Datasheet AD8302 (Analog Devices) - 20

AD8302 Reflectometer
The measurement accuracy can be compromised if board The AD8302 can be configured to measure the magnitude ratio level details are not addressed. Minimize the physical distance and phase difference of signals that are incident on and reflected between the series connected couplers since the extra path from a load. The vector reflection coefficient, ⌫, is defined as, length adds phase error to ⌫. Keep the paths from the couplers to the AD8302 as well matched as possible since any differences Γ = Reflected Voltage / Incident Voltage = (Z − Z )/ (Z + Z (17) L O L O ) introduce measurement errors. The finite directivity, D, of the where Z couplers sets the minimum detectable reflection coefficient, i.e., L is the complex load impedance and ZO is the charac- teristic system impedance. | ΓMIN(dB)|<|D(dB)|. The measured reflection coefficient can be used to calculate the
SOURCE
level of impedance mismatch or standing wave ratio (SWR) of a particular load condition. This proves particularly useful in diag-
INCIDENT REFLECTED ZLOAD WAVE WAVE
nosing varying load impedances such as antennas that can degrade
20dB 1dB
performance and even cause physical damage. The vector reflectometer arrangement given in Figure 13 consists of a pair of directional couplers that sample the incident and reflected sig- nals. The attenuators reposition the two signal levels within the
R2 R1
dynamic range of the AD8302. In analogy to Equations 15 and 16, the attenuation factors and coupling coefficients are given by:
C5 C6 C4 C1 C3
C + L = P − P (18)
R4
B B IN OPT
VP C7
C + L = P + Γ − P (19) A A IN NOM OPT where ⌫NOM is the nominal reflection coefficient in dB and is negative for passive loads. Consider the case where the incident signal is 10 dBm and the nominal reflection coefficient is –19 dB.
AD8302 C2
As shown in Figure 13, using 20 dB couplers on both sides and
1 COMM MFLT 14
–30 dBm for POPT, the attenuators for Channel A and B paths
2 INPA VMAG 13
⌫ are 1 dB and 20 dB, respectively. The magnitude and phase of
R5
the reflection coefficient are available at the VMAG and VPHS
3 OFSA MSET 12
pins scaled to 30 mV/dB and 10 mV/degree. When ⌫ is –19 dB,
4 VPOS VREF 11
the VMAG output is 900 mV.
5 OFSB PSET 10 6 INPB VPHS 9
⌫
R6 7 COMM PFLT 8 C8
Figure 13. Using the AD8302 to Measure the Vector Reflection Coefficient Off an Arbitrary Load –20– REV. B Document Outline FEATURES PRODUCT DESCRIPTION FUNCTIONAL BLOCK DIAGRAM AD8302-SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS CAUTION Equivalent Circuits Typical Performance Characteristics GENERAL DESCRIPTION AND THEORY Basic Theory Structure BASIC CONNECTIONS Measurement Mode Interfacing to the Input Channels Dynamic Range Cross Modulation of Magnitude and Phase Modifying the Slope and Center Point Comparator and Controller Modes APPLICATIONS Measuring Amplifier Gain and Compression Reflectometer CHARACTERIZATION SETUPS AND METHODS Gain Phase OUTLINE DIMENSIONS Ordering Guide Revision History