Electronics > Beginners

Oscilloscope probe adjustment

(1/4) > >>

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.

If you do want a fast enough pulse to test them this is a good generator

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

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.

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.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod