Oscilloscope Probe Quality - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: March 25, 2024, 02:54:33 am
content: This past weekend I went through my box of oscilloscope probes; prior it was a box of tangled probe wires with no clue which ones I have; other than two or three I have hanging in arms reach.Until recently, my oscilloscopes were basic models, maybe 100MHz, but now I have an Agilent 500MHz and a (hopefully soon) repaired LeCroy (I think 500MHz), so I never paid attention to the bandwidth of oscilloscope probes.One probe is a 10074C 150MHz Agilent (the one I use) and then I have Tektronix probes with large plastic boxes on the BNC connection end (some have a pin that tells the scope they are x10 and some don't).My initial assumption was the gray Tektronix ones with large plastic boxes at the BNC end were old and junk, however, I'm finding the bandwidth to be quite high. The Agilent one doesn't have the plastic box, but appears newer and assumed to be better quality.Then I have a few that look like junk with nothing more than a basic BNC connector and a nifty x1 and x10 switch along with a ref position.My question is: what makes a good oscilloscope probe? Is it just the BNC end, the cable, the probe itself, etc...? I'm sure it's a combination, but I'm seeing those Agilent to be expensive, and it's nothing more than 150MHz whereas I have a Tektronix that's 350MHz (P6138) but is much older looking.Since I have higher end scopes, now I'm wondering if I need/should invest in probes with 500MHz plus bandwidth.

author: bdunham7
author_rank: Super Contributor
timestamp: March 25, 2024, 03:38:13 am
content: Quote from: bostonman on March 25, 2024, 02:54:33 amThe answer to your question is: low input capacitance and a properly designed compensation network. The special low-capacitance input resistor at the probe tip end of the P6138 is the magic that makes it work. The box at the BNC end is just a mundane but properly set up compensation network. You don't need "500MHz" probes, just good ones with a reasonable BW like the P6138 and P6139. Actually looking at a 500MHz signal with a high-impedance probe is actually somewhat problematic even with probes rated for it. The complex impedance of even an 8pF probe is quite low at 500MHz and getting a short low-inductive ground connection is tricky.

author: tggzzz
author_rank: Super Contributor
timestamp: March 25, 2024, 08:13:10 am
content: Quote from: bostonman on March 25, 2024, 02:54:33 amFitness for the purpose, of course. Corollary: without defining the purpose, the question should not be answered.There are many classes of scope probe, for a reason. They are not interchangeable.As for the traditional *10 "high" impedance probes, they arent. If they have a 15pF tip capacitance, calculate the impedance at 10/100/500MHz. Thenunderstand"low" impedance "resistive divider" "Z0" probes.FFI, have a look at the refs inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/You will note that probing technique becomes vital at UHF and above. Start by looking athttps://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/and the linked articles.

author: Gyro
author_rank: Super Contributor
timestamp: March 25, 2024, 07:09:07 pm
content: Quote from: bostonman on March 25, 2024, 02:54:33 amYes, be careful of that X10 identifier pin, I remember them causing havoc on HP scopes that had the BNC mounting nut on the outside, they would chew up an lock solid. They will probably scratch up the plastic front panel overlays of other scopes too. Checkbeforeplugging.

author: bostonman
author_rank: Super Contributor
timestamp: March 26, 2024, 02:29:24 am
content: Interesting feedback.Thankfully I don't expect to measure anything close to 500MHz anytime soon. As mentioned, some of the probes I have look like nothing more than a BNC connector and a over molded metal pin at the probe end. My personal feeling was these are low quality and cheap probes that aren't worth using. Then I looked at the old Tek ones with large "boxes" at the BNC end compared to the streamlined Agilent ones I hvae.After looking at the prices for scope probes, I began wondering if they are just fluff such as Agilent selling probes for hundreds of dollars (a co-worker has a similar Agilent scope and said the probes for his model run around $500).Thankfully now I at least have my scope probes in zip lock bags stored in totes. Prior they were in totes and all tangled together with no idea which models are inside.

author: tggzzz
author_rank: Super Contributor
timestamp: March 26, 2024, 09:20:52 am
content: Quote from: bostonman on March 26, 2024, 02:29:24 amSearch DigiKey for "oscilloscope probes", then sort on price.Prepare to be surprised. Once you understand why people buy them, you will have become enlightened.

author: DimitriP
author_rank: Super Contributor
timestamp: March 26, 2024, 05:50:06 pm
content: QuoteSince you asked.....If you actually "need" to use a 500MHz scope it would also "need" a 500MHz probe.And you would not be asking the questionMaintaining the signal's integrity while observing it can get tricky.Depending on the signals you are working with or playing with and the consequences the degree of importance varies.

author: bostonman
author_rank: Super Contributor
timestamp: March 26, 2024, 06:01:25 pm
content: QuoteI had done that and saw the price range. it all makes more sense and certainly answers my question. Previously I also thought companies assume if you're rich enough to afford a >$20k scope, you're rich enough to buy a fancy probe that is ideally equivalent to an average model.Now that I know the cost for these probes, I'm going to be much more careful with them rather than use them to jump rope

author: MarkT
author_rank: Frequent Contributor
timestamp: March 26, 2024, 07:08:06 pm
content: Quote from: DimitriP on March 26, 2024, 05:50:06 pmWell if you are measuring everything at 50ohms, no you wouldn't need a special probe, just a standard BNC cable.

author: DimitriP
author_rank: Super Contributor
timestamp: March 26, 2024, 09:42:34 pm
content: Quote from: MarkT on March 26, 2024, 07:08:06 pmLot's of if ..then.else.. I tried to keep it short