Electronic Design

RF/IF VGA Chip Does It All

According to Maxim Integrated Products, the MAX2065 fully programmable, multistate, analog and digital IF/RF variable-gain amplifier (VGA) aims to solve a number of automatic gain control (AGC) design problems in GSM/EDGE, CDMA, WCDMA, LTE, and WiMAX receiver applications (see the figure). But what does that mean?

In explaining the thinking that went into the design of the chip, Maxim says its engineers started by looking at today’s gain trim and AGC circuits in those apps. Typically, they involve a lot of separate chips—analog voltage and digitalcontrolled attenuators, RF/IF amplifiers, and associated control circuitry. In lieu of all that, Maxim’s engineers packed a linearly controlled 31-dB voltage-variable attenuator, a 31-dB digital step-attenuator, a 22-dB gain driver amplifier, an on-chip 8-bit control digital-to-analog converter (DAC), and a simple SPI-compatible interface into one chip.

Designers can use the MAX2065 as either an IF or RF all-purpose VGA, interfacing directly with 50-O systems operating anywhere between 50 and 1000 MHz. Each of the three independent RF stages has its own RF input and RF output, so the chip can be configured to optimize either noise figure or linearity.

If a designer were to follow the chip’s digital attenuator with its driver amplifier, thereby optimizing linearity, the total gain range would be on the order of 62 dB, with a maximum gain of 19.4 dB, and a 6.5-dB noise figure. The thirdorder intercept would be +42 dBm, second order, +63 dBm, and first order, +19 dBm. Harmonic distortion would be –67 dBc (second order) and –83 dBc (third order).

The SPI can be used to control the 5-bit digital attenuator and the analog attenuator through the on-chip 8-bit DAC. With that combination, it’s possible to adjust the analog attenuation in 0.12-dB increments. It’s also possible to disable the DAC.

The MAX2065 is designed for fastattack AGC functionality as well by limiting amplitude overshoot/undershoot during attenuation transitions to a maximum of 0.05 dB between any two attenuation states over an elapsed time of only 40 ns. To capitalize on this capability, Maxim included a supplemental 5-bit, parallel-control interface that avoids SPI latencies.

Beyond that, an even faster “rapid-fire” gain selection enables even faster toggling among four preprogrammed attenuation steps. “Assume that the AGC application requires a static attenuation adjustment to trim out gain inconsistencies within a receiver lineup. The same AGC circuit may also be required to dynamically attenuate an unwanted blocker signal that could de-sense the receiver and lead to an ADC overdrive condition,” Maxim explains.

“In this example, the MAX2065 would be preprogrammed (through the SPI bus) with two customized attenuation states: one state to address the static gain-trim adjustment, and the other state to counter the unwanted blocker condition,” the company continues.

“The user can also program two additional attenuation states by using the STATE_B control bit as a second I/O pin. These additional attenuation settings are useful for software-defined radio applications where multiple static gain settings may be needed to account for different operating frequencies, or where multiple dynamic attenuation settings are needed to account for different blocker levels (as defined by multiple wireless standards),” Maxim says.

The MAX2065 is available in a 40-TQFN (thin quad flat no-lead) lead-free package. Prices start at $7.48 in quantities of 1000 and up.

DON TUITE

MAXIM INTEGRATED PRODUCTSwww.maxim-ic.com

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