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My issue is regards to viewing the FM spectrum using USRP N210 & GNU Radio. I did try to use several blocks of GNU Radio to view an FFT plot of the FM band using the USRP N210 device. My blocks and results are as below,

enter image description here

I used my mobile phone to listen to 90.5MHz FM channel and confirm that there was indeed a working FM channel at that. Actually, there has to be multiple channels in this band.

In many FM radio examples, it is possible to observe multiple bands of FM channels. Unfortunately for me, I cannot see a single one. I feel that there is something wrong with my setting in GNU radio. I post the settings I have applied to each box below.

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As Marcus Müller has requested I ran the command to find the daughter board I am using. My output is stated below.

user@userpc:~$ uhd_usrp_probe
linux; GNU C++ version 5.3.1 20151219; Boost_105800; UHD_003.009.002-0-unknown

-- Opening a USRP2/N-Series device...
-- Current recv frame size: 1472 bytes
-- Current send frame size: 1472 bytes

UHD Warning:
    Unable to set the thread priority. Performance may be negatively affected.
    Please see the general application notes in the manual for instructions.
    EnvironmentError: OSError: error in pthread_setschedparam
  _____________________________________________________
 /
|       Device: USRP2 / N-Series Device
|     _____________________________________________________
|    /
|   |       Mboard: N210r4
|   |   hardware: 2577
|   |   mac-addr: 00:80:2f:0a:d5:64
|   |   ip-addr: 192.168.10.2
|   |   subnet: 255.255.255.255
|   |   gateway: 255.255.255.255
|   |   gpsdo: none
|   |   serial: F384DA
|   |   FW Version: 12.4
|   |   FPGA Version: 11.1
|   |   
|   |   Time sources: none, external, _external_, mimo
|   |   Clock sources: internal, external, mimo
|   |   Sensors: mimo_locked, ref_locked
|   |     _____________________________________________________
|   |    /
|   |   |       RX DSP: 0
|   |   |   Freq range: -50.000 to 50.000 MHz
|   |     _____________________________________________________
|   |    /
|   |   |       RX DSP: 1
|   |   |   Freq range: -50.000 to 50.000 MHz
|   |     _____________________________________________________
|   |    /
|   |   |       RX Dboard: A
|   |   |     _____________________________________________________
|   |   |    /
|   |   |   |       RX Frontend: 0
|   |   |   |   Name: Unknown (0xffff) - 0
|   |   |   |   Antennas: 
|   |   |   |   Sensors: 
|   |   |   |   Freq range: 0.000 to 0.000 MHz
|   |   |   |   Gain Elements: None
|   |   |   |   Bandwidth range: 0.0 to 0.0 step 0.0 Hz
|   |   |   |   Connection Type: IQ
|   |   |   |   Uses LO offset: No
|   |   |     _____________________________________________________
|   |   |    /
|   |   |   |       RX Codec: A
|   |   |   |   Name: ads62p44
|   |   |   |   Gain range digital: 0.0 to 6.0 step 0.5 dB
|   |   |   |   Gain range fine: 0.0 to 0.5 step 0.1 dB
|   |     _____________________________________________________
|   |    /
|   |   |       TX DSP: 0
|   |   |   Freq range: -50.000 to 50.000 MHz
|   |     _____________________________________________________
|   |    /
|   |   |       TX Dboard: A
|   |   |     _____________________________________________________
|   |   |    /
|   |   |   |       TX Frontend: 0
|   |   |   |   Name: Unknown (0xffff) - 0
|   |   |   |   Antennas: 
|   |   |   |   Sensors: 
|   |   |   |   Freq range: 0.000 to 0.000 MHz
|   |   |   |   Gain Elements: None
|   |   |   |   Bandwidth range: 0.0 to 0.0 step 0.0 Hz
|   |   |   |   Connection Type: IQ
|   |   |   |   Uses LO offset: No
|   |   |     _____________________________________________________
|   |   |    /
|   |   |   |       TX Codec: A
|   |   |   |   Name: ad9777
|   |   |   |   Gain Elements: None

user@userpc:~$ 

To support Marcus's second answer, I post an image of the inside of my USRP N210 below.

enter image description here

It would be really great if my mistake can be spotted.

