Electronic Design
Passive Mixers Offer Wideband Coverage With Superior Noise And Linearity

Passive Mixers Offer Wideband Coverage With Superior Noise And Linearity

The mixer is key to building critical wireless equipment like cellular basestations that cover the latest bands, Long-Term Evolution (LTE), and other 4G technologies. Many active mixer ICs are available to provide the needed wide bandwidth and gain. But for some applications, the noise and linearity specifications don’t live up to the need.

Passive mixers do a better job with noise and linearity, but they offer less gain and narrower bandwidths. The Analog Devices ADL5811 single mixer circuit and ADL5812 dual mixer circuit, though, are wideband passive FET mixers that surpass those limitations (see the figure). Made with biCMOS silicon germanium (SiGe), they provide the superior noise and linearity characteristics desired in mission-critical high-end mixers.

On-chip tunable input baluns and IF filters provide the wide bandwidth characteristics, allowing one device to cover any input frequency range from 700 to 2800 MHz. The local oscillator (LO) frequency can be above or below the input frequency from 250 MHz to 2760 MHz. The IF output range is 40 to 450 MHz.

The mixer inputs are single-ended with a 50-Ω impedance and programmable for the desired frequency range. The input balun can be programmed via a digital serial peripheral interface (SPI). The mixer output feeds a programmable low pass filter on-chip and an IF amplifier. The LO input is single-ended with a 50-Ω input impedance, and it can be as low as 0 dBm.

These new mixers have two very useful additional features. First, their mixer bias adjustment input allows designers to tune the bias for optimizing the tradeoff between third-order intercept (IP3) and noise figure (NF). Second, a control pin is provided to shut down each mixer if it is not being used.

Critical specifications include a power conversion gain of 7 dB, IP3 of 24 dBm over the full RF bandwidth, and input compression (P1dB) of 11 dBm again over the full RF bandwidth. The single sideband (SSB) noise figure is 11 dB.

The ADL5811 and ADL5812 are available in sample quantities now with full production available in the third quarter of 2011. The ADL5811 costs $8.03 and the ADL5812 costs $10.98, both in 1000-unit lots. Evaluation boards are also ready for shipment today.

Analog Devices Inc.  
www.analog.com

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