3V 300MHz TO 450MHz SUPERHETERODYNE RECEIVER WITH INTEGRATED IMAGE REJECTION
SUNNYVALE, CA-September 7, 2001-Maxim Integrated Products introduces the MAX1470, the industry’s first 300MHz to 450MHz superheterodyne ASK receiver designed with both -112dB of sensitivity and 50dB of integrated image rejection. For many applications, this level of image rejection eliminates the need for an external SAW filter. For power savings, the device operates from 3V supplies and draws less than 1.5µA in shutdown mode. During receive mode, the part draws 5.5mA. Depending on the components selected, the MAX1470 is capable of receiving data rates up to 25kbps.
After the LNA, the RF signal is fed though an I/Q mixer that down-converts to a 10.7MHz IF where the RF image is rejected, typically in excess of 50dB. Not only does this integrated image rejection eliminate the need for an external SAW filter in many applications, it also dramatically reduces the complexity of the antenna-matching network.
The output of the mixer and the input of the IF section are a 330Ω impedance to match an inexpensive 10.7MHz IF filter. From the IF filter, the signal is boosted by a chain of limiting amplifiers, producing an overall gain of approximately 65dB, with a bandpass-filter-type response centered near the 10.7MHz IF frequency with a 3dB bandwidth of approximately 11.5MHz. The limiter output is used to derive the receive signal-strength indicator (RSSI) signal with a slope of ~15mV/dB.
The output of the RSSI is fed into a second-order Sallen-Key filter with user-selectable poles. Altering the values of the capacitors moves the corner frequency of the filter, allowing sensitivity to be optimized for the transmitted data rate. From the data filter, the signal is passed through a data slicer with self-adjusting threshold. The data slicer design is optimized for Manchester-encoded data.
The MAX1470 also includes a fully self-contained PLL requiring no external components. The PLL block contains a phase detector, charge-pump/integrated loop filter, voltage-controlled oscillator (VCO), asynchronous 64x clock divider, and crystal oscillator. The quadrature VCO is centered at the nominal LO frequency of 304.3MHz. For an input RF frequency of 315MHz, a reference frequency of 4.7547MHz is needed for a 10.7MHz IF frequency (low-side injection is required). The MAX1470 is available in a 28-pin SSOP package and is offered at $1.65 in quantities of 10,000 pieces.
Because the MAX1470 is designed for power-critical applications, Maxim has implemented a fast-on oscillator (FOX) feature to cut the startup time of the receiver out of shutdown mode. The MAX1470 transitions from shutdown to valid data out in less than 250µs. In power-cycled applications, this quick transition minimizes the time the system needs to be in receive mode.
The MAX1470 architecture is designed for easy application into any 300MHz to 450MHz RF system. The LNA is a cascode amplifier that achieves 16dB of power gain with a 2.0dB noise figure and an IIP3 of -18dBm. The input impedance to the LNA is easily adjustable by varying the off-chip inductive degeneration, accommodating a wider range of antennae designs.
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