Electronic Design

Quadrature Demodulator ICs Achieve Linearity On Par With Passives

I/Q DEMODULATOR ICs (or quadrature demodulators) have been available for some time, but they've generally lacked the high linearity required in some high-performance radio designs. Yet this has changed with Linear Technology's LT5515/5516 direct conversion quadrature demodulators.

These parts boast an input IP3 of +20 dBm at 1.9 GHz, which may be nearly 10 dB better than other readily available quadrature demodulator ICs. In addition, input IP2 is 50 dBm. And, these demodulator chips offer a choice of input frequency ranges: 1.5 to 2.5 GHz (LT5515) or 0.8 to 1.5 GHz (LT5516).

By overcoming the linearity roadblock, the LT5515/5516 will make it possible for designs that currently must rely on passive demodulator designs to experience the integration and performance benefits of the active demodulator approach. The demodulator ICs combine I and Q mixers with a local oscillator (LO) buffer, a signal splitter, a quadrature generator, input amplifiers, and channel filters on the I and Q outputs. This level of integration is achieved in a 4- by 4-mm QFN package (see the figure).

But small size isn't the only benefit of the active demodulator approach. Unlike passive demodulators, the demodulator chips can operate with a low LO drive level of just −5 dBm. Also, the I and Q outputs are tightly matched with respect to gain and phase. Gain mismatch is 0.3 dB max between channels, while phase mismatch is typically 1°.

High port-to-port isolation is another advantage of the active approach. Linear's quadrature demodulators specify a typical LO to RF leakage of −46 dBm. Meanwhile, isolation in the other direction (RF to LO) is specified at 46 dB typical. Although not shown on the data sheet, Linear says that the LO to IF leakage is so low, the company can't measure it. Isolation from the RF to IF ports is similarly high. (On passive I/Q demodulators, these parameters would be measurable.)

One tradeoff of the active demodulator approach may be current consumption. The ICs draw 160 mA max (125 mA typical) at 5 V. Nevertheless, this value can be reduced to 20 µA or less in the shutdown mode.

In lots of 1000, unit pricing starts at $6.75 for the LT5515 and $7.40 for the LT5516.

Linear Technology
[email protected]
(800) 4-LINEAR

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