Datasheet ADL5519 (Analog Devices) - 34

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ADL5519 Data Sheet MEASURING VSWR
Each ADL5519 detector has a nominal input range from −5 dBm to −55 dBm. In this example, the maximum forward power of Measurement of reflected power in wireless transmitters is a critical +50 dBm is attenuated to −10 dBm at the detector input (this auxiliary function that is often overlooked. The power reflected attenuation is achieved through the combined coupling factor back from an antenna is specified using either the voltage standing of the directional coupler and the subsequent attenuation). This wave ratio (VSWR) or the reflection coefficient (also referred to puts the maximum power at the detector comfortably within its as the return loss). Poor VSWR can cause shadowing in a TV linear operating range. Also, when the HPA is transmitting at its broadcast system because the signal reflected off the antenna lowest power level of +20 dBm, the detector input power is reflects again off the power amplifier and is then rebroadcast. In −40 dBm, which is still within its input operating range. wireless communications systems, shadowing produces multipath- like phenomena. Poor VSWR can degrade transmission quality;
50dBm
the catastrophic VSWR that results from damage to a co-axial
40dBm FORWARD REVERSE
cable or to an antenna can, at its worst, destroy the transmitter.
POWER POWER 30dBm RANGE RANGE
The ADL5519 delivers an output voltage proportional to the log
55dB 20dBm ATTENUATION
of the input signal over a large dynamic range. A log-responding
10dBm DECTOR A/B INPUT RANGE
device offers a key advantage in VSWR measurement applications.
0dBm 60dB ATTENUATION
To compute gain or reflection loss, the ratio of the two signal
–10dBm
powers (either OUTPUT/INPUT or REVERSE/FORWARD)
–20dBm POWER AT INPUT A
must be calculated. An analog divider must be used to perform
–30dBm POWR AT INPUT B
this calculation with a linear-responding diode detector, but
–40dBm
only simple subtraction is required when using a log-responding
–50dBm
detector (because log(A/B) = log(A) − log(B)). 5
–60dBm
-07 98 A dual RF detector has an additional advantage compared to 61 0 Figure 70. ADL5519 VSWR Level Planning a discrete implementation. There is a natural tendency for two devices (RF detectors, in this case) to behave similarly when they Careful level planning should be used to match the input power are fabricated on a single piece of silicon, with both devices having levels in a dual detector and to place these power levels within similar temperature drift characteristics, for example. At the the linear operating range of the detectors. The power from the summing node, this drift cancels to yield a result that is more reverse path is attenuated by 55 dB, which means that the detector temperature stable. is capable of measuring reflected power up to 0 dB. In most appli- In Figure 71, two directional couplers are used, one to measure cations, the system is designed to shut down when the reflection forward power and one to measure reverse power. Additional coefficient degrades below a certain minimum (for example, attenuation is required before applying these signals to the 10 dB). Full reflection is allowed when using the ADL5519 because detectors. The ADL5519 dual detector has a measurement range of its large dynamic range. In the case of very little reflection of 50 dB in each detector. Care must be taken in setting the attenua- (a return loss of 20 dB) and the HPA is transmitting +20 dBm, the reverse path detector has an input power of −55 dBm. tion levels so the reflection coefficient can be measured over the desired output power range. The application circuit in Figure 71 provides a direct reading of The level planning used in this example is graphically depicted return loss, forward power, and reverse power. If the forward and in Figure 70. In this example, the expected output power range reverse phase difference (phase angle) is needed to optimize the from the HPA is 30 dB, from 20 dBm to 50 dBm. Over this power delivered to the antenna, the AD8302 should be used. power range, the ADL5519 can accurately measure reflection It provides one output that represents the return loss and one coefficients from 0 dB (short, open, or load) to −20 dB. output that represents the phase difference between the two signals. However, the AD8302 does not provide the absolute forward or reverse power. Rev. B | Page 34 of 39 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION USING THE ADL5519 BASIC CONNECTIONS INPUT SIGNAL COUPLING TEMPERATURE SENSOR INTERFACE VREF INTERFACE POWER-DOWN INTERFACE SETPOINT INTERFACE—VSTA, VSTB OUTPUT INTERFACE—OUTA, OUTB DIFFERENCE OUTPUT—OUTP, OUTN DESCRIPTION OF CHARACTERIZATION BASIS FOR ERROR CALCULATIONS DEVICE CALIBRATION ADJUSTING ACCURACY THROUGH CHOICE OF CALIBRATION POINTS TEMPERATURE COMPENSATION ADJUSTMENT ALTERING THE SLOPE CHANNEL ISOLATION OUTPUT FILTERING PACKAGE CONSIDERATIONS OPERATION ABOVE 8 GHz APPLICATIONS INFORMATION MEASUREMENT MODE CONTROLLER MODE AUTOMATIC GAIN CONTROL GAIN-STABLE TRANSMITTER/RECEIVER MEASURING VSWR EVALUATION BOARD CONFIGURATION OPTIONS EVALUATION BOARD SCHEMATIC AND ARTWORK OUTLINE DIMENSIONS ORDERING GUIDE