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Transmitter handles two frequencies with one crystal

Posted: 03 Jun 2008 ?? ?Print Version ?Bookmark and Share

Keywords:VHF/UHF transmitter? OOK/ASK/FSK? capacitors? power supply? power amplifier?


Maxim Integrated Products has introduced the MAX7058 crystal-referenced, phased-locked loop (PLL) VHF/UHF transmitter for transmitting OOK/ASK/FSK data at 315MHz or 390MHz. This device supports 315MHz/390MHz switched operation with a single 15MHz crystal, and can operate at any two frequencies in the 300MHz to 450MHz range with the use of different crystal frequencies. To enable this flexibility, the MAX7058 integrates a wideband VCO with internal tuning capacitors that can be programmed for matching the impedance of the power amplifier to the antenna. This topology ensures highly efficient transmission at multiple operating frequencies, thus making the MAX7058 well suited for power-sensitive applications such as home security systems, remote controls and garage door openers.

To improve further performance, the MAX7058 integrates an envelope-shaping resistor between the output of the power amplifier and the power supply. This resistor slows the turn-on/off time of the PA in ASK mode, reducing the spectral width of the power amplifier's modulated output signal. Additionally, the MAX7058 features a toggle pin that allows the device to switch automatically between two frequencies, thus eliminating the need for a microprocessor or switch.

The MAX7058's crystal-based architecture eliminates many common SAW transmitter problems by providing greater modulation depth, faster frequency settling, tighter transmit-frequency tolerance and reduced temperature dependence. This device thus provides better overall receiver performance when using a superheterodyne receiver with a narrow IF (i.e., lower noise) bandwidth.

The MAX7058 operates from a single, 2.1V to 3.6V supply, and draws less than 1?A in standby mode and less than 8mA in transmit mode (50 percent duty cycle OOK). Available in a small, 4mm x 4mm, 24-pin TQFN package, this device is fully specified over the -40C to 125C automotive temperature range. Prices start at $1.32 (1,000-up).

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