Re: gEDA-user: Trouble with an usb oscilloscope

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

Robas, Teodor wrote:
> Hello everybody,
> 
> 
> I need some advice if there is somebody on this list and
> has experience with ISP1581 chip or DMA buses.
> 
> Basically I made an usb based oscilloscope with the above
> mentioned usb 2.0 transceiver. This chip has a 16 bit wide
> DMA bus that I had connected directly with the outputs
> from the A/D converter. It is the DMA (master MDMA
> command 06) that drives the sampling.
> 
> The maximum sampling speed should be about 12 Msps.
>  From what I measured on the pc, it is only sampling at
> about 300Ksps. It is very frustrating: it is working, but not
> at the sampling speed the chip shall be capable of.
> 
> Now I want to know if maybe I did something stupid and I
> have conflicts on that bus, or I just forgot a setting. I tried
> other timing settings there but the sampling rate is like a
> stone, always 300 Ksps. And even with the highest timings,
> there should be at least 2 Msps.
> 
> I put the schematic here
> http://deep-blue.ro/tmp/main.pdf


Oops, possibly this is the culprit: R105 and R107 are 100K. If the 4066, 
opamp input and traces tally up to, say, 50pF then this forms an RC 
lowpass that rolls off around 500kHz. Try 10K or less and see if things 
improve.


> I do not have another oscilloscope around so I can only
> guess that the sampling is very low by measuring average
> voltage on the CLK inputs of the converter. And that is
> quite low about 0.5V when sampling is on, and 0V when off.
> 

Use other low cost "test signals" then. For example the oscillator of an 
AM radio. It usually tunes from about 1MHz to 2MHz. Over there in 
Romania your radios might also have a longwave band where it tunes from 
600kHz to 700kHz.

-- 
Regards, Joerg

http://www.analogconsultants.com/



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