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A schematic diagram of a positive step counter is shown in view (A) of figure 445. For step counting, the load resistor of the positivecounting circuit is replaced by capacitor C2. This capacitor is relatively large in comparison to C1. Each time D2 conducts, the charge on C2 increases as shown in view (B). The steps are not the same height each time. They decrease exponentially with time as the voltage across C2 approaches the input voltage.
Figure 445A.—Basic step counter and waveforms.
Figure 445B.—Basic step counter and waveforms.
As long as C2 has no discharge path, the voltage across its terminals increases with each successive step until
it is equal in amplitude to the applied pulse. The voltage across C2 could be applied to a blockingoscillator
circuit to cause the oscillator to pulse after a certain amount of voltage is applied to it.
The circuit
in figure 446, (view A) and (view B), may be used as a frequency divider. When used in this manner, Q1 is used as
a singleswing blocking oscillator that is triggered when the voltage across C2 becomes great enough to forward
bias Q1. At other times, the transistor is cut off by the bias voltage developed in the section of R2 that is
between the ground and the slide.
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Figure 446A.—Step counter as a frequency divider and waveforms.
Figure 446B.—Step counter as a frequency divider and waveforms.
The action of the counter can best be understood by referring back to figure 445. Assume C2 is 10 times larger than C1 and the peak voltage is 10 volts. C1 will assume 9/10 of the positive input voltage at T0, while C2 will assume only 1/10, or 1 volt in this example. At T1 the input will drop in a negative direction and D2 will be cut off. The cathode of D1 will become more negative than its anode and conduct, discharging C1. The charge on C2 will remain at 1 volt because it has no discharge path. At T2 the second pulse will be applied. The 1volt charge on C2 will oppose the 10 volts of the second pulse, and the applied voltage for the capacitors to charge will be 9 volts. C2 will again charge 10 percent, or 0.9 volt. This is in addition to the initial charge of volt. At the end of the second pulse, the voltage on C2 will be 1.9 volts. At T3 the third pulse will be 10 volts, but 1.9 volts will oppose it. Therefore, the applied
452
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 3.8volt 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 to1 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 4to1 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 6to1 counter and a 4 to1 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
SERIESPOSITIVE 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 SERIESPOSITIVE 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.
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The SERIESNEGATIVE LIMITER limits the negative portion of the input pulse. The difference between a seriesnegative limiter and a seriespositive limiter is that the diode is reversed in the negative limiter.
A SERIESNEGATIVE LIMITER with bias is the same as the seriespositive limiter with bias, but the outputs are opposite. When bias aids forward bias, only a portion of the negative input is removed. When bias aids reverse bias, all of the negative and a portion of the positive input is removed.
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In a PARALLEL LIMITER, a resistor and diode are connected in series with the input signal. The
output is taken across the diode.
In the PARALLELPOSITIVE LIMITER, the positive portion
of the input signal is limited when the diode conducts.
The PARALLELNEGATIVE LIMITER diode is reversed from that of the parallel positive limiter to limit only a portion of the negative input signal.
The DUALDIODE LIMITER combines the parallel negative limiter with negative bias (reverse bias) and the parallel positive limiter with positive bias (reverse bias). It will remove parts of the positive and negative input signal.
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A CLAMPING CIRCUIT effectively clamps or ties down the upper or lower extremity of a waveform to a fixed dc potential. Clamping does not change the amplitude or shape of the input waveform.
A POSITIVE CLAMPER will clamp the lower extremity of the input waveform to a dc potential of
0 volts.
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A NEGATIVE CLAMPER will clamp the upper extremity of the input waveform to a dc potential of 0 volts.
A COMMONBASE TRANSISTOR CLAMPER clamps the collector voltage to a reference level. A waveform
other than a sine wave is called a COMPLEX WAVE.
If the odd harmonics of a sine wave are added algebraically,
the result is a square wave. A PERFECT SQUARE WAVE is composed of an infinite number of odd harmonics in phase
with the fundamental wave.
457
A SAWTOOTH WAVE is made up of different harmonics, both odd and even.
A PEAKED WAVE is made up of odd harmonics that are in phase and out of phase with the fundamental.
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INTEGRATION takes place in an RC circuit with the output taken across the capacitor. The amount of integration is dependent upon the time constant of the circuit. Full integration takes place when the time constant of the RC circuit is at least 10 times greater than the duration of the input pulse. An RL circuit is also used as an integration circuit. The output is taken across the resistor and the time constant of the circuit is 10 times greater than the input pulse.
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DIFFERENTIATION is the opposite of integration. In the differentiator, the output is
taken across the resistor. Full differentiation takes place when the time constant of the circuit is 1/10 that of
the input pulse.
A COUNTING CIRCUIT receives uniform pulses, representing units to be counted, and produces a voltage output proportional to its frequency.
ANSWERS TO QUESTION Q1. THROUGH Q27.
A1. Negative.
A2. Positive.
A3. Biasing.
A4. The diode.
A5. Conducting, cutoff.
A6. Short time
constant.
A7. Long time constant.
A8. Most negative.
A9. Positive potential.
A10. Positive clamper with negative bias.
A11. Most positive.
A12. Negative potential.
A13. Positive
bias.
A14. 5 volts.
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A15. It is composed of an infinite number of odd harmonics in phase with the fundamental.
A16. It is composed of odd harmonics some of which are out phase with the fundamental.
A17.
All the odd harmonics are in phase with the fundamental in the square wave. This is not true of the odd harmonics
in the peaked wave.
A18. The time constant is long and the output is taken across the
capacitor in an RC circuit.
A19. A pure sine wave cannot be integrated; it contains no
harmonics.
A20. The ability of the inductor to oppose a change in current.
A21. The
timeconstant value of a long time constantcircuit is 10 times the value of the input pulse duration. The short
timeconstant circuit has a time constant of 1/10 of the pulse duration.
A22. A more complete
integration of the waveform would result from the long time constant.
A23. In an RC circuit
the output is taken across the resistor. In the RL circuit the output is taken across the inductor.
A24.
Frequency counters or frequency dividers.
A25. The frequency of the voltage input.
A26. To provide a quick discharge path for C1.
A27. The load resistor in a
positive counter is replaced by a capacitor in a step counter.
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