Probes for checking voltage references

[RandallMcRee]

Silver-plated teflon wire is great when you need it, you really need it:

http://www.apexjr.com/wire.html

(Satisfied customer, no affiliation, etc)

I use Mogami cable, its great stuff. Markertek is a good source.

Randall

[rhb]

Thanks for the Amazon link.  It's pricey, but not too bad.  I like books which is why I have 5000+.

[macboy]

I have read online that oscilloscope cables are typically 200 ohms to reduce reflections at the 1 Meg scope input.  But I have been unable to find a source of 200 ohm coax.  The highest impedance I could find was 95 ohms.   Moreover, the magnitude of the reflection coefficient is almost unity with 200 ohm cable.  So it seems rather silly even if scope makers have secret sources of cable.

You got it all wrong. The characteristic impedance of the coax isn't 200 ohm, but the end-to-end resistance of the center conductor is around 200 ohm (roughly 100 ohm per metre length). This is due to a very fine, relatively high resistivity center conductor. The high resistance makes the cable a lossy transmission line, which is what reduces the reflections. The extremely fine center conductor also has a vanishingly small surface area, which reduces the capacitance to ground/shield to improve high frequency characteristics (reducing cable capacitance will reduce the need for additional probe tip capacitance to compensate the frequency response).

[rhb]

Thanks for the explanation.  That makes a *lot* more sense than what I read on the internet.  I don't normally use Wikipedia, but they have a reference to the 1959 patent by Kobbe and Polits on the idea.

What's a suitable cable and who sells it?  I can find nothing.  In fact I have not found any specification of the resistance per foot  anywhere.

Edit: I measured a Rigol and an Instek probe center conductor at around 230-240 ohms  Surely I don't have to buy probes and behead them to get cable.

[Conrad Hoffman]

I've never seen that probe cable available anywhere. You also can't solder it, as the center conductor is Evenohm, nichrome or some similar alloy. That's the reason you always find crimps when you take a scope probe apart. The properly terminated 50 ohm coax is fine, so long as the circuit can handle a 50 ohm load. No good for much DC or any high impedance circuits.

[rhb]

It looks as if this is a machining problem making widgets that fit my probe tips.  Or buying probes and cutting off the tips.  Likely a combination of the two.

I made  wire springs per Bob Pease and put a 0.10" header socket on them, but I still have ~0.75 mV of noise picked up from the LED lamp overhead.  Some of this is probably pickup from the board traces, so I need to put the DUT in a shielded enclosure.  In cany case a huge improvement over 12 mV.

[hwj-d]

Hm, again,

if you want to measure references for noise, think about, what a buffered or unbuffered reference can drive. And that's definitely not 50 , 200 or something like that. A buffer like ltc2057, that's used in some forum references like kx, px, etc drives about 20mA, unbuffered about max 2mA. The 50 impendance from RGB-58 cables should closed with a 50 termination resistor to prevent reflections. So, don't do that to measure references directly.

I don't see anyone in the forum, who want to measure a reference with a scope that way. Correct me if I see something wrong.

As David Hess and others mentioned above, you need a LNA for decoupling and measure noise correct with a scope. Pipelie developed a LNA for that. kj73 and others show it in action, for example:
https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1462484/#msg1462484

And even here, the caps of this LNA should preloaded, to prevent damage an unbuffered reference:
https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1461828/#msg1461828

So, i don't know the characteristics of your reference, but note his driving possibilities.

[TiN]

hwj-d have good points. Just lil remark, as my FX REF can drive over 50 mA and don't have problems with charging pipelie's cap in few seconds.

[rhb]

Thanks. 

I was connecting to the output op amp, not the reference itself.   I would not consider a direct connection to the reference with a probe.  Even 1 Meg is too much load.

I've tracked down the noise to the power cable coming from my PS.  I've ordered 20 AWG shielded twisted pair PTFE wire to make PS leads and 22 AWG 4 wire PTFE for resistance measurement cables.

The source of the noise is the LED lamps I just installed when a  fluorescent tube went out.  I really needed more light, so I installed a pair of 4' LED lamps.   I'm going to get some metal screen to cover the lights to see if that helps.

Having a multitude of nearby RF noise sources is unavoidable for me right now.  So I'm going to have to put a bunch of work into suppressing EMI at the source and at the DUT.

