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Designing a wireless base station transmitter chain? Check out ADI's broadband VGA

Posted: 19 Jan 2005 ?? ?Print Version ?Bookmark and Share

Keywords:analog devices? adi? adl5330? variable gain amplifier? vga?

Analog Devices Inc. (ADI) has a reputation for solid, industry-leading design work, and its new ADL5330 variable gain amplifier looks to follow in that tradition. Unlike its other VGAs, this is the company's first variable gain amplifier/attenuator (VGA) specifically made for transmitter applications. This type of VGA is designed to work with a mostly constant input power level from a modulator or a mixer, and then it adjusts the output power level/gain depending on how much is needed.

With an eye on performance and broad application appeal, the team at ADI opted for a wide 1MHz to 3GHz frequency range and a 60dB gain control range. The challenge, of course was to achieve these specifications without getting too close to the noise floor.

"One of the most difficult aspects of designing this chip was to cascade multiple functions within a single IC," notes Phil Halford, Product Marketing Manger for RF. "We had to consider the noise floor issue and keep an eye on output linearity because we were close to the final stage of the transmitter card. We didn't want to sacrifice any dynamic range."

The chip integrates an amplifier stage, a variable attenuator, and an output amplifier stage. Alternatives to its use include discrete solutions or multi-chip modules, which must contend with cost, power and board space issues.

In order to achieve the output power levels (22dBm at 1dB compression) and its 60dB dynamic range, the chip's designers chose to use a differential input and output, requiring the use of an external balun when used in common single-ended 50-ohm environments. (It can be used single-ended without a balun, but the performance will not be as good.)

A key to cellular base station transmitter design is having the ability to control output power and gain, and it is easier to do this before it goes into the power amplifier stage. "Use of this product allows designers to meet the tolerances required and adjust gain to deliver the signal to the output PA," says Halford. "Perhaps they need to adjust gain to set a cell site radius or to account for changes over temperature in the signal chain."

Many designers use voltage variable or digital attenuators followed by fixed gain amplifiers to realize this function. About two years ago, the team at ADI decided to get the functionality on a single chip.

When characterizing the ADL5330, the design team tested this part to ensure relevant signal quality and performance for some of ADI's customers who were developing wideband-CDMA base stations. They also showed it to some customers working with lower-frequency point to point microwave radio links. "They are particularly excited because it offers them a level of integration and precision control that only an analog variable gain amplifier can provide," adds Halford.

The VGA offers up to 20dB of gain, which ADI believes is right on the money for what their customers want in a single stage. If you are looking for a space-saving, low-noise VGA that offers a wide gain control range for your basestation transmitter design, then be sure to give this one a closer look.

- Janine Love


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