I/Q Demodulator Spans 0.7 To 3.0 GHz

March 14, 2012
This miniature quadrature demodulator offers outstanding linearity for communications receivers from 0.7 to 3.0 GHz, featuring extremely tight amplitude and phase balance.

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.

Linear Technology
1630 McCarthy Blvd.
Milpitas, CA 95035-7417
(408) 432-1900
FAX: (408) 434-0507

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