quency increase or decrease as the control voltage input receives a more positive voltage?
Note: “RFC” is an acronym standing for Radio-Frequency Choke, an iron-core inductor whose purpose it is to block radio frequency current from passing through.
This is a schematic for a simple VCO:
The oscillator is of the RC “phase shift” design. Explain how this circuit works. Why does the output frequency vary as the control voltage varies? Does the output frequency increase or decrease as the control voltage input receives a more positive voltage?
Hint: the JFETs in this circuit are not functioning as amplifiers!
FM tends to be a far more noise-resistant means of signal modulation than AM. For instance, the “crackling” form of radio interference caused by natural lightning or the “buzzing” noise produced by high-voltage power lines are both easy to hear on an AM radio, but absent on an FM radio. Explain why.
Predict how the output frequency of this voltage-controlled oscillator (VCO) circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):
- Capacitor C1 fails open:
- Inductor L1 fails open:
- Resistor R1 fails open:
- Resistor R2 fails open:
- Inductor L2 fails partially shorted:
For each of these conditions, explain why the resulting effects will occur. Note: the voltage-dependent capacitance of a varactor diode is given by the following equation:
CJ = Junction capacitance
Co = Junction capacitance with no applied voltage
V = Applied reverse junction voltage
The oscillator circuit in this diagram generates a square wave with an adjustable duty cycle:
A student desires to use this circuit as the basis for a pulse-width modulation (PWM) power controller, to vary the amount of power delivered to a DC load. Since the oscillator circuit is built to produce weak signals and not deliver power directly to a load, the student adds a power MOSFET to switch heavy load currents:
Correlate the duty cycle of the oscillator’s output signal with motor power. In other words, describe how increases and decreases in signal duty cycle affect the amount of power delivered to the electric motor.
Explain why it is important for the final power transistor(s) in a PWM power control circuit to operate at full cutoff and full saturation, and not in the linear (active) mode in between those two extremes. What might happen if the power transistor(s) were to be less than cut-off or less than saturated when carrying load current?
If a pulse-width modulated (PWM) signal is sent to a passive integrator circuit from a circuit capable of both sourcing and sinking current (as is the case with the dual-MOSFET output stage), the output will be a DC voltage (with some ripple):
Determine the relationship between the PWM signal’s duty cycle and the DC voltage output by the integrator. What does this suggest about PWM as a means of communicating information, such as analog data from a measuring device?
Determine the frequency spectrum for a high-frequency, sine wave “carrier” signal that is amplitude-modulated (AM) by an audio-frequency sine wave signal, as the following block diagram shows:
The spectra for these respective waveforms are shown individually:
Plot the modulated signal spectrum here:
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