TA0027 Datasheet, PDF(2/4 Page) RF Micro Devices – Intergrated, Single-Step, 10dB Attenuator for Power Control Applications

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TA0027 Datasheet, PDF (2/4 Pages) RF Micro Devices – Intergrated, Single-Step, 10dB Attenuator for Power Control Applications

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TA0027

Fig. 2. RF2421 VSWR over Frequency

Even in the attenuation mode, the effect on noise figure

would be minimal since the attenuator's noise figure

would be the third term in the cascade NF equation.

In PCS systems, the base stations may be fed from

multiple picocells that are physically separated from it

by up to a hundred feet or more of coaxial cable. The

signal levels coming into the base station will vary

depending on the cable length and individual transpon-

der power. It is desirable to keep the signals at a uni-

form level at the base station, and to do so, it may be

necessary to attenuate the stronger signals. Again, a

step attenuator can be easily dropped in to perform this

function.

The upper end of a receiver's dynamic range is deter-

mined by the largest signal it can handle without being

overdriven and producing unacceptable levels of distor-

tion caused by nonlinearity. Switching in an attenuator

before a low noise amplifier (LNA) in the presence of

strong, in-band signals allows reception by preventing

them from overdriving the receiver front end. This

effectively shifts the dynamic range upward by the

amount of the attenuation. For this application, it is

important that the third-order intercept point (IP3) per-

formance of the attenuator in the off-attenuation mode

is better than that of the LNA so it does not worsen the

receiver's linearity.

As a gain control element at the input of an amplifica-

tion stage, the step attenuator would offer the added

benefit of reducing the VSWR by reducing the return

loss. In many cases, only one or two dB of additional

isolation is needed to improve amplifier instability or to

help isolate a poorly matched load from a source.

When the attenuator is active, it is actually a pad since

it has the same input and output characteristic imped-

ance. It presents itself to the signal as a lossy trans-

13-128

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