I would like to know what will show up if I put bluetooth or wifi carrier signal (2.4GHz family) in a say 50MHz oscilloscope? Am I going to see something like a rectangular block? And the only information I will get is the amplitude?
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2\$\begingroup\$ possible duplicate of What does it mean when they say 50MHz oscilloscope with 250MS/s sample rate? \$\endgroup\$– user39382Jun 2, 2014 at 6:11
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\$\begingroup\$ Duplicate to a much deeper level than the question title suggests, actually. \$\endgroup\$– Ben VoigtJun 3, 2014 at 0:50
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3 Answers
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If it's a good scope, you'll see nothing at all, just a flat line representing DC..
Ironically if it's an inferior scope with incompetently designed input amplifiers and the signal strength is high enough, you may see a step in the flat line when the poorly filtered RF carrier is demodulated by nonlinearities in the amplifier, but that will only tell you the presence of carrier and nothing else. And if the carrier is continuous, you won't see the steps, so you'll never know it's there.
This is the same phenomenon as AM radios and landline telephones picking up a distinctive "dud - dududud - dududud" noise from a GSM phone on the same table.
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\$\begingroup\$ In case people wonder that last bit is talking about: youtube.com/watch?v=h1mlponX_jw \$\endgroup\$– FizzNov 12, 2015 at 16:17
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Nothing. That is, you'll just see a flat line, as if there's no signal. It's too high frequency to see.
The input stage on a 50 MHz scope is not configured to pass a 2.4 GHz signal. It'll be filtered out, and you'll be left looking at the (probably nonexistent) low-frequency components of the signal.
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The signal you have described is what you would see in on a spectrum analyzer, which is used for RF signals like the ones you are asking about. (You are then viewing relative power across a frequency band and not the signal itself -- it is important that you realize the difference, I think.) None of the signals in that band are continuous -- they are not dedicated simplex channels. What you will see on a spectrum analyzer is transient power spikes when devices transmit. As those bands are for low powered public use, they are not designed for continuous output because the band would become useless quite quickly and the devices would consume enormous amounts of power in an attempt to communicate in all of the cacophony.
There are inexpensive spectrum analyzers available to use for this type of signal analysis, but oscilloscopes are useless for nearly all RF work. Once you have demodulated the RF signal and extracted an Intermediate Frequency, you can use a storage scope to view the intelligence, or IF, depending on the signal.
A good signal to work up your bench skills is something like USB -- because you can easily create a setup to capture and analyze that stream. RS-232 is even better because the framing is very apparent and the data rate is relatively low.