Turning a digital frequency counter into a digital voltmeter (DVM): which additional block enables voltage measurement?

Introduction / Context:
A digital frequency counter inherently counts pulses in a given gate time to report frequency. If the input voltage can be converted into a proportional frequency, the same counting mechanism becomes a digital voltmeter. This is the principle behind voltage-to-frequency (V-F) conversion DVMs.

Given Data / Assumptions:

• We have a standard frequency counter with gating and counting circuitry.
• We need a block that converts an analog voltage into a frequency.

Concept / Approach:

A voltage-controlled oscillator (VCO) produces an output frequency proportional to an input control voltage. By scaling and linearizing the VCO, the counter’s reading (counts within the gate time) becomes directly proportional to the input voltage. Average-value noise rejection is also improved due to time averaging in counting.

Step-by-Step Solution:

Verification / Alternative check:

Commercial V-F converter ICs (or VCOs inside PLLs) are commonly used to implement integrating DVMs and telemetry systems—confirming the approach.

Why Other Options Are Wrong:

• D/A converter and power amplifier do not perform voltage-to-frequency conversion.
• Operational amplifier alone cannot convert voltage to frequency without additional timing components.
• PLL without a VCO cannot generate frequency proportional to input voltage.

Common Pitfalls:

• Confusing V-F conversion with A-D conversion using successive approximation or dual-slope methods (different architectures).

Final Answer:

voltage controlled oscillator