# What does it mean when they say 50MHz oscilloscope with 250MS/s sample rate?

I want to buy a digital oscilloscope to see how a WiFi module transfers signals and data. But I don't know which specification should I take into account?

Actually I don't know what the difference is when they say, for example, 50MHz oscilloscope with sample rate of 250MS/s?

Now as the Wifi frequency I am interested in is 2.4GHz, does it mean I have to buy a oscilloscope with 2.4GHz input channel?

• That will be very expensive. The cheapest one I've been able to find with a quick search si something like the Picoscope 9201A (£ 6000) – medivh Aug 1 '13 at 7:29
• 50MHz is the analog bandwidth and 250MS/s is the sample rate. You should check out EEVBlog's introduction to digital storage oscilloscopes. – JYelton Aug 1 '13 at 7:35
• You're probably looking for the wrong tool for the job unless you've got very deep pockets. You probably want something that can take the 2.4GHz down to baseband before looking at it. GNU Radio and USRP might be worth a look (although I've never used it personally). – PeterJ Aug 1 '13 at 7:43
• Note that the wifi is quadrature modulated, so it's going to be almost impossible to make sense of it by eye. It will look a lot like a 2.4Ghz sine wave. – pjc50 Aug 1 '13 at 8:17
• If you just want to measure something like amplitude or see "bursts", you can use the 50 MHz scope along with a "crystal detector" (in quotes because I use that as a blanket term for detector here but there are multiple types). Agilent's 8472B is a good example. Basically, the diode does half wave rectification of RF frequencies and the total amplitude envelope will display on your output. This can be used for basic troubleshooting of your RF signals for power and/or switching speed. – scld Aug 1 '13 at 12:41

To say "50MHz oscilloscope" is to say that it'll show signals accurately up to that frequency, but beyond that signals will be attenuated or not visible. This is a soft transition. No scope's electronics is perfect.

As with audio amps, the upper frequency limit is often defined where a sine wave input diminishes by 3dB, that is, loses half its power, compared to lower frequencies, when going from input to display. Read the scope's specs carefully to be sure.

Note that for complex signals or signals with sharp edges such as sawtooth waves, square waves, I2C signals, bluetooth signals, whatever, the upper frequency limit applies to the Fourier spectrum of the signal. Sharp transitions get mushy. The higher the upper frequency limit, the less mushy. A square wave right at 50MHz may appear so rounded off, but probably not quite as smooth as a sine wave.

For sampling rate, if the scope's amplifier and display are good up to 50MHz, and it's not an analog scope (dusty, old, vacuum tubes, ah good ol' days), the signal needs to be sampled at something like 4x or 5x that frequency to actually show a sine (or sine-ish) wave.

Nyquist says something about 2x, but think about it - if you sample a fast sine wave at 2x its frequency, you could by chance be sampling it once as it crosses zero on the way up, and again as it crosses zero going down. Then you'd see flat zero. So sampling is more than 2x. Nyquist still applies, but in having the response of the scope drop greatly before half the sample frequency. For your 50MHz / 250MHz example, there's probably a fast drop in response somewhere between 100MHz and 120MHz. Beyond that range, you won't see anything.

• thank you for your answer. I am not very pro but what I got is that with this Oscilloscope I can just see the frequency up to 120MHz. am I right? So What kind of oscilloscope shall I purchase to see the WiFi frequency? more than 500MHz? – Mehrdad Kamelzadeh Aug 1 '13 at 7:17
• @MehrdadKamelzadeh: As I understand it, to view a 2.4 GHz signal, you'll need a scope with at least 2.4 GHz bandwidth and a sample rate greater than 12000 MS/s. – RedGrittyBrick Aug 1 '13 at 10:03
• There may well be some analog filtering above 50MHz. You should not count on being able to use the scope at 120MHz – Scott Seidman Aug 1 '13 at 20:58

Oscilloscopes, analog or digital, both have an analog bandwidth, which is the limit at which they can display waveforms with a degree of accuracy. You wouldn't get anything useful measuring a 2.4GHz signal with a 50MHz scope.

You need a scope that has at least the analog bandwidth of the signal you want to analyze, ideally 2X. But, as you may soon discover, oscilloscopes get very expensive as analog bandwidth increases. A 3.5GHz Tek scope is more than $30,000 new. You would need such an instrument if you actually wanted to look at the wireless radio waveform in detail. Chances are that you just want to see the relative strength of the RF signal, and for that, a spectrum analyzer would be appropriate. They're still expensive, but a cursory check shows units capable of 2.4GHz or more start at$4000 or so.

To be of any more help, you would need to provide some more information about what exactly you are wanting to look at. Wireless networking is complicated!