How much should I pay for a 500MHz 10x passive oscilloscope probe? - Page 1
Forum section: electronics / general

author: rs20
author_rank: Super Contributor
timestamp: September 17, 2014, 11:49:02 am
content: Call me a n00b, but I've always just used the probes that come with oscilloscopes. However, I've just ordered a oscilloscope online, and whether probes were included isn't really mentioned (call me a n00b...). So now I'm thinking I might need to buy some, which is fine, but I have no clue what I'm looking for, whether cheap eBay stuff works well enough, etc. I guess I'm after decent ergonomics and a reasonably flat response? Where would you go to get your probe? Good value brands? Brands to avoid? Ideally I'm looking for 500MHz+ (to match the scope), fixed 10x is fine.I tried a search for "500MHz probe" but didn't find anything relevant on the forum.

author: Carrington
author_rank: Super Contributor
timestamp: September 17, 2014, 04:15:26 pm
content: I have a pair of n2890a for sale.Link.

author: Fsck
author_rank: Super Contributor
timestamp: September 17, 2014, 04:19:56 pm
content: I just bought 6 used Lecroy PP006's at 30$/u USD shipped.There are some new agilent 10073C's for 45$ USD on ebay if you can live with funky 2.2M system impedence.http://www.ebay.ca/itm/221544077399?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2014, 06:21:07 pm
content: High impedance probes like the PP006 don't work over 100MHz on real circuits. Their capacitive loading is too high. For probing signals over 100MHz you need active FET probes or low-Z passive divider probes like the Tektronix P6156.

author: tggzzz
author_rank: Super Contributor
timestamp: September 17, 2014, 06:35:23 pm
content: Quote from: nctnico on September 17, 2014, 06:21:07 pmI have some HP10431 10:1 1Mohm probes with a 6.5pF tip capacitance. While not as good as my HP10020 10:1 Z0 probes (<0.7pF), the 10431sareusablewellabove 100MHz provided you don't over-interpret what you are seeing.As with all tools, understand and work with their limitations.

author: nctnico
author_rank: Super Contributor
timestamp: September 17, 2014, 07:27:34 pm
content: That still means it is better to invest in low-Z probes than waste money on 500MHz probes.

author: Fsck
author_rank: Super Contributor
timestamp: September 18, 2014, 10:28:41 pm
content: Quote from: nctnico on September 17, 2014, 07:27:34 pmconsidering it's possible to get a set of 4 high-z quality probes for the price of 1 low-z probe, both types being secondhand, I'm not sure your argument holds for every situation (just because you have 500MHz probes doesn't mean you're going to be needing that bandwidth). I actually got my PP006's for 20$ each + shipping, I just got shafted by ebay's global shipping program.

author: saturation
author_rank: Super Contributor
timestamp: September 19, 2014, 11:47:23 am
content: $80-100. From previous threads:https://www.eevblog.com/forum/reviews/cheap-china-p6xxx-probes-good-bad/msg485656/#msg485656https://www.eevblog.com/forum/reviews/scope-probe-what-the-difference-between-100mhz-and-500mhz/msg395742/#msg395742.. and also +1 for the low Z probes, it can't hurt to have one and compare performance.http://www.emcesd.com/1ghzprob.htm

author: tggzzz
author_rank: Super Contributor
timestamp: September 19, 2014, 12:31:07 pm
content: Quote from: Fsck on September 18, 2014, 10:28:41 pmOne of the key factors in making 500MHz probes (especially high-Z probes) work well, is how they are connected to the circuit under test. From what I can see looking at pictures of low cost scope probes, they typically come only with 6" croc(k)-clip ground leads -which is pretty disastrous at 500MHz.For general purpose probing in the absence of other constraints, my preference is to use either:a 0.5-1" bayonet/spear (see attachment image of 6.5pF hp10431)the soft equivalent, exemplified byhp/agilent/keysight 2870the traditional pair of couple of wire loops directly soldered to the signal and ground, with probe slipped into themthe usual short wire spring slipped over the ground, if the ground plane is sufficiently closeBut I still like Z0 probes, and am inordinately attached to my pair of HP10020s

author: rs20
author_rank: Super Contributor
timestamp: September 19, 2014, 11:12:38 pm
content: Thanks everyone for the advice, and especially for pointing out that a 500 MHz passive probe is probably not actually what I want. Who'd have thought that 13 pF at 500 MHz works out to 25? of reactance? I'd be better off running 50? coax straight down to the board (only partially kidding; that would actually work better, right? If my signal consisted only of components above 250 MHz. As long as I terminated properly at the scope.)I'll see what comes with the scope, but if there are no probes, I'll probably just get some cheap, lower-bandwidth passive probes and wait until I come across a more demanding application (which may never happen, to be honest) and re-assess then. In the mean time, I'm learning a lot from all the links and applications notes here, so thank you!Just out of interest, while on topic, roughly how much would one pay for anactive500 MHz probe? Feeling around US$100 on eBay for 2nd hand Tek probes?

