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1940556 No.1940556 [Reply] [Original] [archived.moe]

I want to measure the transmit power of a radio (at say 20MHz) using pic related. The scope's inputs have 1 MegaOhm impedance. Can I just connect the output of the radio directly to the scope, or do I need some kind of 50 Ohm adapter? I expect the radio to output approx 10 mW.

>> No.1940561
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1940561

also, do i need to use a T adapter with a 50 ohm dummy load, or can i connect radio to scope directly?

>> No.1940594

>>1940556
>Can I just connect the output of the radio directly to the scope
"directly" as in a series connection? No i dont think so. As far as i know, ossilloscopes only detail voltage, so you can only ever connect in parallel to the circuit/element you want to measure. Other than that connect it to whatever the fuck you want unless you live in an industrial plant.

>> No.1940595

https://www.ab4oj.com/test/pwrmeas.html

>> No.1940615

>>1940595

Thanks. The article doesn't mention the scope's impedance (50Ω, 1MΩ, etc). Does this mean it's irrelevant to the measurement?

>> No.1940839

>>1940615
>scope's impedance
Irrelevant as long as it's high compared to the voltage divider output impedance (1K in this example). 1 Meg is high.

>> No.1940850

>>1940556
maybe you are getting mixed up op, with power transmission often impedance matching is talked about to maximise the power transfer into the driven load.
an oscilloscope you don't want to suck a huge power load, you want it to draw as little current as possible and the reason is because you don't want it to have an effect on what you are measuring, the worst thing you can do when measuring a circuit is to change it by measuring it, because then you aren't measuring what the circuit is doing, you are measuring what the circuit PLUS the oscilloscope is doing which isn't how you are going to be using it at the end of the day.
so a relatively high impedance means it won't draw much current means it won't make so much difference to the circuit, which is what you want.
you want the impedance of any voltage measuring device to be as large as possible
if you think about it, measuring current you split the circuit, and so in this configuration you want as low an impedance as possible

>> No.1941056

>>1940850

thanks

>> No.1941081

>>1940556
You get maximum power transfer when you match impedances. Since whatever circuitry your presumably 50Ω antenna connects to in its normal application likely is 50Ω terminated as well then if you want to get an accurate power measurement you will also need to terminate it into a 50Ω load at your scope as well. This can be done with either a T-adapter with a 50Ω terminator or some scopes have a selectable 50Ω mode which terminates them into an internal 50Ω load and you could hook the antenna up to the scope directly then. BEWARE: It should state somewhere on your scope what the max allowed voltage, or some may state the max power, instead for 50Ω is, it's a lot lower than the max for the typical 1MΩ input. If you exceed this rating you may seriously damage your input scope's input circuitry!

>> No.1941091

>>1940839
For low frequency maybe, not so simple at RF

>> No.1941628
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1941628

>>1941081

Unfortunately I don't have a 50Ω terminator and the scope can't be set to 50Ω. My understanding is that if there is an impedance mismatch between the radio (50Ω) and scope (1MΩ), then current and voltage will not be in phase. So I cannot use P = Vrms^2/R to calculate power. If I can't do anything about the impedance mismatch, is there a formula I can use to calculate power?

>> No.1941636

>>1941628
the transmit power depends on the transmitter and the antenna. connect the transmitter and antenna and then connect the scope across the antenna and because it is high impedance it won't affect the result too much by adding it.

if you have just the transmitter the output power is 0 right because there is no load.
if you have the transmitter and the scope then you are only measuring the power that the transmitter can drive into the scope, if the scope had a 50 ohm setting and you connected the transmitter to it the scope would probably fucking explode because its probably not designed to handle that kind of power input.

if you don't have an antenna then get a 50 ohm power resistor (something you know can handle roughly the power output without exploding) and connect the resistor to the transmitter and then measure the voltage across the resistor using the scope.
the 1Mohm scope and 50 ohm load presents as a 49.9975 ohm load which is close enough and well within what resolution your resistor will be or any kind of changes due to room temperature even.

20MHz on a 50MHz scope will look like dogshit probably and 10mW yeah ok maybe the scope could handle it.

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