O-scope Probes

Discussion in 'Electronics Resources' started by mbxs3, Mar 18, 2010.

  1. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    I just purchased a Tektronix 2215A Dual Channel, 60 MHZ O-scope on ebay. It comes with 1 Tektronix P6106 probe. I bought it for educational purposes, as I am a hands on type of person and need to be working with things in order to grasp the concepts. Anyways, I would like to have at least one more probe if not a couple extra incase one goes bad. Is there a certain rating of probe I should go with or would I be fine to pick up the same model that is coming along with my o-scope? Also, does anyone have any tips or comments about this piece of equipment based on personal experience? The good, the bad, the ugly? Thanks for any information.

    -Mike
     
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    I saw some real *beep* in a catalog last week...85 picofarad probes for $9. Stick with the originals if you don't know what you're doing. There are very few products that can keep up with Tektronics quality.

    If you do know what to look for, I am perfectly happy with my Phillpis brand scope and it's probes are what make the aftermarket stuff look like trash. Bandwidth, capacitance, and connector are about all you have to learn. Now watch somebody get on here and tell you something I didn't think of.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Look for a branded x10 or x1/x10 switchable probe with a frequency or bandwidth rating well above that of the 'scope.

    Always use the probe on the x10 setting, that isolates the probe tip from the capacitance of the cable.

    The probes should be calibrated. The scope should have a calibration signal output somewhere, which is usually a square wave at about 1KHz.

    With the probe set to x10, hold it on the test point and set the scope gain & scan so you get a couple of cycle on screen. Adjust the calibration trimmer *in the probe* (using the supplied insulated tool or a plastic 'trimtool') to get the square wave actually square; the leading edges will either peak or droop if the calibration is off.

    It should only be needed once, just to match the probe to the 'scope.

    (The probes have a resistive divider and a capacitive divider in tandem so the gain does not change with frequency. The combination of a fast edge and steady level in a square wave allows the dividers to be properly matched to the 'scope input circuit.

    Note that if it's an analog 'scope, it will be useable to far above it's rated frequency. The ratings are what it's gain calibration and waveform reproduction are good to.
    I have an old Solartron '30MHz' scope that I have regularly used when working on 145MHz ham gear; the display at that frequency is probably down to about a tenth the amplitude it should be, but still very useful.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I would also like to throw in my .02 cents:

    The probes you buy should match the intended usage. IF you are going to be measuring high voltages, you will want a probe that can attenuate the signal before the scope to keep from damaging the scope. You can get probes to 100x (Maybe more these days) that divides the signal by a factor of 100 so if you measured a 500v signal it would appear on the scope as 5v.

    If you are going to be working with hi-fi sound and inductance you will want to get a probe that is rated nice and low in parasitics. TEK probes are nice. If you can afford the, go for it. Treat them well, and you will not be buying them again. Your grandkids may, but not you.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Pay attention to the information in rjenkin's post -- it's important for careful work (what he calls "calibration" is usually called "compensation"). And, ideally, you should do it whenever you put a probe onto a given scope channel (although a dual channel scope's two channels are usually enough alike that you don't need to compensate again).

    I prefer to have two identical probes from the same manufacturer, purchased at the same time. This helps you when you want to subtract two signals.

    I've purchased some scope probes from B&K/Cal Test (they're sister companies). I've been happy with the ones I've purchased. I got two that switch between ground, 1X and 10X (the need for the ground switch is not as important with digital scopes as it was with analog scopes, as the digital scopes usually mark where 0 volts is). I also got a higher voltage probe that will work over 1 kV -- occasionally useful.

    I recommend buying a probe with a bandwidth roughly 2 times over the bandwidth of your scope. The main reason is that you'll have these probes a long time if you take care of them and it's likely you'll eventually upgrade your scope -- then you'll still have probes that are usable at the higher frequencies. Even though it's a bit more expensive for the higher bandwidth probes, the incremental cost is probably not as much as the cost of a new probe down the road. YMMV...

    A fairly expensive tool but occasionally-needed one is a differential amplifier. I bought the $330 Cal Test CT2593-2 and I have been happy with it (these are made in China and you can find other vendors selling the same or similar units). It's important when you just have to see two signals, one of which needs to be differentially measured, but you only have a two channel scope. For lower voltage and lower frequency stuff, I prefer to use the Tek AM502 differential amplifier (these are obsolete, but can be found used on ebay).
     
  6. mbxs3

    Thread Starter Active Member

    Oct 14, 2009
    141
    3
    I appreciate all the great and thorough information everyone. I will definitely refer to the advice from these posts when I decide to make a purchase.
     
  7. endolith

    Member

    Jun 21, 2010
    27
    0
    How's the CMRR on that? I bought a "Texas DP3035" (18149 TE) and returned it because the CMRR was 45 dB or worse >20 kHz and it had 120 Hz wiggles in the output when plugged into the wall wart. The front looks similar to yours, but not identical specs (35 MHz vs 25 MHz, for instance).

    Some others that look identical to the Cal Test CT2593-2: Link Instruments or TPI ADF25A, BK Precision PR-60, Pico TA041.
     
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