What is a reason for radio receiver noise?

  1. Dear colleagues.
    please help in solving the actual problem:
    Our company operates in the field of space communications .
    For reception and transmission of signals via satellite is used parabolic antenna with a diameter of 12 m
    Frequency of received signal is 4000 MHz.
    The receiver is a Low noise amplifier with an effective temperature of noise 40 K.
    At times, the noise level in the receiving circuit increases sharply in 8 times, then after some time, it returned to normal.
    The same antenna near shows no increase in noise.
    What might be the cause of the noise ?
    How can we remove it ?
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    Sounds like a faulty LNA. Have you tried replacing it with a new LNA?
     
  4. Yes, we have tried to replace LNA. The result is the same
     
  5. davenn

    davenn 3,370
    Science Advisor
    Gold Member
    2014 Award

    so to save us all playing 20 questions .... tells us all the steps you have taken to isolate the noise problem
    start at the antenna and go right through to the signal out

    tell us more about your system ... the feed system on the dish, the feedline type how the signal is being processed etc etc

    cheers
    Dave
     
  6. So, this is the antena and feed system:
    [​IMG]
    1- 12 m parabolic antena
    2- counterreflector
    3- horn
    4 -polarizer, Tx- LHCP, Rx- RHCP
    5 -Tx/Rx coupler
    6 -Tx/Rx filter or izolator
    7 -Tx waveguide
    8 - Rx coaxial feed
    9 - Rx waveguide
    LNA - "LNR" company, 3600-4500 MHz, 60 dB gain, 38 K
    Tx C-band: 5900 - 6400 MHz
     
  7. How

    How exactly are you measuring the noise. (When you say "8 times more noise" do you mean 9dB higher noise floor at LNA output, 9dB worse SNR after DSP etc ...)

    Have you tried disconnecting the TX path, terminating the TX input to the coupler?

    Have you tried installing a waveguide termination on the antenna side of the LNA and verifying the noise figure of the LNA without antenna?

    How long is the RX coaxial feed (8). If it is lengthy you may want to drag your spectrum analyzer out to the LNA.

    You say that you have another system that does not exhibit this problem, how far away is it (miles, feet...)?
     
  8. Trying to reply Your questions:

    "8 times more noise" = 9 dB higher noise floor at LNA output, and 9 dB worse SNR after D/C,
    9 dB worse Eb/No for modem operation.

    We have tried to put a piece of beyond waveguide at TX input to the coupler
    ( Beyond waveguide has very big loss for Rx signals).

    We have tried to replace the LNA

    RX coaxial feed (8) is 15 m.

    Another system that does not exhibit this problem, is 100 m far .
     
  9. jim hardy

    jim hardy 4,328
    Science Advisor
    Gold Member
    2014 Award

    100 meters away......

    It's always worth a look at the physical layout. Is this on a rooftop or alone in a field far from any buildings?

    And you say this is intermittent? What else is close to the troubled system?

    I once solved a similar problem with a metal detector, the kind you walk through like in airports.
    Microwaves from a nearby communication link somehow got into the airconditioning ductwork on roof of the building and poured out of the ceiling vent just above the metal detector, upsetting its electronics when the microwave link was active.

    I wouldn't be a bit surprised if a nearby kitchen microwave oven has a bit of leakage around a door seal. If there is one nearby, brush a small flourescent tube around its door with room lights darkened. Leakage will make the tube glow.
     
  10. jim hardy

    jim hardy 4,328
    Science Advisor
    Gold Member
    2014 Award

    ps - all we had to do was move the metal detector out from under the airconditioner vent.
     
  11. berkeman

    Staff: Mentor

    Amazing detective work Jim! :smile:
     
  12. I had a situation where motor noise from an elevator caused problems.

    I still don't understand what is common between the problem antenna and the one that 100m away. Is that a complete system 100m away? or is there a feed from each antenna? Is there an LNA at each antenna? etc

    EDIT: Sorry --- somehow I missed the picture
     
    Last edited: Feb 22, 2014
  13. davenn

    davenn 3,370
    Science Advisor
    Gold Member
    2014 Award

    OK thats some answers :smile:

    how regular/irregular is this change in signal ? several times an hour ? several times a day ?

    what happens if you disconnect the coax at the RX divider ( exactly what is that? is the RX signal being split to other places other than the SA you have shown ?) and replace the coax feed with a dummy load ?
    try that and see if the problem remains. That will localise it to the receiving system or to the coax/LNA and rest of the feed system

    if the problem still remains then look at your RX system/SA for a fault
    if the problem disappears, disconnect the LNA and try the dummy load on the end of the coax at the LNA end

    You need to be methodical with your fault finding .... work your way through the system section by section, eliminating each stage as the source of the problem
    Once you have proven no problems with your system, the only real choice left is intereference from some other RF source nearby
     
  14. vk6kro

    vk6kro 4,058
    Science Advisor

    There are two obvious sources of radio noise in the sky.

    The sun is one and the reflected sun noise from the moon is another.

    If the moon is visible behind the satellite, then the dish may pick up reflected noise.

    Far weaker, but still there, is the noise generated by the planet Jupiter.
    This noise was the beginning of Radio Astronomy.

    So, if you are confident that your equipment is not faulty, then maybe you could check on the position of the moon and sun.
     
  15. Yes, we have good predictions for sun and moon interference, certainly we take it into account
     
  16. Well, dear colleagues, thank you for your help.
    Actually we already eliminated this problem. And it was even 10 years ago. For the first time when the failure appeared, we do not know what to do. Thank God quickly found treatment. Conditionally it looked like this: our operating channels worked at frequency of 6200/3880 MHz. When there is an obstacle, we spent 10 minutes to fine-tune antenna, check the power of the HPA. Then someone offered to send power transmission on the next frequency 6150/3820 MHz and a hindrance was gone ! We did not change frequencies of operating channels, only used "empty" channel on the next frequency. Further we had time, slowly deal with the problem. It took almost a month. So the source of the noise, we found it in our opinion were noises by the LNA. The LNA has an input impedance of 50 Ohm and according to the Boltzmann law emits the white noise with a temperature of 300 K in the direction of feed and horn. This noise somewhere in the feed is reflected and returned to the input of the LNA.
    After the antenna has been modernized by replacing feed, horn and others, the accident never reappeared.
    Left with two questions:
    Why the noise of LNA is reflected in the feed ?
    Why adjacent channel power elliminates noise ?
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook