Author Topic: How do I determine what probes to buy for a scope?  (Read 953 times)

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Offline Aurora900

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How do I determine what probes to buy for a scope?
« on: September 17, 2013, 09:19:30 AM »
Well as the subject says... I'm trying to find information on what I need to know about buying probes.  I just bought my first oscilloscope off ebay... a Hitachi V-212 so I'm kind of a noob to scopes.  I tried searching for information but couldn't really find out what actually matters as far as the probe goes.  I see probes that say 100mhz, but the scope I got is 20mhz.  Is it that the probe can handle up to 100mhz or is it that its specifically designed in some way for 100mhz?  So basically, do I need to get one that says its for 20mhz or is anything higher just as good?

Thanks,
Danielle

Offline AG6QR

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Re: How do I determine what probes to buy for a scope?
« Reply #1 on: September 17, 2013, 09:42:05 AM »
Higher is fine.  100MHz probes should work well with a 20MHz scope.

For what it's worth, lower will also work, but you'll be limited by the probe bandwidth.  You're not likely to find many new general purpose voltage probes with a bandwidth lower than 20MHz, but you might occasionally find more esoteric probes that have a lower bandwidth than your scope.  For example I've got a cheap Hall Effect current clamp that's only rated to 20kHz.

Offline mikeselectricstuff

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Re: How do I determine what probes to buy for a scope?
« Reply #2 on: September 17, 2013, 09:51:45 AM »
Some cheap probes like these will be more than adequate
http://www.ebay.co.uk/itm/New-Two-Oscilloscope-Scope-Clip-Probe-100MHz-Kit-/221270029586?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3384b8b512

One issue with these is they are switchable x1/x10 - you generally only use x1 for low-level stuff where you need max sensitivity - normally you use x10 to minimise loading and maximise bandwidth - the switch is easy to operate accidentally which can lead to confusion until you realise it...
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Offline AndrejaKo

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Re: How do I determine what probes to buy for a scope?
« Reply #3 on: September 17, 2013, 10:02:50 AM »
First, we're using unit called hertz to measure frequency and it is marked by Hz. Lower case m is for mili and upper case M is for Mega.

Next, you already noticed the first important parameter of a probe: It's frequency rating. The second important parameter is the compensation range for oscilloscope input capacitance. Your scope's input capacitance needs to be within that range, so that you can compensate for the scope's capacitance.

For very basic use, you should match the frequency rating and input capacitance with those of the scope and you can end the story here.

A little bit about more "advanced" use:
 One more parameter of a scope probe is the attenuation ratio. Basically the probe reduces the input signal that many times. On the scope, you should select appropriate setting so that scope internally increases the signal amplitude to negate the attenuation. We need attenuation for two most basic reasons: To increase the impedance of the probe/oscilloscope system at higher frequencies and to be able to measure larger voltages.

Basically that input capacitance of the scope I mentioned earlier acts as if it is in parallel with the scope's input resistance of one megohm. The result if that high frequencies will be shunted to ground, so you'll basically have a low-pass filter in front of the scope. The probes with attenuation ratio greater than one have a large resistor connected in series with the tip. It will  reduce the effect of the scope's input capacitance. Also it will reduce the loading on the circuit.

Most common probes, from what I can see, are X1, X10 and adjustable X1/X10 probes. Adjustable probes are handy, because you can view signals with very low amplitude using them in X1 position. This way, there's (almost) no signal attenuation before reaching the scope.  Their down-sides are the fact that you can accidentally switch it from one mode to another and that they have a bit larger input capacitance.

Next we have the input capacitance of the probe. It acts as a capacitor in parallel with the probe tip and ground and it attenuates high-frequency signals. A little bit like the input capacitance of the scope. Generally, lower it is, the better the probe is. 

Another potentially important part of probe selection are the miscellaneous features that the probe may have. Usually, there's a hook cover which goes over the probe tip. Size and shape of that accessory can be important, since some can be easily attached to DIP packages, while others may be too thick to be used on them. Others may have too small hook diameter to be used on thicker cables, should you ever need to handle that.

Next, you have the ground adapters for the probe. If you want to probe high frequency signals, you should use as short ground lead as possible, in order to avoid problems with its inductance. Pay attention to the type of insulation on alligator type ground clips. Some may have a square piece of stiff insulation material around the alligator clips, while others have round insulation. I like the square insulation better, since ofter in round insulation, the alligator clip can start rotating in the insulation as you try to press it. The square type insulations prevent the alligator clip from moving in the insulation. Some probes may come with a very short spring type ground connection as well, so consider them if you plan to work with signals of a bit higher frequency.

Then there are also special tip types for probes. For example, I noticed that I often bend probe tips by stabbing them into a breadboard,  so I took probes with replaceable tips.  Some probes may have little tip adapters for modern SMD packages available, if you need to use them or with tiny grabbers, so you can connect probes close together on a DIP package. Then there's the "minor" point of coaxial cable quality and length. Length usually increases probe capacitance. The "feel" of coaxial cable can affect use. Some stiffer cables may make probes move by themselves, as the cable attempts to move to its preferred position. This can make probing more difficult, especially if you can't use a hook or grabber type tip accessory.

Some oscilloscope probes come with little plastic rings that can be placed on the probe's BCN connector and the probe body itself. This way, you can easily see which probe is connected to which channel on the scope. If you ever get a digital scope with color screen, you can even match ring colors with channel colors on screen to make it even easier to see which probe is connected to what.


