Analysis of fetal Doppler 2 MHz for sonar experiments

Fetal Doppler Teardown and Modification: From Continuous Wave to Pulse Sonar

A fetal Doppler is a device for listening to a fetus's heartbeat at a frequency of 2–3 MHz. The actual operating frequency, measured on a disassembled unit, is 2 MHz, despite the display indicating 3 MHz. The principle is based on the Doppler effect: the transmitting piezoelectric element continuously generates an ultrasonic signal, and the receiving element picks up the signal reflected from moving surfaces, such as heart walls.

The signal is amplified, compared in phase with a reference (using a quadrature detector method), and converted into a low-frequency audio signal via chips like the LM324. A microcontroller (with markings erased) counts the pulse rate and displays it, while also outputting sound to a speaker.

The sensor circuit includes two piezoelectric elements: a transmitter with a generator on a ceramic resonator (AB42 chip) and a receiver with a preamplifier. A low-pass filter cuts out noise.

Teardown and Schematic

The board contains a microcontroller, power driver, audio amplifier, and display. The sensor holds transmitting and receiving piezoelectric elements connected via inductance L1. The schematic of the transmitting part:

[AB42 Generator] --> [Buffer] --> L1 --> Piezo TX
Piezo RX --> Amplifier --> LPF

The actual 2 MHz frequency was confirmed with an oscilloscope. In continuous wave mode, the low power is sufficient for audio detection of pulsations at distances up to 10–15 cm in tissue.

Modification for Pulse Mode

The goal is conversion into a sonar for registering reflections. A buffer SN74AHC1G125D was soldered into the L1 break to control transmission. Control is from an oscilloscope generator: pulse bursts of 5 µs duration (10 periods at 2 MHz), with a repetition period of 2–3 ms.

Distance calculation: one period ~0.7 mm (sound speed ~1400 m/s in tissue/water). The circuit remains unoptimized: without dampers and with resonant circuits, causing artifacts.

Connection:

• Amplifier output to oscilloscope for reception.
• Buffer controlled by a logic signal.

Pulse shape on TX: rectangular bursts with amplitude sufficient to excite the piezoelectric element.

Experiments in Pulse Mode

Testing on water containers (good acoustic conductor):

• Vertical signal feed into a glass: reflection time from the bottom matches calculated (depth ~10 cm).
• Lateral feed through a basin wall with gel: clear reflections from a moving hand underwater.
• The sensor's directional pattern is narrow; signal is registered along the propagation path.

Applying to the body yielded no reflections: low power, unsuitable pulse envelope, attenuation in tissue and fat layers. The oscilloscope is not optimal for weak signals.

Key points:

• Actual frequency is 2 MHz, not 3 MHz as indicated.
• Continuous wave mode is only suitable for Doppler shift, not for echolocation.
• Pulse modification gives rough reflections in water but is useless for tissue without power amplification.
• The battery-powered device is not suitable for a full ultrasound scanner.
• Potential applications: demonstrator, aquarium fish finder, toy sonar.

Potential Applications and Limitations

The modified Doppler demonstrates basic sonar principles but is limited by power and circuit design.

• Demonstrator: for teaching ultrasonic echolocation.
• Toy sonar: for models in a bathtub.
• Aquarium fish finder: detecting objects in small volumes.

A full ultrasound scanner requires a different architecture: high-power pulse generators, damped transducers, TGC amplifiers, and high-resolution A/D converters.

— Editorial Team