Measure Outpur Power With Your Oscilloscope (866)

[@Frankh77]

Your subject matter timing is impeccable. Seems like every time I have a new project going on, you post a helpful video about it! I'm throwing Alexa out of the window tomorrow ;)

[@AC3HT]

Thanks Dave for the presentation. Always appreciate the sit-down discussion topics with the board diagram illustrations and real world application examples. 73, AC3HT

[@subramanianr7206]

Hi Dave, the question is how to measure 100w of RF power on the oscilloscope that the individual would borrow from his friend. You showed a qrp TX power measurement using long coaxial cables, the "cantenna" RF dummy load (!!??) and the probe in ×1X position. The individual will have to use a proper RF dummy load, 30dB attenuator (min) and good quality RF cables. The probe has to be frequency compensated as well. Let him not damage the front end of the DSO!! De VU2RZA

[@BaldurNorddahl]

100 watt is 71 volt rms and the cheap rigol scope can cope with 100 volt rms. So he wont blow up the scope. But he will not get a usable measurement either because max is 10 volts per division with 8 divisions on the screen. To clarify: 100 watt will create a 200 volt peak to peak signal but the oscilloscope can maximum display a 80 volt peak to peak signal. The scope will survive a 100 volt rms (or 300 volt rms) signal but it will not measure it. It is therefore impossible to make the measurement as shown in the video for a power level of 100 watt using a cheap Rigol ds1000 oscilloscope. I would suggest a 40 dB 100 watt attenuator and a spectrum analyser. That is the correct tool for the job and also displays the wanted measurement directly.

[@stargazer7644]

While it is always good to think about these types of things before connecting anything to your test equipment, you really do need to take the next step and do the math to see if there's actually a problem before posting a comment like this. That scope will handle 300V RMS on the scope input (as displayed next to the connector). That means it can handle (300V^2)/50R=1800W. A 100W signal is no problem at all for the scope, no attenuator required, though you could certainly put the probe on 10X if you want. Be sure to also check the probe's voltage and frequency specs for what you're measuring. The cheap probes that come with this level of Rigol scope might need to be on 10X for accuracy. We're talking about HF here at less than 30 MHz, not microwaves. The quality of the cables is pretty much irrelevant at that low of a frequency - there's almost no loss over 3 feet. And if you compensated the probes when you got them, they'll be fine for HF use. That Cantenna is rated for 1500W as I recall. And as for the second comment about displaying only an 80v peak signal, you simply turn the vertical scale up. That scope will go to 100V per division, so it can display an 800V p-p signal which is just shy of the max input limit of 848V p-p.

[@BaldurNorddahl]

@@stargazer7644 The datasheet clearly states on page 1 the following: "Low noise floor, vertical scale range: 1 mV/div to 10 V/div". So no, it will not be able to go to 100 V/div. Which is quite logical given that the scope will blow up long before reaching the 800 V of such a setting. You could use 1:10 probes and the included probes are rated for 300 V rms. It is just not what is demonstrated in the video. With the 1:10 setting activated the scale will of course go to 100 V/div. The next problem is that the scope is not very accurate for this purpose.

[@stargazer7644]

@@BaldurNorddahl The scope inputs are rated for 300V RMS (it is right there on the front of the scope at @3:37, written above CH1 ). 300V RMS is 424v peak, or 848V p-p. You aren't going to blow it up. Whether the probes will do it or not is another question. I don't use the cheap probes that come with these things.

[@BaldurNorddahl]

@@stargazer7644 read my first comment again - I calculated that you would not be blowing it up so no need to repeat me 🙂 But it is also true that you are not going to be able to do the measurement as Dave demonstrates. The measurement will be clipped at 80 V peak to peak trying to measure a waveform that is 200 V peak to peak. To repeat you could use 1:10 probes, cheap or otherwise, to bring the voltage down with a factor of 10 and thereby be able to do the measurement. But Dave does not demonstrate how to do that.

[@MichaelALoberg]

That’s really helpful Dave, thanks!!!