[Conrad Hoffman]

FWIW, I installed a bunch of LED lights in my lab. They save lots of money compared to the 6 flood lamps that were there before. Still, when I do FM alignments or any very sensitive measurements, I have to turn them off and resort to an incandescent in a desk lamp.

[Echo88]

I got rid of the led-lamps in my measurement room and now use halogen-incandescent lights instead. So now i dont have to fear led-lamp-noise but can see spikes on my resistor measurement when i switch the lights on...so, not a perfect solution. 
Might need an input current impulse limiter for the lights.

[hwj-d]

That's my little workbench corner with selfmade ledlamp. The lamp is made quick and dirty with some aluprofiles, very cheap 4x 10W led modules, a cheap 3A buck converter inside the aluprofile, silicon wires, an old laptop power supply and a sonoff module. In the right corner are some servers, synology, odroid xu4, cubitruck, pizerow's, lan switches, under the table fritzbox router, wlan, dect, laptop, monitor on the left, bluethoth, cell phone ...

Believe me or not, i didn't see excessive spikes  from that all if i measure my resistors, references on that table with my 34461a (nlpc 10-100).
Ok, that's not a profi like lab, were I don't implement that all in a spot like that.

[eurofox]

That's my little workbench corner with selfmade ledlamp. The lamp is made quick and dirty with some aluprofiles, very cheap 4x 10W led modules, a cheap 3A buck converter inside the aluprofile, silicon wires, an old laptop power supply and a sonoff module. In the right corner are some servers, synology, odroid xu4, cubitruck, pizerow's, lan switches, under the table fritzbox router, wlan, dect, laptop, monitor on the left, bluethoth, cell phone ...

Believe me or not, i didn't see excessive spikes  from that all if i measure my resistors, references on that table with my 34461a (nlpc 10-100).
Ok, that's not a profi like lab, were I don't implement that all in a spot like that.

Did you put 0.01% resistors in your lamp? 

[hwj-d]

Did you put 0.01% resistors in your lamp? 

psst, that's the trick ... 

[tautech]

Reg
Check out this great old book about Tek probe design:
https://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

[rhb]

Thanks to all.

My Dad left me a bunch of large aluminum chassis' which are large enough to have left me wondering what to do with them.  Based on input from a number of people, I'm going to use  a couple to make a Faraday  cage test box.  Given the prices I've seen for mu metal I'll make  a  smaller Faraday cage with magnetic shielding.

[ArthurDent]

Regarding the discussion of what type of cable/wiring to use, years ago I had an electrometer which had an obscenely high input impedance. I made an input cable for the electrometer using what I thought would be a great piece of cable which looked like RG-58 size shielded coaxial cable. Under the black outer jacket was a silver plated braided shield, under that was a polyethylene tube, and fitting loosely inside the tube was a small teflon insulated stranded silver plated wire. Maybe it looks like a little overkill but I had a piece of the cable on hand and this seemed like a perfect application for it. 

I noticed I was having a lot of trouble getting stable readings and found that it was the triboelectric effect caused by me moving the cable. I found that if I just had the tip of my probe disconnected and wiggled the wire slightly the analog meter needle would wildly zing from one end of the scale to the other. The slight flexing and/or movement of the different insulating materials used in the cable was producing the triboelectric effect, like rubbing a balloon on your hair to make it stand up. 

If you’re trying to make precision measurements everything from LED lighting, SMPS, to wiring may have an effect.

[splin]

Instead of using costly to run incandescents, why not modify suitable LED lamps/striplights etc to run from a mains transformer + linear regulator(s)? Some lamps use a relatively high voltage so might not be ideal from a safety point of view but certainly many cheap Chinese linear light fittings should be suitable. Shouldn't be expensive either given the low power levels required.

Obviously you couldn't use the existing mains light fittings and wiring but in a lab environment it shouldn't be too unsightly to install replacement or suplementary noise free lighting.

[zhtoor]

That's my little workbench corner with selfmade ledlamp. The lamp is made quick and dirty with some aluprofiles, very cheap 4x 10W led modules, a cheap 3A buck converter inside the aluprofile, silicon wires, an old laptop power supply and a sonoff module. In the right corner are some servers, synology, odroid xu4, cubitruck, pizerow's, lan switches, under the table fritzbox router, wlan, dect, laptop, monitor on the left, bluethoth, cell phone ...

