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Question about scope ac-dc diagram

Discussion in 'Electronic Basics' started by [email protected], Oct 8, 2006.

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  1. Guest

    I would like to ask about the ac coupling in a scope, what sort
    of capacitor do I need to use?
     
  2. Why do you need to know? What purpose do you intend it to serve?
     
  3. john jardine

    john jardine Guest

    0.047uF, 400V, non-polarised.
     
  4. Guest

    Hello,

    So why this value then? (I want to understand a bit
    what I am doing)
     
  5. Phil Allison

    Phil Allison Guest


    ** The standard input resistance for a scope is 1 megohm - so using a
    coupling cap of 0.047 uF passes AC signals with frequencies down to around
    20 Hz with little loss. The - 3dB frequency is at 3.5 Hz.

    A smaller cap value would offer no advantages while a much larger one would
    take an annoyingly long time to charge up and settle when a high DC voltage
    was applied.

    The maximum DC voltage that can be displayed on a scope screen is generally
    around 300 volts - without the use of a 10:1 or 100:1 divider probe. A
    plastic film cap with a 400 or 600 volt DC rating does the job OK.



    ........ Phil
     
  6. Guest

    Great stuff, now after this coupling capacitor one design from
    veleman scopes uses another capacitor of 2.2nF in parallel
    to 100R resistor. What could be the reason for that?
     
  7. Phil Allison

    Phil Allison Guest


    ** Need to see the whole schematic to say for sure.

    But a 100 ohm resistor would limit charging current flow in the coupling cap
    to a safe value whenever the scope's input is *grossly* over driven with AC
    or DC voltage causing input device protection diodes ( to supply) to
    conduct.




    ....... Phil
     
  8. Guest

    Ok, so 100R protects against some unforseen circumstances
    (maybe against failure of the 47nF coupling)
    Then, the 2.2n excludes the 100R protection for f > 1MHz, correct?

    Now, the next stage there is a 3 stage resistor divider.
    91K, then into 10K and to signal ground. (signal ground
    is raised, and is 2.5v compared to real ground.)
    So in total this is 1M as you said.
    Now the not so clear thing is that in parallel with the
    910K we have a varicap 22pF, then in parallel with the
    91K we have 150pF and a varicap 30pF, then in parallel
    with the 10K we have 2.2nF in series with 3R3. Please can
    you explain the purpose of all the capacitors, and the 3R3.
    The only clue I can give you is that in the calibration process
    I am supposed to connect the input to test point 1, and then
    adjust CV1 (22pF) untill a test square wave is flat on top and
    bottom, and does not have rounded or pointy edges.
    Then I do the same with CV2 (30pF) for test square wave 2.
    If you carefully follow my text you can draw the schematic.
     
  9. john jardine

    john jardine Guest

    Give us a break Joakent!. It feels like we're being asked to analyse a
    whole 'scope circuit with the proviso we are spoon fed only one component
    at a time.
    Scan the Y amplifier circuit, put it up someplace where we can all take a
    look.

    As the input signal moves into the heavy circuitry, a huge number of valid
    component arrangements become available to the original designer. Hence
    individual components can only be examined in context of the particular
    circuit block of which they form an essential part.

    The 2n2 and 100R, I've no idea why they are there. If you'd said "100K" and
    2n2, then they most surely would have been overvoltage protection
    components for the first FET.

    All those cap's strung across the resistor string are perfectly normal and
    quite essential to allow a flat frequency response from the range switch. If
    this capacitor 'frequency compensating network' wasn't there, the frequency
    response of the scope would be from DC to about 3kHz.
    Notice if you multiply the component values of each C-R shunt section
    (trimmers notionally at half their range), you'll find they give pretty much
    the same 'time constant' number. If you wanted to extend the ranges out then
    it's a case of adding extra resistors to the chain and then adding shunt
    cap's that calculate out to give this same value.
    They're a kind of balancing act, used in a precise, controlled manner to
    swamp out numerous, havoc causing stray capacitances, that would otherwise
    act with the range resistors, to form unwanted RC low pass filters.

