ltc5587 Linear Technology Corporation, ltc5587 Datasheet - Page 16

• Manufacturer Part Number: ltc5587
• Description: Ltc5587 6 Ghz Rms Power Detector With Digital Output Features
• Manufacturer: Linear Technology Corporation

LTC5587
APPLICATIONS INFORMATION
For a given RF modulation type, WCDMA for example,
the internal 150kHz filter provides nominal filtering of the
residual ripple level. Additional external filtering happens
in the log-domain, which introduces a systematic log-er-
ror in relation to the signal’s crest factor as shown in the
following equation in dB
Error|
Where CF is the crest factor and r is the duty cycle of the
measurement (or number of measurements made at the
peak envelope divided by the total number of periodic
measurements in the measurement period). It is important
to note that the CF refers to the 150kHz low-pass filtered
envelope of the signal. The error will depend on the statis-
tics and bandwidth of the modulation signal in relation to
the internal 150kHz filter. For example: simulations have
shown for the case of WCDMA that it is possible to set
the external filter capacitor corner frequency at 15kHz and
only introduce an error less than 0.1dB.
Figure 10 shows the output AC modulation ripple as a
function of modulation difference frequency for a 2-tone
input signal at 2140MHz with –10dBm input power. The
resulting deviation in the output voltage of the detector
shows the effect of the internal 150kHz filter.
Figure 10. Output DC Voltage Deviation and Residual Ripple vs
2-Tone Separation Frequency
1. Steve Murray, “Beware of Spectrum Analyzer Power Averaging Techniques,” Microwaves
& RF, Dec. 2006.
16
dB
= 10 • log
30
25
20
15
10
0.001
5
0
T
A
2-TONE FREQUENCY SEPARATION (MHz)
= 25°C
10
0.01
(r + (1-r)10
1
:
0.1
–CF/10
1
) – CF • (r-1)
5587 F10
10
0
–0.5
–1.5
–2.0
–2.5
–3.0
–1.0
The output voltage noise density and integrated noise are
shown respectively in Figures 11 and 12 for various input
power levels. The noise is a strong function of input level
and there is roughly a 10dB improvement in the output
noise level for an input level of 0dBm versus no input.
Figure 12. Integrated Output Voltage Noise
Figure 11. Output Voltage Noise Density
1.0
2.0
1.8
1.6
1.4
1.2
0.8
0.6
0.4
0.2
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
0.1
0.1
T
T
A
A
= 25°C
= 25°C
0dBm
–10dBm
–20dBm
–30dBm
NO RF INPUT
0dBm
–10dBm
–20dBm
–30dBm
NO RF INPUT
1
1
FREQUENCY (kHz)
FREQUENCY (kHz)
10
10
100
100
5587 F12
5587 F11
1000
1000
5587f