#46: Basics of amplitude and distortion measurements on a scope and spectrum analyzer

This video (by request) takes a look at making signal amplitude and distortion measurements on an oscilloscope and a spectrum analyzer. It runs through a quick example of measuring the signal amplitude on the scope in volts peak-to-peak, then converting this measurement to dBm and verifying that this is what is measured on the spectrum analyzer. The importance of maintaining a common load impedance for these measurements is discussed. The second half of the video discusses distortion measurements, and what you can and cannot see on a scope compared to a spectrum analyzer. It shows that while a scope is really good at making amplitude measurements (and can be used to measure frequency as shown in a previous video), it isn't necessarily the best tool to measure distortion at levels that are required to meet FCC Part 97 Emission requirements for amateur radio use.

[@christheother9088]

It's amazing how much you can learn during a cup of coffee.

[@Alucard99999]

Thanks so much for the wealth of knowledge you share by making these videos!

[@worldwide1376]

You are one smart cookie! Great demonstration on a great use case for a spectrum analyzer. Cheers.

[@richpaul8132]

Far over my head, (not even on the same planet), but for some strange reason that I can't explain, it seems to make sense to me... I've always been fascinated by microelectronics and when I look at something like a high-performance PC motherboard, I feel like the designer had to be an alien in order to come up with the way it all comes together, (and don't get me started on the how a CPU knows what to do and when to do it). The entire theory, development and operation of these incredible systems is mind boggling. I watched this video three times and I can honestly say that I couldn't come close to setting up a test like this. However, it does take some of the 'mystery' out of my predominantly blacked-out understanding of electronics as a whole! Thanks for the the great explanation! Rich

[@cbradiomizfit7562]

I've seen several of your videos and find them a very useful part of my hobby and testing procedures. I decided to subscribe

[@kissingfrogs]

Thank you so much I am thoroughly enjoying your collection of videos.

[@KB4QAA]

Great demonstration! That definitely gets the idea across.

[@JohnRaschedian]

This is the perfect place to learn electronics. Thank you!

[@ruhnet]

Superb video with some great advice. Thanks!

[@jimadams2473]

Another great instructional video. THANKS!!

[@577buttfan]

This dude is so smart I love his videos!!

[@Myccsf]

Thanks for the video. It help me a lot while doing my own THD.

[@WPF465B]

Another great video, thanks.

[@scotthovey4828]

Very well done! Look forward to watching more of your videos. Also, Thank-you for numbering your videos.

[@w2aew]

The numbering also comes in handy for my video index file.

[@PawelGrodkowski]

@@w2aew Fucked Up It is a wonder what FUCKING CANADA can do to some of the real countires that count!!!!!😀✊✊✊✊✊

[@keithostertag]

Good question, great answer! #2 will be very useful to me (wished I had thought of it myself...!) Thanks!

[@nibaron_13]

Hidden gem in YT

[@stevepence9869]

Learning a lot.

[@electronicatutorial]

The power suite program that comes free with that SA it has a one bottom Harmonic Distorsion measurements that does allows you to see all the harmonics power levels, does that measure take into account the analyzer own distorsion characteristics? if not how would you know it?

[@kevingray707]

Love the video.

[@electronicatutorial]

To properly drive a micophone (600 Ohm) to a 1 Mohm imput, can I just use a 600 Ohm Feed-thru termination so my transfer of energy would be at maximum right?

[@Pwaak]

Very Interesting! Thank You!

[@zloben9000]

great vid. man

[@FUNKLABOR_DL1LEP]

Great show! 73

[@KozmykJ]

Oh I get it. I was thinking of a variation on viewing carrier and modulation by switching the timebase but of course the time difference isn't enough for the 2nd, 3rd, 4th harmonics. Then of course there's the diminishing amplitude as you pointed out. Thanks for the quick reply.

[@electronicatutorial]

Is it possible to use a Lock-in Amplifier to see the levels of distorsion?

[@petopeter4832]

my best teacher

[@hugoribeiro8281]

Very good 👌

[@subhasarkar8823]

Thank you Alan :)

[@jimfraziernm]

I am just learning to use my spectrum analyzer. What were the spectrum analyzer settings for RBW and video BW for this demonstration?

[@PatentSTM]

very good vid.

[@gbowne1]

If you only had certain components in stock what would you build if you were into building RF circuits.

[@xyloeye]

Alan, great video as always. When measuring LF audio signals on a spectrum analyzer, what's the best way to attenuate the signal to avoid damage to the mixer? If I measure the signal at clipping, the voltage could approach the rail value. Thanks

[@w2aew]

xyloeye Apply the signal to an appropriate load, then stick a resistor in series to the Spectrum Analyzer. This resistor will form a voltage divider with the 50 ohm input impedance of the spectrum analyzer (provided the analyzer goes down to audio frequencies. For example a 450 ohm series resistor will attenuate the signal by a factor of 10.

[@xyloeye]

***** Oh, duh. Too simple. Thanks, Alan. Ken

[@spectralcodec]

Thanks for the great video! If the distortion was bad enough to see on the scope what would it look like?

[@hyperboogie]

Since most of us can't afford such spectrum analyzers, is it possible to see these harmonics using the FFT function on a digital scope?

[@786rabdul]

I didnt got the meaning of your sentence that "scope timebase is in its calibrated position" plz explain........today also i tried but still got the same result ,i am not getting that frequency which i m giving from function generator.and i m totally clear about my procedure of givng input ders no mistake....My scope is APLAB DUAL TRACE .Thnx for reply.

