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dumb question #53647 - scope probes

Discussion in 'Electronic Design' started by Paul Burridge, Feb 11, 2006.

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  1. Hi all,

    I bought this probe off e-bay:

    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7587004154&rd=1&sspagename=STRK:MEWN:IT&rd=1

    I thought it would be just the job for my 350Mhz analogue tek scope,
    but in small print on the probe it says it's for use with Tek TDS400
    series scopes only. Why the limitation?
    The other thing I've noticed is that I can't seem to compensate this
    probe as there doesn't seem to be a conventional adjustment screw
    beneath the (tiny) access hole visible in the photo. The 1k square
    wave resembles dragon's teeth so it really does need the scope
    capacitance tweaked a fair bit before it's usable. Does this probe
    require some special service tool to tweak for compensation?
    Finally, there's a spring-loaded pin that's pressed home when the BNC
    connector is plugged into the scope. What's that do?
    THanks.
    More in the "dumb questions" series later.
    p.
     
  2. Rich Grise

    Rich Grise Guest

    I don't know the answer to this one, but I'd think a little bitty tweaker
    could fit through that hole - like one of those plastic TV tools.
    That's what makes it specific to the TDS300 (and possibly others - I've
    seen that type of thing before), is that there's a contact ring around
    the outside of the BNC on the scope, that that pin contacts, and tells
    the scope "I'm a 10X probe."

    Hope This Helps!
    Rich
     
  3. Rich Webb

    Rich Webb Guest

    There should be a compensation adjustment screw under that tiny hole
    in the "box" at the base, adjustable with the usual tweaker. If it's
    missing, that could be why it was on eBay...

    The spring-loaded pin on the connector allows the probe to tell the
    scope what its impedance ratio is, i.e., 10:1. IIRC, you can't "lie"
    to the TDS400 scopes by manually setting the ratio for a given channel;
    they "know" based on the coding that comes back from the probe via
    that pin.

    The coding pin fits into a recessed collar around the base of the BNC
    connection on the scope. The limitation on "for use only with" *may* be
    because the depth of the pin interferes with connecting the probe to
    some scopes.
     
  4. Jim Yanik

    Jim Yanik Guest

    what's the charts on TEK's website say for that model probe?
    There may be a bandwidth limitation.

    The required tweaker may not be the usual straight screwdriver-style bit;it
    may be one of those ultra-miniature square drive types.
    The TDS scope BNCs also have probe power contacts for active probes,I
    believe.
     
  5. Sounds like a perfect description of this probe's adjustment! I guess
    I can't use the tiny steel watchmaker's screwdrivers I have for this
    purpose as they'll introduce their own capacitance. What is the proper
    tool to use for this?
     
  6. Hi,

    next in this series of Dumb Questions:

    I've been offered a 1000Mz FET active probe for my 350Mhz analogue Tek
    scope. I like the super-low loading feature of these probes for
    delicate RF circuits, but will I have a problem compensating it? The
    probe's input capacitance is just under 2pf whereas the scope's is
    15pf. This difference has made proper compensation impossible for low
    capacitance passive probes; will I face the same problem with an
    active?
     
  7. I have a 6139 probe which looks similar, only has
    a single pin to notify the scope of 10:1.

    The compensation adjustment is a long way inside - only
    just visible and requires a crosspoint (philips) tools.

    ---Paul
     
  8. Definitely no power connection; not sure about the plug. The sockets
    on the scope are the usual bnc female type.
     

  9. The first question is: Does your scope have a power connector for the
    probe, and does it match the plug on the probe?

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. The second question: Does your scope have the ability to switch its
    input impedance from 1Mohm to 50ohm? Active probes usually expect a
    50 ohm load. Since you are using 50 ohm coax to connect the probe to
    the scope, there is no need for the sort of compensation that's
    required for a hi-Z passive probe.

    Regards,
    Allan
     
  11. The FET probe is useless without power, and the only source I ever
    found for the power connected was from a junked TEK scope.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  12. Keith

    Keith Guest

    Tek makes FET probe power supplies too (1101A is one for the P6201 probes).
     
  13. Jamie

    Jamie Guest

    no, in fact it sounds like the exact setup i have ! :)
    i have a set of P6201 active fet probe kit.
    to be used on my Tek 350 Mhz scope.
    it works very good.
    i have a 1.5 Pf tip for mine. :)
     
  14. Jamie

    Jamie Guest

    you don't have a 485 ?
    the power is not in the bnc., is little half
    moon looking thing.
    small round hole with a half moon key.
     
  15. The scope in question is the 2465A; it has no such facility, I'm
    afraid...
     

  16. The company I worked for wouldn't buy it, and they had plenty of junk
    scopes. They even swiped a few from the rear of working scopes that were
    too far away from the BNC input connector to use the FET probe.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  17. Rich Grise

    Rich Grise Guest

    If you have a phillips jeweler's screwdriver, I'd say, go ahead and try
    it. The adjustment screw on those things is _supposed to be_ at ground
    potential. Give it a shot, and if, when you pull the tool out, the
    waveform changes, then stick the tool back in, and overadjust it the
    other way - it may take a few minutes to get it spot-on, but it could
    be both instructive and possibly, fun. :)

    Good Luck!
    Rich
     
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