for repeater operation which is equivalent to the div~sion ratio of 137 f 60. MC74416 is a decrement-
ing counter which counts in the order of 0, 4, 3, 2, 1 , 0 ( 5 ) , 4, 3, ..... receiving input pulses, assuming that preset value is 5 and PE is "0" ( L level). But output becomes "1" ( H level) only when the count is 0. I t means that five input pulses make one output pulse and the frequency is divided t o 115. With three ICs connected in cascade, the division ratio can be raised up to 999. IClO is a high speed D-flip-flop which improves the operating frequency of MC74416, 8 MHz (min.), by
B. a factor of two or more with the aid of gates A and Fig. 4 shows the case where the least significant digit
of the actual division ratio, Ns, is 7. Although resetting should be done at the rise of input pulse and presetting should be done at the decay of the input pulse when the count has become three, the level at A is set to L at the count of five and i t becomes the output of IC10-1 at the next pulse. This output ( 0 1 ) resets the MC74416 and presets it t o N at the same time, but counting is not performed since PE remains at the L level during the next
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Fig. 4
Block Diagram of PROGRAMMABLE COUNTER and TX-OFFSET Circuit
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