Filter fundamentals: what is the defining action of a high-pass filter on signal frequencies?

Introduction / Context:
Filters shape spectra to meet bandwidth and noise objectives. High-pass filters are ubiquitous in audio coupling, sensor signal conditioning, and communications front ends. Recognizing their basic pass/reject behavior is foundational before tackling cutoff frequency and order.

Given Data / Assumptions:

• Linear time-invariant filter behavior.
• “Pass” means relatively low attenuation; “reduce” means attenuate strongly.
• No special equalization or active gain stages considered.

Concept / Approach:
A high-pass filter attenuates frequencies below its cutoff while allowing higher frequencies to pass with less attenuation. The simplest RC high-pass uses a series capacitor and shunt resistor; at low frequencies, the capacitor’s reactance is large (blocking), and at high frequencies, it is small (passing).

Step-by-Step Solution:
1) Recall RC high-pass impedance: Xc = 1 / (2 * pi * f * C).2) At low f, Xc is high, so output is reduced (attenuated).3) At high f, Xc is low, so the signal passes with minimal loss relative to the passband.4) Therefore, the correct description is “passes higher-frequency signals and reduces lower-frequency signals.”

Verification / Alternative check:
Bode magnitude of a first-order high-pass shows +20 dB/decade slope below cutoff and ~0 dB flat passband above the corner f_c = 1 / (2 * pi * R * C).

Why Other Options Are Wrong:

• Passes low and reduces high: That is a low-pass filter.
• No effect on AC: False; filters are defined by frequency-dependent effects.
• Increases all frequencies: Describes a broadband amplifier, not a filter.
• None of the above: Incorrect because the high-pass behavior is well defined.

Common Pitfalls:
Confusing high-pass with low-pass due to series vs shunt placement of components is common. Always check the behavior of Xc versus frequency.

Final Answer:
passes higher-frequency signals and reduces lower-frequency signals.