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AD8302 Reflectometer-Beginners questions

Question asked by ku637 on Dec 8, 2010
Latest reply on Apr 1, 2017 by EDCIV



Ive some doubts in using ad8302 as a reflectometer described in datasheet to use for a lab experiment to mesure VSWR.Im completely new to RF and this is my first learning experiment.So please forgive if im asking some silly doubts..


I need to measure the VSWR in a signal line 0dBm@500MHz fixed (generated from a source)  that is terminated to a variable resistive load(0-50Ohm).

Idea is to manually vary the load and measure the VSWR for each.


Im using a dual directional coupler narda 3020A having 20dB coupling to sample the incident and reflected power & the AD8302 Evaluation board from Analog Devices to measure the reflection.


I ve read from the datasheet that the input range for ad8302 is -60dBm to 0dBm.


So i added an additional 10dB attenuator to the coupled incident(C) and relflected power (D), so that when 0dBm input signal (A)comes the coupled incident power(C)will be -20dBm and the input to the ad8302 INPB pin will be -30dBm

Similar attenuation is given to the sampled relflected power,so that for a full reflection -30dBm will reach at the INPA pin of ad8302 and it will result in a 900mV output at VMAG pin.


1)Could some one tell me if there is any problem with the method approched?


2)I ve read that the ad8302 can have a +/30dB gain measurement range, so for a full reflection in my application the INPA will be -30dBm equalling the INPB,so exisiting 60dB range is not used and only 30dB (ie from -30dBm to -60dBm )in INPA will be used right?


3)Is it really possible to measure upto -60dBm at INPA in my application , ive seen datasheet mentioning about directivity of coupler limiting the minimum measurable reflection coefficient.The directivity of Narda 3020A i measured equal 35dB approximately.(The reflected power at coupled port(C) measured for a 50Ohm termination load at output(B) is approximately -55dBm ,so i calculated directivity =isolation-coupling ie . 55-20=35dB).Can somebody explain me how this gonna affect the minimum measurable Reflection coeficient?



4)Ive seen the Voltage at VMAG is a function as V=Vslp*log(Va/Vb)+900mV

Im using the default values in the evaluation board , so i think the slope is 30mV

so V=0.030*log(Va/Vb)+0.9

4.a)Is Va & Vb in dBV?

4.b)Can we take log(Va/Vb) directly as Reflection coefficient? Or Can somebody tell me how to exaclty convert this voltage(VMAG) into reflection coeffiecnt?.. one that ranges from (0-1) or to reflection coefficient in dB?


Thanks Again..