Oscilloscope Suggestions

[teknoman2]

Finally I am able  to buy a new digital oscilloscope.
After looking on the net I found that Rigol DS1074Z-S is probably the best way to go.

70 MHz bandwidth, 4 channels, 1G samples/s and built in 25 MHz waveform generator.
Its a bit pricy but I believe worth the extra money.

Do you reccomend something else?

Cheers,
Panos.

[RobA]

I did a bunch of research towards choosing which DSO to buy a couple of months back. For me, it came down to being between Rigol, Siglent, and Tektronix all in about that same level as the one you are looking at down to the level of those around $400US with two channels and around 100MHz.

I didn't really see a use for 4 channels. Having at least two does make sense though. On the DSO's you need to be a bit careful anyway because the cheaper models all seem to multiplex the channels and thus reduce the real bandwidth. Which brings up bandwidth, for the DSO's the bandwidth numbers are misleading in several ways. The reality seems to be that you need a spec'd bandwidth on the scope that's about ten times what you intend to measure.

My take on the different brands was that the Rigol and Siglent scopes both seem to be OK. I did get the feeling that there was a ton of astroturfing of forums and reviews going on and that made it hard for me to get a real feel for them. I also thought that the Tek scope was actually probably a better scope even though the forum and Youtube reviews tended to say the opposite. I say that simply because of the specs and design of the scopes. The Tek scopes are actually using custom designed ASIC and the specs really do look to be better. On the other hand, it did also feel to me that the low-end Tek scopes were being underpowered so as not to compete against the higher end Tek scopes.

It's all a bit of a pain, but it really comes down to what features you need and how the advertised specs really relate to what you need. Id guess that for most pedal work, they'd all be OK and mostly overkill. But, there are things that all of them are going to be bad at.

Which is why in the end, I didn't get any of them. I ended up buying a cheap analog scope. I found out that there are actually a couple of companies making them. I got the Gw Instek GOS-630FC. The reason I went with it was that the combination of the sampling rate and the bit depth on all of the lower end DSO's is low enough (8-bit) that I wouldn't be able to make the noise calculations/observations that I needed. I'm actually really happy with the analog scope. It's only a 30MHz bandwidth, but it's an analog 30MHz and that really does mean something different. Also, with an analog scope, there's no dead time in the displayed sampling or triggering. That can be an issue for trying to see some types of noise. (On the DSO side, I had the impression that the Tek scopes would have an advantage over the other DSO's, but I can't remember the specifics).

Another way to go is by looking at some of the used Tek and HP analog scopes that are around. The reason I didn't go this route is that finding a calibrated scope that came with probes proved difficult around me. I really wanted to stick with something that I knew was calibrated correctly to start off with and had all the parts functioning when I got it. But, depending on what's available around you, a used analog scope could be a really good and cheaper alternative.

After having said all of that, I think that I could have been mostly happy with either the Siglent or Rigol scopes in the 70MHz to 100MHz ranges for most everything that I would be doing with testing effects.   

[teknoman2]

Hey mate,

that was a very nice and thorough explanation for choosing DSO.
I noticed as you said, that RIGOL has BW limitations when you are using multiple channels.
Probably I will search for a two channel DSO with minimum 1G per sample.
Another thing that in on my mind is the build in waveform generator.
Do you think should I choose an integrated generator in DSO?

Also I agree with analog scopes but the thing that concerns me is the callibration.
In Europe sometimes callibration is even more expensive that the scope.
Thats why I went down the road of analog.

Again mate,
thank you for your suggestions.

Cheers,
Panos.

[RobA]

To clarify a bit, it's not just the Rigol that has bandwidth limitations when you are doing multiple channels. It's actually a bit hard to get the details, but my impression is that all of the lower end DSO's multiplex their channels. An important note to this is that it will only happen when you are actively using multiple channels. If you are only using one channel, then it will get the full bandwidth.

I love the idea of having an integrated signal generator. I almost went with the Red Pitaya for that reason and a couple of others. The only thing that stopped me from going that route was that I couldn't get a feel for how good the web based interface was with it.

