Filters and Frequency Control Flashcards

A T-network low-pass filter uses which arrangement of components?

Answer: Two series inductors with a shunt capacitor between them

A T-network low-pass filter consists of two series inductors (forming the top of the T) with a shunt capacitor connected between them to ground.

Which type of filter would be most appropriate for attenuating harmonic emissions from a transmitter final amplifier?

Answer: Low-pass filter

A low-pass filter installed at the transmitter output passes the fundamental frequency while attenuating harmonics, which are higher in frequency.

What is frequency synthesis in modern radio equipment?

Answer: Using a phase-locked loop (PLL) to generate precise frequencies from a reference oscillator

Frequency synthesis uses a phase-locked loop referenced to a stable crystal oscillator to generate a wide range of precise frequencies across the operating spectrum.

In a phase-locked loop (PLL), what is the function of the voltage-controlled oscillator (VCO)?

Answer: It produces the output frequency, controlled by a DC voltage from the loop filter

The VCO generates the output signal at a frequency proportional to its input control voltage; the loop adjusts this voltage to keep the VCO locked to the reference.

What is the rolloff rate of a single-pole (first-order) RC low-pass filter beyond the cutoff frequency?

Answer: 6 dB per octave (20 dB per decade)

A first-order (one-pole) filter rolls off at 6 dB per octave or 20 dB per decade above the cutoff frequency.

What causes spurious oscillations (parasitic oscillations) in RF amplifier stages?

Answer: Unintended feedback paths created by stray inductance and capacitance at the operating frequency

Stray inductance and capacitance in component leads and circuit board traces can create unintended resonant feedback loops, causing unwanted oscillation.