Electronics > Projects, Designs, and Technical Stuff

DIY oscilloscope (yet again)

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Yansi:
The HMCAD1511 is a nice chip, however pretty darn expensive, for just a 10 or 20 MHz frontend. For 8 channels, you would need also 2 of them.

For the simple lowMHz DSO a dedicated VGA and ADC may become cheaper.  Although, with 2pcs HMCAD1511, the whole thing may look much simpler on the PCB.

If it'd be me and cheaping out on some project cobbling for fun, I'd opt for external voltage controlled VGA such as AD8330 from the far east sources ($2-3 a pop) and a 100MSps ADC, such as AD9288 (also dirt cheap from far east), four times.  Leaving me at about $30 for the whole VGA+ADC saga.  Of course this does not likely make much if any sense for a commercial design, at least I won't be quite worried about damaging a $70 IC in the process.

There may be even cheaper possibility than AD8330, as a DC to 20MHz or so VGA, this is still a bit overkill.

And also, I've never worked with LVDS on an FPGA, hence a lot of parallel buses from the smaller ADCs are more familiar to me as to what I can do :)

PMA:
Won't you need DAC with VGA to set gain, or have I missed something? To my (quite limited) understanding, I prefer PGA. Small gain adjustments can be done on digital side if needed.

Yansi:
Well, at the time I was interested in building my own single channel DSO (with the sole purpose to learn VHDL), I couldn't find any cheaper gain selector other than the AD8330. Any PGA I came across was either way to slow, way to fast (hence extremely expensive) or didn't provide enough gain steps.  AD8330 as a continuously variable gain stage was easy to set the gain using a DAC. I already had available two 12bit D/A channel in the MCU I wanted to control the system, so that was easy (one DAC for the VGA setting, the other for input offset setting, with a couple of offset range switches from analog MUXes). MCP4921/4922 are also very cheap external SPI DACs.

/EDIT: Small gain adjustments on the digital side would mean fractional multiplication of high bandwidth data, which was a nono for me. I wanted to use a cheap CPLD (Altera MAX II), where such would become impossible. I had a 480x272px display, so I didn't have to scale the 8bit data, just place directly on screen.
 

PMA:
Thanks for the clarification.

I am much more comfortable in digital domain than in analog domain, so I probably see things in different light. That's why I prefer integrated solutions (where somebody else have solved analog issues for me), instead of more discrete solutions. I wouldn't say that money is no option, but in any case this exercise will cost more than commercial product so cost effectiveness isn't highest priority.

I am planning to use HDMI output and external screen with the scope - I know, it is a bit unusual configuration. External display enables enough of real estate to display multiple signals, but in the same time it would be responsive because it is connected to the scope. PC mouse could be used to control the UI. In my case, raw data can be read to the memory and scaled when it is drawn on the display, which eliminates scaling of high bandwidth data.

roy5:

I have used an Arduino as an oscilloscope and signal generator in the past. It works great as long as 5V is sufficient for you. I suppose it probably wouldn't work so well at very high frequencies either, but again, for my application it was great. It actually worked much better than a $50 oscilloscope I bought from China which connects by USB to my computer and runs via software on the computer.

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