Combined oscilloscope and function generator any good?

[Moset]

I've been idly shopping around for an oscilloscope over the past few days, for audio signal testing (ringing, etc.) and also some perhaps some simple power supply stability/noise testing, logic signal checking etc.

I came across the Velleman PCSGU250, which is PC-based and comes with a signal generator. I realize it will likely not be up to par with a proper analog oscilloscope, which would cost about the same though second-hand.

I could use a proper function generator as well, though that's harder to find cheap. Does this combination tool seem at all useful, or are the individual parts to crappy to be of any use? Would I be better off spending my money on a proper scope and just building a simple square-wave generator out of a PIC or something?

[tomchr]

Depends... Some people love the PC-based tools, others hate them. Personally, I prefer stand-alone test equipment -- especially the fundamental ones (multimeter, oscilloscope, signal generator) - over PC-based equipment. I like having a real front plate with knobs and indicators. Turning a knob with my fingers is an intuitive process to me. Clicking on a knob on a computer screen to turn the image of the knob is not.

In general, inexpensive PC-based tools tend to be of rather low performance. For oscilloscopes, the input sensitivity is limited and maximum input voltage is typically pretty low. In addition their bandwidth tends to be pretty low so you may not see the aberrations on the square wave indicating circuit trouble.

As for the signal generator, there are programs out there (TrueRTA is one I think) that can generate the common waveforms through your sound card. Add an op-amp buffer (LME49710 comes to mind) and you're set. Maybe throw in an attenuator and volume control for finer control of the signal level.
Or pick up a used HP 3312A signal generator for $100-ish on eBay.

~Tom

[Moset]

I was afraid the soundcard wouldn't be completely up to it at higher frequencies. Did some quick test playing and recording the same soundcard and I got terrible ringing. Could be the recording part that wasn't up to par, though. The 'scope above has a max input voltage of 30V, which seems pretty low. That's too low for even one rail of my amp.

Thanks for the input, I'll start chasing used analog scopes again.

[grenert]

I can't comment on the specific scope you're looking at, but most oscilloscope probes are 10X, or switchable 1X/10X, so you really could go up to 300V.

[tomchr]

Originally Posted by grenert

I can't comment on the specific scope you're looking at, but most oscilloscope probes are 10X, or switchable 1X/10X, so you really could go up to 300V.

The probes are only 1x, 10x, 100x, etc into a 1 Mohm load. I don't know what load a PC scope represents, but if it's different from 1 Mohm or not specified at all, I'd shy away from it just for that reason.

~Tom

[tomchr]

Originally Posted by Moset

I was afraid the soundcard wouldn't be completely up to it at higher frequencies. Did some quick test playing and recording the same soundcard and I got terrible ringing.

If a square wave is what you want, you could run a sine wave from the sound card into a fast comparator. Follow that up with a buffer such as the LME49600 or a fast op-amp configured as a buffer and you're in business. The amplitude of that square wave is fixed, though. I suppose you could put in a "volume control" between the comparator output and the buffer...

~Tom

[Plammox]

Hi Moset.

I own a PCSGU250 and in general it's pretty useful for the beginning hobbyist with no budget for a scope and a function generator.

The GUI is quite well made and responsive and packs a few things you can't do with normal scopes/function generators, such as bode/FFT plots. And saving the measurement snapshots. I'm annoyed by the lack of a Linux port. I'm not investing in a Win7 license just to run this thing on my Eee. (on Win7 RC1, this worked fine by the way). I'm still irked by the fact you need to click around in the GUI, instead of just grabbing the buttons of a normal analog scope.

The whole thing is powered via USB, so make sure to have you power supply plugged in, to not drain the batteries (should you use a portable)

You get one 60Mhz probe with accessories in the package. (along with manuals, the software and a USB cable)

Please note that the function generator is rated ~1% THD, so I guess it is not the obvious choice for precision measurements.

I just did get my hands on a second hand calibrated 100 MHz scope that was only slightly more expensive than the PCSGU250. Since you are in Europe, you should check out: Helmut Singer Elektronik - Mess- und Kommunikationstechnik - Electronic Test- and Measurement Equipment. They test the stuff before shipping it and are very reasonable in general. (If you can't trust the Germans, then who can you trust?)

They mostly have the pricey second hand equipment on, as the hobbyist-level priced stuff is sold very fast. I was on the lookout for a few weeks until this turned up.

Good luck.

Lars