My audio interface for my computer has a signal generator built in that works for almost everything I do. Having a signal generator is really helpful for debugging and looking at frequency responses. If I want a really quick visual feedback on what's happening with a pedal, I can push a white noise signal in and look at the response on the audio interface's spectrum view and get a really good picture of what's going on. I've used a sine wave generator several times to help track down where bugs were in something I was working on. I think it's really pretty useful.

I really agree with you about your concern with calibration and used scopes. I did find one place that was selling nice used Tek analog scopes that were refurbished and calibrated with certificates, but they ended up being more expensive than the one I bought. I think that if you were using the scope for professional work, you'd probably need to get the DSO recalibrated periodically too, but at least you know a new one comes calibrated. The analog scope I got did come with a calibration certificate and the measurements I've made with it do look to be within spec. But, if I could've picked up one of the nice Tek or HP analog scopes with calibration docs, it would have been really tempting.

One of the nice things you get on the DSO's that you won't get on an analog scope is the ability to store waveforms for comparison. That could be pretty powerful for some applications. There are lots of tradeoffs and it's pretty much impossible to do an actual comparison to figure out what to get. It makes it pretty hard to choose. On the good side, I think that almost any of them would be a useful tool.

[teknoman2]

I totally agree about storing waveforms for comparison. This is a big disadvantage of analog scopes.
Also another thing with the analog ones from what I remember on when I was in university is that the display doesnt have enough energy
to display your trace when your trace beam is faster cause the frequency is very high, correct me if I am wrong.

After searching a little on the web I found this scope with built in 1MHz generator but I am not really sure if the 20MHz is enough for audio
apllications. I am pretty sure that you can callibrate an BBD delay with this one but for other applications would be enough?

http://www.gwinstek.com/en/product/productdetail.aspx?pid=3&mid=6&id=74

[RobA]

I got this one here, http://www.gwinstek.com/en/product/productdetail.aspx?pid=3&mid=6&id=70.
It's at 30MHz and has been fine for everything I've needed so far. I calibrated the Flintlock Flanger with it and it worked really well. It doesn't have a built in signal generator though. It does have a frequency counter built in and that was useful for doing the flanger calibration. I used a sine wave generator from my MOTU audio interface to do the signal generator for that calibration.

The Flintlock was at something like 2.6MHz, so the BBD stuff should be well within the range of the scope. Other than that, up to this point, the fastest signals I've looked at relating to audio circuits were in the 1 to 4MHz range when I was looking at the noise coming off of an MCU based board I'm working on. I can't think of anything working with audio circuits that would need anything more than the 20MHz version could do. For the digital and MCU applications, I'd think you'd need to go to a logic analyzer instead.

As far as the brightness goes, the brightness of the display does depend on the time division setting. It also gets dimmer when you have both channels on and tracking in time mode. That's because these cheaper analog scopes have to do this thing where either it alternates between the two channels or it chops up the two signals into tiny segments to display. In both cases, both signals are displayed all the time, but it's using the time of the phosphor decay to smooth the display and this causes the traces to be dimmer when the signal is fast. But, how bright or dim depends on the brightness of the scope. The one I have isn't tremendously bright, but I've pushed it all the way up to test it and I could always see the traces.

One thing I've been thinking about doing is making a function generator out of the XR-2206 IC. It wouldn't be as convenient as having the built in generator, but it would still be pretty handy and cheap.

[teknoman2]

I saw the 30MHz model of your and it is quite interesting.
I think retail price is 450 uds which quite really good I must say.
Probably I will search a little bit more but from what I see its a very nice tool.
Also with the rest of the budget I can buy a decent waveform generator.

Thanx again RobA your suggestion was very helpfull.

Cheers,
Panos.

[RobA]

Glad I could help. I went back and forth a bunch trying to decide. It's not any easy choice and it's hard to get information.

One more thing on the DSO versus analog scope question. The analog scopes are a lot bigger and heavier than the DSO and have smaller screens than lots of the DSO's. If you are tight on space, that could be a point in the DSO's favor too.

Good luck on figuring out which one is going to work best for you. They're all pretty good really and can be very useful, especially when you are working on seeing what new circuits are doing.

[teknoman2]

yeah I know that analog vs digital have the disavantage of space.
Still I ve got plenty of room on my bench.
I am doing a little plan about my small lab on what I am going to use.
Hope everything will go well....

[RobA]

Good luck getting it all set up. It's pretty fun just playing around with the scopes and stuff.