Help explain waveform? - Page 1
Forum section: electronics / rf-microwave

author: kenw232
author_rank: Contributor
timestamp: October 23, 2024, 11:16:01 pm
content: I have a signal generator. It will module an AM signal over a carrier wave. From the internet this is what the scope should show:But as shown on the scope.jpg in the attachment I do not get that. Why? Its like the wave on the scope is out of phase by 180 degrees? Even the AWG shows what the scope should show (the same thing), but it doesn't.

author: radiolistener
author_rank: Super Contributor
timestamp: October 23, 2024, 11:30:34 pm
content: Quote from: kenw232 on October 23, 2024, 11:16:01 pmBecause you watching it wrong way. It shows one period of high frequency carrier, but you need to watch for one period of low frequency message signal in order to see picture from the book.You need to change time scale setting from 50 ns/div (your current setting on the screen) to ms range. For example try 1 ms/div.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 24, 2024, 04:32:08 am
content: What frequency is your message?What frequency is your carrier?

author: Ian.M
author_rank: Super Contributor
timestamp: October 24, 2024, 07:05:09 am
content: The AWG photo says its a 500 Hz 100% modulation sinewave on a 3.1 MHz carrier. The diagram from the internet and the image on the SDG1032X function generator screen are cartoons not real life. There are 6200 carrier waves in one cycle of the modulation, but the cartoon WWW image has only nine carrier waves in one modulation cycle and that on the function generator screen about 22 just to show the concept of AM modulation as a visual aid for the hard of thinking.Get the carrier and modulation frequencies within two orders of magnitude of each other, set the timebase time/div to approx 1/5 of the modulation period, and either use the oneshot trigger for the timebase or set the function generator to output the modulation frequency on its other channel and trigger off that, and your scope display will look more like the cartoon . . .Explanation for timebase setting: Most scopes show 8 or 10 major divisions horizontally, so to see one full cycle of a waveform the time/div must be 1/8 or 1/10 of its period accordingly. However as it is often desirable to keep the timebase calibrated (i.e. not to use variable time/div), take the next greater time/div setting and see a bit more than one cycle. The suggested 1/5 will show two cycles on 10 div wide screens and about one and a half cycles on 8 div wide screens.

author: Miek
author_rank: Regular Contributor
timestamp: October 24, 2024, 08:15:29 am
content: What you're seeing is the effect of waveform persistence on your scope and being zoomed in to a really short timebase.Each time it triggers, it captures two cycles of the carrier and it draws many separate captures all overlaid upon one another. (see attached pic)You see that shape because sometimes it's capturing at low amplitude, sometimes it's capturing at maximum amplitude, and everything in between all drawn at once.If you hit the "single" button on your scope, you'll get one capture on its own and it should be clearer what's happening. Tap it a few times and you'll see the sinusoid vary in amplitude.If you zoom out on the timebase and hit single again, you'll start seeing the overall shape as you expected from the diagram.

author: kenw232
author_rank: Contributor
timestamp: October 24, 2024, 12:44:29 pm
content: Quote from: Andy Chee on October 24, 2024, 04:32:08 amIt shows in the screenshots. Message is 500Hz, carrier is 3.1MHz.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 24, 2024, 01:02:15 pm
content: The conventional way to display an Amplitude Modulated waveform is to set your time/div to the same setting you would use to display several cycles of the modulating waveform.That will give you a display of the modulated waveform which looks similar to that on the generator.With such a display, you can determine a number of useful characteristics of the modulated waveform, such as modulation percentage, modulation linearity & various others.The display shown in the OP appears all over the Internet, is "pretty" & looks "high tech", but I have never seen it used for anything practical.

author: Andy Chee
author_rank: Super Contributor
timestamp: October 24, 2024, 03:15:44 pm
content: Quote from: kenw232 on October 24, 2024, 12:44:29 pmIf you are trying to replicate the visualisation in the first image sketched diagram, then the frequency ratio is about 1:10 e.g. Message 500Hz, carrier 5kHz. With your 3.1MHz, you won't be able to view the exaggerated zero crossings of the carrier, unless you zoom in as demonstrated here at time index 8:45