Electronics > Projects, Designs, and Technical Stuff
DIY oscilloscope (yet again)
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PMA:
Although I am a bit Arduino fan boy, it isn't right tool for this. I want to have moderate bandwidth, which means that FPGA based solution is the only option (in practice).
fmkit:
if you need GHz bandwith isn't  easier to shift portion of spectrum dwn tofit existing scope bandwith?
 just like SDR and older receivers do.  2 years back  I made 0-2GHz spectrum analyzer using  STM32F103(HY-Mini32v 3.2" TFT) and now thinking to add DSO function to existing setup,that is adding 2nd mixer and IF  amp:

 
Yansi:
 Jeeeez... read at least the first post of the thread!
PMA:

--- Quote from: fmkit on November 21, 2019, 10:14:11 am ---if you need GHz bandwith isn't  easier to shift portion of spectrum dwn tofit existing scope bandwith?
 just like SDR and older receivers do.

--- End quote ---

This is mostly learning experience for me. Scope is just good excuse to learn more about analog electronics and FPGA development.

I don't think that there is any simpler way to acquire full waveform (0-50 Mhz, 500 MS/s per ch) than proper analog front end and FPGA, since there isn't too many oscilloscope specific ICs on the market.

David Hess:

--- Quote from: gf on November 18, 2019, 11:05:53 pm ---
--- Quote from: David Hess on November 17, 2019, 06:53:49 pm ---Many low cost commercial designs use the extra resolution provided by a higher resolution converter as a free programmable gain stage.

--- End quote ---

You mean the polular HMCAD1511?
--- End quote ---

No, I mean many low cost USB and "toy" DSOs use a 12 bit ADC but only return 8 bit results except in high resolution mode.  The extra 4 bits are only used in place of a PGA.

What the HMCAD1511 does relies on the error correction included in its pipelined subranging ADC design which is different as it is not designed to return 12 bit results.


--- Quote from: Yansi on November 19, 2019, 10:51:43 pm ---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.
--- End quote ---

High bandwidth integrated solutions suitable for oscilloscope gain control have been largely proprietary until relatively recently.  There are ways to do it with commonly available parts, and you can learn the details by studying old oscilloscope design, but the techniques are not well known because it is a rare and difficult application.


--- Quote from: fmkit on November 21, 2019, 10:14:11 am ---if you need GHz bandwith isn't  easier to shift portion of spectrum dwn tofit existing scope bandwith?
 just like SDR and older receivers do.  2 years back  I made 0-2GHz spectrum analyzer using  STM32F103(HY-Mini32v 3.2" TFT) and now thinking to add DSO function to existing setup,that is adding 2nd mixer and IF  amp:
--- End quote ---

That is possible and some very wide bandwidth DSOs now work that way but in general that method is not suitable for a baseband (time domain) applications because it requires a huge amount of postprocessing to merge the frequency ranges.

Sampling and equivalent time sampling oscilloscopes could be considered to operate that way.  Sampling and mixing are essentially the same process and may use the same parts and circuits.  The same diodes used in microwave mixers are used in microwave samplers.  Direct coupled mixers make fine sampling elements.
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