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Aliasing/Out of Band Signal with AD9364

Question asked by spraju87 on Oct 4, 2016
Latest reply on Oct 7, 2016 by sripad

Hello,

 

I am presently evaluating AD9364 with FMCOMMS4+Zed board set-up with No-OS software. When I set the sampling rate to 60 MHz and Rx Bandwidth to 56 MHz, I see aliasing of out of band signals back into base-band. For example, when the Rx LO frequency is 2400 MHz and Rx BW is 56 MHz (2372 to 2428 MHz), then signal beyond 2430 MHz aliases back to 2370 MHz. As the signal is moved farther away, the aliasing signal rolls off towards 0 (like the transition band of a low pass filter).

 

For the above settings, the effect is pre-dominant when the signal is between 2430 MHz and 2440 MHz, beyond which it is completely suppressed. Due to this, the region between 2372 MHz and 2380 MHz is rendered useless. I also see the same effect on the other side too, when the input signal is between 2370 MHz or 2360 MHz, effectively rendering the 2420 to 2428 MHz useless.

 

Due to this, the effective bandwidth that I can measure without interference/aliasing is 40 MHz only. Could someone tell me how to eliminate this aliasing effect, so I could use the complete bandwidth of 56 MHz ? Is this happening because of the 3 dB bandwidth of "Butterworth Rx anti-aliasing filter" which is by default set to 1.4*BBBW (1.4*56/2 MHz = 39.2 MHz) ? If yes, then can I somehow modify this to have more sharper roll-off like 1.15*BBBW ?

 

Thanks,

Raju

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