Design FAQs - RF Detectors For Wireless Devices

1 Sponsored by analog DevicesWhat is an RF detector?An RF detector monitors or samples the output ofan RF circuit and develops a dc output voltage pro-portional to the power at that do you do with an RF detector?RF Detectors are used primarily to measure andcontrol RF power in Wireless are power measurement and control soimportant?RF power, rather than voltage, is the primarymeasure of a Wireless signal. In a receiver, signalstrength is a key factor in maintaining reliable com-munications. In the transmitter, the amount ofpower transmitted is critical because of regulatoryguidelines. It s also important for maintaining therange and reliability of the radio is the unit of power measurement inRF applications?The unit of power is the watt. However, it is com-mon in most RF and Wireless applications toexpress power in terms of dBm or decibels relatedto 1 mW:dBm = 10log [power(mW) / 1 mW]The table shows the relationship between absolutepower and dBm.

2 This unit of measurement is usu-ally referenced to an impedance of 50 .What is the main application of RF Detectors ?Transmitter output power measurement is the pri-mary application. It is essential to know the RFoutput power because the application specifies it inmost cases, and certain maximum values must notbe exceeded according to Federal CommunicationsCommission regulations. In many cases, the trans-mitter power is controlled automatically. As aresult, the output power is measured and comparedto a set point level in a feedback control circuit sopower can be adjusted as required. In receivers, power measurement is usuallyreferred to as the received signal strength indicator(RSSI). The RSSI signal typically is used to controlthe gain of the RF/IF signal chain with an auto-matic gain control (AGC) or automatic level con-trol (ALC) circuit to maintain a constant signallevel suitable for analog -to-digital conversion anddemodulation.

3 What are some other uses of RF Detectors ?Voltage standing-wave ratio (VSWR) measure-ment and control is another popular application inhigh-power RF amplifiers. Impedance mismatches(high VSWR) at the antenna cause reflections andlead to loss of transmitted power. Furthermore,high VSWR can damage an amplifier or a trans-mission line. When two logarithmic Detectors are used, thepower gain of a circuit can be measured by sub-tracting the input reading from the output , a gain calculation calls for dividing theoutput power reading by the input reading. This isa difficult math operation in analog circuits. Butwhen the quantities are logarithmic, the division canbe performed using a simple subtraction. Poweramplifier linearization is another common there different types of RF Detectors ?There are two basic types: the logarithmic typeand the rms type.

4 The log type converts the inputRF power into a dc voltage proportional to the logof the input, making the output directly related todecibels. The rms detector creates a dc output pro-portional to the rms value of the does the output response of a log RFdetector look like?In a typical response curve of a log detector, theoutput is linear over the logarithmic decibel inputrange (Fig. 1). The slope of the curve is typically inthe 20- to 25-mV/dB are the general criteria for selectingone type of RF detector over another?The type of RF signal to be measured is the mostimportant determining factor in the type of detec-tor to use. For most general power measurementand control applications, the log type is the mostuseful. For pulsed RF signals, the log type is alsobest because of the fast response times available.

5 Inthose applications where the signal has a high crestfactor or a widely varying crest factor, the rms typeis generally better. The crest factor is the ratio of the peak to rmsvalue of the signal. For example, higher-order quad-VOUT(V) level (dBm) 80 60 40 20020 60 50 40 30 20 100 Input amplitude (dBm)OUT [A, B] (V)Error (dB) 40 COUTB 40 COUTA 85 COUTB 85 C1. In a typical output curve for a loga-rithmic RF power detector, the output islinear over the logarithmic decibel This output voltage and temperature error curve of a dual logarithmicRF power detector includes variations from 40 C to 85 C at Detectors ForWireless DevicesFREQUENTLY ASKED QUESTIONSL ouis E. FrenzelCommunications/Test Editorrature amplitude modu-lation (QAM) signals( , 16 QAM, 64 QAM,and 256 QAM) havehigh crest factors.

6 In thecase of spread-spectrumsignals such as thoseused in CDMA andWCDMA cellular sys-tems and orthogonal fre-quency-division multi-plexing (OFDM) signalssuch as WiMAX andWiBro, the high crestfactor (typically 10 to 13dB) will change dynami-cally. In such applica-tions, an rms detector isgenerally more about temperature stability?Temperature stability is an expression of thevariation of the measurement accuracy versustemperature. Temperature stability is generallyexpressed in dB, that is, the voltage variation atthe output of the detector converted into Devices have a worst-case temperature sta-bility of dB over their full power Detectors , though, achieve 0-dB temperaturestability at the top end of their input range. Figure2 shows a typical temperature error graph for adual detector, where the 0-dB crossover point is atan input amplitude of 13 can designers take advantage of the 0-dB crossover point?

7 The output of a power amplifier (PA) is sam-pled with a directional coupler. With the PA atmax power, the coupler output should be attenu-ated down to the 0-dB crossover point of the RFdetector. The detector output value is then digi-tized in an analog -to-digital converter (ADC)and sent to an embedded controller that calcu-lates the power level based on previously storedcalibration coefficients. The power level is compared to a set pointvalue. If the measured value is higher or lowerthan the set point, the controller uses a digital-to- analog converter (DAC) to control the gain of avariable gain amplifier (VGA). This results in achange in the output power at the PA. The near 0-dB temperature drift of the detector at thecrossover point enables the ALC loop to veryaccurately control the PA s output ONLINE 17106 The industry s most comprehensive, award-winning, RF power detector portfolioThe dual RF power detector/controller precisely measures and controlsthe gain across the transmitter and receiver signal path within wirelessinfrastructure equipment.

8 The dual detector is an integration of two detec-tors to provide accurate, independent, decibel-scaled voltage outputs ofboth RF input channels. In addition to the two matched output voltages,the dual detector provides a difference output that effectively measuresthe power gain or loss between the two inputs, and some even provide anaccurately scaled temperature output. With two matched Detectors , theprecise matching of both channels reduces temperature- and part-to-partvariations in comparison to discrete designs. With the difference output,the dual detector can accurately control the gain across a transmitter orreceiver signal chain as each of the two individual Detectors can be set tomeasure input signals at different frequencies over a 60-dB dynamicrange. Specific applications include PA (power amplifier) control and lin-earization, antenna VSWR (voltage standing-wave ratio) monitoring, transmitter power control, and AGC (automaticgain control) circuits.

9 Better control over the power and gain within a basestation PA allows designers to significantlyreduce cost and improve operating 1 W100 mW10 mW1 mW100 W10 W1 W100 nW10 nW1 nW100 pW10 pW1 pWdBm+30+20+100 10 20 30 40 50 60 70 80 90 POWER LEVELS AND THEIR DBMEQUIVALENTSTo view our complete selection of RF power Detectors , go to NumberAD8361AD8362AD8364(Dual)ADL5501 Max RF Freq(GHz) (dB)30606030 Temp Drift(dB) inVoltsLinear in dBLinear in dBLinear inVoltsVoltageSupply (V) to to to to (mA) ; SOIC16-Lead SOP32-Lead CSP6-Lead SC-70 PartNumberAD8302(Dual w/Phase)AD8317AD8318AD8319 ADL5519(Dual)Max RF Freq(GHz) (dB)6050704064 Temp Drift(dB) (ns)6081088 VoltageSupply (V) to to to to to (mA)1920682256 Package14-LeadTSSOP8-Lead CSP16-Lead CSP8-Lead CSP32-Lead CSPTruPwrTMRMS DetectorsLogarithmic Detectors