Wireless keyless entry and tyre pressure monitoring systems based on SAW resonators are catching on in a big way in automotive engineering.
Apart from automotive applications, the number of radio-based services in general is steadily growing, especially in the ISM (industrial, scientific and medical) band with centre frequencies from 433,05 to 434,79 MHz.
Although many applications require ISM bandwidth only momentarily, mutual interference can result if different signals are transmitted simultaneously on the same frequency. One remedy for dynamic in-band interference is to retransmit the data telegram on different frequencies within the ISM band: the transmitter and receiver alternately use different frequencies. As interference usually affects only one channel, the message can be received and evaluated via the second channel. Channels are selected in the intermediate frequency with the aid of an IF filter. To avoid additional unwanted channel attenuation, both channels must be located within the passband of the RF filter. The transmitter frequency is stabilised with a phase-locked loop or by switching between two SAW resonators. The advantage of the SAW solution of Epcos is its simplicity of design and thus low cost, as it represents merely an extension of the familiar SAW transmitter.
For this application in particular, Epcos has developed two-in-one SAW resonators R770 (433,81/434,06 MHz) and R771 (314,875/315,125 MHz). Each type contains two electrically independent, high-precision single-port SAW resonators in a QCC8C SMD ceramic package with a footprint of 5 x 5 mm. Both single-port resonators are implemented on the same chip and fabricated simultaneously in an identical process. This results in superior synchronism against temperature and infrequency response. Almost any desired frequency difference between the two resonators can be designed. In the send oscillator, the two resonators are alternately connected to the oscillator circuit. Switching is done via PIN diodes.
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