Fifty Ohm Probes - Page 1
Forum section: electronics / testgear

author: German_EE
author_rank: Super Contributor
timestamp: February 03, 2015, 09:35:53 pm
content: For normal use the scope probes that we buy work just fine, they are manufactured for 1M ohm plusxpF scope input impedance and we all know the advantages and pitfalls of their use. For RF use, especially when measuring low levels of RF power, it is possible to switch a scope to 50 ohm input impedance but here's where the problem starts.There are no 50 ohm probes.Do a Google search and ONE applicable link comes up where a brave fellow makes a 50 ohm probe from some hardline. After spending a few hours looking I can't find anybody who makes 50 ohm probes or, even, a standard probe that can be disassembled and rewired with thin 50 ohm coax. Note that a 50 ohm or 'straight through' probe does have other uses. It could be used on a frequency counter where scope probes work but only just and it could be used on an audio or RF voltmeter.There must be a market for such a device so where are they? Do these probes exist or do I need to get some brass tube and coax so I can make some?

author: tautech
author_rank: Super Contributor
timestamp: February 03, 2015, 09:49:13 pm
content: Seem to remember a recent post where a member modded a BNC 50 Ohm socket with tips that could be connected to a std BNC cable.Lets hope he spots this and links his imagery.

author: electr_peter
author_rank: Unknown
timestamp: February 03, 2015, 10:17:05 pm
content: Quote from: tautech on February 03, 2015, 09:49:13 pmDo you mean this post?https://www.eevblog.com/forum/testgear/scope-probes-as-supplied-by-rigol-for-the-low-end-scopes-a-discussion/msg594599/#msg594599Regarding probes, it is a question of compromise between bandwidth, circuit loading, price, etc.Passive spectrum covers mostly:1x/10x 9 MOhm probes with 1 MOhm scope - good performance with low BW, high circuit loading in higher frequencies10x 450 Ohm probes with 50 Ohm scope - good performance with med-high BW, low circuit loading in higher frequencies50 Ohm coax with 50 Ohm scope - high circuit loading, wide BWThe reason why 50 Ohm scope probe is not popular because it offers no real benefits when compared to other options. If you need 50 Ohm probe, just connect a coax. And typical coax is not that good compared to high BW scope coax.

author: tautech
author_rank: Super Contributor
timestamp: February 03, 2015, 10:25:47 pm
content: Quote from: electr_peter on February 03, 2015, 10:17:05 pmNo Peter, not that one.Member IIRC used SMD resistors soldered in the socket for impedence matching.There was close-ups of his probe.

author: Lightages
author_rank: Unknown
timestamp: February 03, 2015, 10:38:39 pm
content: I don't understand where and when you would want a 50 ohm probe. Perhaps this is my ignorance speaking but if the source is a 50 ohm source then just stick a coax with proper termination and probe it with a normal probe. If the source is 50 ohm and it already is in circuit with a 50 ohm load, then adding another 50 ohm load in parallel will change the loading and change what you are trying to see. If the output is a transmitter output then it needs a proper load at all times when outputting and can be damaged with the incorrect load so it needs to be loaded correctly before you probe it. If the transmitter is 1000W, for example, then your 50 ohm probe would need to dissipate 1000W!

author: skipjackrc4
author_rank: Regular Contributor
timestamp: February 03, 2015, 11:12:43 pm
content: Just solder a thin piece of coax with an SMA connector on it. That's effective a 50 ohm probe.Something to consider, though, is that when you attach your 50 Ohm probe to a 50 Ohm trace, that trace is no longer 50 Ohm...

author: nctnico
author_rank: Super Contributor
timestamp: February 03, 2015, 11:45:26 pm
content: Quote from: German_EE on February 03, 2015, 09:35:53 pmYes there are. They are called 'FET probes' or 'low-z passive divider probes'. The latter are slightly less expensive. Both offer superior performance compared to standard probes.

author: donmr
author_rank: Regular Contributor
timestamp: February 03, 2015, 11:46:41 pm
content: Did you recall one like this?http://www.emcesd.com/1ghzprob.htmIts passive, not an FET probe.I'd like to find an affordable FET probe, or an amplifier to use a 1M probe with a 50 ohm input.

