lt5570 Linear Technology Corporation, lt5570 Datasheet - Page 11

• Manufacturer Part Number: lt5570
• Description: Fast Responding, 40mhz To 2.7ghz Mean-squared Power Detector
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The LT5570 is a mean-squared RF power detector, capable
of measuring an RF signal over the frequency range from
40MHz to 2.7GHz, independent of input waveforms with
different crest factors such as CW, CDMA, WCDMA, TD-
SCDMA and WiMAX signals. A wide dynamic range is
achieved with very stable output within the full temperature
range from –40˚C to 85˚C.
RF Inputs
The differential RF inputs are internally biased at
V
These pins should be DC blocked when connected to
ground or other matching components. The impedance
vs. frequency of the differential RF input is detailed in the
following table.
Table 1. RF Differential Input Impedance
FREQUENCY
CC
(MHz)
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
100
200
400
600
800
– 1.224V. The differential impedance is about 200Ω.
40
DIFFERENTIAL INPUT
IMPEDANCE (Ω)
202.8 –j 10.9
201.8 –j 14.5
200.6 –j17.9
199.1 –j21.3
197.3 –j24.7
195.4 –j27.9
193.2 –j31.1
190.8 –j34.2
188.2 –j37.4
185.3 –j40.4
181.9 –j43.5
178.3 –j46.4
174.4 –j49.3
203.5 –j 7.3
204 –j 0.6
204 –j 1.8
204 –j 3.6
0.606
0.606
0.606
0.606
0.605
0.604
0.603
0.602
0.601
0.599
0.598
0.596
0.593
0.591
0.589
0.586
0.582
MAG
S11
ANGLE (°)
–0.1
–0.3
–0.5
–1.1
–1.6
–2.1
–2.7
–3.2
–3.8
–4.4
–5.0
–5.6
–6.2
–6.9
–7.6
–8.4
–9.2
The LT5570’s differential inputs are optimally driven from
a fully balanced source. When the signal is from a single-
ended 50Ω source, conversion to a differential signal is
required to achieve the maximum dynamic range. This
is best achieved using a 1:4 balun to match the internal
200Ω input impedance as shown in Figures 1 and 3. This
impedance transformation results in 6dB voltage gain. At
high frequency, additional LC elements may be needed
for input impedance matching due to the parasitics of the
transformer and PCB trace.
The approximate RF input power range of the LT5570 is
60dB at frequencies up to 900MHz, even with high crest
factor signals such as a 4-carrier W-CDMA waveform.
However the minimum detectable RF power level degrades
as the input RF frequency increases.
Due to the high RF input impedance of the LT5570, a
narrow band L-C matching network can be used for the
conversion of a single-ended to balanced signal as well.
By this means, the sensitivity and overall linear dynamic
range of the detector remain the same, without using an
RF balun.
The LT5570 can also be driven in a single-ended con-
fi guration. Figure 5 shows the simplifi ed circuit of this
single-ended confi guration. The DEC Pin is preferably ac-
coupled to ground via a capacitor rather than left fl oating.
INPUT
RF
Figure 5. Single-Ended Input Confi guration
100
1nF
1nF
IN
DEC
IN
+
–
100
100
LT5570
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