Question

Which of the following statement is false?

• A. Transformer voltage develops in electromagnetic flowmeters employing AC magnetic current with sine wave excitation
• B. The best technique to minimize the transformer voltage effect is to employ quadrature suppression circuit
• C. In sine wave blood flowmeters, the signal-to-noise ratio is enhanced by employing low-noise FETs at amplifier output stage and half-wave demodulators
• D. In sine wave blood flowmeters, the signal-to-noise ratio is enhanced by employing low-noise FETs at amplifier input stage and full-wave demodulators

Hint:

In any low-level physiological signal chain, noise performance is dictated by the first amplification block. According to Friis' Formula for Noise Factor, noise introduced by later stages is divided by the gain of the preceding stages: \[ F_{total} = F_1 + \frac{F_2 - 1}{G_1} + \frac{F_3 - 1}{G_1 G_2} \] Consequently, high-impedance, low-noise active devices (such as FETs) must be positioned at the input stage ($F_1$), never at the output stage, to maximize signal-to-noise ratio.

Answer 1

The Correct Option is C

Concept: Electromagnetic blood flowmeters measure the velocity of blood flow in an intact vessel based on Faraday's Law of Electromagnetic Induction. When an electrically conductive fluid (such as blood containing ionic species) flows through a transverse magnetic field ($B$) generated by an electromagnet around the vessel, an electromotive force (EMF) is induced across the vessel diameter. This voltage is collected by two small pick-up electrodes and is defined by: \[ e_f = B \cdot d \cdot v \] where $d$ is the vessel diameter and $v$ is the instantaneous velocity of blood flow. To prevent electrode polarization artifacts caused by direct current (DC) fields, manufacturers use alternating current (AC) excitation (such as sine wave currents) to drive the electromagnets. However, using an AC magnetic field introduces an unwanted noise component known as transformer voltage.

Step 1: Analyzing options (A) and (B) regarding transformer voltage.
When a sine wave current excites the electromagnet, the magnetic flux ($\phi$) varies continuously over time ($\frac{d\phi}{dt}$). The input circuit loop—formed by the probe lead wires, pick-up electrodes, and conductive tissue—acts as a single-turn secondary winding of a transformer. Even when blood flow velocity is zero ($v = 0$), a parasitic noise voltage is induced in this loop, defined by: \[ e_t = k \cdot \frac{d\phi}{dt} \] Since the magnetic field varies as a sine wave ($\sin(\omega t)$), its derivative yields a cosine wave ($\cos(\omega t)$), introducing a $90^\circ$ phase shift (quadrature noise). This confirms that statement (A) is true. Because this noise signal is exactly $90^\circ$ out of phase with the flow signal, designers use a quadrature suppression circuit or synchronous phase-sensitive detectors to block the noise while passing the in-phase flow signal. This confirms that statement (B) is also true.

Step 2: Evaluating statements (C) and (D) regarding SNR enhancement.
To capture the tiny microvolt-level signals generated by a flowmeter sensor while maintaining a high signal-to-noise ratio (SNR):
• Low-noise field-effect transistors (FETs) must be placed at the input stage of the preamplifier chain. Placing them at the output stage provides no benefit, as front-end thermal and flicker noise would already dominate the signal.
• The AC modulated signal must be converted back to DC using a high-efficiency full-wave demodulator. Full-wave demodulators preserve signal energy from both halves of the AC cycle, yielding superior noise performance compared to a basic half-wave demodulator. Evaluating both options shows that Statement (D) is true, which means Statement (C) is false. Since the question asks for the false statement, Option (C) is the correct answer.