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2014, 11:53:58 pm
content: Quote from: rs20 on September 19, 2014, 11:12:38 pmI guess you need to spend a couple of hundred for one from Ebay. The passive low Z probes are around $100 if you are patient.

author: tggzzz
author_rank: Super Contributor
timestamp: September 20, 2014, 12:26:58 am
content: Quote from: nctnico on September 19, 2014, 11:53:58 pmOr, if you are more impatient, or if you want to have fun hacking something semi-reasonable - you could make your own Z0 probe.http://koti.kapsi.fi/jahonen/Electronics/DIY%201k%20probe/

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 10:52:43 am
content: Quote from: tggzzz on September 19, 2014, 12:31:07 pmWhere I can get away with it, I prefer using a coaxial probe tip adapter but I get almost as good a result with the wire loops soldered into place. The coaxial probe tip adapters which can be soldered into place used to be more common but now they want like $1 or more for each one and they are usually unavailable anyway.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 10:59:26 am
content: Quote from: rs20 on September 19, 2014, 11:12:38 pmIt is not quite that bad but you have identified the problem. Probe bandwidth is specified with a 25 ohm source from a terminated 50 ohm transmission line which is unrealistic in many cases.QuoteI often end up just designing in a small JFET or bipolar follower which can drive a 50 ohm coaxial cable or the probe with a coaxial adapter.I have always been leery of used active probes because they are fragile and difficult to repair. If I was really desperate, I might make one with a bandwidth in the 100s of MHz. Jim Williams and Robert Pease wrote about simple implementations. I am a little surprised nobody has made a kit for a relatively low performance one.On the other hand, x10 500 ohm or x100 5000 ohm low-z passive probes are pretty easy to make.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 11:04:18 am
content: Quote from: rs20 on September 17, 2014, 11:49:02 amI have had good results with a pair of Texas 250 MHz probes I bought off of Ebay but have heard mixed results about their 500 MHz ones. My fastest high input impedance oscilloscopes are 200 and 300 MHz so I have no way to test the 500 MHz ones at their rated speed even if I wanted to.One thing you might consider is finding a reference level flat pulse generator which can be used to verify probe performance. Not all 500 MHz probes are going to perform well with all 500 MHz oscilloscopes because of high frequency compensation requirements.

author: woodchips
author_rank: Frequent Contributor
timestamp: September 26, 2014, 08:26:53 pm
content: As has been mentioned, make your own.With 50 ohm BNC and coax just solder the smallest 470R resistor you can find to the centre core and the screen to earth and you have a 10X probe for nothing. This is why any scope over about 100MHz has a 50 ohm input.With a standard probe wrap the earth lead around the probe tip, you are minimizing inductance and hence maximizing bandwidth.

author: tggzzz
author_rank: Super Contributor
timestamp: September 27, 2014, 04:18:54 pm
content: Quote from: woodchips on September 26, 2014, 08:26:53 pmI doubted that, and have just tested it. As expected it has little effect.Looking at a digital output with a 150MHz *10 probe (Agilent N10074C), the overshoot was reduced from 0.6V to 0.5V.However, one technique does have asignificanteffect, reducing the overshoot to <0.1V (which is probably representative of the unprobed circuit). The techinque is to minimise the inductance by minimising the length of the ground lead by using eitherthe 0.5" wire spring supplied with the scopea homemade version of the bayonet in the attached picture, made from a pogo pin, sheet phosphor bronze, and Fimo PVC modelling clay. (The pogo pin is too fragile - my next version will be with a needle)BTW, the picture is an HP10431 with a tip capacitance of 6.5pF, which is useful to 500MHz. I have used bayonet grounds on HP10020 probes that are good to 1.5GHz (<0.7pF tip), and one Keysight probe appears to use the technique on a 6GHz probe.Don't faff around: use easy techniques that work well!