2-30
Then, by dividing the time for 1 cycle (1 microsecond) into gate length (500 microseconds), you will
get the number of cycles (500).
There are several different varieties of pulsed oscillators for different applications. The schematic
diagram shown in figure 2-25, view (A), is an emitter-loaded pulsed oscillator. The tank circuit can be
placed in the collector circuit, in which case it is referred to as a collector-loaded pulsed oscillator. The
difference between the emitter-loaded and the collector-loaded oscillator is in the output signal. The first
alternation of an emitter-loaded npn pulsed oscillator is negative. The first alternation of the collector-
loaded pulsed oscillator is positive. If a pnp is used, the oscillator will reverse the first alternation of both
the emitter-loaded and the collector-loaded oscillator.
You probably have noticed by now that feedback has not been mentioned in this discussion.
Remember that regenerative feedback was a requirement for sustained oscillations. In the case of the
pulsed oscillator, oscillations are only required for a very short period of time. You should understand,
however, that as the width of the input gate (which cuts off the transistor) is increased, the amplitude of
the sine wave begins to decrease (dampen) near the end of the gate period because of the lack of
feedback. If a long period of oscillation is required for a particular application, a pulsed oscillator with
regenerative feedback is used. The principle of operation remains the same except that the feedback
network sustains the oscillation period for the desired amount of time.
Q-20. Oscillators that are turned on and off at a specific time are known as what type of oscillators?
Q-21. What is the polarity of the first alternation of the tank circuit in an emitter-loaded npn pulsed
oscillator?
HARMONICS
From your study of oscillators, you should know that the oscillator will oscillate at the resonant
frequency of the tank circuit. Although the tank circuit is resonant at a particular frequency, many other
frequencies other than the resonant frequency are present in the oscillator. These other frequencies are
referred to as HARMONICS. A harmonic is defined as a sinusoidal wave having a frequency that is a
multiple of the fundamental frequency. In other words, a sine wave that is twice that fundamental
frequency is referred to as the SECOND HARMONIC.
What you must remember is that the current in circuits operating at the resonant frequency is
relatively large in amplitude. The harmonic frequency amplitudes are relatively small. For example, the
second harmonic of a fundamental frequency has only 20 percent of the amplitude of the resonant
frequency. A third harmonic has perhaps 10 percent of the amplitude of the fundamental frequency.
One useful purpose of harmonics is that of frequency multiplication. It can be used in circuits to
multiply the fundamental frequency to a higher frequency. The need for frequency-multiplier circuits
results from the fact that the frequency stability of most oscillators decreases as frequency increases.
Relatively good stability can be achieved at the lower frequencies. Thus, to achieve optimum stability, an
oscillator is operated at a low frequency, and one or more stages of multiplication are used to raise the
signal to the desired operating frequency.