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    $\begingroup$ What daughterboard are you using? (if in doubt: run uhd_usrp_probe) $\endgroup$ – Marcus Müller Nov 3 '17 at 22:04
  • $\begingroup$ @MarcusMüller I have updated the question itself with the information you requested as the output was longer. The antenna I tried earlier was a 868MHz antenna. As strongly suggested by yours and Kevins answer I did plug in wire of long enough length to the USRP and tried its both slots. Still no difference. I have three USRP devices and I checked with two. Both produce the same result :-( Please let me know if you need further debug information. $\endgroup$ – Dina Nov 4 '17 at 7:35
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Your uhd_usrp_probe output shows:

|   |   |   |       RX Frontend: 0
|   |   |   |   Name: Unknown (0xffff) - 0

and

|   |   |   |       TX Frontend: 0
|   |   |   |   Name: Unknown (0xffff) - 0

this is typically the symptom of a not properly connected daughterboard, or one with broken power rails; less often, sign of a corrupted daughterboard eeprom.

Please try to take out and plug back in your daughterboard (you still haven't told us which daughterboard model you're using!), and try again.

The antenna I tried earlier was a 868MHz antenna.

So, no chance you get good reception at 90 MHz, then, right?

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I don't see anything wrong with your basic GNU Radio flow graph, so it is likely a hardware configuration issue.

I'm not familiar with the USRP N210 in particular, so this is a bit generic:

  • In my experience, the most common software reason for getting no visible signal is incorrect gain settings. Is 15 dB an appropriate middle-of-the-road setting or is it too low or too high? I don't know what the minimum and maximum supported by the hardware are, but the FFT plot looks more like an overload than below-the-noise-floor, so try reducing it.

    In extreme cases, such as if you live near a broadcast station, you might even need an external attenuator.

  • You have selected antenna port RX2. Is your antenna actually plugged into that port, or a different one?

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    $\begingroup$ excellent! By the way, when you select "relative" as "gain type" and use "0.9", you get "high gain", no matter what daughterboard you use. $\endgroup$ – Marcus Müller Nov 3 '17 at 22:01
  • $\begingroup$ Dear Kevin, Thanks for your time. I did try different gain settings but my results were same. I also tried by fixing a wire to the USRP to mimic a FM antenna as I was using a 868MHz antenna earlier. Both my USRPs give same result. I can guarantee there is no FM transmitter near where I live. May be its my installation? Perhaps the GNU radio version and the UHD ETTUS client dont match. But then, how do I see some spectrum anyway. I'm a little lost as I cant even do this simple thing. Please do tell me if you need any other additional information. $\endgroup$ – Dina Nov 4 '17 at 9:39
  • $\begingroup$ Dear Kevin, you were absolutely right! the problem was hardware. I didn't know that USRP does not work without a daughter-board. And I hadn't plugged in one. Thank you for your time. $\endgroup$ – Dina Nov 4 '17 at 11:07
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Other common sources of problems:

  • This happens more often than one would think: Are you sure your antenna is suited for the band? I've talked to way too many people that complained about bad reception, then to proceed to show they were using a 2.4 GHz antenna for 433 MHz.
  • Many broadcast antennas you can buy have a 75Ω impedance, whereas all USRP daughterboard have 50Ω. That mismatch shouldn't pose a significant problem, though (you lose a few dB, but broadcast is usually "screamingly loud" compared to a USRP's sensitivity)
  • If you really see nothing but this ca 0.005 amplitude peak in the middle, I'd argue you've got LO leakage. However, you have a very wide "hill" around that, as if lots of interference gets mixed down there. That is strange.
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  • $\begingroup$ Dear Marcus, none of the USRPs I received from lab came with any daughter-boards. When I opened the original packaging of the USRP, it was just a plain single board inside the white metal box. No daughter-board was plugged in. I apologize for this question, but do I need a daughter-board even to receive ? I will attach a photo of the inside of the USRP below. $\endgroup$ – Dina Nov 4 '17 at 10:50
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    $\begingroup$ Yes, you will need a daughterboard. $\endgroup$ – Marcus Müller Nov 4 '17 at 10:52
  • $\begingroup$ I updated the question with a photo of inside of the USRP N210. So is there no way i can even receive a simple spectrum like FM without a daughter-board? Thank you so much Marcus. Can you please guide me to ETTUS documentation I must read prior to using the USRP N210. I really don't want to do any more stupid mistakes like this again. $\endgroup$ – Dina Nov 4 '17 at 11:01
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    $\begingroup$ The daughterboard contains practically all of the analog electronics you need for reception. Yes, a daughterboard is necessary for any kind of usage of an N210. $\endgroup$ – Marcus Müller Nov 4 '17 at 11:04
  • $\begingroup$ I really appreciate your efforts. A sincere thank you. And apologize for wasting time on a problem this obvious. $\endgroup$ – Dina Nov 4 '17 at 11:06

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