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LMV232TLX Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMV232TLX Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page 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 ap- plications. 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 1. The output power of one PA is measured at a time, depending on the bandselect pin (BS). If the BS = High RF IN1 is used for measurements, if BS = Low RFIN2is used. The measured output voltage of the LMV232 is read by the ADC of the baseband chip and the gain of the PA gain 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 attenua- tor. The setup can be adjusted to various PA output ranges by selection of a directional coupler or insertion of an addi- tional (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 conver- sion gain from RF IN 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 op- eration is not based on peak detection, but on direct deter- mination of the mean square value. This is the most accurate power measurement, since it exactly implements the defini- tion of power. A mean-square detector measures V RMS 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 = V PEAK 2 for all waveforms, while it should measures P = V PEAK 2/2 (forR=1 Ω) for a sine wave and P = V PEAK 2/3 for a triangle wave for instance. For a CDMA signal, the mea- surement error can be in the order of 5 to 6 dB. For many wave forms, specially those with high peak-to-average ra- tios, 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 deter- mined as a measure for the accuracy of the detector. 20127801 FIGURE 1. Typical Application www.national.com 9 |
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