[@radiotestfr1232]

Hi, Can i mesure it with simple one oscilloscope and FFT ? How can i do ? Thanks for all your work. ;)

[@w2aew]

Yes, you can. I already have a video or two discussing the use of FFT on a scope. Just beware that a scope channel has more inherent distortion than a spectrum analyzer - so don't be surprised if you see harmonics that are 40-50dB down on a perfect signal which may be distortion from the scope.

[@tronotytronoty1530]

I just saw your Rohde Schwarz SML01 generator in this video. Can you make a video showing this instruments and its functionality?

[@w2aew]

+tronoty tronoty I'll add that to my list.

[@jackofalltrades3727]

Have you done a video on measuring total harmonic distortion using a oscilloscope on FFT and calculating the sum of the harmonics ;if this is possible the calculation looks complicated to me Thanks

[@w2aew]

I have now - here's the link: kzbin.info/www/bejne/qZDGh4NrnNpnibs

[@MrMac5150]

Your the Best

[@786rabdul]

Hello i am giving 1K frequency from function generator but on CRO after calculating i am geting only 256Hz ,whats wrong here ,why i didnt got 1K freq or near about of it anybody plz Help.........

[@keithostertag]

Slightly OT- I notice that when you measured the trace on your scope, you used the "inside" edge of the trace. Not sure, but it made me wonder if you instead used the "outside" edge of the trace that may have impacted your final scope dBm calculation. Is that standard/conventional practice to use the inside edge of the trace for the most accurate measurements? I realize this makes no difference in the relative harmonic distortion component of the signal.

[@ernieschatz3783]

Question: You used a "50 to 75 ohm min loss pad... " on the spectrum analyzer. On our oscopes that don't have selectable input impedance, we use a 50ohm resistor on the scope end for probless measurements using 50ohm coax. What is the advantage to using the 50 to 75 at your specan vs just a 50 ohm pad? Is this needed because the specan input impedance is much lower than an O scope?

[@w2aew]

That specan had a 75 ohm input impedance (designed for the CATV industry). That’s why I used the pad.

[@ernieschatz3783]

@@w2aew Okay, so I guess what I'm asking is that since you chose that 50 to 75 ohm pad for a specan with a 75 ohm input inpedance. Does the much higher input impedance (1 megohm) of an O scope negate the need for using say a 50ohm to 1 megohm pad. It's a theory question.

[@w2aew]

@@ernieschatz3783 When bringing low frequencies in (lower than RF), you can usually get away with just the direct connection to the 1Mohm input. For RF, you would want the coax to be terminated into 50 ohms - either using the scope's internal 50 ohm termination if it has one, or through the use of a 50 ohm feedthru termination.

[@ernieschatz3783]

@@w2aew Yes, we use a 50ohm feedthrough termination for our fixed input impedance oscopes regardless. I'm still not understanding why you don't just use a 50ohm feedthrough vs the 50 to 75 pad for the specan even though it has a 75ohm input impedance. We don't use a 50ohm to 1megohm pad for oscopes.

[@w2aew]

@@ernieschatz3783 If I put the 50 ohm feedthrough on the specan, then the 50 ohm would appear in parallel to the 75 ohm specan input, resulting in a 30 ohm termination. There is no such thing as a 50 ohm to 1Mohm pad, because the 1Mohm is not a transmission line termination.

[@karielf8947]

This is very intresting thanks a lot, I guess that the SA corrects the FT of signals, cause in this case you have a sin in a temporal window, and TF would have a constant of T/2 for voltage, is my analysis true? thank you

[@w2aew]

I don't understand your question.

[@karielf8947]

The fft of à sinus in à T Windows gives à fourier transform with amplitude T/2. This has not bien s'en here, is because the sa correct the amplitude ?

[@w2aew]

@@karielf8947 Yes, SA and FFTs in instruments are designed to give you an accurate amplitude measurement.

[@karielf8947]

thank you@@w2aew

[@mdzacharias]

I own a Sound Technology 1000A FM generator, and I've always wondered WHY the output is calibrated for 50 ohms when a standard FM receiver antenna impedance is 75 ohms - the very intended purpose of this machine is to test FM receivers, yes? Perhaps you could shed some light? I just use it "as-is" with no adapter; I'm just using it for a source and not for measurements.

[@WestCoastMole]

Mark way back when, when radio was in its infancy research found that cable with a 30 Ohm characteristic impedance was the optimum for power handling requirements. Cable with a 70 ohm characteristic impedance worked best to minimize signal loss. So a compromise was struck to settle on 50 ohms. The Cable TV industry selected 75 Ohms in the early 1970s. During the 1970s one could also find 52 ohm Coax but that has fallen away over the last 30 some odd years and 50 ohms has become the standard. What's a couple of ohms among friends. ;-)

[@KozmykJ]

If you didn't have a SA to hand couldn't you go searching for the harmonics with the scope?

[@kissingfrogs]

I was pondering this. As we know the frequency of the harmonics we have a start but I suspect at 40dB down the scope would be struggling to detect it.

[@ernestb.2377]

...and of course always paying attention what level in dBm is allowed at the input of your measurement equipment. Scoop is probably more "forgiven" but most spectrum analyzers would not tolerate much on the input, as low as 1dBm! Otherwise you can blow up your very expensive equipment.

[@jimmyc451]

300mV p-p is NOT 150 rms, that is 150 mV peak. P-P X 0.3535=Vrms or V peak x .707 .

[@w2aew]

Yeah, I mis-spoke, but I did handle the number properly, converting 150mVpeak to rms by dividing by sqrt(2)... ...same as multiplying by 0.707.

[@peteleoni9665]

....................... Useful info starts @ 6 min