oscilloscope

[clkaczor]

Are four channel scopes practical? How often does someone use all four channels?

[Phaedrus]

I work in the ATX power supply industry, and our ripple measurement application requires monitoring 3-4 voltage rails at once, so four channels is essential for our needs. They're also great for comparing multiple signals side-by-side, like this:




For a hobbyist two channels will probably work 99% of the time, but it's nice to have the extra flexibility four channels gives you. It all comes down to what *you* need out of your scope.

[pickle9000]

The convenience factor is very high, being able to leave a hookup in place saves time.   

[AG6QR]

The number of channels needed depends on what you're doing. 

To take the example of the ATX power supply which Phaedrus brought up.  For some purposes, it might be possible to monitor different rails separately.  If you're just wanting to measure the ripple on each rail independently, you might even be able to get away with a single channel.  But for example, what if you were interested in finding out how the other rails behave as you suddenly put a transient load on the 5V rail?  If you had enough channels, the fastest way to do the experiment would be to probe all the parameters simultaneously as you added and removed the load on the 5V rail.  If you only had two channels, you might be able to probe the 5V rail's load and then probe each of the other rails separately, one at a time, moving the probe around and repeating the application of a load on the 5V rail.  If your time is worth much and you do that job often, you'd probably buy a four channel scope, but for an occasional hobbyist, you might eventually be able to get the information you needed with two channels.

Multiple channels are very useful when several things are varying at once, and interacting with one another, and you want to see the phase or timing relationships between them.

Fully characterizing an audio amplifier might require four channels.  You might be interested in the input voltage and current, and the output voltage and current.  That could be four channels right there.  Or maybe you'd be interested in the voltages at a few intermediate stages along the amplifier, in addition to the input and output voltages.  Or if you have a stereo system with some potential coupling or interaction between channels, you might be interested in left and right inputs and outputs.  There are plenty of ways to use up four or more channels.

Suppose a microcontroller is sending and receiving serial data from a computer, and also producing analog PWM output, and maybe receiving analog input, and something's not working quite right, and you need to debug the microcontroller's firmware.  It might be helpful to see all the relevant inputs and outputs on the same stream of data to see the sequences involved in the malfunction.  When some signals are digital, maybe you don't need the full resolution of an analog channel.  This starts getting into the area where a MSO or logic analyzer becomes useful.  In digital systems, you may want many more than four channels.  For example, it's sometimes handy to watch all the lines on a parallel data bus or address bus.

It's hard to say exactly how much of a premium four channels is worth over two channels.  More channels are always better, but exactly how much better depends on the circumstances.  I'll just point out that I have never heard anyone say, "My four channel scope is OK, but I just wish it didn't have so many channels."

[extide]

If you are thinking about buying a low end hobby scope, then get the Rigol DS1054Z. It has 4 channels, useful stuff like an intensity graded display, and all at an amazing price similar to older low end 2-ch scopes. (~$400 USD). Honestly, at that price, there is no point worrying about getting a 2 or 4ch scope. Now, if you need something more specific, like higher b/w, or higher sample rate, then the price goes up and the options change and well I am not even sure if you are looking to get a scope or not so we can leave that for another time. For the nitpickers, yes the DS1054Z has some shortcomings, BUT for the price it is absolutely the best scope you can get period, seriously a game-changer in that price range.

[Arlen moulton]

I would still get an analogue scope though because they will always have their uses for stuff

[DanielS]

How often does someone use all four channels?

Most of the time?

Even for simple things like debugging an active analog filter, having four channels is nice: you can have one channel permanently connected to the input, another permanently connected to the output and still have two channels left to probe nodes in the feedback path. This spares you the trouble of having to continuously move your input/output probes around to take a peek at something else.

For power supply applications, this allows you to take a look at correlated input and output voltages, which is really handy for multi-rail power supplies. Just like the analog filter bit above, it also enables you to keep an eye on input/output voltages while probing around other nodes for troubleshooting.

Same goes for digital stuff where you might want to keep an eye on a reference signal or two (clock, sync or other) along your other signals of interest.

Four channels is massively convenient if you can get a four-channel scope that meets all your other requirements within whatever budget you can spare.