Lime USB - Failed Loopback Test

Right out of the box from Crowdsupply a couple of months ago. I followed the instructions to install LimeSuite on Ubuntu I updated the board and when LimeQuickTest…Any thoughts on why?

->Start time: Thu Mar 4 11:09:17 2021
->LimeSuite version: 20.10.0-g1480bfea

->Device: LimeSDR-USB, media=USB 3.0, module=FX3, addr=1d50:6108, serial=00090726074F2D11, HW=4, GW=2.23
Serial Number: (BLANK)
Chip temperature: 27 C

[ Clock Network Test ]
->FX3 GPIF clock test
Test results: 28731; 32487; 36243 - 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 : 5113013 (min); 5113146 (max) - PASSED
->Clock Network Test PASSED

->Read data: 12 07 1A 12 07 1A 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:(-14.8 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=800.0MHz, SXT=805.0MHz): Result:(-16.2 dBFS, 5.00 MHz) - PASSED
->Run Tests (TX_1 -> LNA_W):
CH0 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-19.7 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-22.6 dBFS, 5.00 MHz) - FAILED
->Run Tests (TX_2-> LNA_H):
CH0 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-17.2 dBFS, 5.00 MHz) - PASSED
CH1 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-13.3 dBFS, 5.00 MHz) - PASSED
->RF Loopback Test FAILED

=> Board tests FAILED <=

Elapsed time: 1.74 seconds

It looks like (TX_1 -> LNA_W) CH1 is the one that caused the failure. Any thoughts on the cause and fix?


My very basic understanding of that test is that it is a loopback test where the transmitter of channel 1 in this case is tuned to a frequency of 1805 MHz, and told to transmit a signal (CW at a guess), and the RX channel 1 is told to receive at 1800MHz. And the result is what signal level is seen by RX and at what frequency offset. In this case -22.6dBFS is the signal level seen at a frequency offset of +5.00MHz. A pass at all the frequencies used in the loopback tests is defined as a signal level of -9.00 to -21.00 dBFS (or -15 dBFS with a tolerance of +/- 6 dB)

So your LimeSDR is working, but on those ports it means one of a two things the TX power is either a bit less than expected at that frequency, or the RX sensitivity is a bit lower than expected at that frequency. The loopback as far as I’m aware, on the ports that are 1.6dB outside specification (TX_1 -> LNA_W channel 1), is internal to the LMS7002M chip.

Gotcha. Thanks for the Info. I guess, leave as is? or should i get an RMA from CrowdSupply? @andrewback

It’s working, but as noted by @mzs, a little bit out of spec. Assuming that you had nothing connected to the RF ports — as this can skew results — you may want to consider RMA.