Electronic Design

Quadrature Modulator Advances Basestation Performance

Perhaps the most critical circuit in a broadband wireless system today is the modulator that translates the I and Q baseband signals into RF and upconverts them to the final operating frequency. Such circuits must have extremely low intermodulation distortion and noise but produce some useful gain. This requirement is especially critical in 3G cell-phone basestations where multiple wideband signals must be combined.

Any quadrature modulator for new systems must exhibit some serious specifications related to frequency range, dynamic range, linearity, and stability. Analog Devices' F-MOD modulators have such specs to meet equipment vendors' requirements for multibands/ multistandards, lower costs per channel, and lower manufacturing costs.

The F-MOD series targets infrastructure equipment for 2.5G and 3G basestations, broadband wireless systems like WiMAX, and Cable Modem Termination System (CMTS) cable TV head-end products. Designated as the ADL537x series, these chips perform direct conversion in infrastructure transmitters. The I and Q baseband signals from the baseband digital-to-analog converters (DACs) feed filters that drive the modulators.

For direct conversion, the local oscillator will run at the desired output frequency. Yet the modulator also may be used in a more traditional architecture where the modulation is performed at a lower intermediate frequency and then upconverted to the final output frequency.

Here, the I and Q signals are mixed with the local oscillator (LO) signal from a frequency synthesizer. The analog input frequency range is an amazingly wide dc to 4000 MHz. Five parts are optimized for specific frequency ranges. The ADL5370 operates from 250 to 1300 MHz, the ADL5371 covers 700 to 1300 MHz, the ADL5372 covers 1600 to 2400 MHz, the ADL5373 covers 2400 to 3000 MHz, and the ADL5374 covers 3300 to 4000 MHz.

Besides the very low output noise floor, this series offers best-in-class output power. Operating at 2140 MHz, the ADL5372 has an adjacent channel power ratio (ACPR) of less than -75 dB with an output power of -13 dBm. These parts exhibit high linearity with high output power of P1 dB + 12 dBm and IP3 + 26 dBm. Noise is a low -158 dBm/Hz. The LO power required is +3 dBm.

The ADL537x series is an upgrade of Analog Devices' previous AD8345/49 line of modulators. The ADL537x series is pin-compatible with the AD8345/49. The semiconductor technology is complementary bipolar silicon-germanium (SiGe) silicon-on-insulator. Operating voltage is 4.75 to 5.5 V.

The accompanying ADL5322 and ADL5323 RF pre-driver amplifiers fit in the signal chain after the modulator. They're designed to be used as the first stage of the transmitter power amplifier. The gain is fixed at 20 dB ±0.25 dB, and input and output impedances are 50 ½. The thirdorder intercept (IP3) is 42 dBm, the high output power P1dB point is 28 dBm, and the noise figure is 4 dB. The ADL5322 covers 700 to 1000 MHz, while the ADL5323 covers 1700 to 2400 MHz.

These amplifiers' internal impedance matching and biasing components negate the need for external inductors, capacitors, resistors, or strip-line sections. Everything is built in, except the input and output coupling capacitors. The 3- by 3-mm lead-frame chip-scale package offers 24 pins.

Samples are available now, with production by July. The ADL537x costs $4.98 each and the ADL5322 and ADL5323 cost $3.48, all in lots of 1000.

Analog Devices Inc.

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