4-53
voltage will be 10
-
1.9 volts, or 8.1 volts. C2 will charge to 10 percent of 8.1 volts, or .81 volt. The
voltage on C2 will become 1 + .9 + .81, or 2.71 volts. Successive input pulses will raise C2 by 10 percent
of the remaining voltage toward 10 volts until the blocking oscillator works. If the oscillator bias is set so
that Q1 begins conduction at 3.8 volts, this will continue until 3.8 volts is exceeded. Since the fourth step
is 3.5 volts and the fifth is 4.1 volts, the 3.8-volt level is crossed at the fifth step. If the oscillator goes
through 1 cycle of operation every fifth step and C2 is discharged at this point, this circuit would be a 5-
to-1 divider.
The circuit can be made to divide by 3, 4, or some other value by setting the bias at a different level.
For example, if the bias is set at 2.9 volts, conduction will occur at the fourth step, making it a 4-to-1
divider.
The counting stability of the step counter is dependent upon the exponential charging rate of
capacitor C2. As C2 increases to higher steps, the voltage increments are less and less. If the ratio
becomes too great, the higher steps become almost indiscernible. For this reason, accuracy decreases as
the ratio increases. When you desire to count by a large number, 24 for example, a 6-to-1 counter and a 4-
to-1 counter are connected in cascade (series). A more stable method of counting 24 would be to use a
2:1, 3:1, 4:1 counter connected in cascade. Most step counters operate on a ratio of 5 to 1 or less.
Q27. What is the difference between a positive counter and a step counter?
SUMMARY
This chapter has presented information on wave shaping. The information that follows summarizes
the important points of this chapter.
A LIMITER is a device which limits or prevents some part of a waveform from exceeding a
specified value.
In a SERIES LIMITER, the diode is in series with the output. It can limit either the negative or
positive alternation of the input signal.
In a SERIES-POSITIVE LIMITER, the diode is in series with the output which is taken across the
resistor. It removes the positive alternation of the input signal.
In a SERIES-POSITIVE LIMITER WITH BIAS, the bias potential will either aid or oppose the
flow of current. When aiding forward bias, only a portion of the positive input pulse is removed. When
the bias aids the reverse bias, all of the positive and a portion of the negative pulse is removed.
