lt5538 Linear Technology Corporation, lt5538 Datasheet - Page 8

• Manufacturer Part Number: lt5538
• Description: 40mhz To 3.8ghz Rf Power Detector With 75db Dynamic Range
• Manufacturer: Linear Technology Corporation

LT5538
APPLICATIONS INFORMATION
The LT5538 is a 40MHz to 3.8 GHz logarithmic RF power
detector. It consists of cascaded limiting amplifi ers and
RF detectors. The output currents from every RF detector
are combined and low-pass fi ltered before applied to the
output buffer amplifi er. As a result, the fi nal DC output
voltage approximates the logarithm of the amplitude of the
input signal. The LT5538 is able to accurately measure an
RF signal over a 70dB dynamic range (–68dBm to 2dBm
at 2.1GHz) with 50Ω single-ended input impedance. The
slope of linear to log transfer function is about 17.7mV/dB
at 2.1GHz. Within the linear dynamic range, very stable
output is achieved over the full temperature range from
–40°C to 85°C and over the full operating frequency
range from 40MHz to 3.8GHz. The absolute variation over
temperature is typically within ±1dB over 65dB dynamic
range at 2.1GHz.
RF INPUT
The simplifi ed schematic of the input circuit is shown in
Figure 1. The IN
V
20pF capacitor. An external capacitor of 1nF is needed to
connect this pin to ground for low frequency operation.
The impedance between IN
RF input pin IN
to ground or other matching components. A 56Ω resistor
(R1) connected to ground will provide better than 10dB
input return loss over the operating frequency range up
to 1.5GHz. At higher operating frequency, additional LC
8
CC
–0.5V. The IN
Figure 1. Simplifi ed Schematic of the Input Circuit
IN
IN
+
–
+
+
should be DC blocked when connected
–
and IN
pin is internally coupled to ground via
–
+
pins are internally biased to
5.3k
394
and IN
20p
5.3k
–
5538 F01
is about 394Ω. The
V
CC
+
–
matching elements are needed for a proper impedance
matching to a 50Ω source as shown in Figure 2. Refer to
Figure 6 for the circuit schematic of the input matching
network. The input impedance vs frequency of the RF input
port IN
Table 1. RF Input Impedance
Figure 2. Input Return Loss with Additional LC Matching Network
FREQUENCY
(MHz)
40
100
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
3600
3800
+
is detailed in Table 1.
–25
–30
–10
–15
–20
–5
RF INPUT
IMPEDANCE (Ω)
47.3 + j129.7
246.6 + j210.7
408.7 – j37.8
192.9 – j190.9
105.6 – j158.4
69.3 – j127.4
51.8 – j106.2
41.5 – j90.9
34.2 – j78.7
29.2 – j60.0
25.4 – j60.7
22.6 – j53.8
20.5 – j47.7
18.9 – j42.4
17.9 – j37.6
17.1 – j33.4
16.4 – j29.5
16.1 – j26.0
15.9 – j22.8
15.9 – j20.0
15.9 – j17.5
0
0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4.0
FREQUENCY (GHz)
W/O L1 AND C8
L1 = 1.5nH, C8 = 1pF
C4, C11 = 12pF, C8 = 0.7pF
0.800
0.790
0.785
0.772
0.756
0.737
0.720
0.707
0.697
0.687
0.678
0.669
0.659
0.649
0.638
0.627
0.615
0.602
0.589
0.574
0.560
MAG
S11
5538 F02
ANGLE(°)
–101.5
–108.2
–114.7
–121.0
–127.0
–132.8
–137.9
–14.9
–25.3
–34.4
–42.7
–50.6
–58.2
–65.6
–73.1
–80.4
–87.7
–94.6
38.5
11.5
–1.5
5538f