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AD9361 TX and RX bandwidth setting effects

Question asked by mario_pasquali on May 15, 2014
Latest reply on May 16, 2014 by rgetz

Hello all,

 

I am seeing some surprising behaviors with bandwidth settings while doing characterization of the AD9361.  I searched the forum to see if this is already documented but didn't find anything related.

 

My setup:

  • ZedBoard with AD-FMCOMMS2-EBZ FMC module
  • Latest Linux image from Analog device with IIO scope
  • Spectrum analyzer
  • Cabled connection between AD-FMCOMMS2-EBZ port TX1A and spectrum analyzer

 

After the boot, IIO starts automatically with a 100 MHz bandwidth configuration.  I started doing measurements that way and I noticed a very bad LO rejection as well as pretty strong image.  I then found out that if I configure the TX bandwidth at 53MHz or less, the LO and image go away completely, but if I go above 55MHz, then the LO shows up again.  Here are some screenshots to illustrate this:

 

TX bandwidth at 53MHz

TX 20140508 1208 Setting DDS 14 and 20MHz RF BW 53MHz.jpg

TX 20140508 1208 DDS 14 and 20MHz RF BW 53MHz.JPG


TX bandwidth at 55MHz

TX 20140508 1207 Setting DDS 14 and 20MHz RF BW 55MHz (or above).jpg

TX 20140508 1207 DDS 14 and 20MHz RF BW 55MHz (or above).JPG

 

 

Even if the IIO GUI enables setting the TX bandwidth up to 100MHz, this is well outside the specified limit, but this looks surprising though.  Maybe just a software bug, but probably worth fixing as this could lead to wrong conclusions.

 

Similarly, we need issue when configuring the RX bandwidth, but more annoying, this time we are within the specification.  The TX LO and image are at reasonable levels when the RX bandwidth is configured at 10MHz, but the situation gets bad with a BW of 20MHz, and ugly with a BW of 45MHz. Looks a bit suspect that the RX bandwidth has such a large impact on the TX.  See the following screenshots.

 

RX bandwidth at 10MHz

RX 20140508 1452 Setting DDS 15 and 17MHz RX BW 10MHz - OK.jpg

RX 20140508 1452 DDS 15 and 17MHz RX BW 10MHz - OK.JPG


RX bandwidth at 20MHz

RX 20140508 1454 Settings DDS 15 and 17MHz RX BW 20MHz - bad.jpg

RX 20140508 1454 DDS 15 and 17MHz RX BW 20MHz - bad.JPG


RX bandwidth at 45MHz

RX 20140508 1455 Settings DDS 15 and 17MHz RX BW 45MHz - worse.jpg

RX 20140508 1455 DDS 15 and 17MHz RX BW 45MHz - worse.JPG

 

Questions:

  • Is the described behavior of the TX bandwidth on the TX signal expected?
  • Is the described behavior of the RX bandwidth on the TX signal expected?

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