The AD831 is a low distortion, wide dynamic range, monolithic mixer for use in such applications as RF to IF down conversion in HF and VHF receivers, the second mixer in DMR base stations, direct-to-baseband conversion, quadrature modulation and demodulation, and doppler-shift detection in ultrasound imaging applications. The mixer includes an LO driver and a low-noise output amplifier and provides both user-programmable power consumption and 3rd-order intercept point.
The AD831 provides a +24 dBm third-order intercept point for -10 dBm LO power, thus improving system performance and reducing system cost compared to passive mixers, by eliminating the need for a high power LO driver and its attendant shielding and isolation problems.
The RF, IF, and LO ports may be dc or ac coupled when the mixer is operating from ±5 V supplies or ac coupled when operating from a single supply of 9 V minimum. The mixer operates with RF and LO inputs as high as 500 MHz.
The mixer"s IF output is available as either a differential current output or a single-ended voltage output. The differential output is from a pair of open collectors and may be ac coupled via a transformer or capacitor to provide a 250 MHz output bandwidth. In down-conversion applications, a single capacitor connected across these outputs implements a low-pass filter to reduce harmonics directly at the mixer core, simplifying output filtering. When building a quadrature-amplitude modulator or image reject mixer, the differential current outputs of two AD831s may be summed by connecting them together.
An integral low noise amplifier provides a single-ended voltage output and can drive such low impedance loads as filters, 50 W amplifier inputs, and A/D converters. Its small signal bandwidth exceeds 200 MHz. A single resistor connected between pins OUT and FB sets its gain. The amplifier"s low dc offset allows its use in such direct-coupled applications as direct-to-baseband conversion and quadrature-amplitude demodulation.
The mixer"s SSB noise figure is 10.3 dB at 70 MHz using its output amplifier and optimum source impedance. Unlike passive mixers, the AD831 has no insertion loss and does not require an external diplexer or passive termination.
A programmable-bias feature allows the user to reduce power consumption, with a reduction in the 1 dB compression point and third-order intercept. This permits a tradeoff between dynamic range and power consumption. For example, the AD831 may be used as a second mixer in cellular and two-way radio base stations at reduced power while still providing a substantial performance improvement over passive solutions.
