PC Oscope

Discussion in 'The Projects Forum' started by Wendy, Jun 6, 2008.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I'm thinking of building a front end for a PC Oscope. Some info and ideas can be found in the AAC eBook at http://www.allaboutcircuits.com/vol_6/chpt_4/13.html . I'm thinking of something a bit more comprehensive and complex, for basic amplitude, gain control, and input protection for the stereo input of the PC. Ultimately I will add this to my blog.

    My first thought is input protection. What is the maximum input a PC stereo input can take and be safe? If it is around ±½V then I could use to regular diodes back to back, otherwise two zeners back to back might work.

    I'm also thinking of a simple 555 calibration circuit for peak to peak calibrations.

    A basic PC Oscope package and my current graphics files can be downloaded here.

    Ideas anyone?
     
    Last edited: Aug 12, 2008
  2. SgtWookie

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  3. snarf

    Active Member

    Jun 7, 2007
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    Hi,

    This is an interesting topic and I would like to know more about it.

    I have also seen another of your replies to ‘howdoesthatworkguy’ dated September 2007 recommending the same Xoscope. You also mentioned a subsequent version Zyscope. I have seen the Xoscope but have been unable to find the Zyscope, maybe a link would help.

    Also at http://xoscope.sourceforge.net/ it says that it's for Linux, will it also run under Windows?

    What changes would need to be made to the circuit to opt for one of your recommended opamp?

    Are these Winscopes that good or are there better ones? Always keeping in mind that the cost is always an important factor.

    Would it be better to have one connecting to the parallel or serial ports then to the sound card?

    I have seen Velleman PC Oscilloscope kits, K8031 which is one channel and connecting to the parallel port, and the PCSU1000 which is two channel and connecting to USB, but these are too expensive.

    All advice appreciated.

    Thanks.
     
  4. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I like the basics, such as the op amp and several features of the schematic. For a 20Khz max speed (and I suspect with a 44Khz sampling that is pushing it) it is major overkill. Given that it is a drop in replacement for a 1558 as far as pin outs go it is worth using a 1558 with a socket for later upgrading.

    The basic design leaves me uncomfortable in that, if I read it correctly, it still allows up to ±12V into the mic input. I still need to come up with a number that is a safe level of input for this microphone input. It does buffer the input, which is a plus.

    http://www.allaboutcircuits.com/vol_4/chpt_13/10.html

    This section brings up a point which is definitely pertinent, and is incorporated in the first buffers schematic. That is a low pass filter. It is probably a must in this design, and 20Khz roll off seems as good as anywhere.

    I bought several data switching and telephone line switching boxes for $3 each (the guys dumping them). The box and rotary switch are worth the money, I'm thinking of building an attenuator to allow for ranges on this buffer. I'm thinking of using 2 9V batteries for the power source for a ± power supply, plus it will provide 10V for the calibrator, which will also have voltage divider outputs.

    I'll bring up other types of faster pc oscopes in a later thread, I'm doing this one because fundimentally, no real extra hardware is required. I am curious how it will perform, as I get some results I'll post pictures.

    I still need to determine a safe input level for the mic input for my own piece of mind.
     
  5. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    OK, here is what I have at the moment. Reviewing active filters I really have to wonder if I want to bother, without going to a really high order filter the flatness and phase shift is questionable. Any input on that thought?

    [​IMG]

    Yes, I know a 82MΩ resistor may be an impossible value to get, however, I'll cross that bridge when I come to it. I also plan on tweaking the calibrator design when I have a working circuit, I'm not sure I like the ranges, but I threw them in there with the intent on modifying them when I saw the end result.
     
    Last edited: Jun 9, 2008
  6. SgtWookie

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    For your resistors, try using 12 + 36 instead of 820 + 9.1. You'll wind up with more realistic values in the high end; like 1.2M + 3.6M.
     
  7. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Sound good for the high end. My original thought was to tweak the values using a ohm meter for more precision, which I will still do. The values shown are preliminary. It would even be worth doing on the 1KΩ resistor on the bottom thinking about it, that way I could get good precision from 5% parts.

    So what is your opinion on the low pass filters? Think it would be a good thing or not?


    ********************

    Dumb luck beats careful planning, every time. I made as complete a 5% ¼W resistor kit I could about a year ago, I have 10MΩ, 15MΩ, 22MΩ, and 82MΩ. Of course, to tweak to right value, I'll probably need the 75MΩ or 91MΩ, which I don't have. Given the number of possible solutions, including the preliminary ones if they work out, I'll worry about it when I build it.
     
    Last edited: Jun 8, 2008
  8. Wendy

    Thread Starter Moderator

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    OK, after waffling for a while I picked some arbitrary values, and added a Butterworth Low Pass active filter with unity gain. I'll aim for around 30Khz as the cut off to minimize amplitude variations, though I still wonder if this isn't a case of major overkill and couldn't be eliminated entirely.

