GROL Filters and Frequency Control 2 — Questions and Answers
Question 1: What component is primarily responsible for setting the resonant frequency in a crystal-controlled oscillator?
- Inductor
- Capacitor
- Quartz crystal (Correct answer)
- Resistor
Correct answer: Quartz crystal
The quartz crystal's mechanical resonance determines the oscillator frequency with very high stability, overriding the effects of LC components.
Question 2: A Butterworth filter is characterized by which response?
- Maximally flat passband with no ripple (Correct answer)
- Ripple in the passband but steeper rolloff
- Equiripple in both passband and stopband
- Resonant peaks outside the passband
Correct answer: Maximally flat passband with no ripple
A Butterworth filter provides a maximally flat (no ripple) frequency response in the passband, at the expense of a less steep initial rolloff compared to other designs.
Question 3: What is the cutoff frequency of a filter defined as?
- The frequency at which output power drops to zero
- The frequency at which output power is 3 dB below the passband level (Correct answer)
- The highest frequency the filter can handle
- The frequency at which the filter resonates
Correct answer: The frequency at which output power is 3 dB below the passband level
The cutoff (or -3 dB) frequency is the point where output power falls to half (−3 dB) of the passband level, marking the boundary of the filter's pass region.
Question 4: In a parallel LC circuit at resonance, the impedance is:
- Minimum (near zero)
- Maximum (very high) (Correct answer)
- Equal to the resistance only
- Equal to the source impedance
Correct answer: Maximum (very high)
At resonance, a parallel LC (tank) circuit presents maximum impedance because the circulating current between L and C is self-sustaining, drawing minimal current from the source.
Question 5: What happens to the selectivity of a filter as its Q factor increases?
- Selectivity decreases
- Selectivity increases (Correct answer)
- Selectivity is unaffected by Q
- The filter becomes a low-pass type
Correct answer: Selectivity increases
Higher Q means a narrower bandwidth relative to the center frequency, resulting in greater selectivity (ability to distinguish closely spaced signals).
Question 6: What is the main advantage of using a crystal filter in a superheterodyne receiver's IF stage?
- It provides amplification
- It offers very narrow, stable bandwidth for improved adjacent-channel rejection (Correct answer)
- It converts the IF frequency to audio
- It detects AM signals directly
Correct answer: It offers very narrow, stable bandwidth for improved adjacent-channel rejection
Crystal IF filters provide extremely narrow and stable passbands, greatly improving the receiver's ability to reject adjacent-channel interference.
GROL Filters and Frequency Control 2
What component is primarily responsible for setting the resonant frequency in a crystal-controlled oscillator?