Tuesday, December 4, 2007

More Scope Research

I've been looking for a way to look at my circuits without spending large sums of money on a reasonably capable digital sampling oscilloscope.

I want to do analog circuits, mixed signal circuits, and microcontroller work. I want to drive LCD displays and eventually learn how to generate VGA and NTSC signals for displays.

I have big goals, and big dreams, but I'm a husband and a father of young girls and I have a full time day job and I can't afford all this damn electronics equipment.

So I made it my mission to find cheap-ass solutions to expensive test equipment. So far, I'm having only limited success. This post is to try and consolidate the information I've gathered and relate some of my experiences.

Here's what I've got so far:

Parallel Port Oscilloscopes



I built this, and it works. I don't have real scope probes, but I cannibalized a $10 radio shack multimeter I had retired and mounted everything in that enclosure. This works really well for low speed digital signals. The resolution isn't so great, and the software is pretty simple (I use the free version.)

Sound Card Scopes



with this hardware:

http://xoscope.sourceforge.net/hardware/hardware.html (or bought from http://www.jaycarelectronics.com/productView.asp?ID=KA1811)

Virtins Sound Card Multi-Instrument


I built the virtins sound card input protection circuit and I use it with the Zelscope software. The circuit is just some additional impedance on the line and some overload protection:


Serial Port Scope

I also discovered, in my surfing around, a serial-port based FPGA-based idea:


which shows some promise, though I haven't done any kind of FPGA work in the past and this would be another huge learning experience. The high-speed aspect of it looks promising, but the software seems feature-sparse.

USB Scope

There are many USB scope setups. Many of them way too expensive for me (I'm looking in the under-$200 range) but there are a few standouts.

Hobby Lab USB Scope


Po.Labs Scope


(Same device, different company?)

Hantek DSO-2090



The sound-card based scopes may be fine for some analog stuff, but I encountered strange behavior that I understand is related to capacitors on the input lines inside the sound card itself (noise reduction?) that make it very hard to use for digital circuits. I can't seem to see a nice square wave, I get drop-off towards 0 volts. I also don't ever see a real voltage level on the sound card scope... it always seems to taper. It's disconcerting, and has put me off of that solution.

The Parallel-port based Scope2k4 seems to work pretty well for digital circuits, but the sampling frequency is pretty low and low resolution (8 bit ADC, max of like 4kHz sampling rate?) Which makes me think it's impractical for any real microcontroller work.

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