Scope question...

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psycho
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Scope question...

Post by psycho » Thu Jul 26, 2012 6:24 pm

I am looking at getting a new scope since my old one is old and tired.

I am considering this one :

http://www.saelig.com/PSBE100/PSPC017.htm

and this one :

http://www.saelig.com/PSBE100/PSBE100008.htm

I am leaning towards the OWON since it has a 10Mpt memory depth. The reviews are pretty good on it too. The Rigol one is loved but a number of people but it only has a 1Mpt sample memory.

Anyway, my main question is... In both of the manuals I noticed a little note that says if you have the probe set for 1x (vs. 10x), the max speed it NOT 100MHz but rather 5MHz (OWON) or 6MHz (Rigol). What gives? Just a cheap probe or is there a legitimate reason for this?

Thanks!
Kevin

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Re: Scope question...

Post by Robert Reed » Thu Jul 26, 2012 9:13 pm

It may be that the reduction in bandwidth of the 1X position rather than 10X is the cause of this. This is normal in even the best of probes.

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haklesup
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Re: Scope question...

Post by haklesup » Fri Jul 27, 2012 4:40 pm

Probably has something to do with the gain-bandwith product though the relationship you state seems to be inverse of what the GBWP would do at say an op amp output.

http://en.wikipedia.org/wiki/Gain%E2%80 ... th_product

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Bob Scott
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Re: Scope question...

Post by Bob Scott » Fri Jul 27, 2012 6:16 pm

psycho wrote:Anyway, my main question is... In both of the manuals I noticed a little note that says if you have the probe set for 1x (vs. 10x), the max speed it NOT 100MHz but rather 5MHz (OWON) or 6MHz (Rigol). What gives? Kevin
There is a resistive attenuator inserted into the probe circuit when it is switched to 10X. The attenuator has a lower output impedance driving the capacitance of the probe than when the 1X positon is selected.

Using the equation for finding the rolloff frequency f = 1 / (2*pi*r*C) where "r" is the probe switched attenuator output impedance and "C" is the probe capacitance, you can see that by lowering the value of "r" the rolloff frequency will rise and vice-versa.
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Re: Scope question...

Post by Dean Huster » Sat Jul 28, 2012 6:14 pm

I didn't see that comment on their site. Still, they're talking about system bandwidth: probe + scope. As mentioned, a 10X probe will give the best system bandwidth figures IF the probe has a bandwidth that is a LOT higher than that of the scope. A 100MHz probe on a 100MHz scope DOES NOT give you a 100MHz system bandwidth. And the "adjustable" description I saw did not mean that the attenuation ratio was adjustable. It simply mentioned that there was a probe compensation adjustment for the 10X position that the 1X position does not have (or need). They are describing the probes that come with their scope. Here's an article I wrote concerning system bandwidth:

OSCILLOSCOPES, PLUG-INS, PROBES
AND SYSTEM BANDWIDTH



Many folks have the misconception that if they have an oscilloscope with a 50MHz bandwidth (upper frequency limit), a probe with a 50MHz bandwidth is a good match, as long as the probe can be compensated to the scope's input capacitance.


Probe Compensation

Yes, probe compensation is important. A feature of all attenuator probes (not non-attenuating 1x probes) is that they have a compensation adjustment to match the probe's attenuating capacitance to the input capacitance of the scope. A properly made adjustment will keep the system's frequency response flat throughout the system bandwidth. An improperly compensated probe will cause an erroneous fall in amplitude or increase in amplitude as the frequency is increased.

Poor compensation is often noticed with poor waveshape on square waves have a frequency in the 100Hz to 2kHz area. Unfortunately, most non-rectangular waveforms will not show any problems at all because only the amplitude is affected, not the waveshape.

The best way to compensate a probe is to simply use the probe compensation signal available on the front panel of every* scope. It's usually a square wave of around 1kHz and the probe is adjusted (see probe manual) for the flattest-topped** square wave possible.