    One of the reasons I went for the 9.1 to 820 ratio is if I up or drop the 820 to the next standard value the total resistance change is around 1%, in other words a very fine increment of change. I'll almost certainly have to change it for the high end, unless I find a source of high value resistors.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Don't forget trim pots. Fixed resistors just never have the right ratios without some tweaking.

    The oft spoke-of 2 watt 1 ohm resistor plus a triangular file can also make those odd resistor values.
     
  10. Wendy

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    The 820 resistors are test select, I'll replace them with 750 or 910, or more/less as needed. The 1KΩ on R11, R21, and R31 probably need something similar. I assume that is what you're talking about.
     
  11. Wendy

    Thread Starter Moderator

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    OK, I've backed up a little, and come up with a finished print that I will build for testing purposes. I may add changes later, you can always tell when I plan on this when I start adding Rev letters. I've decided to eliminate the filters, and I got feedback from the Creative forum about Sound Blasters, which is a standard in my book. They said the input is 10-200mv at 600 Ω, so back to back diodes it is.

    [​IMG]

    I'm also planning on living with the 5% on the resistors, I'm not test selecting this version. It occurred to me I have no idea how well this will work, period, so why waste time if precision doesn't pay off (in other words, it works badly).
     
    Last edited: Jun 10, 2008
  12. SgtWookie

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    Well, I guess I'm wondering why you aren't using the feedback in the opamps to limit output voltage, instead of having a resistor network up front?
     
  13. Wendy

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    Care to show an example? I might go with it. This is basically something I threw out there, and it took a life of its own. I'm sticking with stuff I know well at this point. I figured the gain of the buffer amps is a total unknown, so allowed for a range of 11X or so.
     
  14. Wendy

    Thread Starter Moderator

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    I've spent the last couple of weekends making these prints. I know I'm going to change things around, which is why I'm using breadboard, but I'm also going to make several if the design works. If it does work you will see a new post in the Projects collection, with either these prints or their successors.

    [​IMG]

    [​IMG]

    Couple of notes on the conventions.

    All resistors are color codes left to right or top down, and all resistors are color coded, though you may have to zoom up on it to see it.

    I've put cathode strips on diodes on end, again you may have to zoom in to see it.

    Pads marked in blue are connect points for wires going to the case.

    Wires on top are green, wires in blue are on the bottom, blue pad connects are usually the wires from the components bridging the adjacent pad.

    I've drawn a separate layout without components to show hidden wires, usually cross connects on pads.

    Next step, draw the hole pattern in the case to be used. I mentioned this before, but I've found a source of boxes that have a 4 pole (minimum) 4 position rotary switch for $3 each. These were RS232 data and phone line switch boxes.

    The grind continues...
     
    Last edited: Jun 22, 2008
  15. Wendy

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    One last note, I included the calibrator because you should be able to set this sucker up with just a DVM, which will be the calibration standard. Just in case you wondered why I was going to the effort. It is also the area mostly likely to be modified after I have a working unit and am able to see how it works.

    Any suggestions or ideas, it's not too late... but getting close.
     
  16. Wendy

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    Minor tweaks to the design, Rev C is the prototype, I'll be making a Rev. D for the later versions, basically getting rid of the cap on the calibrator.

    [​IMG]

    [​IMG]


    And here is what Rev D will look like, not much different.

    [​IMG]

    [​IMG]


    All that's left is drilling out the case. Both these Revs will change (without notice) after I figure out what the real calibrator schematic will look like.
     
    Last edited: Aug 3, 2008
  17. SgtWookie

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    You're not really going to use 1458's, are you?
     
  18. Wendy

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    I'm using sockets, my plan is to use the absolute worst case, LM1458's, then upgrade to best case (for me). Not sure what that is yet, probably LM358's. I could use LM353's, since with the input diodes phase reversal would never be an issue. What do you think?

    If there isn't real difference between them then I'll keep the 1458's in the plans. Input impedance alone will probably drive a change though.

    I'm planning on posting reports when in the projects forum when I start running tests. Amplitude accuracy and freq response is my 2 big parameters I'm intersted in, along with waveform accuracy. One nice thing, I should be able to post real screen shots, which if this works (I give it less than 50% odds overall of being satisfactory) will be a real plus.

    I hope this works, I just don't see a PC making a good oscope. Would still be worth doing just for audio experiments though, and I know one guy who would like it just to troubleshoot his audio amp setup.

    There is one proviso I need to add on the next thread, you are putting your PC at risk if you are dumb enough to use this for high power applications, such as AC.

    Funny thing about IC2B, it was a matter of convienence to connect it that way, just easier to go with on the layout.
     
    Last edited: Aug 3, 2008
  19. SgtWookie

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    Well, the LF353 is better than the LM358, which is better than the 1458.
    If you wanted something even nicer, try the LMV2011 - it's a single opamp though.
    You could even try a TL072.
     
  20. Wendy

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    The board is already wired, I'm about to drill the case. Given how unique this breadboard is I could have gone the PCB route, but close enough.
     
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