6 Transformer and Transducer

A matched transformer-transducer pair is included on the PGA450Q1EVM.

C20 is a temperature compensation capacitor for the XDCR. Match C20 to the selected XDCR. If XDCR is Murata MA58MF14-7N, 2000-pF capacitance is installed, and 1500-pF capacitance is provided as an alternative. If XDCR is Murata MA58AF14-0N. 1500-pF capacitance is installed. Alternative specifications include: 1500 pF, 0805, 250 V, 20%, NPO

Figure 2. Transformer and Connector for the Transducer

A matched transformer-transducer pair is included on the PGA450Q1EVM. Table 4 lists closed-top transducers and tunable push-pull transformers from various manufacturer numbers that can be matched with a tuning capacitor to create a sensor pair. By default, the transducer and transformer provided with the EVM are the Murata MA58MF14-7N and Mitsumi K5-R4, respectively.

Murata has provided the following note with regard to the availability and ability to order their ultrasonic sensors:

• For small quantity sample requests, inquire online at the Murata website: https://www.murata.com/en-us/contactform.
• For consumer-grade applications exposed to controlled environments, consider open-structure sensors, such as the Murata MA40H1S-R. Distributors will typically have these sensors readily available for purchase online.
• For automotive-grade sensors (such as the transducers listed in Table 4 for applications exposed to harsh environments, consider closed-top waterproof sensors, and inquire with distributors. The distributors will then request an order with Murata. Lead times for these sensors may vary up to several weeks.

Key ultrasonic sensor specifications are frequency, sensitivity, and directivity. The transformer is used to excite the transducer. The transformer is center tapped to double the voltage. Typically, a tuning capacitor is needed to match the resonant frequency between the transducer and transformer.

Equation 1.

C_PT, R_T, L_T, and C_T are characteristics of the transducer, L_SEC is the secondary inductance of the transformer, and C_TUNE is an external capacitor placed across the terminals of the transducer.

Figure 3. Equivalent Circuit of Transformer-Transducer Sensor Pair