Oscilloscope probe adjustment

[torch]

I am expecting delivery of my new Rigol 1052 any day now (it's cleared customs and in the mail). One thing I noticed in the huge Rigol hack thread are comments that the supplied probes are of questionable performance at higher frequencies. So when I spotted a pair of probes, complete with all the accessory tips at a yard sale and discovered they were 250mHz probes according to the included manual, I figured they were worth the $10 asking price.

Now, perusing the manual in greater depth at home, I discovered that they actually have two compensation adjustments -- the traditional one at the tip that is adjusted to the 1khz calibration signal of the scope, which n my very limited previous experience, is all I have ever seen before. But these also have a second, high frequency, adjustment hidden in the BNC connector, revealed only when a shroud is removed. According to the manual, this is factory adjusted to match a 250MHz scope with input capacitance of 20pF but "The probe can however be adjusted to match other combinations of input capacitance and bandwideths, any further adjustments only being necessary if the probe is then transferred to a different type of oscilloscope".

The adjustment procedure necessitates the use of a fast rise pulse generator with a rise time of <1nS, which I don't have. The Rigol inputs are 15pF and of course, I have no idea if the P.O. adjusted these to a particular scope in the past.

So my question is: how important or useful would it be to adjust these to the Rigol scope (post-100MHz hack)?

Edited to add: These are Coline model M12SW probes, if anyone is interested.

[DaveW]

If you do want a fast enough pulse to test them this is a good generator
http://www.i9t.net/fast-pulse/fast-pulse.html

[tyblu]

I wonder if you could use that adjustment procedure to tune the 50MHz - 100MHz (note capital M -- the units millihertz or mHz is often used) band, or if this would just skew the factory calibration lookup table. How exactly does Rigol characterize its inputs? I imagine with a "perfectly resistive" 50-ohm probe, so any probe calibration, wherever on the spectrum, would be beneficial. However, if it is factory calibrated with a high-Z probe it would then also depend on the high-frequency characteristics of the probe... If I can figure this out in 2 minutes, I imagine Rigol figured it out years ago and hopefully now use "perfect probe" calibration

So yeah, use that to tune the high end. 1-ns pulses tunes the probes to ~350MHz rise times, or ~1.4X the 3dB point -- may be more useful to tune to a lower frequency, like ~150MHz, using 2.3ns pulses, just in case the passband isn't perfectly flat when tuned higher. Then again, the analog 3dB point of the Rigol's inputs is supposedly around 160MHz, so you could just as well go for 160MHz*1.4=224MHz, using 1.56ns pulses (0.35/224MHz -- if this is confusing, lookup "rise time" on WikiP).

Here's a homemade pulse generator: http://www.i9t.net/fast-pulse/fast-pulse.html

[torch]

Thanks for the link guys. I'm not sure if that would work though -- don't you need a square wave to adjust a probe? How would you identify over-/under-shooting with just a pulse? Going by the pictures in the manual, it looks like the procedure is just like tuning the low frequency response with the 1k calibration output on the scope.

[Wim_L]

Correct. Hameg supplies probes like that with its scopes (some even including a double adjustment for the high frequencies), and some of their scopes have both a low- and a high- frequency square wave generator as calibrator output. You don't need ultrafast risetimes though, the Hameg manual specified the fast square wave as having a risetime below 4ns. That should be sufficient to calibrate a 250MHz probe. (The probe can detect faster risetimes, but compensating for over/undershoot can be done with such edges)

If your scope only has a low frequency calibration output, you'll need to find another fast square wave generator, or perhaps improvise an oscillator with a fast logic chip.

[torch]

What about trying to meter the clock pin on a computer? Should be plenty fast, and is readily available. Anyone know off hand how square the signal would be?

[tyblu]

DSOs are great for catching pulses. One of their principle advantages over CROs is their complex trigger capabilities.

[alm]

I don't think an avalanche pulser is of much use, you want to look for a fast, clean edge with little overshoot or aberrations, not a semi-Dirac pulse. Using unknown signals is probably a bad idea, you don't know if the over/undershoot you're correcting is in your signal or in the probe. If you don't have access to the proper signals, I'd rather leave it alone, you'll likely make it worse.

I think adding tons of capacitance (hi-Z probe) to a fast clock line will severely distort it, plus I wouldn't count on them being nice square waves, no point in working hard to preserve all those odd harmonics, it's only the fundamental that counts as a clock signal. The rise times will probably be just fast enough for the logic to work, to reduce EMI.

[metalphreak]

What kind of probes did everyone else get with their Rigols? I got 150Mhz probes with mine which is pretty good considering its officially only a 50mhz scope.

[torch]

I came across this circuit:

http://www.maxim-ic.com/app-notes/index.mvp/id/1866

Note that there is an error in the actual schematic, which calls U1 and U2 "MAX6644". From the text, U1 and U2 are actually "MAX4644".

I got all exited -- until I noticed the rise time was 4ns.  Still, from what Wim_L says, that could be close enough for the girl I go with.

I was thinking of powering it with 3 button cells -- in series for V+, and V_High, and tap 2 cells for V_Low -- to avoid any power supply noise issues.

Comments? Suggestions?

[alm]

How will introducing another unknown element help with adjustment? What's your reference signal, do you know what the output signal should look like? Do they specify aberrations with your layout and load? What problem are you trying to solve?

