LMV232

SNWS017C – DECEMBER 2004 – REVISED MARCH 2013

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APPLICATION NOTES

The LMV232 mean square power detector is particularly suited for accurate power measurement of RF

modulated signals that exhibit large peak to average ratios, i.e. large variations of the signal envelope. Such

noise-like signals are encountered e.g. in CDMA and Wide-band CDMA cell-phones. Many power detection

circuits, particularly those devised for constant-envelope modulated signals as in GSM, are based on peak

detection and provide accurate power measurements for constant envelope or low-crest factor (ratio of peak to

RMS) signals only. Such detectors are therefore not particularly suited for CDMA and WCDMA applications.

TYPICAL APPLICATION

The LMV232 is especially suited for CDMA and WCDMA applications with 2 Power Amplifiers (PA’s). A typical

setup is given in Figure 21. The output power of one PA is measured at a time, depending on the bandselect pin

(BS). If the BS = High RFIN1 is used for measurements, if BS = Low RFIN2 is used. The measured output voltage

of the LMV232 is read by the ADC of the baseband chip and the gain of the PA is adjusted if necessary. With an

input impedance of 50

Ω, the LMV232 can be directly connected to a 20 dB directional coupler without the need

for an additional external attenuator. The setup can be adjusted to various PA output ranges by selection of a

directional coupler or insertion of an additional (resistive) attenuator between the coupler outputs and the

LMV232 RF inputs.

The LMV232 conversion gain and bandwidth are configured by a resistor and a capacitor. Resistor R1 sets the

conversion gain from RFIN to the output voltage. A higher resistor value will result in a higher conversion gain.

The maximum dynamic range is achieved when the resistor value is as high as possible, i.e. the output signal

just doesn’t clip and the voltage stays within the baseband ADC input range. The filter bandwidth is adjusted by

capacitor C1. The capacitor value should be chosen such that the response time of the device is fast enough and

modulation on the RF input signal is not visible at the output (ripple suppression). The

−3 dB filter bandwidth of

the output filter is determined by the time constant R1*C1. Generally a capacitor value of 1.5 nF is a good

choice.

PEAK TO AVERAGE RATIO SENSITIVITY

The LMV232 power detector provides an accurate power measurement for arbitrary input signals, low and high

peak-to-average ratios and crest factors. This is because its operation is not based on peak detection, but on

direct determination of the mean square value. This is the most accurate power measurement, since it exactly

implements the definition of power. A mean-square detector measures VRMS

2 for all waveforms. Peak detection is

less accurate because the relation between peak detection and mean square detection depends on the

waveform. A peak detector measures P = VPEAK

2 for all waveforms, while it should measures P = V

PEAK

2/2 (for R

= 1

Ω) for a sine wave and P = VPEAK

2/3 for a triangle wave for instance. For a CDMA signal, the measurement

error can be in the order of 5 to 6 dB. For many wave forms, specially those with high peak-to-average ratios,

peak detection is not accurate enough and therefore a mean square detector is recommended.

MEAN SQUARE CONFORMANCE ERROR

The LMV232 is a mean square detector and therefore should have an output voltage (in Volts) that linearly

relates to the RF input power (in mW). The input referred error, with respect to an ideal linear mean square

detector, is determined as a measure for the accuracy of the detector.

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