Reaching receiver high linearity and wide dynamic range calls for a high-quality quadrature demodulatorone boasting outstanding amplitude and phase balance in the in-phase (I) and quadrature (Q) signal paths. This is difficult to achieve with a discrete component but, using integrated-circuit (IC) technology such as in the LTC5585 I/Q demodulator from Linear Technology, sterling performance is possible across the wireless and cellular band of interest from 700 MHz to 3 GHz. This direct-conversion quadrature demodulator can actually be used over frequencies extending as low as 400 to 700 MHz and as high as 3 to 4 GHz, with somewhat reduced performance.
The LTC5585 is housed in a 24-lead, 4 x 4 mm QFN surface-mount package. The I/Q demodulator chip includes balanced I and Q mixers; local oscillator (LO) buffer amplification; I and Q filters; a precision quadrature phase shifter; and on-chip input-second-order-intercept (IIP2) and DC offset calibration circuitry. The demodulator provides four analog control voltage interface pins for IIP2 and DC offset correction.
In addition, the IC features an integrated on-chip broadband transformer that couples input RF signals from a receiver's input port to the demodulator's internal circuitry using only simple off-chip inductive-capacitive (LC) impedance matching. The demodulator is designed to work with a receiver's low-noise amplifier (LNA), input filtering, and analog-to-digital converters (ADCs) and digital-to-analog converters (DACs; see figure).
The LTC5585 quadrature demodulator is capable of baseband output demodulation bandwidth of greater than 530 MHz. It can handle LO and input RF power levels as high as +20 dBm with typical input 1-dB compression of +16 dBm. The quadrature demodulator boasts typical input third-order intercept point (IIP3) of +25.7 dBm, with unadjusted IIP2 of typically +60 dBm and optimized IIP2 of typically +80 dBm. It sports a typical double-sideband (DSB) noise figure of 12.7 dB.
The quadrature demodulator IC features closely matched components for outstanding internal signal-processing characteristics. For example, the I/Q amplitude mismatch between channels is typically only 0.05 dB while the I/Q phase mismatch between channels is typically only 0.3 deg, both measured at 1.95 GHz. The demodulator, which delivers 2.4-dB typical conversion gain, exhibits -64 dBm typical LO-to-RF leakage from 700 to 3000 Mhz, with 45-dB typical image rejection and 60-dB typical RF-to-LO isolation.
The LTC5585 is well suited for any receiver within its frequency range that requires a wide, spurious-free dynamic range with outstanding linearity. it can be used for both commercial and military receiver designs and in a variety of different broadband communications systems. P&A: $5.98 each (1000 qty.); stock.
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