Automotive ultrasonic ranging: Gain increase may not boost detection distance

As ultrasonic-based distance ranging becomes more widely employed in applications such as blind spot detection, objects at distances greater than six meters (20 ft) have to be detected. The amplitude of the echo signal reflected by objects at far distances is very small. So there is a temptation to increase the amplifier gain, K, in order to detect objects at such distances. In this article, we show that increasing the amplifier gain, K, may not always result in the ability to detect objects at farther distances.

Background
One application for advanced driver assistance systems (ADAS) in a passenger car is ultrasonic-based distance ranging. Ultrasonic sound wave time-of-flight (TOF) is used to calculate distances to objects to assist the driver in parking the car, identifying parking spots, or detecting objects in the driver's blind spot.

In ultrasonic-based ADAS, piezoelectric transducers typically are used to convert the ultrasonic waves into electrical signals. The receiver sensitivity of piezoelectric ultrasonic transducers usually is small, resulting in very small voltages. Figure 1 shows a typical signal chain used to process the echo voltage.

This echo signal, which is an AM signal, is corrupted with noise. The noise in figure 1 is input-referred noise and is the sum of noise from external environment and from all components in the signal chain. This corrupted signal is then amplified by an amplifier with gain K. The amplified signal is digitized using an analog-to-digital converter (ADC). The digitized AM signal is bandpass-filtered.

The bandpass filter (BPF) primarily is used to improve the signal's signal-to-noise ratio (SNR). The filtered signal level is compared against a threshold, L, to detect the presence of an object. Bandpass filters typically are followed by an amplitude demodulator. However, for the purpose of this article, the demodulator is not relevant.

Threshold analysis

Observations
Equation 8 can be used to analyze the smallest detectable distance by considering noise from components upstream and downstream from the amplifier. The components upstream from the amplifier include environment noise, transducer noise, noise from any current limiting resistors, and noise the amplifier itself. Downstream components include the ADC quantization noise and filter calculations errors.