[@jasonbrown467]

i have had a 4 channel 350mhz scope, with built in sig generator for about 1 year now and barely know how to use it, videos like this should help move me in the right direction. appreciate the work!!

[@owenvinall9970]

Thanks David, another excellent video. Your explanation was so easy to understand Owen Vk5kov

[@stargazer7644]

@7:30 This is incorrect. RMS=Peak divided by 1.414 not multiplied.

[@glenmartin2437]

Thank you, Dave. N0QFT

[@oasntet]

Some decent-quality multimeters can also read Vrms for sine waves; my $50 DMM from China manages just fine. You do need to be careful terminating the coax in such a way that you can probe either side of the load, preferably with alligator clips. Most DMMs can also handle much higher voltage, like well above 240, so 100W will be trivial.

[@stargazer7644]

Be careful. Those multimeters are not likely to measure true RMS V up into the megahertz. A Fluke 179 for example, can only measure true RMS up to 1 kHz. It is going to give a very incorrect reading above that. Even a Fluke 87V (a $600 meter) can only do 20kHz below 6V, and 1kHz above that.

[@DavidSmith_W6DPS]

You do have to take care with the DMM bandwidth. Most DMMs are not designed for RF measurements. A signal above the maximum frequency for the DMM will usually read low, if at all.

[@bart99gt]

You'd have to look at a VTVM to measure up into the MHz range. They put a diode either in the probe, or use a diode tube that is effective at much higher frequencies.

[@BaldurNorddahl]

If you are going to borrow the instrument anyway, then please borrow a spectrum analyser such as the tiny SA. The new tiny SA ultra is very nice. And of course remember to use attenuators. This way the instrument will display the power directly in addition to displaying harmonics and other signal quality measurements. Use the correct tool for the job.

[@stargazer7644]

But then you're also going to need a 50dB 100W attenuator (as you mentioned), which will cost you more than the tiny SA. If someone just hooks their 100W rig to a Tiny SA and keys down to try to measure power, the smoke is going to pour out of it and they'll be buying someone a new tiny SA. The max input on a tiny SA is +5dBm or 3.2 milliwatts. I'd say using an SA to simply measure power output is way overkill.

[@BaldurNorddahl]

@@stargazer7644 the old HAMs will tell you to just get a Bird 43 power meter. But those are probably more expensive than the tinysa Ultra plus attenuator.

[@stargazer7644]

@@BaldurNorddahl Just the H series 100W HF slug for a Bird 43 will cost about what the tinySA and attenuator will cost, never mind the Bird meter itself (though you can get them used on eBay for a fraction of that). But you'd have a high accuracy meter that you could use from near DC to a couple of GHz and from milliwatts to 25kW depending on the slug. And no batteries! If you just need a somewhat accurate power indication on HF, MFJ will work fine for between $60 and $250, depending on model. I have all 3 (SA with attenuators, Bird 43, MFJ AWM-30) and if I'm just looking to measure transmitter power output, I do reach for the Bird.

[@SteveBrace]

I would never lend my measurement equipment to someone who doesn't have experience using it. That said, I'd be happy to take the measurement for them... I'm a bit precious with my lab equipment! 73 DE 2E0PTY.

[@j.r.3215]

Also Vp x sin(45) = Vrms.

[@lyfandeth]

Glad your tooth is better today.

[@jeromegrzelak8236]

Mike get a power meter kg6mn

[@markusberg2770]

Vp x .707 = Vrms Vrms = 1.11 x Vavg 1.414 x Vrms= Vp Vavg= .637 x Vp Thank you, Dave.

[@AndrejaKostic]

Sorry, but the statement that the scope has high input impedance, in the context of this video, is simply incorrect: It's 1 MΩ in parallel with 13 pF, which is not very close to 1 MΩ at HF. Trying such measurement with higher power can easily damage Rigol DS1000Z series scopes.

[@paulgrodkowski5839]

Lot of food 😀

[@jeromegrzelak8236]

YEP

[@procerpat9223]

sqrt(2)=1.414 not 1.404