RF Loopback Test FAILED, but TX and RX seems works


Before the first run of LimeSDR I’ve performed a test and everything was ok. After some time i got the RF Loopback Test FAILED.

I’ve checked both TX 1, LNA_W of CH 0 (RX 1_W and TX1_1) and they are both seems working. No any noticeable difference between RX 1_L or TX1_2 for example. But test is still failed.

Could you please advise how this loopback is working? There is no any connection between RX 1_W and TX1_1. I can see there are some connection between TX 1/2_2 and RX 1/2_H on the schematic diagram but only for these two ports. Maybe it’s possible to found a faulty element and to replace it. Of course if it’s not a faulty IC.

Nothing was connected to the board.

Dear LimeSDR team could you please explain how the loopback works and could you please suggest what I can do with this error.

Logs are below:

->Start time: Sun Jan 26 20:52:11 2020

->Device: LimeSDR-USB, media=USB 3.0, module=FX3, serial=00090726074F2410, index=0
Serial Number: 00090726074F2410

[ Clock Network Test ]
->FX3 GPIF clock test
Test results: 34642; 38398; 42154 - PASSED
->Si5351C test
CLK0: 17554 / 17554 - PASSED
CLK1: 17554 / 17554 - PASSED
CLK2: 17554 / 17554 - PASSED
CLK3: 17554 / 17554 - PASSED
CLK4: 17554 / 17554 - PASSED
CLK5: 17554 / 17554 - PASSED
CLK6: 17554 / 17554 - PASSED
->ADF4002 Test
Result: 10 - PASSED
->VCTCXO test
Results : 5112962 (min); 5113093 (max) - PASSED
->Clock Network Test PASSED

->Read data: 12 07 19 12 07 19 02

[ LMS7002M Test ]
->Perform Registers Test
->External Reset line test
Reg 0x20: Write value 0xFFFD, Read value 0xFFFD
Reg 0x20: value after reset 0x0FFFF
->LMS7002M Test PASSED

[ RF Loopback Test ]
Note: The test should be run without anything connected to RF ports
->Configure LMS
->Run Tests (TX_2-> LNA_L):
CH0 (SXR=800.0MHz, SXT=805.0MHz): Result:(-14.3 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=800.0MHz, SXT=805.0MHz): Result:(-16.4 dBFS, 5.00 MHz) - PASSED
->Run Tests (TX_1 -> LNA_W):
CH0 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-22.0 dBFS, 5.00 MHz) - FAILED
CH1 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-17.3 dBFS, 5.00 MHz) - PASSED
->Run Tests (TX_2-> LNA_H):
CH0 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-16.0 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-13.7 dBFS, 5.00 MHz) - PASSED
->RF Loopback Test FAILED

=> Board tests FAILED <=

Elapsed time: 1.73 seconds

Thanks in advance.

The TX_1 -> LNA_W is chip’s internal loopback.
CH0 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-22.0 dBFS, 5.00 MHz) - FAILED
It is working, it just received a slightly lower signal. Looking at the source code, the expected level is -15±6dB, so it’s not that far from the lower threshold. Additionaly these values that you are getting can change depending on the chip/board temperature.

Before the first run of LimeSDR I’ve performed a test and everything was ok.
I assume the chip was cold at that point so that’s why it passed. And after it warmed up it started getting different values.

1 Like

Thank you for the answer!

Yes, I’ve noticed that this value (dBFS) depends on the chip/board temperature. But not so strong. It’s about 2-3 dBFS. And in the every beginning this value was around 16 -18 dBFS. Now it’s around 22dBFS.
I hope that the absence of a heat sink will not affect the chip. The parameters will not degrade while using the device.

And another question is why the results of the test is changed?

I’m not hardware engineer so don’t know answers to that.

Hello @Zack. Could you please comment on my issue. Why after some time RF Loopback would fail?