LMV225 datasheet(17/21 Pages) NSC

17 / 21 page

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

1 like the

LMV225. Though a resistor is seen on the coupler side

COUPLER).

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

COUPLER =50

Ω 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 “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(t)=VIN [1 + µ(t)] cos (2 ·

π ·f·t)

(4)

In which V

IN is the amplitude of the carrier frequency and the

amplitude modulation µ(t) can be between -1 and 1.

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

IN • (1+µ) and a minimum amplitude VIN • (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

Y 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

Y 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

OUT of

the LMV225/LMV228 is linear in dB, or proportional to the

input power P

IN 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

OUT 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

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