I don't have access to the manual for your probes, but the HF adjustment procedure in the Tek P6133 manual uses a dedicated calibration generator (with very clean edges), and uses a 50ohm connection to establish a reference signal. In the Tek P6137 manual, they mention that HF compensation rarely requires adjustment, unless you replace the cable or the probe has excessive aberrations/not enough bandwidth. As long as neither is the case, I would leave it alone.

[saturation]

Hi!

Its a standard issue with Rigol, I think its product # is RP2200.

What kind of probes did everyone else get with their Rigols? I got 150Mhz probes with mine which is pretty good considering its officially only a 50mhz scope.

[torch]

How will introducing another unknown element help with adjustment? What's your reference signal, do you know what the output signal should look like? Do they specify aberrations with your layout and load? What problem are you trying to solve?

All good points. I don't have access to any other scope or equipment to check the finished circuit against. The only "reference" would be Fig.2 in that link, and of course, is merely what they achieved with their circuit. However, it seems to me that I could use a short co-ax cable with bnc connectors to connect this circuit directly to the scope, take a screen shot, and use that as a reference to ensure the probe performance is adjusted to be no worse than a direct cable, no?

As for the problem I'm trying to solve: perhaps none whatsoever. I really don't know enough about all this, which is why my original question was "how important or useful would it be to adjust these to the Rigol scope (post-100MHz hack)?" The Rigol scope inputs are 15pF, the cables came from the factory adjusted to 20pF. Does that mismatch matter? Of course, the Previous Owner may have adjusted them to his particular equipment already, so I really have no way of knowing what they are currently set to anyway.

If the truth be told, this is as much of a learning exercise as anything. I will probably never have a need for a 100MHz scope. (see, I got the right 'M' this time -- I'm learning already  ). My last scope was an old Tek 20 MHz unit that was hopelessly inaccurate and out of calibration with one working pre-amp. Most of what I do is automotive and motor control. I could probably get away with one of those hand-held or USB scopes. However, for the price, I figured I may as well upgrade to the Rigol and if I've got it, I may as well ensure it is operating at it's best.

Who knows where this learning process may take me? Electronics have always intrigued me. I used to play around making various beginner-type circuits, but then I had to raise a family and access to components dropped off sharply around here anyway when Radio Shack went out of business (I'm in a very rural part of Canada). Flash forward 30 years, I have some free time on my hands and this past summer a shop opened only a few hours away stocked with all sorts of goodies...

[alm]

However, it seems to me that I could use a short co-ax cable with bnc connectors to connect this circuit directly to the scope, take a screen shot, and use that as a reference to ensure the probe performance is adjusted to be no worse than a direct cable, no?

Connecting an unterminated coax cable will present a huge capacitive load (look up capacitance per meter/foot, and multiply by length), like a 1x probe, but much worse (they worked hard to minimize this and other effects). 1x probes usually have a bandwidth of 15MHz or so, and in reality are often much worse, because the output impedance is higher than the 25ohm impedance used for bandwidth testing.

In the P6133 adjustment procedure, they connect the generator (which has a 50ohm output impedance) via a 50ohm coax cable, and connect this with 50 ohm in the scope. The Rigol does not have a 50ohm setting, you could use a feed-through terminator, although that won't be a perfect 50ohm terminator (~15pF of scope in parallel). Your source isn't 50ohm either.

As for the problem I'm trying to solve: perhaps none whatsoever. I really don't know enough about all this, which is why my original question was "how important or useful would it be to adjust these to the Rigol scope (post-100MHz hack)?" The Rigol scope inputs are 15pF, the cables came from the factory adjusted to 20pF. Does that mismatch matter?

Minor differences in scope input capacitance usually only require LF adjustment. The fact that it's under the BNC shell indicates that it's not something that requires regular adjustment.

Of course, the Previous Owner may have adjusted them to his particular equipment already, so I really have no way of knowing what they are currently set to anyway.

Sure, but I'd verify the specs to see if this is actually the case before adjusting anything.

If the truth be told, this is as much of a learning exercise as anything. [...]

Nothing wrong with experimenting, you're unlikely to hurt anyone or damage anything by doing this. Just keep in mind that you may make it much worse by adjusting without proper equipment. Performance verification is fine, obviously.

[torch]

In the P6133 adjustment procedure, they connect the generator (which has a 50ohm output impedance) via a 50ohm coax cable, and connect this with 50 ohm in the scope. The Rigol does not have a 50ohm setting, you could use a feed-through terminator, although that won't be a perfect 50ohm terminator (~15pF of scope in parallel). Your source isn't 50ohm either.

Ok, is there a better way to match the impedances? (impedancii? I wonder what the plural form is...)

Minor differences in scope input capacitance usually only require LF adjustment. The fact that it's under the BNC shell indicates that it's not something that requires regular adjustment.

Understood. The manual basically said the same thing -- adjust once, don't worry about it again unless using it with a different scope.

Sure, but I'd verify the specs to see if this is actually the case before adjusting anything.

Is it as simple as connecting to a capacitance meter to see what it is? Or is it frequency dependent, like impedance?

Nothing wrong with experimenting, you're unlikely to hurt anyone or damage anything by doing this. Just keep in mind that you may make it much worse by adjusting without proper equipment. Performance verification is fine, obviously.

Hey, there's ten bucks riding on getting this right, eh? Can't afford to make any major mistakes now!  

[DJPhil]

Ok, is there a better way to match the impedances? (impedancii? I wonder what the plural form is...)

impedances