RMS Detectors Scan 75-dB Dynamic Range

Dec. 20, 2010
This dynamic duo of single-ended detectors delivers the wide detection bandwidth from DC to 3.9 GHz needed for 4G wireless applications such as WiMAX and LTE.

Power detection in communications receiver and transmitter signal chains is essential, especially for applications that depend on tight control of power levels, such as wireless communications systems. Different types of power detectors are available for this purpose, including monolithic models with differential input ports. Although they offer power measurement ranges of 40 dB or more, they must be connected to a balanced-unbalanced (balun) transformer for use with a single-ended input. The new HMC1020LP4E and HMC1021LP4E root-mean-square (RMS) power detectors from Hittite Microwave Corp. offer a simpler approach and with higher performance. They have single-ended inputs, so the balun is not needed. Both operate from DC to 3.9 GHz, the HMC1020LP4E with a 76-dB power measurement range (-68 to +8 dBm), and the HMC1021LP4E with a dynamic range of 73-dB (-68 to +5 dBm). Both deliver 0.1 dB RMS measurement accuracy with respect to continuouswave (CW) response over those dynamic ranges, regardless of modulation format. The HMC1021LP4E adds an envelope detector with 100-MHz bandwidth for instantaneous power measurements, when the peak (rather than the average) power must be known at any given time.

RMS detectors such as the HMC1020LP4E and HMC1021LP4E devices include a number of different function blocks, including full-wave rectifiers, log/antilog circuits, and an integrator. The RMS output voltage (in V) is directly proportional to the average input signal power (in dBm). The average power indication from an RMS detector is well suited for signal gain and level control. In spite of the number of circuit functions, these RMS detectors are small, supplied in plastic surfacemount- technology (SMT) packages measuring just 4 x 4 mm (Fig. 1). They can tackle average power measurements on signals with complex modulation, including those with peak-to-average power ratios (crest factors) as high as 15 dB. In fact, the HMC1020LP4E and HMC1021LP4E maintain high accuracy independent of the modulation of the RF carrier. Both RMS detectors achieve less than 0.1 dB detection deviations with respect to CW response under all modulation schemes, including LTE, HSDPA, CDMA, W-CDMA, TD-SCDMA, and WiMAX/WiBRO.

Although measuring average power is vital for signal gain and level control in a communications system, it is sometimes also important to know details about the envelope of a signal. For example, continuous monitoring of peak signal power is often necessary to prevent saturation of components in a signal-processing chain. Similarly, monitoring signal crest factor (CF) for a radio transmitter provides details needed to optimize power-amplifier efficiency in that part of a communications system. Traditional CF monitoring solutions have employed two power detectors in parallel: an RMS detector and an envelope detector. The HMC1021LPE puts both detectors on the same integrated circuit (IC), in the same package, to simplify this application.

The HMC1021LPE is capable of extracting envelope information from a modulated RF signal with modulation bandwidths exceeding 100 MHz. It provides two detection outputs: one is a voltage representing average power and one with a voltage that provides instantaneous envelope information. The instantaneous envelope output is suitable for implementing high-speed RF power protection systems as well as efficiencyenhancing envelope-tracking power amplifier implementations.

The HMC1020LPE and HMC1021LPE detectors are both designed for ease of use compared with traditional differential RMS power detectors. Baluns are not needed with the HMC1020LPE and HMC1021LPE detectors and, when driven by a singleended 50-O modulated source, both detectors deliver a detection range of better than 70 dB (Fig. 2). The singleended models HMC1020LPE and HMC1021LPE operate from DC to 3.9 GHz using a standard DC blocking capacitor in a simple circuit arrangement that avoids signal degradation for the sake of power detection. The simple single-ended interface eliminates the cost of a balun along with the additional printed-circuit-board (PCB) real estate needed for mounting the balun. The simpler circuitry possible with a single-ended interface also contributes to increased measurement repeatability.

The proprietary RMS detection core common to both the HMC1020LPE and the HMC1021LPE is optimized to accurately detect the RMS power of modulated RF signals with high crest factors. Compared with a CW response, the devices deliver detection accuracy of better than 0.2 dB even when processing signals with complex modulation formats having crest factors as high as 15 dB. When evaluated with fourth-generation wideband-codedivision- multiple-access (WCDMA4) signals, the crest-factor-immune architecture of these RMS detectors results in detection accuracy of better than 0.1 dB over the entire operating frequency and temperature range, compared with the CW response .

The HMC1021LP4E combines accurate RMS detection with fast, wideband linear envelope detection in a small package. The HMC1021LP4E's on-chip envelope detector extracts the instantaneous envelope information of the RF signal in parallel with the RMS detector. This envelope output is buffered and available at the HMC1021LP4E's ETOUT pin, alongside the HMC1021LP4E's RMS detection output RMSOUT pin. The ETOUT output provides a linear representation of the instantaneous envelope of the RF signal for modulation bandwidths in excess of 100 MHz.

The model HMC1021LP4E with its two detection outputs provides the capability to measure peak signal power, average power, peak-to-average power ratio, and RF wave shape. Based on traditional design approaches, this would require two separate detectors with separate sets of complex calibration procedures over temperature to correlate the detector outputs. The HMC1021LP4E's ETOUT output can also be used to implement fast-responding over-power protection and envelope tracking power amplifier systems. The table provides a summary of the key performance specifications for the HMC1020LP4E and the HMC1021LP4E RMS detectors. Earlier models HMC909LP4E and HMC1010LP4E RMS detectors are also included for comparison.

Driven by demands for higher data rates, newer wireless communications standards, such as WiMAX and the 3GPP Long Term Evolution (LTE) system, rely on complex, bandwidth-efficient modulation schemes. Such complex modulation approaches require higher crest factors and wider modulation bandwidths, motivating power amplifier manufacturers to develop more efficient amplification systems.

The envelope tracking power amplifier (ET-PA) is one of these next-generation power amplifier systems (Fig. 4). The ET-PA designs aim to boost the efficiency of the overall system to reduce the wasted DC power by modulating the power supply of the amplifier to scale with the envelope of the RF carrier. Detecting the envelope of the RF carrier and modulating the power supply are two key requirements of such a system. The HMC1021LP4E with a linear envelope detector capable of 100-MHz modulation bandwidth (Fig. 5) is an ideal solution for implementing ET-PA designs. The RMS detector included in the HMC1021LP4E can also monitor or control the average transmitted power in such systems.

The HMC1020LP4E and HMC1021LP4E RMS power detectors are suitable for broadband, highdynamic- range applications requiring repeatable measurement of average signal power; especially where detection of RF and intermediate-frequency (IF) wave shape and/or crest factor change is crucial. Both devices feature RMS detection cores that operate from DC to 3.9 GHz and achieve input sensing ranges to 76 and 73 dB, respectively (with accuracy of 1 dB), with a simple single-ended input interface. The HMC1021LP4E, which integrates a fast envelope detector core with detection bandwidth better than 100 MHz, also serves applications that require instantaneous envelope measurements of RF carriers with wide modulation bandwidths. The HMC1020LP4E and the RMS power detectors and their evaluation boards are available from stock and can be ordered via the company's e-commerce site or direct purchase order. Hittite Microwave Corp., 20 Alpha Rd., Chelmsford, MA 01824; (978) 250- 3343, FAX: (978) 250-3373, e-mail: [email protected], www.hittite.com.

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