Signal source question

--- 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.

-- John Fields

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.

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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.