LimeSDR Tx - Low and unstabel power

Hi guys,

I am trying to characterize the Tx of my limeSDR mini. I connected the limeSDR_Tx directly into a spectrum analyser, transmitted a sine wave by using GNURadio and measured the power [dBm] using the spectrum. I encountered 2 issues:

  1. Max power is -17dBm as oppose to the expected 10 dBm.
  2. The power output is very unstable, get a peak every 100 MHz approximately and then the signal falls back down.
  3. I repeated a similar procedure with limeSDR (not mini) and encountered similar values, so it’s probably not the specific device that is broken.

Here are two images, the right one is a scan over the peak frequencies, to see the overall performance and the left one is a scan with a smaller step of 5 MHz around 900 MHz to demonstrate the unstable amplitude I’m seeing.

I am wondering, is there a way to fix this?

First things that come to mind:

  • make sure the Lime gets enough power from the USB
  • make sure the Lime doesn’t get too hot when transmitting (eg use a fan)

Thanks Karl. Tried those, don’t think it’s the issue.

The temp looks good, don’t know the exact value, but the chip is hardly warm.
The USB voltage reading is 4.7-4.93 volts (tried few different ports). Also, I checked the current difference between the transmitting and the idle modes, got a delta of 40 mA, so 10log10(4.9x0.04/1e-3) = 23 dBm is going into the device.

I also tried calibrating the SDR using limesuit and afterwards loading this from gnuradio, this gave me even worst performance :frowning: and changing the Tx band which also didn’t help.