The Linear Technology LTC5585 wideband high-linearity I/Q demodulator targets LTE and 4G basestations. Its 530-MHz bandwidth and adjustable IIP2 and dc offset let designers optimize their receiver designs, including those for DPD service. An integrated balun transformer eliminates most external components.

Modern basestation designers face increasingly difficult decisions because of the need to deal with a mix of modern needs such as wide operating frequency range, multimode signal handling, superior linearity, and wide bandwidth. To handle Long-Term Evolution (LTE) signals as well as older 3G technologies, the receiver must have a mix of capabilities heretofore unavailable. Yet Linear Technology’s LTC5585 I/Q demodulator brings a real solution with just the right mix of wide bandwidth and linearity to satisfy any designer (see the figure).

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This ultra-wide-bandwidth direct conversion I/Q demodulator operates from 700 MHz to 3 GHz, covering virtually all cellular basestation frequency bands. Its baseband output demodulation bandwidth tops 530 MHz, meaning it can support the bandwidth requirements of the latest-generation wideband LTE multimode receivers as well as the stringent requirements of digital pre-distortion (DPD) receivers. And, the LTC5585 has outstanding linearity. Typical performance figures include a third-order intercept (IIP3) of 25.7 dBm and a second-order intercept (IIP2) of 60 dBm at 1.95 GHz.

The LTC5585 also has two unique built-in calibration features. First, its advanced circuitry enables you to optimize the receiver’s IIP2 performance, increasing from a nominal 60 dBm to an unprecedented 80 dBm or higher. Second, its on-chip circuitry nulls out the dc offset voltages at the I and Q outputs. Both can enhance receiver performance. In addition, the LTC5585 provides an excellent 1-dB gain compression measurement (P1dB) specification of 16 dBm.

Another key feature to enhance its use in direct conversion receiver applications is its very low I/Q amplitude and phase mismatch. The amplitude mismatch is typically 0.05 dB, while the phase error is typically 0.7°, both measured at 1.95 GHz. This combination produces a receiver image rejection capability of 43 dB. Other specifications include a conversion gain of 2.4 dB and a noise figure (NF) of 12.7 dB. The LTC5585 also has an on-chip RF transformer to reduce external components.

Because of its very wide bandwidth capability, the LTC5585 is especially well suited for multimode LTE, W-CDMA, and TD-SCDMA basestations, and DPD receivers as well as for main receiver applications. The LTC5585 is particularly suited to DPD receiver service because its wide bandwidth allows up to fifth harmonic distortion to be passed and compensated for.

Most new basestations use DPD to help linearize the power amplifiers. A separate receiver is needed in a feedback path to detect and process the harmonic distortion. This pushes the demodulation bandwidth requirement of more than 300 MHz. Besides basestation applications, the LTC5585 is a good receiver solution for applications in military receivers, broadband communications, point-to-point microwave data links, image reject receivers, and long-range RFID readers.

The LTC5585 is powered from a single 5-V supply, drawing a total supply current of 200 mA. It provides a digital input to enable or disable the chip. When disabled, the IC draws typically 11 μA of leakage current. The demodulator’s fast turn-on time of 200 ns and turn-off time of 800 ns enables it to be used in burst-mode receivers.

Packaged in a 24-lead, 4- by 4-mm quad flat no-lead (QFN) package, the LTC5585 and is specified for a case operating temperature from –40°C to 105°C. Prices start at $5.98 each in 1000-piece quantities. Production quantities are immediately available.

Linear Technology Corp.
www.linear.com/product/LTC5585