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newbie question HP8657A

Discussion in 'Electronic Basics' started by jim, Sep 27, 2003.

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  1. jim

    jim Guest

    Hi,
    I'm pretty new to electronics,and have just purchased a HP8657A
    signal generator. The trouble is,the voltage readings don't seem to
    make any sense. When I connect the generator directly into my scope
    ,through a 1K load,the voltage readings are way off. For example,
    generator reads 300mV the scope reads 1.52V (pk/pk),gen reads 600mV
    scope reads 3.5V (pk/pk). So (not having a manual ) my first question
    is, are the readings of amplitude on these old generators assumed to
    be pk to pk or just straight amplitude from baseline., or RMS? The
    second question is why the discrepancy in readings. The scope is
    reading an increase of approx. 500mV (pk/pk) for every 100mV increase
    in generator output.. I'm using a 10X probe but the tek scope
    compensates for the readings,I checked that with my power supply.
    Any help would be appreciated. Thanks jim
     
  2. John Larkin

    John Larkin Guest

    The generator indicates RMS, and assumes a 50 ohm load. Without the
    load (ie, into a high-Z scope) the voltage will double. Factoring in
    the RMS-to-p/p ratio (2.828) then another 2:1 for the load thing, 300
    mV RMS becomes 1.70 p/p, close to what you're seeing.

    John
     
  3. jim

    jim Guest

    Yep, I put a 47 ohm resistor in and switched to rms readings and sure
    enough it tracked,about 15mv off due probably to the resistor. As this
    generator seems not to be an ideal source,next step is to calc.
    Thevinin,I guess. thanks again jim
     
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