Finally, we need to take a look at the oscilloscope and the probe together as a test system. Oscilloscope's bandwidth is the so called "-3 dB point", that is to say the point at which you only see around 70% of signal's amplitude. If I remember correctly, same thing applies for oscilloscope probes as well. The result of that is that you basically have probe doing low-pass filtering on your signal and then you have the scope itself doing low-pass filtering on your signal. So if you want to get the most from your scope at high frequencies, you should get probes which have greater bandwidth than your scope, so that the scope isn't limited by the probes. I've seen an article from Agilent that explains how to calculate the bandwidth of scope and probe together as a system, but I can't remember its name now and I can't recall the formula :(.

There's an interesting article from Tektronix called ABC of probes available on the Internet if you want to go into more detail on probe selection.
« Last Edit: September 17, 2013, 10:10:27 AM by AndrejaKo »

Offline Aurora900

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Re: How do I determine what probes to buy for a scope?
« Reply #4 on: September 17, 2013, 12:00:10 PM »
Thanks for the replies... Especially Andrejako... holy crap that's a lot of info :P  But its exactly the kinda stuff I wanted to know :)  (I'm a sucker for details and knowing exactly how things work...)

I think some of it went a bit over my head though lol

And sorry, I should have known better than to be lazy about the capitalization of the M on an engineering board :P

From what I found the v-212's capacitance is 23pF... So looking at something like this: http://www.amazon.com/100MHz-Oscilloscope-Clip-Probes-Accessory/dp/B0030KWM30/ref=sr_1_1?s=industrial&ie=UTF8&qid=1379382596&sr=1-1

They say the compensation range is 15 to 40pF, so this should be good for my scope I'm assuming.

Offline Seg

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Re: How do I determine what probes to buy for a scope?
« Reply #5 on: September 17, 2013, 12:33:03 PM »
The rule of thumb seems to be, you want probes rated at least twice the bandwidth of your scope if you want to achieve anywhere near its rated performance.

Cheapie 100mhz probes direct from Hong Kong go for ~$10 USD on eBay so I see no excuse to go less than that.

I just ordered me a pair of 200mhz probes for $13.80 USD each... http://www.ebay.com/itm/320847586124

Offline nanofrog

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Re: How do I determine what probes to buy for a scope?
« Reply #6 on: September 17, 2013, 12:55:23 PM »
They say the compensation range is 15 to 40pF, so this should be good for my scope I'm assuming.

Yes.

You'll have to adjust the compensation trimmer on the probe body to get it set (you want nice square edges to the cal signal generated by the scope).

How-To for adjusting probe compensation:

Oscilloscope operation 10X probe compensation

Offline Aurora900

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Re: How do I determine what probes to buy for a scope?
« Reply #7 on: September 17, 2013, 01:13:20 PM »
Great video, thanks nanofrog!  So... seeing as I don't have a square wave generator... I guess I'll throw something together with an arduino.

Offline nanofrog

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Re: How do I determine what probes to buy for a scope?
« Reply #8 on: September 17, 2013, 01:25:54 PM »
Great video, thanks nanofrog!  So... seeing as I don't have a square wave generator... I guess I'll throw something together with an arduino.
No, not at all.

Your oscilloscope has one built in, specifically for the purpose of compensating your probes. What you hook the end of the probe to, is the metal tab to the immediate bottom right of the CRT on your unit.

Look for where it says CAL .5V just above it.

Offline Aurora900

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Re: How do I determine what probes to buy for a scope?
« Reply #9 on: September 17, 2013, 01:28:00 PM »
Great video, thanks nanofrog!  So... seeing as I don't have a square wave generator... I guess I'll throw something together with an arduino.
No, not at all.

Your oscilloscope has one built in, specifically for the purpose of compensating your probes. What you hook the end of the probe to, is the metal tab to the immediate bottom right of the CRT on your unit.

Look for where it says CAL .5V just above it.

Oh... I figured that was just some fancy thing that guy in the video had...  Good to know haha.  It would have been easy with an arduino anyways... the tone function just outputs a square wave.

Offline nanofrog

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Re: How do I determine what probes to buy for a scope?
« Reply #10 on: September 17, 2013, 01:44:19 PM »
Oh... I figured that was just some fancy thing that guy in the video had.
Nah. It's on the front panel of any scope I can recall (could be missing on something from the Vacuum tube era though, not sure).

Where exactly it's located at is another matter however...  ;) For example, the Tek 2245B I have, it's located between the CH. 3 & CH. 4 BNC connectors.

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Re: How do I determine what probes to buy for a scope?
« Reply #11 on: September 17, 2013, 02:02:55 PM »
Nah. It's on the front panel of any scope I can recall (could be missing on something from the Vacuum tube era though, not sure).
Some Tek 500 scopes from the vacuum tube era would have a probe calibration output with variable frequency and amplitude up to 80 V or so. Way over-designed as a calibration output, but probably also useful as a signal gen in a pinch.

Offline AG6QR

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Re: How do I determine what probes to buy for a scope?
« Reply #12 on: September 17, 2013, 02:59:05 PM »
Oh... I figured that was just some fancy thing that guy in the video had...  Good to know haha.  It would have been easy with an arduino anyways... the tone function just outputs a square wave.

The one that's built in to your scope was specifically designed to have a fast enough rise/fall time, and to be flat enough at the top and bottom to do a good job of probe compensation, considering the bandwidth of your scope.  Something you throw together with an Arduino might be very good, but there's also a chance that it could have some overshoot, undershoot, ringing, low slew rate, or other artifacts that could make compensation tricky.

Use the one built in to your scope as long as it's not totally broken.  But yeah, for fun some time, throw something together with an Arduino and see how it compares.


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