PC Based Digital Storage Oscilloscope Mk3

Yes, it's finally here !. The DSOA Mk3 has been a long time comming (many years in fact), but it was finally published in the October/November 98 issues of EA.

Here are the basic specs :

- 20MS/s
- 5MHz analog bandwidth (*see below)
- Dual channels
- Optional external trigger input.
- PC-Based, connects to standard parallel port.
- Easy to build on one single sided pcb.
- Vertical position adjustments.
- Fully software controlled timebase and attenuation rages.
- 100ns/div to 50ms/div timebase in 1-2-5 sequence.
- 100mV/div to 5V/div vertical ranges in 1-2-5 sequence.

Latest News :

Well, it looks like I was way off the mark with the design of this Mk3. Hundreds of previous users aid they would love something like this, and would pay anything to get it. Well, while this may be true, it looks like no one wants to even think about building it if there isn't a full kit available. A few people have expressed interest in a short form kit, but overall the entire project is a flop :(

The major kit suppliers in their infinite wisdom have decided not to kit up for the project, apparently because it is too expensive. There has been an expression of interest from other people about doing a short form kit, but this may or may not eventuate. No matter how good this design is, it is a complete flop simply because there is no kit available.

Current Status :

This is my current stance on this project, please do not email me (or call me) and ask what the current status is, because this is the current status. If anything changes, I will change this :

- I am NOT selling a kit (either full or short form), the PCB, or any components, and have no plans on doing so.
- I do NOT know anyone, anywhere in the world, who is selling any sort of kit.
- As far as I know, the PCB is available from RCS radio, for something like A$58.
- ALL of the parts are available from either RS Components, Farnell Components, Jaycar Electronics, Altronics, Dick Smith etc. RS and Farnell stock the "hard to get" chips. I live in Australia, and these are all Australian companies, I have NO IDEA where you can get the components in other countries.
- I have no plans to finish the Windows software to a point where I would be happy to sell it commercially. Sorry about that, but because of the lack of interest, it is not worth my while.
  However if you want the Windows software here it is. It is not complete, very much an early beta copy and I do not support it and will not enter into any correspondance about it. I won't even say if it actually works or not!. The source code for this version is NOT available, don't even bother asking.
- I am *NOT* providing comprehensive support for this project (or any project for that matter). I will answer general questions, but I do not have the time to support every aspect of it's construction with you. Basically, you build it at your own risk, the same as every other published magazine project, I don't know why people think this one is any different. This is an advanced project and if you do not have the technical skills to troubleshoot the project if it does not work then you shouldn't even attempt it.
- The Windows Source code IS *NOT* AVAILABLE - PLEASE STOP ASKING!!!!!!!!. Only the executable is available. The Full DOS version source code is available below, that's all I have.

The simple DOS based software and source code are available here for free.

What is the Bandwidth ?

There seems to be a lot of confusion about the bandwidth of a real time digital cro such as this project and the previous Mk1 and Mk2 designs. A lot of people keep asking what the bandwidth is, and seem to forget the basics of digital cro's. There is a big difference between sample rate, analog bandwidth, and effective bandwidth. I will clarify this situation now :

Sample Rate :

A real-time digital cro samples in real-time, ie, it takes one sample after the other. In the case of the Mk3 this is 20Mhz. This 20MHz is NOT the bandwidth of the cro, it is the sample rate of the analog to digital convertor.

Analog Bandwidth :

The analog bandwith is the -3dB bandwidth of the analog input amplifers and associated circuitry BEFORE the ADC. In the case of the Mk3 design, it is roughly 5MHz.

Effective Bandwidth :

The effective bandwith is a rule of thumb, which is basically 10 times less than the sample rate, or 2MHz in this case. This comes about because you need at least 10 samples per cycle of the signal you are measuring to get any decent waveform.

Now, to answer everyone's question, the effective bandwidth of this Mk3 design is roughly 2MHz, which gives you 10 samples per cycle. The analog bandwidth may be 5MHz or more, but if you try and measure a 5MHz signal you will only get 4 samples of your waveform which is next to useless. If you measure a sine wave you will see something like a triangle wave!.

If you want a real-time 20MHz bandwidth digital cro, then you need a sample rate of at least 200MHz, which quadruples the price and complexity, that's just the way it is.

For example, the new Tektronix TDS200 series real-time CRO's have a 100MHz effective bandwidth by virtue of a 1GHz sample rate. All the manufacturers are heading this way, as the interleaved sampling method (which allows high analog bandwidth with a small sample rate) is a dogdy way to do it, and manufacturers are finally learning this.

Files :

The complete construction article in text form.

The DOS control software including source code.

The Windows control software (early beta version) - No instructions, no support, NO SOURCE CODE, supplied as-is.

The schematic of the analog section.

The schematic of the digital section.

The schematics in Protel format.

The operational timing diagram.

The PCB file in Autotrax format.

View the PCB in 300dpi GIF format.

Component Overlay in GIF format.

Return to Project Page