AP 3302 Pt. 3
Section 2
CHAPTER 11
Frequency-Dividing & Counting Circuits
By
using components of different values in the two blocking oscillators we can
make the ten-mile cal pips from the frequency divider larger than the five-mile
cal pips from the cal pip generator. When they are both applied to the c.r.t.
the display is as shown in Fig 2. By using frequency dividers in this way we
can obtain an elaborate calibration display where the trace itself is marked
off like a ruler in whatever units of range we require. For example, a unit
which supplies 5-, 10- and 50-mile cal pips may use three separate circuits,
each circuit being triggered by the one that precedes it. The 10-mile circuit
operates at half the frequency of the 5-mile circuit and the frequency of the
50-mile circuit is one-fifth that of the 10-mile circuit.
Frequency Division in Multivibrator
The basic circuit of an astable multivibrator used as a frequency divider is illustrated in Fig 3. The positive-going input trigger pulses are applied to the grid of V1 but these have no effect until the grid voltage has risen almost to cut-off. Pulse 4 at time t1 causes V1g to rise above cut-off as shown in Fig 3 and initiates the usual multivibrator avalanche. With V1 conducting V1a falls almost to zero volts and remains there whilst V2g is rising to cut-off in the normal manner. When V2g reaches cut-off V2a falls towards zero; V1g is then driven below cut-off and V1a rises to h.t.+. Pulses 1, 2 and 3 have no effect on the circuit action, pulses 1 and 2 because V1 is already conducting when they appear and pulse 3 because V1g is too far below cut-off to be affected. Pulse 4 however at time t2 again raises V1g above cut-off before it would normally do so and the action is repeated. The output from V2 anode is differentiated by C3R3 and then negatively-limited by the diode. Fig 3 shows that the system delivers one output pulse coincident with every fourth input pulse. The circuit is thus counting down by a ratio of 4:1.
