Question about gain definitions and such

Discussion in 'Wireless & RF Design' started by rangeralex10, Aug 16, 2016.

  1. rangeralex10

    Thread Starter New Member

    Aug 16, 2016
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    Hello all. I've been working in the EE and RF field for a few years now, and currently I'm testing out some amp boxes and rf control units. I'm having an issue with some definition of Gain among my coworkers. We can't agree on how to calculate it for a specific purpose. The test procedure our MEs and SEs came up with asks us to give them the Gain of an RF pathway in the RFCU, but doesn't define what "gain" they're talking about.

    Here's a description of the test we run:

    Hook up an Agilent Specan input to the RF output on the RFCU with 50db atten in line and the output of a 6GHz siggen straight to the RF input on the RFCU(standard I know, bear with me). Pipe in a 2.4 GHz signal at about 7db input power, it runs through our LNAs, Diplexer, and Bias Tees, through the 50db atten, and reads about -29db on the Specan.

    We've been asked to calc gain as (Atten + Specan db) - input power, so the above example would be (50 + -29) - 7 = 14. The spec sheets for the amps we're using state it should be around 35db +/- 5, not 14. So would that be the gain we should be calculating? Or is there a different formula?

    I'm sure this is a simple question for some people, but we've been having it out over a week about this and no one can agree. Degrees and experience not withstanding, we all feel pretty dumb about this for some reason.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    From your description I believe they are interested in Power Gain; but your description is difficult to follow without a diagram. One dead giveaway for a state of confusion is the "7 dB input power reference". Anything measured in dB is relative to something else. Is it possible you mean 7 dBm, which is 7 dB with respect to 1 milliwatt? Next 50 dB of attenuation should have a minus sign associated with it. What I think is that you start with +7dBm out of the signal generator, there is an unknown attenuation(gain) X due to the LNA, diplexer, bias tees and such followed by 50 dB in the attenuator and finally you have -29 dBm on the specan.

    +7\;\text dBm\; + X \text dBm\; -\;50  \text dBm = -29 \;\text dBm

    So just like you I get

    \text X\;=\;+14\; \text dBm
     
    Last edited: Aug 16, 2016
  3. rangeralex10

    Thread Starter New Member

    Aug 16, 2016
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    Correct, I mean +7dBm, and yes -50dB of attenuation. Thanks for the concurrence! It's good to have some feedback from an outside source.

    I will try to upload a diagram when I get one...The sys eng guys still haven't given us testing folk a diagram, because you know, we're too neanderthal to use one right? :-/
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    I don't suppose it implies you are too Neanderthal to make one.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    It just occurred to me that if -29 dBm was actually -2.9 dBm then the gain X would be 40.1 dBm which is just outside the range of 35 ± 5 dBm. Is it possible somebody dropped a decimal point somewhere?
     
  6. rangeralex10

    Thread Starter New Member

    Aug 16, 2016
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    No, no one dropped a decimal.

    As for making a diagram, I don't have the software to make one, outside of a very basic sketch.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    It is the way we made them when I was a young engineer. It's the only way to go when there are impediments, such as the ones you face. MSPaint is something you probably have access to, give it a try.
     
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  8. rangeralex10

    Thread Starter New Member

    Aug 16, 2016
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    It looks something like this. Again, sorry for the mspaint and thnaks for the help.

    RFCU diagram.png
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    That's not bad. I kinda like the creative symbology.
     
  10. rangeralex10

    Thread Starter New Member

    Aug 16, 2016
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    Ha thanks.
     
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