1. Home
  2. Hobby Electronics Basics
  3. Signal source question

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.

John

Reply to
John Larkin
Loading thread data ...

--
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,
 Click to see the full signature
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.

Reply to
Mjolinor

an.

half

in

with

this

dissipated

because

impedance

impedance.

sense

sig

in

get

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.

Reply to
Mjolinor

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.