I'm using a PicoZed SDR module (Rev. C) on the FMC carrier board (Rev. C) and I find that when I connect the board to the network using the Ethernet 1 port, there are significant spurious sidebands on the transmission at ~ +/- 12.5 kHz from the carrier. At a Tx frequency of ~1400 MHz, they are about -38 dBc. The sidebands are absent if I use the Ethernet 2 port.
The sideband level varies with Tx frequency. For example, at 200 MHz they are only about -60 dBc. The relative level to the carrier remains the same when the Tx attenuation is increased. The sidebands are present for all Tx ports: 1A, 1B, 2A and 2B.
Has anyone else observed this?