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  1. #1
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    oscilloscope probe circuit

    hi all
    may be this sound unusual that some one ask for scope probe circuit !!
    i didn't know if something like this is available and easy to get or to built , but i think there are many othe well experienced guys and experts who can help in this.
    i'm totally new to scope using , just got a used 20MHz dual trace used scope without probes and found the probe price very high to it's look and simplecity
    i thought it could be built if i can see the construction details and components also the circuit diagram and wiring.
    may be this sounds very silly and dummy , but i need to know if it's available or no and why?
    i'll be waiting for ur help and response and all responses are greatly appreciated.
    thank u all in advance
    best regards

    •   Alt7th March 2005, 18:38

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    oscilloscope probe equivalent circuit

    Hi Sawwa !

    Once I faced this same problem. I purchased the GBDSO (gameboy disgital storage scope) board from Elektor and I have not the probes (3.5mm stereo jack connector for ach channel). I decided not to adapt (destroy) a commercial probe so i built one (two).

    All you have to do is: buy good quality flexible coaxial cable (around 1.5m for each probe); BNC connectors, several resistors (5Mohm and 1Mohms), if possible a small (as possible) variable capacitor trimmer (up to 20pf) or if it is hard, some low capacitance smds capacitors (1pF to 10pF).

    1) Discover the input impedance of scope (usually 1Mohm).

    2) Measure the capacitance of the coax cable. Let´s say it measures 45pF.

    3) To build a 10x probe (more usual) you should build an array of 9Mohm (5Mohm resistor + 4 x 1Mohm resistor in series). Connect in parallel to the 9Mohm resultant resistor a trimmer/capacitor of 5pf (9 times smaller than the coax cable capacitance). If you have the trimmer, you can adjust the best capacitance.

    4) What you are doing is a voltage divider (both for resistance and capacitances) where the tip of the probe is connected to one tip of the resistors // trimmer and this is soldered to the centre of the cable. The shielding wire is connected to a wire with a hook (ground). The other side of the coax cable is soldered to the BNC connector. Of course you have to find a way to pack the resistor array + trimmer and the solders (I fit it in a pen tube).

    5) To calibrate the probe, hook the probe tip to the 1kHz square wave signal available in all scopes (or from a function generator). The signal must be perfectly square. By adjusting the trimmer you can round or sharp the edges of the signal to get the best signal shape.


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    •   Alt7th March 2005, 20:07

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  3. #3
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    oscilloscope probe circuit model

    Dear rkodaira
    thanks alot man for that simple effective explaination u did, i was thinking it's simple but now i'm sure and wondering : why this high price for the ready made probes?
    well, i have just a little questions if u don't mind , even some of them didn't relate to the construction method, but get themselves popup in mind :)
    ok , let's ask the important things that related to the construction methode
    shold the resistors be in array?
    i mean should i use the values u mentioned in series or i can use a 9 mega ohm resistor if available ?
    i think it's better to have a little number of resistors , so if i could manage a one or two equivalents 9 Mega Ohm, it's better .. am i right?

    i think if i got only one resistor and used one trimmer in parallel to it , the size would be greatly small to fit into the probe head
    for the trimmer i think it's easy to get but 5pf is so small value , i'll try to and when i manage all the required things i'll let u know
    finally i'd like to thank u very much again for ur help and support
    best regards



    •   Alt8th March 2005, 03:13

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    homemade oscilloscope probes

    Hi again !

    Answering your questions:

    - The high price for commercial scope probes is due to the high quality material used in its building. Think about a top Tektronix scope with large bandwidth more than 200MHz. Its probe coax cable should be very good to not have high parasitic capacitance, the adjusting trimmer small and precise packed inside the probe body, good hooks, etc.
    Notice that what you are going to do is very amateur, with components available in stores and not customised, not adequately fit to the professional probes.
    I think that for entry level scopes (with 20MHz bandwidth) the homemade probe can be done, but it doesn´t work for advanced scopes.

    - Yes the resistor can be only one 9Mohm if you can find one (I think 9.1Mohm is easily find - but there is an error). The important is to have the 9Mohm equivalent resistor (maybe 3.3Mohm+4.7Mohm+1Mohm or any other combination resulting 9Mohm).

    - About the 5pF trimmer, I did just an example. If you got a 0 - 20pF trimmer it should be adequate to your probe because in fact you don´t know the cable capacitance and you have to adjust the trimmer to compensate the cable´s capacitance. If you find a 20pF, you can adjust it from 0 to 20pF.

    Good luck !



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    oscilloscope probe home made

    There are many instructions with pictures on the web for building your own scope probes.

    Take a look at:

    http://www.emcesd.com/1ghzprob.htm
    http://www.sigcon.com/Pubs/straight/probes.htm
    http://www.xilinx.com/products/spart...pes/rec015.htm
    http://www.elexp.com/t_probe.htm


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  6. #6
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    homemade oscilloscope probe

    Basically, placing cap trimmer in parallel to 9MΩ resistor requires high voltage trimmer, which is big, unless you intend to measure low voltages, up to 50Vpp. Usually, 9MΩ resistor is paralleled with fixed high voltage capacitor and the trimmer cap is placed across the cable connection.



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