Voltage-controlled oscillators: an oscillator whose output frequency is intentionally controllable by an external analog “control voltage” input is commonly referred to by which acronym?

Introduction / Context:
Many communication and synthesis systems require oscillators whose frequency can be tuned by a control signal. This is central to frequency modulation (FM), phase-locked loops (PLLs), and tone generation. The standard term for such a device is widely used across datasheets and textbooks.

Given Data / Assumptions:

• Output is a periodic waveform (e.g., square or sine).
• An analog control voltage continuously adjusts the oscillation frequency.
• No digital clock input is required for the basic function.

Concept / Approach:
A voltage-controlled oscillator (VCO) produces an output frequency f_out that is a function of an input control voltage V_ctrl, typically f_out = k_vco * V_ctrl + f_offset in the linearized region. In contrast, a PLL uses a VCO within a feedback system to lock phase/frequency, while Schmitt triggers and latches are thresholding and storage elements, not tunable oscillators by themselves.

Step-by-Step Solution:

Verification / Alternative check:
Block diagrams of PLLs always include a VCO block; it is the oscillator subcomponent whose frequency is steered by the loop filter voltage. Standalone VCO chips or function generator ICs advertise tuning sensitivity in Hz/V or rad/s per volt.

Why Other Options Are Wrong:

• PLL: a control system that typically contains a VCO, not the oscillator itself.
• Schmitt trigger: a comparator with hysteresis; may be used within oscillators but is not voltage-controlled frequency by itself.
• S-R latch: a bistable storage element; not an oscillator.
• VBR: not a standard oscillator acronym.

Common Pitfalls:
Confusing the VCO component with the full PLL system; assuming any oscillator with a tuning knob is necessarily crystal-based (VCOs are often RC or LC based with varactors).

Final Answer:
VCO