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Old 7th May 2007, 07:28 PM   #1
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Default Choosing oscilloscope probes

I bought two "oscilloscope+FFT analyzer+logic analyzer" ISA cards recently on eBay without probes. These are my first "'scopes" and I'd like to know what kind of probes I should buy. With these cards, I can either choose to have one single 200MHz channel or two 100MHz channels per card.

But, since they are not linked together, if I put both in 200MHz mode, they act as two separate 'scopes, so comparing between both signals is not possible.

I know absolutely nothing to probes, what should I get?
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Old 8th May 2007, 11:44 AM   #2
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You shold be able to use virtually any scope probe, but best to get them both for 200 Mhz since your cards will go up to 200 Mhz sample rate. This is assuming that the cards or some part of your system have two BNC connectors, one for each probe, one on to each channel input.

There are always loads of Scope probes available on Ebay at about 5- 10 each.

Hope this helps
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Old 8th May 2007, 11:44 AM   #3
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Further to my last regarding Scope probes with BNC connectors, I have realized that there are available scope probes for sound card scopes and these have Phono plug connectors so these should be available also.

There should be no reason why you should not be able to cut off the BNC connectors and fit Phono's instead but the BNC is better at high frequency than Phono. There may be a difficulty with getting suitable plugs to fit the thin wire that is used, definately better to get the correct type in the first place. It all depends on your card input requirements.

There is another post regarding PC sound card scope probes just down the page, click on the link to see Phono connector scope probes.
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Old 8th May 2007, 03:07 PM   #4
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Well, it's not a sound card, it's some real dedicated scopes with BNC connectors.

The thing is, I know I can buy almost any probes out there, but there is also is the fact that there are X1, X10, X100 ones, different coax cables, different connectors, and a big price difference between the probes.

I'd like to narrow the amount of choice to more specific things / what's the most useful, etc.
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Old 8th May 2007, 03:13 PM   #5
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Probes with phono connectors are definitely not suitable for a 200 MHz sampling scope.

Tektronix has at least one good app note on their web site about choosing scope probes. I suggest you start there.

Decent scope probes have a trimmer capacitor in the little box with the BNC connector. The trimmer is adjusted to match the probe capacitance to the scope's input capacitance using the reference 1 KHz clock on the scope. You typically adjust the capacitance to make the square wave look square. Don't even think about a high frequency measurement without trimming the probe.

I_F
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Old 8th May 2007, 03:45 PM   #6
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Well, I can't seem to find any interesting info on Tek's site, but I found these:
http://www.ezprobe.com/mm5/merchant....tegory_Code=PP
http://www.ezprobe.com/mm5/merchant....tegory_Code=PP

Much cheaper than Tektronix's $100+ 200MHz probes!
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Old 8th May 2007, 03:51 PM   #7
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you can make a phono plug which has decent impedance to 150 MHz or so -- it takes a little bit of 5/32" brass tubing and RG-188 coax -- http://users.easystreet.com/w7zoi/rca_plugs.html

if you aren't going to roll your own connector with an RCA plug and brass tubing -- this is the effect of one RCA phono connector from 100kHz to 200 MHz -- the trace was normalized at 50 ohms -- the vertical scale is 0.05dB/interval.

"A bad probe is worse than a bad wife..."
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Old 8th May 2007, 04:03 PM   #8
Hartono is offline Hartono  Indonesia
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1.get a cheap x10 probe first, you will always need spare probe later on
x10 is good compromise for start, I'll always have at least 2 of those

typical worst case you will get at least 25 mhz bandwith with cheap 10x probe. make sure it has adjustment.

since higher quality probe is prohibitively expensive, In practice I use the cheap much more to prevent damage from overvoltage, or mishandling.

2.Next step will be high impedance active probe,. you will get at least 100Mhz bandwith for decent price + high impedance input, or you can even make it yourself if you can find a schematic/guide.

3.Differential probe is nice, but expensive.

4.DC+AC current probe is nice to have but extremely expensive.

get number 1 first and study to use it maximally, higher speed probe is useless if not properly used, learn how to use grounding near the tip to get best measurement result at high speed circuit ( 20Mhz ++).
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Old 8th May 2007, 07:11 PM   #9
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Again, the cards use BNC, not RCA.

I'll look into what you say Hartono.
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Old 8th May 2007, 08:48 PM   #10
EC8010 is offline EC8010  United Kingdom
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Decent 100MHz probes are cheap. As has been pointed out, it's essential to calibrate them. Avoid switched x1, x10 probes like the plague. Most x10 probes have a voltage limit of only 300V, sometimes less. Worse, there are some x100 probes with the same voltage limit. Only buy a x100 probe if it is good for 2.5kV or more.
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