LMV228SD National Semiconductor, LMV228SD Datasheet - Page 23

• Manufacturer Part Number: LMV228SD
• Manufacturer: National Semiconductor

Specifications of LMV228SD

• Pin Count: 6
• Screening Level: Industrial
• Package Type: LLP EP
• Lead Free Status / Rohs Status: Not Compliant

Application Notes
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
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
to the turn-on time of the device.
The LMV226/LMV228 do not use a resistor R
LMV225. Though a resistor is seen on the coupler side
(R
LMV226/LMV228 LF corner frequency, where R
with the coupler output impedance (R
With R
frequency is 50 MHz.
The output voltage is proportional to the logarithm of the
input power, often called “linear-in-dB”. Figure 3 shows the
typical output voltage versus PA output 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:
In which V
amplitude modulation µ(t) can be between -1 and 1.
FIGURE 3. Typical power detector response, V
COUPLER
COUPLER
IN
). Therefore a similar equation holds for the
V
is the amplitude of the carrier frequency and the
IN
(t) = V
= 50Ω and C = 100 pF, the resulting corner
PA output Power
IN
[1 + µ(t)] cos (2 · π · f · t)
(Continued)
COUPLER
).
1
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is replaced
1
OUT
like the
2
, adds
IN
vs.
(4)
to
23
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
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:
where V
is the modulation index. Equation (5) can be reduced to:
Consequently, the ripple is independent of the average input
power of the RF input signal and only depends on the
logarithmic slope V
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
the LMV225/LMV228 is linear in dB, or proportional to the
input power P
modulation in the order of 5 to 6 dB. Since the transfer is
linear in dB, the output voltage V
about 5 to 6 dB in the curve (Figure 5).
Y
IN
is the slope of the detection curve (Figure 5) and µ
FIGURE 4. AM Modulated RF Signal
• (1+µ) and a minimum amplitude V
IN
in dBm. As discussed earlier, CDMA has a
Y
and the ratio of the maximum and the
OUT
will vary linearly over
www.national.com
IN
• (1-µ),
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OUT
(5)
(6)
of