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參數(shù)資料
| 型號： | LMV228 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | RF Power Detector for CDMA and WCDMA |
| 中文描述： | RF功率檢測器的CDMA和WCDMA |
| 文件頁數(shù)： | 17/21頁 |
| 文件大?。?/td> | 1090K |
| 代理商： | LMV228 |
Application Notes
(Continued)
number. The goal is to have a magnitude transfer, which is
sufficiently flat in the used frequency range; capacitor C
should be chosen significantly larger than capacitor C
IN
to
assure a proper performance of the high resistive tap. Ca-
pacitor C shouldn’t be chosen excessively large since the
RC-time, it introduces in combination with resistor R
2
, adds
to the turn-on time of the device.
The LMV226/LMV228 do not use a resistor R
like the
LMV225. Though a resistor is seen on the coupler side
(R
). Therefore a similar equation holds for the
LMV226/LMV228 LF corner frequency, where R
1
is replaced
with the coupler output impedance (R
COUPLER
).
With R
= 50
and C = 100 pF, the resulting corner
frequency is 50 MHz.
The output voltage is proportional to the logarithm of the
input power, often called “l(fā)inear-in-dB”.
Figure 3
shows the
typical output voltage versus PAoutput power of the LMV225
setup as depicted in
Figure 1
.
OUTPUT RIPPLE DUE TO AM MODULATION
A CDMA modulated carrier wave generally contains some
amplitude modulation that might disturb the RF power mea-
surement used for controlling the PA. This section explains
the relation between amplitude modulation in the RF signal
and the ripple on the output of the LMV225/LMV228. Expres-
sions are provided to estimate this ripple on the output. The
ripple can be further reduced by lowpass filtering at the
output. This is realized by connecting an capacitor from the
output of the LMV225/LMV228 to ground.
Estimating Output Ripple
The CDMA modulated RF input signal of
Figure 3
can be
described as:
V
IN
(t) = V
IN
[1 + μ(t)] cos (2 ·
π
· f · t)
In which V
is the amplitude of the carrier frequency and the
amplitude modulation μ(t) can be between -1 and 1.
(4)
The ripple observed at the output of the detector equals the
detectors response to the power variation at the input due to
AM modulation (
Figure 4
). This signal has a maximum am-
plitude V
(1+μ) and a minimum amplitude V
(1-μ),
where 1+μ can be maximum 2 and 1-μ can be minimum 0.
The amplitude of the ripple can be described with the for-
mula:
(5)
where V
is the slope of the detection curve (
Figure 5
) and μ
is the modulation index.
Equation (5)
can be reduced to:
(6)
Consequently, the ripple is independent of the average input
power of the RF input signal and only depends on the
logarithmic slope V
and the ratio of the maximum and the
minimum input signal amplitude.
For CDMA, the ratio of the maximum and the minimum input
signal amplitude modulation is typically in the order of 5 to 6
dB, which is equivalent to a modulation index μ of 0.28 to
0.33.
A further understanding of the equation above can be
achieved via the knowledge that the output voltage V
of
the LMV225/LMV228 is linear in dB, or proportional to the
input power P
in dBm. As discussed earlier, CDMA has a
modulation in the order of 5 to 6 dB. Since the transfer is
linear in dB, the output voltage V
will vary linearly over
about 5 to 6 dB in the curve (
Figure 5
).
20076016
FIGURE 3. Typical power detector response, V
OUT
vs.
PA output Power
20076017
FIGURE 4. AM Modulated RF Signal
L
www.national.com
17
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LMV228SD | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:RF Power Detector for CDMA and WCDMA |
| LMV228SD/NOPB | 功能描述:射頻檢測器 RoHS:否 制造商:Skyworks Solutions, Inc. 配置: 頻率范圍:650 MHz to 3 GHz 最大二極管電容: 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 封裝 / 箱體:SC-88 封裝:Reel |
| LMV228SDEVAL | 功能描述:BOARD EVALUATION FOR LMV228SD RoHS:否 類別:RF/IF 和 RFID >> RF 評估和開發(fā)套件，板 系列:- 標準包裝:1 系列:- 類型:GPS 接收器 頻率:1575MHz 適用于相關(guān)產(chǎn)品:- 已供物品:模塊 其它名稱:SER3796 |
| LMV228SDX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:RF Power Detector for CDMA and WCDMA |
| LMV228SDX/NOPB | 功能描述:射頻檢測器 RoHS:否 制造商:Skyworks Solutions, Inc. 配置: 頻率范圍:650 MHz to 3 GHz 最大二極管電容: 最大工作溫度:+ 85 C 最小工作溫度:- 40 C 封裝 / 箱體:SC-88 封裝:Reel |
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