Signal source question

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 inside.

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.

0.4472v--------Rint50--------+-----------Vout | | | Rload | | gnd

so it's just like a DC voltage divider.


Reply to
John Larkin
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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,
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Reply to
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.

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