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ADL5501 Datasheet(Fiches technique) 18 Page - Analog Devices

Numéro de pièce ADL5501
Description  50 MHz to 4 GHz TruPwr Detector
Télécharger  28 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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ADL5501
Rev. 0 | Page 18 of 28
INPUT COUPLING USING A SERIES RESISTOR
Figure 43 shows a technique for coupling the input signal into
the ADL5501 that can be applicable where the input signal is
much larger than the input range of the ADL5501. A series
resistor combines with the input impedance of the ADL5501
to attenuate the input signal. Because this series resistor forms
a divider with the frequency dependent input impedance, the
apparent gain changes greatly with frequency. However, this
method has the advantage of very little power being tapped off
in RF power transmission applications. If the resistor is large
compared to the transmission line’s impedance, the VSWR of
the system is relatively unaffected.
ADL5501
RFIN
RFIN
RSERIES
Figure 43. Attenuating the Input Signal
The resistive tap or series resistance, RSERIES, can expressed as
RSERIES = RIN (1 − 10ATTN/20)/(10ATTN/20)
(1)
where:
RIN is the input impedance of RFIN.
ATTN is the desired attenuation factor in dB.
For example, if a power amplifier with a maximum output power
of +28 dBm is matched to the ADL5501 input at +5 dBm, then
a −23 dB attenuation factor is required. At 900 MHz, the input
resistance, RIN, is 55 Ω.
RSERIES = (55 Ω)(1 − 10−23/20)/(10−23/20) = 722 Ω
(2)
Thus, for an attenuation of −23 dB, a series resistance of
approximately 722 Ω is needed.
MULTIPLE RF INPUTS
Figure 44 shows a technique for combining multiple RF input
signals to the ADL5501. Some applications can share a single
detector for multiple bands. Three 16.5 Ω resistors in a T-network
combine the three 50 Ω terminations (including the ADL5501).
The broadband resistive combiner ensures each port of the
T-network sees a 50 Ω termination. Because there are only 6 dB
of isolation from one port of the combiner to the other ports,
only one band should be active at a time.
ADL5501
RFIN
BAND 1
50Ω
BAND 2
DIRECTIONAL
COUPLER
16.5Ω
50Ω
16.5Ω
16.5Ω
DIRECTIONAL
COUPLER
Figure 44. Combining Multiple RF Input Signals
SELECTING THE SQUARE-DOMAIN FILTER AND
OUTPUT LOW-PASS FILTER
The internal filter capacitor of the ADL5501 provides averaging
in the square domain but leaves some residual ac on the output.
Signals with high peak-to-average ratios, such as W-CDMA or
CDMA2000, can produce ac-residual levels on the ADL5501
dc output. To reduce the effects of these low frequency components
in the waveforms, some additional filtering is required.
The square-domain filter capacitance of the ADL5501 can be
augmented by connecting a capacitor between Pin 2 (FLTR) and
Pin 1 (VPOS). In addition, the output of the ADL5501 can be
filtered directly by placing a capacitor between VRMS (Pin 6)
and ground. The combination of the on-chip, 100 Ω output
series resistance and the external shunt capacitor forms a low-
pass filter to reduce the residual ac.
Table 4 shows the effect of several capacitor values for various
communications standards with high peak-to-average ratios
along with the residual ripple at the output, in peak-to-peak and
rms volts. Note that large load capacitances increase the turn-on
and pulse response times (see Figure 30, Figure 32, Figure 33,
Figure 35, Figure 37, and Figure 38). For more information on
the effects of the filter capacitances on the response, see the
Power Consumption, Enable, and Power-On/-Off Response
Time section.




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