# Lime USB - Failed Loopback Test

**URL:** <https://discourse.myriadrf.org/t/lime-usb-failed-loopback-test/6996>\
**Category:** LimeSDR\
**Created:** [4 March 2021 16:12 UTC](https://discourse.myriadrf.org/t/lime-usb-failed-loopback-test/6996 "2021-03-04T16:12:38Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![mzs](https://yyz2.discourse-cdn.com/flex034/user_avatar/discourse.myriadrf.org/mzs/32/2678_2.png) [@mzs](https://discourse.myriadrf.org/u/mzs)\
**Post date:** [4 March 2021 19:23 UTC](https://discourse.myriadrf.org/t/lime-usb-failed-loopback-test/6996/2 "2021-03-04T19:23:48Z")

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> [@whzrf](#):
>
> CH1 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-22.6 dBFS, 5.00 MHz) - FAILED

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](https://github.com/myriadrf/LimeSuite/blob/master/QuickTest/LimeSDRTest_USB.cpp#L149) dBFS with a tolerance of +/- [6 dB](https://github.com/myriadrf/LimeSuite/blob/master/QuickTest/LimeSDRTest.cpp#L334))

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.

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