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.
- 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 bandwidth at 55MHz
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 bandwidth at 20MHz
RX bandwidth at 45MHz
- 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?