author: VK5RC
author_rank: Unknown
timestamp: February 04, 2015, 12:08:38 am
content: I am a bit puzzled, don't you normally use a high impedance probe on a circuit that is supposed to be working normally, the probe should have minimal impact on the circuit's behaviour so it should be as high impedance (and low capacitance) as possible (almost impossible at higher frequencies). 50 ohm impedance is a common inter-connection impedance between sections so if that is required a 50ohm connector, either a terminator/attenuator/dummy load and good coax is all that is required? That is usually a lot easier to achieve. Eg I have many cables capable of 10GHz 50ohm but no 10GHz probes (nor matching oscilloscope!)

author: tautech
author_rank: Super Contributor
timestamp: February 04, 2015, 12:11:18 am
content: Quote from: donmr on February 03, 2015, 11:46:41 pmYep that's it.It was linked to by a member.There's also some stuff on 50 Ohm setups here:https://www.eevblog.com/forum/testgear/importance-of-good-terminators-for-rf-pulse-applications/

author: mazurov
author_rank: Frequent Contributor
timestamp: February 04, 2015, 12:11:56 am
content: I have several passive 50 ohm probes made commercially. One is Tektronix P6158, another is P6058. 50 ohm FET probes that I own are P6201 (one of them proudly wearing Agilent asset tag :-)) and P6202. You will need old Tek scope or 1010 power supply for those.Don't know if they are still made though; ppl increasingly prefer makeshift low-Z probes these days. P6158 is called "low capacitance probe", you may want to google around to see if anything with this description is offered for sale.

author: mazurov
author_rank: Frequent Contributor
timestamp: February 04, 2015, 12:15:16 am
content: Quote from: VK5RC on February 04, 2015, 12:08:38 amThat's the probe tip. OP was asking (I hope) about 50 ohm output probe to match 50 ohm input of the instrument.

author: GK
author_rank: Super Contributor
timestamp: February 04, 2015, 03:03:04 am
content: Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.I haven't look too hard yet, but so far I've only found second hand examples of 500 ohm probes on Ebay. I don't know of any contemporary manufacturer of 500 ohm, high frequency probes, but would like to find one.

author: Hugoneus
author_rank: Frequent Contributor
timestamp: February 04, 2015, 04:41:16 am
content: Quote from: GK on February 04, 2015, 03:03:04 amRigol makes them. 500Ohm, 1GHz bandwidth. The model number is RP6150, it has 1.5GHz of claimed bandwidth. It came with my DS6000 series scope.

author: blacknoise
author_rank: Contributor
timestamp: February 04, 2015, 04:48:16 am
content: There is / was even a flexible passive low impedance probe kit with pluggable probe tips to get input impedances beetween 50 ohm (1:1) and 5 kOhm (100:1) - look for HP 10020A in the attached document.And you may easily build your own passive low impedance probe. Assuming your instrument is already equipped with a 50-Ohm terminated input, you only need to place the correct resistor in front of the inner conductor of a 50-Ohm coaxial probe cable.The rule for this resistor is ((attentuation-1) * 50 Ohm) - e.g. 0 Ohm for a (1:1) probe, 450 Ohm for a (1:10) probe or 4950 Ohm for a (1:100) probe. For best high frequency performance you may add a compensation capacitor accross the probe tips input resistor. It's theoretical optimal value is:((capacitance of the scope input) + (capacitance of the coax cable) pF) / (attentuation-1)To reduce the "ringing" of a probe with a capacity-compensated probe tips input resistor, you may add a fractional of the probe tips input resistor without the parallel capacitor. Then this fractional resistor acts as a damping resistor in the LC resonant circuit beetween the probe tip, across the signal source, and the probe's ground connector.Commercial probes i have seen used values beetween about 20 and 100 Ohm for this damping resistor. For the accurate attentuation you have to reduce the main tip input resistor by the value of this damping resistor!For example,a (1:10) probe:tip--50Ohm--(400Ohm || compcap)--coax cable--(50Ohm scope-in)or a (1:100) probe:tip--50Ohm--(4900Ohm || compcap)--coax cable--(50Ohm scope-in)compcap = compensation capacitor (see above)A final note: for best performane (above a few 100 MHz) you should use SMD resistors and capacitors and keep all "wild" wires short.Kind regards

author: tggzzz
author_rank: Super Contributor
timestamp: February 04, 2015, 09:59:06 am
content: Quote from: Hugoneus on February 04, 2015, 04:41:16 amHmmm.Supplied with a 6" (=140nH) ground lead.No specification for tip capacitance.I'd want to measure that before I believed it.OTOH 25 years ago I was using HP10020 probes (1.5GHz, 0.7pF) and getting extremely good performance on a 1GHz scope observing 800ps risetimes.

