lt5581 Linear Technology Corporation, lt5581 Datasheet - Page 14

• Manufacturer Part Number: lt5581
• Description: 6ghz Rms Power Detector With 40db Dynamic Range
• Manufacturer: Linear Technology Corporation

LT5581
Figure 10 shows that rise time and fall time are strong
functions of RF input power. Data is taken without the
output fi lter capacitor.
For a given RF modulation type—WCDMA, for exam-
ple—the internal 150kHz fi lter provides nominal fi ltering
of the residual ripple level. Additional external fi ltering
occurs in the log domain, which introduces a systematic
log error in relation to the signal’s crest factor, as shown
in the following equation in dB.
Where CF is the crest factor and r is the duty cycle of the
measurement (or number of measurements made at the
APPLICATIONS INFORMATION
14
Error|dB = 10 • log
Figure 12. Output Voltage Noise Density
Figure 10. RF Pulse Response Rise Time
and Fall Time vs RF Input Power
2.5
4.0
3.5
3.0
2.0
1.5
1.0
0.5
9
8
6
5
4
0
7
3
2
1
0
–30
0.1
T
T
A
A
= 25°C
= 25°C
–25
0dBm
–10dBm
–20dBm
–30dBm
NO RF INPUT
10
–20
1
INPUT POWER (dBm)
(r + (1 – r)10
FREQUENCY (kHz)
–15
RISE TIME
FALL TIME
10
–10
1
–5
100
–CF/10
0
5581 F10
5581 F12
) – CF • (r-1)
1000
5
peak envelope, divided by the total number of periodic
measurements in the measurement period). It is impor-
tant to note that the CF refers to the 150kHz low pass
fi ltered envelope of the signal. The error will depend on
the statistics and bandwidth of the modulation signal in
relation to the internal 150kHz fi lter. For example, in the
case of WCDMA, simulations prove that it is possible to
set the external fi lter capacitor corner frequency at 15kHz
and only introduce an error less than 0.1dB.
Figure 11 depicts the output AM modulation ripple as a
function of modulation difference frequency for a 2-tone
input signal at 2140MHz with –10dBm input power. The
1
Steve Murray, “Beware of Spectrum Analyzer Power Averaging Techniques,” Microwaves
& RF, Dec. 2006.
Figure 11. Output DC Voltage Deviation and Residual
Ripple vs 2-Tone Separation Frequency
Figure 13. Integrated Output Voltage Noise
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
30
25
20
15
10
0.001
5
0
0
0.1
T
T
A
A
2-TONE FREQUENCY SEPARATION (MHz)
= 25°C
= 25°C
0dBm
–10dBm
–20dBm
–30dBm
NO RF INPUT
0.01
1
FREQUENCY (kHz)
0.1
10
100
1
5581 F11
5581 F13
1000
10
0
–0.5
–1.5
–2.0
–2.5
–3.0
–1.0
5581f