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Using ADL5391 to cancel antenna-to-antenna coupling in phased-array radar

I'm sketching out a phased-array radar design (RFID reader, actually, but it's basically the same thing) that needs accurate cancellation of Tx->Rx feedthrough (largely due to coupling among the antennae).  The Tx signal (915 MHz band) needs to be phase-shifted, scaled, and added to the Rx signal.  Judging from the ADL5391 datasheet, it would be suitable for this use, given a voltage-controlled phase shifter at one input and the appropriate in-system measurement and control paths.

There are various design challenges here, some of them application-specific.  But the first is to get a handle on this new multiplier architecture, and in particular on its impedance matching requirements, which could complicate the implementation of the varactor-based phase shifter.  Is there an ADL5391 application note, or other reference design, that would help?

Thanks in advance,

- Michael

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  • Hello Michael,

    Thanks for the explanation.  I've never seen this application before and it is very interesting.  I am starting to understand your application better!  I am in the process of updating the ADL5562 input impedance on the datasheet so the datasheet you currently see on the website is not 100% correct but very close.  The ADL5561 is exactly the same as the ADL5562 but a lower bandwidth part and that datasheet has the updated input impedance.  With this said, the input impedance of the ADL5562 is closer to 200 ohms.  I understand your analysis concerning the input impedance of the ADL5562 for the "unbalanced" signals. I am not sure if I exactly agree with your analysis.  Since the input signals are "unbalanced" and not in the pure sense "differential", the input impedance will be greater than the gain resistors itself.  The best approach to getting this correct is to provide you with the S4P file for the ADL5562 and you can simulate the circuit and determine the matching circuit.

    The ADL5562 has a common mode amplifier feedback which tries to correct for the common mode at the inputs.  In your application, I may forsee this common mode amplifier injecting some small signal into the opposite input, which may not be there originally, and vise versa given that you have unbalanced signals.  I am guessing the effect of this may be small and it may manifest itself as not having the 12 dB gain as you expected or having a little more gain.

    Have you considered using an active mixer like the ADL5801/5802, which takes in differential inputs and will do the mixing?  It may simplify your design supstantially.  I am not exactly sure if the common mode rejection is as good as the ADL5562.  It may be something you would want to take a look at.

    Qui

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  • Hello Michael,

    Thanks for the explanation.  I've never seen this application before and it is very interesting.  I am starting to understand your application better!  I am in the process of updating the ADL5562 input impedance on the datasheet so the datasheet you currently see on the website is not 100% correct but very close.  The ADL5561 is exactly the same as the ADL5562 but a lower bandwidth part and that datasheet has the updated input impedance.  With this said, the input impedance of the ADL5562 is closer to 200 ohms.  I understand your analysis concerning the input impedance of the ADL5562 for the "unbalanced" signals. I am not sure if I exactly agree with your analysis.  Since the input signals are "unbalanced" and not in the pure sense "differential", the input impedance will be greater than the gain resistors itself.  The best approach to getting this correct is to provide you with the S4P file for the ADL5562 and you can simulate the circuit and determine the matching circuit.

    The ADL5562 has a common mode amplifier feedback which tries to correct for the common mode at the inputs.  In your application, I may forsee this common mode amplifier injecting some small signal into the opposite input, which may not be there originally, and vise versa given that you have unbalanced signals.  I am guessing the effect of this may be small and it may manifest itself as not having the 12 dB gain as you expected or having a little more gain.

    Have you considered using an active mixer like the ADL5801/5802, which takes in differential inputs and will do the mixing?  It may simplify your design supstantially.  I am not exactly sure if the common mode rejection is as good as the ADL5562.  It may be something you would want to take a look at.

    Qui

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