AD8302 calibration

Hi!

Greetings!


I have designed a single tone (150MHz) reflectometer circuit using AD8302. I am struggling since more than two months in just to achieve measured reflection coefficients same as the ideal ones as mentioned in col.5 in the following table.


Please take a quick look at the data mentioned below.

I got following results while applying resistive terminations:

For my application, above error (data mentioned in red) is very significant and cannot be ignored. Please let me know, what is the way to overcome these errors. Suggest me the steps required to calibrate AD8302.

Please note that in actual case, my terminating load is the complex impedance.

8302 Measurement voltage (mag/angle) output range measured= ~ 200mV to 1600mV

Further, it was also ensured that 8302 operates within the input as well as output voltage range as described in the datasheet.

Abstract circuit flow:

15dBm VHF source->35dB attenuation-> Incident wave fed to INPA of 8302

Reflected power->20dB attenuation-> reflected wave fed to INPB of 8302

Please note that high directivity (45dB) minicircuit’s couplers are used

Please guide me in this regard as soon as possible.

Thank you,

Sanket

(RF design engineer)

  • The calibration I have mentioned in col.4 is carried out in following way [reference: D. Pozar’s book on Microwave Engineering]:

    calibrated reflection coefficient may be written as,

    where,

    Please verify whether above calibration method is applicable to my case or not. If not than please suggest any other method/s.

    What I want is the relationship/ mathematical steps/ method to convert (calibrate) reflection coefficients from col.2(measured) to col.5(ideal). One may suggest any literature as well.

    Thanks!

  • 0
    •  Analog Employees 
    on Dec 16, 2015 11:58 PM

    The AD8302 has inherent phase measurement error in and around 0, 180 and -180 degrees:

    You will have to calibrate this error out of your system for each and every part (every par is different).  But i don't think you will really be able to calibrate the extremes out (at 0, 180, -180 degrees) because VPHS will essentially give you the same voltage for 178, 179, 180, -180, -179, -178, etc..  This is due to the response flattening out which introduces ambiguity into the system.  Does all of this make sense?

  • I am aware of the erroneous behavior of 8302 near +/- 180 degree.

    Anyways, the region is not of my concern as you can see in col.2 of the table which I have mentioned, I am operating at around -154 degree!

    Do AD experts normally suggest any processing to reflection coefficients for 8302 users? In AD tests, did 8302 achieve the exact reflection coefficients as measured in typical VNA (considering that 8302 is made to operate in its highest accuracy range)? What was/would be the mean measurement error (in mag < angle) according to you?

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:18 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin