Question

In transformer coupling employed for protection against leakage currents, a carrier signal is employed with

• A. Frequency or pulse width modulation
• B. Amplitude modulation
• C. Phase modulation
• D. Pulse amplitude modulation

Hint:

To safely pass low-frequency biopotentials across an isolation transformer, systems use Frequency Modulation (FM) or Pulse Width Modulation (PWM). These time-domain schemes protect data accuracy from component drift and amplitude variations.

Answer 1

The Correct Option is A

Concept: To ensure patient safety in biomedical instrumentation, electronic circuits must include electrical isolation barriers. These barriers prevent dangerous 50/60 Hz AC leakage currents from flowing through the patient leads to the ground, which could otherwise cause microshock hazards. Isolation amplifiers often use transformer coupling to cross this break in the circuit.

Step 1: Understanding the transformer limitation.
Standard isolation transformers cannot pass low-frequency or direct current (DC) biomedical signals (such as 0.05 Hz ECG signals) due to core saturation and low inductive reactance at low frequencies. To overcome this, the low-frequency biopotential signal is modulated onto a high-frequency AC carrier signal, passed across the isolation transformer barrier, and then demodulated on the safe output side.

Step 2: Evaluating modulation schemes for isolation safety.
The choice of modulation scheme affects how accurately the signal is transmitted across the barrier:
• Amplitude Modulation (AM) Vulnerabilities: AM encodes signal data in the voltage amplitude of the carrier wave. Small variations in the transformer's alignment, core temperature, or component aging can alter the carrier's amplitude, causing calibration drift and measurement errors.
• Frequency Modulation (FM) and Pulse Width Modulation (PWM) Advantages: FM and PWM encode signal data in the time domain (using frequency shifts or pulse durations) rather than the voltage amplitude. Because information is carried by timing transitions rather than voltage levels, any amplitude variations introduced by the transformer barrier do not affect the data accuracy. This makes FM and PWM highly robust against component tolerances and drift, validating Option (A).