author: w2aew
author_rank: Super Contributor
timestamp: February 04, 2015, 06:03:58 pm
content: Tek P6150 is a low capacitance Z0 probe - 500ohm input impedance when terminated at the scope with 50ohms (thus is a 10x probe), and has 9GHz BW. The same probe has "feedthrough" tips which basically make it simply an extension of the coax.Tek P6158 is a 20x probe, thus a 1K input impedance. 3GHz BWBoth of these are current products from Tek.If you look used, you may also find things like the:P6057, 100x 5Kohm 1.4GHzP6056, 10x, 500ohm, 3GHz.

author: darrell
author_rank: Regular Contributor
timestamp: February 05, 2015, 01:20:56 am
content: Quote from: GK on February 04, 2015, 03:03:04 amI have a pair of these from Agilent. I got then used for a fraction of the $3400 price. The 450 and 950 ohm resistors are Caddock non inductive high voltage resistors. Rise time is limited by my scope to about 140 ps. I also have a pair of LeCroy 1.5 GHz 10:1 transmission line probes (PP061, discontinued).http://www.keysight.com/en/pd-37010-pn-54006A/6-ghz-passive-divider-probe-kit?nid=-34025.536883024&cc=US&lc=engWhen I'm doing anything more than verifying the presence and shape of a signal, I tend to solder a 950 ohm 0402 resistor to what I want to probe and a stripped end of a small coax to the other end of the resistor and nearest ground. The 0402 resistor parasitics result in about 3 dB of peaking by 6 GHz for the 950 ohm 20:1 probe. Stacking two or three resistors should improve the performance. I suspect that is part of why Agilent used the inch long high voltage resistors.

author: w2aew
author_rank: Super Contributor
timestamp: February 05, 2015, 01:54:26 am
content: Quote from: GK on February 04, 2015, 03:03:04 amTek still makes them. The P6150 is 500 ohm (thus 10x), 9GHz bandwidth.

author: helius
author_rank: Super Contributor
timestamp: February 05, 2015, 05:05:02 am
content: The Tek P6507 is a 1X 50-ohm probe. It's made for semi-permanent board mounting.

author: GK
author_rank: Super Contributor
timestamp: February 05, 2015, 09:38:41 am
content: lol, like I said, I haven't looked too hard yetI like the Tek 20:1 1k probe and it would go nicely with my 7904A, but ouch on the price for a brand new one (>600 USD).

author: Mark_O
author_rank: Frequent Contributor
timestamp: February 10, 2015, 08:36:39 am
content: Quote from: joeqsmith on February 10, 2015, 01:28:23 amVery clean construction. Nicely done.Thanks for sharing your technique.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2015, 09:53:22 am
content: Quote from: joeqsmith on February 10, 2015, 01:28:23 amThat's the neatest homebrew construction for the resistor I have seen.It does, however, rely on there being an available circuit ground at precisely that distance from the point being probed.One alternative to the sewing needle would be "piano wire" or "music wire", say 0.8mm diameter. It has thepotentialadvantage that it can be bent into any desirable shape using strong pliers, but is also sufficiently stiff to act as a ground bayonet/spear. This would enable construction of a probe tip similar to the old HP10020A 1.5GHz probe (see PDF attached tohttps://www.eevblog.com/forum/testgear/fifty-ohm-probes/msg601417/#msg601417), or the high impedance 500MHz probe shown below.Another alternative to the sewing needle would be the flexible sheet ground used in the Keysight 2870 series.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 11, 2015, 03:26:50 am
content: Thanks.Copper foil cut into strip used to form a clamp. Easy to make but would guess the performance with just a wire wrapped around the end would be better.

