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PDF TA0027 Data sheet ( Hoja de datos )
| Número de pieza | TA0027 | |
| Descripción | 10dB Attenuator | |
| Fabricantes | RF Micro Devices | |
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TA0027
TA0027
RF2421: Integrated, Single-Step, 10dB Attenuator for Power Control
Applications
Wireless communication is continuing to expand into
all areas of the business, government, and consumer
sectors. New markets are opening around the world for
products that will meet specialized requirements. To
build such products, designers rely on having access
to ever more sophisticated and integrated devices.
One particular type of device that has not seen a great
deal of development is the surface mount, integrated
attenuator. They are useful in reducing the power level
of RF signals by a specific amount. There are two gen-
eral classes - step and variable. Step attenuators allow
one or more discrete levels of attenuation and are set
by one or more control inputs. The variable class
allows a specific value of attenuation to be selected
from an overall range via an analog control voltage.
They are more flexible than step attenuators as they
allow any amount of attenuation to be set but the cost
is greater circuit complexity. In both types, when no
attenuation is selected, the attenuation is just the inser-
tion loss of the device. The simplest type, and the focus
of this article, is the single-step attenuator which needs
only one control input.
Integrated attenuators offer several advantages over
solutions using other types of devices. They are small,
consume very little power, and can handle reasonable
input power levels. Their small size makes them very
attractive when trying to preserve or reduce printed cir-
cuit board space. Being designed with field effect tran-
sistor (FET) technology, they can function while
drawing very low levels of current. Other approaches to
providing the same function would include the use of
discrete semiconductors, PIN diodes, or resistive net-
works in combination with active, discrete switches or
relays. Each of these alternatives has drawbacks in
cost, size, or performance when compared with the
integrated solution.
An integrated attenuator is internally composed of a
resistive attenuator network, in either a 'pi' or 'T' config-
uration, and active switches for signal routing. Figure 1
shows an equivalent circuit for a 'pi' attenuator along
with the standard equations for first-order calculation of
the resistor values. One example of this kind of compo-
nent is the RF2421 from RF Micro Devices. It is a
S2
RB
RF IN
RF OUT
RA = RO(N+1)/(N-1)
RB = RO(N2-1)/2N
Where: N = VIN / VOUT
RO = Impedance
RA
S1
RA
RO
S1
Figure 1. Single-Step Attenuator Equivalent Circuit
monolithic, GaAs MESFET, switched attenuator in an
SOP-8 style package. It has a digitally controlled, sin-
gle-step, typical attenuation of 10dB. The supply volt-
age range is 2.7V to 6V DC and no negative supply
voltages are required. The current consumption is less
than 1uA with no attenuation and is typically less than
500uA in the attenuation mode.
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The input and output of the device have a low voltage
standing wave ratio (VSWR) 50Ω match (Fig. 2) and
the RF output can drive up to +16dBm. It has 1dB typi-
cal insertion loss over the specified 500 to 3,000MHz
operating frequency range and good flatness in any
50MHz band (Fig. 3). The resistors are nickel chro-
mium and provide excellent temperature stability. The
RF ports are reversible, i.e., the input signal can be
applied to either port. The attenuation control pin has
an internal pull-down resistor which causes the attenu-
ator to be turned off when it is not connected.
Typical uses for this type of part include local oscillator,
IF and RF level control throughout communications,
measurement, and control circuits. Due to its flat, wide-
band response, it has little effect on phase and group
delay and thus introduces minimal distortion. One
radio designer required a way to reduce the output of
the receive mixer in a CDMA base station prior to the
IF amplifier. He found the RF2421 to be an ideal solu-
tion. Also, to provide one step of transmit level control
with little degradation of the noise figure (NF), it could
be used in a CDMA handset transmit chain between
the upconverter and the bandpass filter (Fig. 4). Since
the device is purely passive, it produces no additive
noise and the NF is, essentially, the insertion loss.
13
Copyright 1997-2000 RF Micro Devices, Inc.
13-127
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