Does a digital multimeter have a square law device on the board?

Discussion in 'General Electronics Chat' started by Henry Sipes, Apr 16, 2012.

  1. Henry Sipes

    Thread Starter New Member

    Apr 16, 2012
    2
    0
    Good Day,

    I have built a radio telescope and I am currently reading the changing voltage across the satellite meter in millivolts. I can produce a very nice drift scan of the Sun. Professional radio telescope receivers use a square law device in the detector. I am using a simple Channel Master satellite meter and a digital multi-meter to read the signals from a dish with LNB. Does my digital multi-meter have a square law device? Is the d-c output voltage proportional to the input-voltage amplitude squared? Or is the square law device on the Channel master board and I am simply reading the output with my multimeter?

    Thanks,
    Henry
     
  2. #12

    Expert

    Nov 30, 2010
    16,338
    6,824
    I have never heard of a multimeter that does anything except measure voltage or current. They don't do math.

    That's 40 years of experience talking, but it is also a person that might be outdated.
    I think I'm right.
    Anybody else know if I'm wrong?
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    The ones with true RMS have something that does some math, I think it's some kind of integral maybe it has a squaring function?
     
  4. Stuntman

    Active Member

    Mar 28, 2011
    181
    47
    Do you mind if I ask what kind of DC values you are trying to read from your satellite meter?

    ETA: Your setup should have some kind of meter (be it digital or analog) for measuring the signal intensity directly from the LNB of the dish. Does your model have some kind of DC output that corresponds with this?
     
    Last edited: Apr 16, 2012
  5. crutschow

    Expert

    Mar 14, 2008
    13,044
    3,244
    A square-law device has an output proportion to the power of the signal (i.e. V squared). A diode biased in its logarithmic region will act as a square-law device. For low level high frequency signals a diode with a low forward drop, such as a germanium, Schottky, or back (tunnel) diode work well.

    No common digital voltmeter does this.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    300
    It is possible that the equipment monitoring the level from your dish may be producing a logarithmic level indication, in decibels or related units (such as "S" points).

    I know that this is often done with received level indications in general, to cope with the relatively big changes in magnitude which are typical over the working range.
     
  7. Henry Sipes

    Thread Starter New Member

    Apr 16, 2012
    2
    0
    Stuntman,

    I am using a fairly new TurboPro HD dish that has 3 LNBs but I am only using the center one. I am passing the signal to a Channel Master meter. I am reading the voltage that the meter sends to the indicator meter (analog display - red bar that rotates back and forth as the signal increases or decreases).

    The LNB itself must have a square law device correct or I would not be able to plot a drift scan of the Sun. I am trying to understand what part of my system is really the receiver. Descriptions of LNB devices describe the signal being fed into a Low Noise Amplifier, a Mixer, and then a L Band Amplifier which outputs the signal to the Channel Master meter. This sounds very similiar to radio telescope receivers which have an RF amplifier, mixer, IF amplifier followed by a detector. Kraus 1986 describes the detector as a Square Law Device in a radio receiver.

    I have found with this radio telescope project that many folks do not understand the electronics on the inside. They can explain the astronomy side and how the signal can tell you the temperature of the source, etc. but not really how the guts work. As an engineer (mechanical), I am trying to understand the guts. Channel Master would not give me the time of day when I asked them to explain their board - no longer supported, etc. etc. I am moving to the next step which is to install a LabJack device and then read the signal on my sound board. However, I wanted to relate my current readings through my multi-meter with the new setup.

    Thanks for your comments. Any further help is appreciated.

    Henry

    Reference:

    Kraus, J., 1986, Radio Astronomy, Durham, N.H., Cygnus-Quasar Books
     
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