I'm using the ZedBoard+FMCOMMS right now, and I notice that as soon as I set the Tx LO, the Tx channel begins to output that LO at about -10 dBm. When I start streaming my data, I get the expected signal output. But as soon as I stop streaming (by destroying the Tx buffer via libiio), the LO immediately comes back at around -10 dBm. This isn’t really the behavior I expect or want. I would expect Tx channel to stop emitting as soon as I stop streaming. I thought it might be a problem with how my software configures things, but I see the same behavior with the ad9361-iiostream (https://github.com/analogdevicesinc/libiio/blob/master/examples/ad9361-iiostream.c) example. As soon as the application exits, the LO pops up at around -10 dBm right at 2.5 GHz. Any idea why?
There is a solution I read here in forum that add internal attenuation by adding internal attenuation , the LO leakage really dropped down but with that the desired frequency level also get lowered.
I'm in FDD mode, btw.
What HDL version are you using? It might be the case that the last sample get's repeated which then causes carrier bleed. Try sending ZEROs at the end of you buffer. In recent HDL versions the behavior should be that as soon as the DMA starves data, ZEROs are being inserted.
I am using 2018_r1 but I am facing this issue more when I inject Rx signal (via Rx) . I received Rx signal and applied modulation on received signal and then transmitted . So I want Tx not transmit LO up to the time of (processing and modulation) .Currently the Tx LO is continuously transmitting although I am not passing any signal to DDS at that time.
Yes, this is LO leakage from the transmitter. If you want to eliminate it, power down the TX LO or increase the TX attenuation when not transmitting. Otherwise, use TDD mode.
Thanks for your suggestion . I tried ( increase the TX attenuation when not transmitting ) this option but this also decrease/attenuated the level of TX signal . So I will explore the first option as well (power down the TX LO) .
1-and a strange behavior I observed that during TX mode , LO (during testing LO=155 MHz , RF BW 56 MHz,leakage is occurred but at the same time I got an other peaks(level almost equal to LO) at 148.5 MHz. ? what would be the reason /???
Spurs and harmonics typically happen when you overdrive the DAC. Try reducing the signal you send to the DAC.