    [ NB: When you tweek the trimmers while looking at the 'squareness' of a low
    frequency square wave, you're actually setting and seeing the amount of
    multi-MHz , harmonic components of that square wave, allowed through into
    the Y amplifier. A neat trick!]

    Attenuator design and input circuitry for scopes and suchlike is often
    overlooked but is a fascinating subject in it's own right and if done badly
    can break a product.
    If you've queries ... Post the circuit somewhere!!.
    john
     
  10. Which is why I asked, "Why do you need to know? What purpose do you intend
    it to serve?"
     
  11. Phil Allison

    Phil Allison Guest


    ** Not the latter, as the cap may be bypassed any time with the DC/AC
    coupling switch.


    ** The 100 ohm resistor feeds the input attenuator's parallel capacitance of
    about 30 pF.

    With no cap in parallel, response would to fall by about 1 dB at 20 Mhz.



    ** Two words - " stray capacitance". The purpose of those capacitors is
    to *swamp* the effect of all the stray capacitances of the resistors, PCB
    tracks and wiring. Although only a few pF each, such strays ruin the
    desired flat response of the attenuator at high frequencies.

    Note how the capacitors impedances at any given frequency are in similar (
    ie 10:1 ) ratios to the resistors and so ( when precisely trimmed ) create a
    matching capacitive voltage divider in parallel with the resistive one.

    All input signals go to the top of this divider chain and so see a load of 1
    megohm in parallel with about 30 pF regardless of the attenuation at the
    input of the scope's electronics.

    BTW 1

    At 10 MHz, the input load impedance is about 500 ohms due to the effect of
    30 pF.


    BTW 2

    Kindly post the schem of the input circuity - what you have chosen NOT to
    describe holds answers to you questions.


    BTW 3

    Do not refer to a " trimmer capacitor" as a " varicap" cos that is a whole
    nother beast.



    ........ Phil
     
  12. Guest


    How do you calculate 30pF? The first capacitor in the chain is 22pF,
    the other capacitors on on the next stage of the divided so they add
    in series, i.e. 22pF will be slightly reduced even.


    Can you give an example of a stary capacitance, i.e. draw it in the
    diagram,
    and show how e.g. the 22pF capacitor act to cancel it?

    Hence what?
    O.k. , so trimmer, as opposed to potentiometer-like, is that what
    you mean?
     
  13. Phil Allison

    Phil Allison Guest

    ** BEWARE:

    Google Groper Psycho Asshole !!



    ** Plus **strays** = " about 30 pF " - ASSHOLE !!




    ** Huh ????

    Better go learn that capacitance exists between all conductors.

    Something an ASD fucked, code scribbling moron like YOU never learns.



    PLUS - What fucking diagram ??

    The one YOU have so far refused to post ????

    Fat chick porn exceeds your Lilliputian intellectual ability - ASSHOLE

    **** the HELL OFF.






    ........ Phil
     
  14. Guest

    100K? that would be almost like scope design suicide
    (and I am not a designer, yet)
    How did you calculate 3KHz?
    Can you explain the importance of the time constant concept?
    Can you show this in a diagram, i.e. introduce a capacitor called
    'stray'
    and show how our capacitors cancel it out?
    Can you explain where these harmonics are coming from,
    and why and how to we need to compensate for them?
    Very true!
    I trust my problems will keep you busy for a while ... :)
    p.s. I will post the diagram, I need to setup a yahoo
    account, hopefully I will manage it today.
     
  15. Phil Allison

    Phil Allison Guest



    ** Piss this pig arrogant Google moron off.






    ......... Phil
     
  16. Puckdropper

    Puckdropper Guest

    @individual.net:

    *plonk*

    Puckdropper
     
  17. Alan B

    Alan B Guest

    On 11 Oct 2006 16:39:48 GMT, in message
    <452d1e54$0$97261$>, Puckdropper
    <> scribed:

    Well now that's playing fast and loose with your entertainment dollar!
     
  18. JoeBloe

    JoeBloe Guest


    As fucked up, retarded and stupid as Phil-baby is, I still think it
    far more retarded to announce one's filter file edits.

    You know... as if anyone gives a shit. Grow the **** up.
     
  19. Guest

    Well?
    I thought you wanted to help?
     
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