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Signal source question

Discussion in 'Electronic Basics' started by Mjolinor, Feb 5, 2005.

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  1. Mjolinor

    Mjolinor Guest

    Think maybe it's been too long since I had to do this sort of thing but I
    can't get my head round it. Maybe a bit of basic help will sort me out.

    I have a 50 ohm sweeping signal generator and a 50 ohm spectrum analyser.

    Set the output of the sig gen to 0 dBm (1mW) and connect it to the spec an.
    I get a flat trace at 0dBm on the spec an.

    Does this mean that the sig gen internals are producing a fixed 2 mW, half
    of which is being dissipated in the 50 internals of the sig gen and half in
    the spec an?

    If that is the case and a third 50 ohm device is connected in parallel with
    the previous arrangement then the spectrum analyser trace will drop. At this
    point is the sig gen still producing 2 mW and there will be 1/3 dissipated
    by each of the devices or is the output of the sig gen unspecified because
    it isn't driving what it was designed to drive.

    I want to connect the sig gen / spec an arrangement to an unknown impedance
    and use the spec an trace to give me an indication of the unknown impedance.
    I crunched the numbers for an equation based on the above but I get no sense
    out of it. The problem is, I think, somewhere around whether or not the sig
    gen is a current source or voltage source, assuming either doesn't give
    sensible results and I reckon the truth is that the sig gen is somewhere in
    between maybe being constant power but even when I assume that I don't get
    what I think I should.
  2. John Larkin

    John Larkin Guest

    It's easier to work in voltage.

    Assume your sig-gen is an ideal sinewave source in series with an
    internal 50 ohms.

    Connect the gen to an external 50 ohm load, which it's advertised to
    dump 1 mw into. So the voltage across the load is 0.2236 volts RMS
    (which makes 1 mW into 50 r)

    So we have a generator feeding a 50:50 ohm resistive voltage divider
    giving us 0.2236 out. So the ideal generator is making 0.4472 volts

    Now just treat situations like you propose as a simple voltage
    divider, do the ordinary stuff, and once you know the voltages you can
    convert any element's voltage back to power and dBm if you like.


    so it's just like a DC voltage divider.

  3. Mjolinor

    Mjolinor Guest

    I did it that way too but still don't get sense out of it. It's probably my
    spreadsheet equation that has brackets in the wrong place :)

    Is it reasonable to treat it as fixed V, I thought that it was better to
    treat it as a fixed power to be dissipated by total |Z| with variable V and
    I from source, it got horribly messy and didn't work :). I suppose it does
    depend on s[ecific design of sig gen though.
  4. John Fields

    John Fields Guest

    It's better if you treat the internal source as a stiff voltage
    source, since that's what it probably is. To find out for sure,
    measure the unloaded output voltage from the generator with a
    high-impedance device. It should be precisely twice what it is when
    it's loaded with a proper load. If you wanted to, you could load it
    with all kinds of non-inductive resistances, calculate what the output
    voltage should be with that load, and then back into what the source
    voltage should be for that load. It should stay constant. If it is,
    and you do that for a few loads above and below the nominal 50 ohms
    and calculate the power dissipated by the load resistance, you'll find
    that it's at a maximum when the load is precisely 50 ohms.
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