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'[EE]: PC Digitizing Storage Oscilliscope'
2001\01\06@215258 by kben

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Hi,
Does anybody have any recommendations for a PC Digitizing
Storage Oscilliscope, or just a regualr DSO.
I have looked at Bitscope and the Link Instruments products,
and also a board by Protek. I have also looked at the Instek brand of
o-scopes. I am relatively new to this and
would like something user friendly. My main use would be with PICs,
so I assume the 20-40mhz range would suffice.
Also, as this is a hobby, I was trying to keep the price <= $400.

Thanks in advance,
                Kevin
Apologies if this should be OT not EE.

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2001\01\06@231556 by Jinx

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> Does anybody have any recommendations for a PC Digitizing
> Storage Oscilliscope, or just a regualr DSO.

I've had my eye on the Electronics Australia Oct 1998 DSO. It
has dual channel 20MSPS @ 8 bits, PC controlled timebase,
vertical attenuation and vertical waveform position controls. It
looks as though it could be built quite cheaply. Let me know if
you want a schematic

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2001\01\08@125242 by Dan Michaels

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Jinx wrote:
>> Does anybody have any recommendations for a PC Digitizing
>> Storage Oscilliscope, or just a regualr DSO.
>
>I've had my eye on the Electronics Australia Oct 1998 DSO. It
>has dual channel 20MSPS @ 8 bits, PC controlled timebase,
>vertical attenuation and vertical waveform position controls. It
>looks as though it could be built quite cheaply. Let me know if
>you want a schematic
>

Thank you for the schematic, sir. I was most interested in the
design of the analog front-end. This is always "the" major
difficulty with this kind of project.

The input amps can only accept Vin to +/-12v in direct mode, and
the protection for overvoltage [470ohm series R + diodes clamping
to +/-12v] is barely marginal. If you OV with 50v, you severely
load down the signal, and can drive large currents into limiter
resistors R20/R27 --> (50-12)/470 = 80mA --> 3W dissipation.
There goes your 1 Mohm input resistance down the hollow pipe.

thanks,
- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
=========================

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2001\01\08@144648 by rottosen

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Dan:

Do mind showing us your front end?

-- Rich


Dan Michaels wrote:
{Quote hidden}

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2001\01\08@160719 by Jinx
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> Dan:
>
> Do mind showing us your front end?
>
> -- Rich

He might do for a meal and a movie ;-)

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2001\01\08@163834 by Jinx

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part 1 146 bytes content-type:text/plain; (decoded 7bit)

> Dan:
>
> Do mind showing us your front end?
>
> -- Rich

R20=R27 (dual channel) is the 470R



part 2 2540 bytes content-type:image/gif; (decode)


part 3 144 bytes
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2001\01\08@164714 by Dan Michaels

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At 12:45 PM 1/8/01 -0700, you wrote:
>Dan:
>
>Do mind showing us your front end?
>
>-- Rich
>

People usually only make requests like this in the summertime,
after being out in the desert for a few hours without a hat :).

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2001\01\08@171803 by rottosen

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I'll show you mine if you'll show me yours  ;-)


           about          +V
       +-- 5pf--+         |
       |        |         |
       |        |      |--
 >--+--+--100K--+--+-->|     JFET (2N5486)
    |           |  |   |--
    |   diodes  ^  V      |
   1Meg         |  |      |
    |           |  |      +----->> to preamp
    |           V  V      |         (0 volts out for 0 volts in)
    V                     |
                       |--
                   +-->|     JFET (2N5486)
                   |   |--
                   |      |
                   |      |
                   +------+
                          |
                          |
                          -V

-- Rich


Dan Michaels wrote:
{Quote hidden}

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2001\01\08@222938 by Dan Michaels

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Rich Ottosen wrote:
{Quote hidden}

Does this mean [ie, because of the clamping diode connections]
that the input dynamic range here is only +/-0.7v ??? Or I am
not understanding it.

[BTW, you don't want to see mine. The reason I never did this
kind of project is because I could never figure out how to get
20 mhz BW, +/-50V input dynamic range, 1 Mohm input impedance,
OV protection, and also NOT have to include a mechanical switch
or relay - so I never did it].

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2001\01\08@223601 by rottosen

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Silvio:
I used this circuit at +/-5V myself. The limiting parameters are the
Gate Source Cutoff Voltage and the maximum peak voltage that you need to
buffer.

For the 2N5486 the Gate Source Cutoff Voltage is 6.0 volts maximum which
is too much for the +/-5V supplies. There is a similar part, the 2N5484
that cuts off at 3.0 volts. This should allow almost 2 volts of peak
input/output swing.

Another factor to consider is the Zero Gate Voltage Drain Current. For
the 2N5486 it is 8ma to 20ma, for the 2N5484, it is lower at from 1ma to
5ma. This matters because this current is the idle current that the
circuit draws. As a tradeoff, the higher operating current can drive a
larger capacitive load at a higher frequency.

My calculations show that the 1ma of the 2N5484 can drive 20pf of load
from -2 volts to +2 volts in about 80ns which is about 4MHz of
bandwidth. The 2N5486 would be several times faster but would need
larger supply voltages.

To keep the load on the buffer down to 20pf it should drive a single
amplifier or comparator circuit.


-- Rich


T_BoNe wrote:
{Quote hidden}

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2001\01\08@224958 by rottosen

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Dan Michaels wrote:
{Quote hidden}

You are right. The circuit only allows about 1 volt peak to peak with
the clamp diades as shown. Several diodes could be seriesed to increase
the input voltage range. This is good because it also decreases the
input capacitance. Another way is to use a pair of clamp voltage souces
to bias the diodes to higher voltage. These voltages have to be less
than the supply voltages.

My needs were for the input of a frequency counter. Large dynamic range
wasn't required since the following stage was a high speed comparator.
The counter works out to about 70 MHz. Limited by the comparator and the
TTL divide by 10 prescaler I used.


> [BTW, you don't want to see mine. The reason I never did this
> kind of project is because I could never figure out how to get
> 20 mhz BW, +/-50V input dynamic range, 1 Mohm input impedance,
> OV protection, and also NOT have to include a mechanical switch
> or relay - so I never did it].


This is why Tektronix gets the big bucks. When I was in tech. school I
obtained a set of books published by Tektronix on how to design
oscilloscopes. They still used tubes in many of their examples but I
would love have those books again. Maybe I need to try E-Bay -- Danger
Will Robinson --.


-- Rich


>
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2001\01\09@065845 by Michael Rigby-Jones

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Bit of a personal question isn't it?

{Quote hidden}

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2001\01\09@144817 by Dan Michaels

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Jinx wrote:
>Attachment Converted: C:\WIN31APP\EUDORA\ATTACH\dsofront.gif

Rich Ottosen  wrote:
{Quote hidden}

I think a good modification for the ckt shown by Sir Jinx-GFW
in his attachment might be to change R20 from 470 ohms to say
100K, and put a 20 pF cap across it, similar to what Rich shows
above. The 100K should pass along DC/low-fequencies well, and
the cap will pass the high end. You also get OV clamping without
excessively loading down the input signal. I don't think I would
want to use the SJ-GFW ckt as originally shown.

- danM

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2001\01\09@182255 by dre Domingos F. Souza

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>>Dan:
>>Do mind showing us your front end?
>>-- Rich

       Wow man...sometimes the air gets very fresh...the people get very lonely...how gay... :oD


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