Scope probes - Page 1
Forum section: electronics / general

author: IvoS
author_rank: Frequent Contributor
timestamp: January 06, 2012, 12:32:24 am
content: Which probe is better? The one with adjustment on BNC connector or the one with adjustment on the probe itself? Does the probe capacitance have to be within the capacitance of the oscilloscope input? Does it mean that I wouldn't be able to use this onehttp://search.digikey.com/us/en/products/IP250/290-1057-ND/274489with hacked Rigol 1052E?How much is the voltage value displayed on the screen accurate? Do I have to do any voltage derating with relation to frequency? I measure mostly low frequency (audible frequency) but low signal levels, around few milivolts on audio equipment. How accurate the scope can be if I read 1mV RMS on the display?Anyway,the supplied Rigol probe looks really cheap. I would like to buy a probe with a hefty thick cable, BW 150-200MHz.I know that wire quality matters. I don't believe in "snake oil"but I can hear the difference if I use good interconnects- OCC (single cast) 99.999999% copper (like Harm. Tech) compared to other "cheaper" cables.Any suggestions?Thanks.

author: slateraptor
author_rank: Frequent Contributor
timestamp: January 06, 2012, 10:10:27 am
content: w2aew, a respected member of the forum, has created an informative video on the basic theory of passive probes, in particular, the why's and how's of probe compensation. It would behoove you to watch and learn from him.At 1mV RMS, you'll be kissing the noise floor all day...and an OCC replacement anything won't make it any better either.As a suggestion, drop the audiofoolery nonsense. That marketing sucker mentality doesn't fly well around here. Besides, you'll need all the money you can scrounge up to replace those perfectly functional Rigol probes with probes that are BW-spec'd 4 orders of magnitude higher than your target frequency range.P.S. It really is funny how any casting process can claim purity exceeding six sigma...or even that achieving purity levels in a metal that's particularly prone to oxidization is significant for such irrelevant, inconsequential consumer applications.

author: ipman
author_rank: Regular Contributor
timestamp: January 06, 2012, 11:22:55 am
content: Few people know what is the precise limit between good cables, with good low resistance and proper section and audiophoolery.Also take into account that design also sells.

author: saturation
author_rank: Super Contributor
timestamp: January 06, 2012, 05:25:38 pm
content: Its good to read on theory on how probes are designed. A classic remains the Tek Series article from 1969. Although probes look sleeker today, and are more compact and lighter, much of the design theory, particularly passive designs, still follow all the principles in this text.http://www.slack.com/TE/TekConcepts/TekProbeCircuits.pdf

author: BravoV
author_rank: Super Contributor
timestamp: January 06, 2012, 07:01:57 pm
content: Quote from: IvoS on January 06, 2012, 12:32:24 amEven a high quality 400 Mhz rated Tek probe has about 4 mm in diameter, if you're expecting good probe cable that is similar to those thick audiophoolery cable, you will be disappointed.Quote from: IvoS on January 06, 2012, 12:32:24 amJust a friendly suggestion, you should be careful in making this kind of claim, especially in this forum here, really worry you might be bullied, seriously.

author: The_Penguin
author_rank: Regular Contributor
timestamp: January 06, 2012, 08:31:59 pm
content: Quote from: IvoS on January 06, 2012, 12:32:24 amLet me guess. I'll bet this "difference" sounds like this:Ka-CHING!!

author: IvoS
author_rank: Frequent Contributor
timestamp: January 06, 2012, 08:52:57 pm
content: Quote from: The_Penguin on January 06, 2012, 08:31:59 pmI am not first day in town. We did several blind listening tests, I know what I am talking about even though it doesn't make sense from engeneering point of view. So you want to tell me that cheap RadioShack interconnects sound the same?Anyway, I wasn't trying to say to use audio interconnects as scope probes. Just giving an example on interconnects quality and differences, that's all. Who is iritated by my posting should just simply ignore it.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2012, 09:05:20 pm
content: Quote from: IvoS on January 06, 2012, 08:52:57 pmThat's not the right comparison. If you compare cheap interconnects with good interconnects the good ones may well sound better. But if you compare good interconnects with expensive interconnects, do you still expect to hear a difference?

author: IvoS
author_rank: Frequent Contributor
timestamp: January 06, 2012, 09:20:06 pm
content: It depends what someone claims to be a good interconnect.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2012, 09:30:11 pm
content: Quote from: IvoS on January 06, 2012, 09:20:06 pmThat's a matter of objective engineering and not subjective judgement.

author: Unknown
author_rank: Unknown
timestamp: January 06, 2012, 09:34:39 pm
content: Quote from: IvoS on January 06, 2012, 12:32:24 amDoesn't matter. Having it near the connector is more convenient and allows for a smaller probe.Quote from: IvoS on January 06, 2012, 12:32:24 amThe other way around. The compensation range of a (10x) probe should include the scope's input capacitance. If you look at the datasheet of the IP250, the compensation range is 10 - 60 pF. I believe the Rigol's input capacitance is something like 15 pF, so it should compensate fine. ReadTektronix' ABCs of Probesor even better the document Saturation referenced to understand why and how to compensate, and why it doesn't matter for a 1x probe. For best quality, I would recommend against a switchable probe, they're usually the weakest performers, although it's unlikely to matter at the low frequencies the Rigol can register.Quote from: IvoS on January 06, 2012, 12:32:24 amAnalog scopes usually have an accuracy of about 3% (it's hard to do much better by eye). Digital scopes are similar. A 10x probe is typically specified to have an attenuation accuracy of about 1%. All these numbers are at DC. Scopes are -3 dB down (-30%) at their rated bandwidth, so a 50 MHz scope with matching probe might be -30% off at 50 MHz. Between DC and 50 MHz, it's off by less than -3 dB, but by how much is not usually specified.If you're handling 1 mV signals, I would suggest a good 1x probe. The bandwidth will be limited to < 10 MHz (realistically to even less if you take the capacitive loading into account), but the signal to noise ratio will be much improved.Quote from: IvoS on January 06, 2012, 12:32:24 amExpensive probes will have thinner and more flexible cables. Only cheap probes have those ugly, stiff and thick cables. It's not like this cable is carrying any significant amount of current. The core is resistive, so low resistance is not a factor. The center conductor is usually extremely thin to reduce capacitance. High frequency connectors also tend to be smaller than the old, clunky BNC. You don't listen to scope probes, you measure with them, so this decision should be 100% down to engineering. Those nice people at companies like Agilent, Tektronix and Lecroy spend a lot of time engineering these probes for best performance. If you're dying to spend money, I would recommend to buy probes from one of these companies, but don't expect to see any difference except mechanical features at those low frequencies.Also pay attention to the accessories you get with the probe. Not much point worrying about the best probe if you use a 6" ground lead with tons of inductance. Use the shortest ground connection possible for best performance. None of this matters at DC or very close to DC (eg. audio).

author: IvoS
author_rank: Frequent Contributor
timestamp: January 06, 2012, 10:59:32 pm
content: Thanks so much "alm" for answering my questions.