GSM Images over FM Band on LNAL

[racerxdl]

Hi, I noticed an issue when making the limefm application. My WBFM Spectrum is poluted with a lot of GSM / LTE Images. I don’t recall that happening when I first tested my limesdr (I think about a year ago), so it might be an issue with the latest firmware.

I’m using the LNAL from Channel A and unplugged all other antenas just in case. My Antena is tuned to 2m band so it should receive FM signals better than GSM band. Also my other sdrs (RTLSDR, Airspy) can receive FM without a antenna plugged in.

So I run LimeQuickTest and it gave me an error:
┌─[lucas@nblucas] - [~] - [Sun Sep 02, 18:00]
└─[$] <> LimeQuickTest
[ TESTING STARTED ]
->Start time: Sun Sep 2 18:00:40 2018

->Device: LimeSDR-USB, media=USB 3.0, module=FX3, addr=1d50:6108, serial=0009062000C5170C
  Serial Number: 0009062000C5170C

[ Clock Network Test ]
->FX3 GPIF clock test
  Test results: 14682; 18438; 22194 - 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
->VCTCXO test
  Results : 5112944 (min); 5113077 (max) - PASSED
->ADF4002 Test
  Result: 10 - PASSED
->Clock Network Test PASSED

[ FPGA EEPROM Test ]
->Read EEPROM
->Read data: 11 05 0F 11 05 0F 02
->FPGA EEPROM Test PASSED

[ 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 ]
->Configure LMS
->Run Tests (TX_2-> LNA_L):
  CH0 (SXR=800.0MHz, SXT=805.0MHz): Result:(-14.0 dBFS, 5.00 MHz) - PASSED
  CH1 (SXR=800.0MHz, SXT=805.0MHz): Result:(-15.8 dBFS, 5.00 MHz) - PASSED
->Run Tests (TX_1 -> LNA_W):
  CH0 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-15.0 dBFS, 5.00 MHz) - PASSED
  CH1 (SXR=1800.0MHz, SXT=1805.0MHz): Result:(-27.4 dBFS, 5.00 MHz) - FAILED
->Run Tests (TX_2-> LNA_H):
  CH0 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-17.8 dBFS, 5.00 MHz) - PASSED
  CH1 (SXR=2500.0MHz, SXT=2505.0MHz): Result:(-15.8 dBFS, 5.00 MHz) - PASSED
->RF Loopback Test FAILED

=> Board tests FAILED <=

Elapsed time: 1.42 seconds

I’m using Gateware 17 revision 2.

Any ideas what can be the cause of the issues?

EDIT: Same band with lower gains:

Same band with Airspy Mini:

EDIT2:

Ok so I changed the antenna between my 2m band antenna and a Baofeng UV-5R antena (which works for FM, VHF and UHF) and the images are bit less noticiable. If I put the antena in certain position, it disappears by complete:

That also causes the SNR of the WBFM stations to be higher (I think the GSM stuff is overloading the frontend). I tried to set the analog filter to something low (like 2MHz), but even so I can see the GSM Images. Is there anything else I can do?

EDIT3: Just for reference, an image at same center frequency as before:

[luigi]

I’m experiencing the same problem with the LimeSDR Mini. Multiple GSM and LTE up-link being visible at random frequencies, including the FM band.

[Zack]

Hi @racerxdl,

Do you use RF filter after antenna?

[racerxdl]

Well you mean at the hardware? No, I dont. In none of my SDRs.

[mzs]

Well technically the RTL-SDR and Airspy do have a really nice internal tracking filter (well probably a whole range of them switched in for various frequency bands) inside the tuner chip. Which I always assume worked something like the below diagram (using varicap/varactor diodes), but there is no public information about how the Rafael Micro tracking filters actually works. All I know for sure is that there are two external inductors (~8n2H to 180nH according to the leaked datasheet each directly connected to two pins on the chip), and everything else to do with this function is internal to the tuner chip.

[M0GLO]

Yeah, you NEED to use external filters on these, they are NOT commercial ready units with all the bells and whistles.
They are dev boards and require quite a few external modules to bring them up to commercial level.
Think of them as really sensitive FET mixers with all the downconversion parts but nothing really up front and you get the idea.
Band pass filters are your friends here.

[racerxdl]

Well that’s sad, but understandable. I though the LMS7002 had tracking filters. So I will try to stick with hardware filters to avoid these issues.

Thanks!

[K6MSM]

It’s mostly sad in that my other SDR’s don’t have this problem.

In the USA, the LimeSDR USB and LimeSDR Mini are plagued with 700MHz LTE images onto the 2m band and there are many other image combinations possible as well. The 1/5th one seems the worst.

Attached is a pic and it gets really bad close to an LTE cell tower. I use cable TV low pass filters to knock this down.

Mike
K6MSM

[M0GLO]

Your other SDR’s more than likely have actual cap and coil front end filters built into them.