Selecting a new probe for your scope must include the determination that the range of capacitance to which it can compensate will include the input capacitance of your scope. Beware of older and lower-bandwidth scopes. They sometimes have higher input capacitances. I've seen as high as 33pF and 47pF and many probes, especially high-bandwidth probes in the 200- and 300MHz area, will only compensate between 10pF and 30pF. 20pF has become pretty much the standard input capacitance for most oscilloscopes.


System Bandwidth

Risetime of a system is directly related to bandwidth. The standard equation for this is:

risetime = 350/bandwidth,

where risetime is in nanoseconds and bandwidth is in megahertz

If you have an oscilloscope mainframe with a risetime of 2ns and a plug-in with a risetime of 2ns, the system risetime (input to CRT) will be the square root of the sum of the squares of the individual risetimes. So the square root of 2² + 2² = the square root of 8 or 2.8ns. A 2ns risetime computes to a 175MHz bandwidth. This means that a 175MHz plug-in installed in a 175MHz mainframe will not have a system bandwidth of 175MHz, but will be 350/2.8 or 125MHz! If you add a probe, it gets even worse.

Let's assume you have a 100MHz mainframe, a 250MHz plug-in and a 200MHz probe. The risetimes of those three will be 3.5ns, 1.4ns and 1.75ns respectively. This calculates to a system risetime of the squareroot of 3.5² + 1.4² + 1.75² or 4.15ns which calculates to a bandwidth of about 84MHz.

If you're using a portable scope with a 50MHz bandwidth, you'll have a system bandwidth of 35MHz with a 50MHz probe, 45MHz with a 100MHz probe and 48.5MHz with a 200MHz probe. If you can outfit your scope with a probe with a bandwidth of at least twice the scope bandwidth, you'll be doing pretty good with system bandwidth.

Don't forget that the better brands of scopes (Tektronix, Hewlett-Packard/Agilent) are pretty lax in their bandwidth specifications. A 100MHz Tektronix 465 can usually be counted on to have an actual bandwidth of around 130MHz or more. If it actually is 130Mhz, a 200MHz probe will give you a system bandwidth of 109MHz, better than the scope's catalog bandwidth specification!

For a "fast" system bandwidth calculation, the equation is "the inverse of the square root of the sums of the inverse bandwidths".

OK. If you have a 500MHz mainframe, a 250MHz plug-in and a 350MHz probe, here's the calculator steps for system bandwidth:

500 [1/X] [X²] + 250 [1/X] [X²] + 350 [1/X] [X²] = [1/X] [SQR X]

which should get you about 188MHz.





* I say "every" here because you don't have much of a worthwhile scope if it doesn't have that signal available. Some lab-grade scopes have it available in several amplitudes and at a fairly accurate frequency.

**Don't quote me or make fun of my term here.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

Robert Reed
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Re: Scope question...

Post by Robert Reed » Sat Jul 28, 2012 9:34 pm

All of what Dean posted is sadly true, and we have not even got into ground lead inductance effects yet. For more in depth reading on the subject of probes, you may want to check out my N&V Nov. 2010 article titled "Oscilloscope Probes and Probing"

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Re: Scope question...

Post by psycho » Mon Jul 30, 2012 12:48 pm

Sorry I couldn't reply sooner. It seems that on this board if you change your email address, you get locked out and the admin has to reactivate your account...

Anyway, I get what you are saying. The users manuals were where I read 5 & 6 MHz. Either way you slice it, it will be better for what I want to do (mostly digital stuff) than my old BK 1530.

I wish I could afford a really nice one but these get pretty good reviews.


Thanks!

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Bob Scott
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Re: Scope question...

Post by Bob Scott » Wed Aug 01, 2012 3:43 pm

Dean Huster wrote:the system risetime (input to CRT) will be the square root of the sum of the squares of the individual risetimes.
-------------------------------------------------------------
500 [1/X] [X²] + 250 [1/X] [X²] + 350 [1/X] [X²] = [1/X] [SQR X]
Thanks Dean! I learned something new today.

The equation looks like it is applicable to a system with multiple single pole filters each rolling off at 6dB/octave. You can calculate the frequency where the resultant -6dB rolloff point actually is. This is SUPER! If you have an audio system where several components in series each roll off at 20Hz , you can calculate real rolloff point. Hmm.
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