Test Equipment

[ch83575]

This is only partly audio related, but I really respect all of the people here and I want to hear what you all think. I need an oscilloscope... mostly for school, but also to use on audio projects. I also need a logic analyzer, I am currently planning on specializing in embedded design. The problem is that I dont really know exactly what I need. Here are some options I am looking at:

Rigol DS1102D : 100MHz MSO. This one has an integrated logic analyser. Pros: good size buffer, relatively high bandwidth, logic and analog time synced! Cons: extreme limit of my price range, only 8-bit vertical resolution.

Rigol DS1052D : Same as above but only 50MHz, so add slightly low bandwidth to the cons.

Bitscope 325 : This one also has the mixed signal as well, but I just have a really bad feeling about a USB oscilloscope... I don't know why. The specs all look ok (except this one says 8bit vert resolution with only 6.5-7.8 effective, whatever that means), I just don't know if it is an effective tool or not.

Analog Scope + 8-bit logic analyser: What I mean here is something like an old Tek 2400 series, maybe 200-300 MHz, perhaps even 4 channel. Then a separate logic analyser, something like a Saleae Logic . In this case I think the scope and analyser would both be better than the equivalent above, but not synced in time.

So, if I go with the Rigol will I need the 100MHz? Or do you think one of the other options better in the long run?

I appreciate any help and comments, I know there are a lot of EEs here.

Thanks,
Chad Huard

[Amphiprion]

Hi Chad,

I'm at work right now, but will definitely post at length later tonight. I just got out of a PCB design review and am kicking back for a few minutes, but not long enough to research your options and make a proper suggestion. I'm a BSEE specializing in military/aerospace electronics design FWIW.

For your embedded designs, do you know what clock speeds you will be pushing?

except this one says 8bit vert resolution with only 6.5-7.8 effective, whatever that means

You are working on EE, yes? You should take the time to learn this Basically it means the INL/DNL of their ADC sucks, and even though it spits out an 8 bit number there's inaccuracy enough that you really only get 6.5-7.8 effective bits (lower as sample speeds go up usually). Read up on INL, DNL, and ENOB, it's good stuff to know, esp for embedded systems design.

http://en.wikipedia.org/wiki/ENOB

[ch83575]

The ENOB thing makes sense. I don't know what I was thinking, it's actually pretty self explanitory. I guess what threw me was that in the specs it looked like a selling piont, when in reality 6.5 bit resolution isn't something to brag about. At least they are honest.

As for speeds, the class I am in is using an HC11 with a 2MHz E clock. But that doesn't mean I won't be working with somthing faster in the future (I am in my 3rd semester). I also have quite a few analog circuit labs that I will be starting this summer and if previous experience holds it would be really nice to be able to do work outside of the lab. Thing is, I really don't know what kinds of things I will need to be able to do, so I am worried that if I pick a scope based on my current project the very next semester it will be useless.

Thanks,
Chad

[Amphiprion]

In my opinion a 100MHz scope is the minimum you want. I myself have a 350MHz DPO at home, so I've already put my money where my mouth is. And that's for my hobby stuff. Work has more serious equipment available.

Things that don't require significant bandwidth: analog in general, embedded digital control and communications (I2C bus runs at 1.7MHz max, SPI usually about 20MHz but can vary a lot), things where you only care if it is a 0 or 1 and nothing about the edge rate. Heck, the fastest I can toggle pins on an 18F PIC is about 10MHz, and that's a decent 8 bit micro running at 40MHz internally.

Things that require significant bandwidth: anything where you care about edges. PWM amplifiers, switching DC/DC power converters, EMI compliance, snubber tuning, etc. Edges edges edges. You don't need a 100MHz scope to look at a microcontroller running at 2MHz, but you would absolutely want one if you were looking at a switching converter running at 2MHz. When I was at Cirrus, we had PWM amps running at a measly 384kHz, but we had to go bust out the 1GHz Tek DPO7000 and the active diff probes to properly tune the output snubbers so we didn't hose the FM band.

My guess is 100MHz will be plenty fine for your education stuff, but once you start doing real work you will want at least that much bandwidth.

In your price range, it is going to be DSO or analog. I would go with something made by a good name like Tek, and I think a good analog 2465 or similar would be a fantastic scope you would keep for a lifetime. I would likely have gone that route had I not the budget of an employed engineer with no family That and a separate logic analyzer would get my vote.

Also, you should check out electro-tech-online.com. It's a great forum for asking questions like this. I'm one of the guys with the audio-related user name if you post there (not uncle scrooge though - you'll understand what I mean).