Receiving Systems
The circuit shown in Fig. 8C10 is the output of a switching power supply. Measuring from the junction of CR6, CR7 and L1 to ground with an oscilloscope, what waveform would you expect to see?
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Filtered DC.
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Pulsating DC at line frequency.
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AC at line frequency.
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Correct AnswerPulsating DC much higher than line frequency.
Switching power supplies operate at 20KHz and up, way above 60Hz US AC, 50Hz European AC and 400Hz ship and aircraft AC.
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With regard to the comparator shown in Fig. 8C11, the input is a sinusoid. Nominal high level output of the comparator is 4.5 volts. Choose the most correct statement regarding the input and output.
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Correct AnswerThe leading edge of the output waveform occurs 180 degrees after positive zero crossing of the input waveform.
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The rising edge of the output waveform trails the positive zero crossing of the input waveform by 45 degrees.
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The rising edge of the output waveform trails the negative zero crossing of the input waveform by 45 degrees.
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The rising edge of the output waveform trails the positive peak of the input waveform by 45 degrees.
A comparator is an electronic device that compares the amplitude of two input signals and produces an output based on their relative levels. In this case, the input to the comparator is a sinusoidal waveform.
When the input waveform is a sinusoid, it goes through positive and negative zero crossings as it oscillates. The "positive zero crossing" refers to the point where the sinusoid changes direction from a negative value to zero and then becomes positive.
For the comparator output, the "leading edge" is the rising edge of the waveform. In this context, it means the point where the output transitions from a low voltage level to a high voltage level.
The statement indicates that the leading edge of the output waveform occurs 180 degrees after the positive zero crossing of the input waveform. This means that the output waveform rises to its high level after the input waveform crosses the positive zero point and continues to rise for half a cycle of the sinusoid.
Mnemonic: "POLE - Positive, Output, Leading, Edge"
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The nominal output high of the comparator shown in Fig. 8C11 is 4.5 volts. Choose the most correct statement which describes the trip points.
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Upper trip point is 4.5 volts. Lower trip point is approximately 0 volts.
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Upper trip point is 2.5 volts. Lower trip point is approximately 2.0 volts.
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Upper trip point is 900 microvolts. Lower trip point is approximately 0 volts.
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Correct AnswerUpper trip point is +1.285 volts. Lower trip point is -1.285 volts.
The formula to calculate voltage division is:
\(V_{tj} = V_{out} \times \frac{R_2}{(R_1 + R_2)}\)
So plugging in the values we can get
\(V_{tj} = 4.5V \times \frac{2Ω}{(5Ω + 2Ω )}\) \(V_{tj} = 4.5V \times 0.2855 = 1.285V\)
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