author: tggzzz
author_rank: Super Contributor
timestamp: February 11, 2015, 10:26:14 am
content: Quote from: joeqsmith on February 11, 2015, 03:26:50 amThat's another neat easy construction. You might like to consider documenting it in some form of blog that is less transitory than this forum, and then submitting it to something like hackaday.com. That would enable people to find it more easily.I'm unsure of the performance of the spear vs wire, and no longer have access to equipment that could measure it.I have used the spear on the ancient HP10020 1.5GHz <0.7pF probe to measure digital risetimes of ~800ps with a 1GHz scope.I presume one advantage of the foil on the modern Keysight 2870 is slightly lower inductance, due to it being wider.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 12, 2015, 03:50:15 am
content: Large probe was very poor. Smaller probe is not the best but much better. The clamp and needle idea is not so great....Swept to 2.7GHz. Looks like the 0603 wedged in there is not so good. Playing with 1206's the inductance was always a problem.BLUE 420MHz was 230 ohmsORANGE 820MHz was 130 ohmsTransmission, ORANGE 1610MHz.

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2015, 01:04:39 pm
content: Quote from: joeqsmith on February 12, 2015, 03:50:15 amI would interpret that as indicating the tip capacitance is approximately 1.5pF.For reference, here's a scan of the interesting bits of my hp10020 probe, 10:1, 500 ohms, 1.5GHz, <0.7pF, millimetre scale.The metal part contains the 450ohm resistor and when inserted into the bayonet only the cable's screw thread is visible. The bayonet makes contact with the wide part of the metal shank (see the diagonal "score" marks) plus the end of metal part's narrow shank.The metal part is light and there are visible external air gaps inside the plastic part - I interpret this as minimising material with Er>1 so as to minimise tip capacitance.

author: Carrington
author_rank: Super Contributor
timestamp: February 12, 2015, 03:33:18 pm
content: In fact some Keysight probes use these non-Inductive resistors (MG660 and MG680):www.sider.com.hk/uploadfiles/files/mg.pdfWhy? Check this out, by Dr. Howard Johnson:How to Make A 1K-Ohm Probe:https://www.sigcon.com/Pubs/straight/probes.htmOthers non-inductive resistors:-https://www.vishay.com/docs/30203/rwrmil.pdf-https://www.vishay.com/docs/30204/rsns.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: February 13, 2015, 01:29:28 am
content: Quote from: tggzzz on February 12, 2015, 01:04:39 pmI measured 1.7pf at 410MHz.Should mention, the waveform is the same in both with same probe. The reason risetime is so improved was higher BW input.I have built some disposable ones using two 1206s (about 1pf ea) in series, but the BW on these were around 1GHz. Keep in mind that the standard ECL logic will have the following input capacitance per unit load:MECL 10KH, 2.9pFMECL 10K, 3.3pFMECL III, 3.3pF

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2015, 12:14:04 pm
content: Quote from: joeqsmith on February 13, 2015, 01:29:28 amIf n=needle, O=dielectric supporting the needle, r=resistor, then your current probe is effectivelynnnnOOOOOOOOrOOOOOOOWould it be possible to remove some of the dielectric so that it is more likennnnOnnnnnnOrOnnnnnnOI suspect that the HP10020's construction is more like that, and it might allow you to halve the capacitance associated with the dielectric.I'm tempted to build one of thesehttp://www.edn.com/design/test-and-measurement/4435414/Sub-picofarad-measurement-with-CMOS-invertersto help measure the tip capacitance.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 14, 2015, 02:13:12 am
content: Quote from: tggzzz on February 13, 2015, 12:14:04 pmTo some degree, yes it could be made this way but I think the problem is going to be supporting the tip.

author: tggzzz
author_rank: Super Contributor
timestamp: February 22, 2015, 11:11:06 am
content: You may find this relevant w.r.t. reducing the probe's tip capacitance, particularly figures 6 and 7.http://m.eet.com/media/1149173/23518-46018.pdf

author: grenert
author_rank: Frequent Contributor
timestamp: February 22, 2015, 05:42:28 pm
content: Here's an old EEVblog thread on making DIY 50 ohm probes:https://www.eevblog.com/forum/projects/lo-z-probe/