Question

In sine wave blood flowmeters, the signal-to-noise ratio is enhanced by employing:

• A. Low-noise FETs at amplifier output stage and half-wave demodulators
• B. Full-wave demodulators and low-noise FETs at amplifier output stage
• C. Half-wave demodulators and proper turns ratio for the step-down transformer
• D. Low-noise FETs at amplifier input stage and full-wave demodulators

Hint:

To optimize SNR in low-amplitude biomedical instrumentation systems: - Always minimize front-end noise by using high-impedance, low-noise active devices (like JFETs or CMOS Op-Amps) at the input stage. - Maximize signal recovery efficiency by utilizing full-wave synchronous rectification/demodulation to capture the signal's full energy envelope while filtering out out-of-phase noise artifacts.

Answer 1

The Correct Option is D

Concept: This question directly targets the optimal circuit design parameters used to maximize signal-to-noise ratio (SNR) in a sine wave electromagnetic blood flowmeter. The raw induced voltage across the pickup electrodes is incredibly small, often buried beneath environmental electromagnetic interference, thermal noise, and $90^\circ$ out-of-phase transformer loop noise. Extracting this signal requires optimized low-noise analog signal processing techniques.

Step 1: Optimizing the Front-End Amplifier Stage.
The output impedance of an electromagnetic flowmeter probe can be high and variable due to changing electrode-to-tissue interfaces. To prevent signal attenuation, the preamplifier must present an extremely high input impedance. Field-Effect Transistors (FETs) provide an ultra-high input impedance at their gate terminal. By selecting specialized low-noise FETs and placing them at the input stage of the preamplifier, the circuit minimizes the overall system noise figure ($F$) right at the source, preventing the amplifier's internal electronics from overwhelming the microvolt flow signals.

Step 2: Selecting the Demodulation Architecture.
The flow information is carried as an amplitude-modulated (AM) signal riding on the sine-wave carrier frequency of the magnetic field excitation. To recover the baseband flow profile, the amplified signal must undergo synchronous detection and demodulation.
• A half-wave demodulator discards half of the AC signal cycle, reducing signal amplitude by half and introducing significant ripple components that require heavy low-pass filtering, which degrades system response speed.
• A full-wave demodulator rectifies and integrates both positive and negative half-cycles synchronously. This preserves the full power content of the signal and effectively cancels out random high-frequency noise components, maximizing the final SNR. Combining low-noise FETs at the input stage with full-wave demodulators provides the optimal circuit configuration, matching Option (D).