Believe me or not, i didn't see excessive spikes  from that all if i measure my resistors, references on that table with my 34461a (nlpc 10-100).
Ok, that's not a profi like lab, were I don't implement that all in a spot like that.

nice and cosy setup 

regards.

-zia

[rhb]

The LED lamps run straight off the mains.  I'm going to install a 1/8" hardware cloth screen today and see how that does.  At least if the cloth arrived as promised.

I'm acutely aware I've started playing in the land where you have to hold your mouth just right to get things to work.

[MisterDiodes]

Just some general food for thought, since RHB seems to be just starting out:

If you run LED lighting, even if you run RAW (directly from mains) be aware there is a small current transient when the diodes switch "on", so pay attention that the power source is quiet DC (and no switch mode power supply allowed near low-ppm test area - batteries or regulated linear + isolation XFMR only).  Also pay attention to the LED CRI - If it's not at least 90+ it becomes more difficult to see the difference between some shades of wire or component code colors - say orange and red.

For those reasons we generally only use halogen incandescent from 12V, 24V or 48V battery banks during critical tests.

As far as cabling goes for chasing low ppm - you're generally not ever going to be measuring a Vref with a 'scope probe (not without an LNA).  Cat5 or even shielded Cat6-7 with faster twist rates is fine in most cases, and a decent DMM is required. CLEAN Copper to Copper connections only, watch out for oxide Copper or tarnished silver plate on that silver-plated wire (that creates thermal EMFs and noise).  Gold plated connectors are easier to keep clean, but add a small amount of noise.  Keep the leads dressed SHORT and use DIRECT wiring, keep power cords away from sensitive circuits, make use of shields / guard circuits as required, and understand ground loops.

Remember:  Every circuit loop that encloses a non-zero area is a potential magnetic pickup point, and every PN junction is a potential demodulator.

Copper / Aluminum shielding is not the best at low freq - for that application look at steel and or MuMetal.  If you're really serious about cable shielding at low freq you start to look at MuMetal braided cable sheathing - depending on your work environment.  In our work environment we have to use that since the 3458a's are being used near some servo drives, and we're trying to grab a low ppm DC measure.  In that case the servo wiring is on one shield system, and the measuring cables are on a separate shield / guard system.

More Suggested reading - Keithley's Low Noise Measurement handbook, and Art of Electronics, 3rd edition.

[cellularmitosis]

even shielded Cat6-7 with faster twist rates

Ah, the fast twist rates should be even better than the "blue" pair in CAT5 https://www.eevblog.com/forum/metrology/probes-for-checking-voltage-references/msg1462766/#msg1462766

[rhb]

Well, 1/8" galvanized screen over the LEDs accomplished very little. :-(   Not really surprised, but I was hopeful.

Unfortunately, I don't know what's in the lamps.  They apparently have some sort of SMPS internally because the noise is definitely not 60 Hz switching transients.  I just ordered an HP 8560A with TG, so with some probe construction I should be able to track down and squelch most of the noise.

I would not consider measuring Vref with a scope probe.  I just wanted to see how much noise there was.  It's high enough to not be measurable without considerably more finesse.

Mu metal is shockingly expensive.  I plan to make a large Faraday cage which is just aluminum and a smaller one which is aluminum clad with mu metal sheet and mount assorted feedthrus.

I have bothof the references, but more time spent reading AoE 2nd than 3rd.  Only glanced briefly at the Keithly monograph.  I have another monograph on low noise electronics circuits.  Clearly time to give it a read.

Thanks to all.
Reg

[MisterDiodes]

Mu metal is shockingly expensive.

Welcome to the world of low ppm and high precision measures...get your wallet out! 

MuMetal and to a certain extent steel are your very true friends at low freq - aluminum / copper shields won't really do it for mag shielding, but they can help for electrostatics.  It depends on what problems you're facing. We use screened isolation transformers in the work areas for bench mains power (keeps out noise coming in on utility mains and the utility ground - usually the worst offender), and sometimes you'll need a dedicated lab ground and/ or a ZSRG system (Zero Signal Reference Ground - usually a copper grounded grid in your lab floor), depending on what you're measuring. 

Low ppm measuring also means you're shielding against air drafts and typically you've got a place with good temperature control.  Pay attention that all your test lead connections / binding posts are at the same temperature for lowest thermal EMF effect.

Here's another one for your homework reading list describing at least some of the grounding techniques - this is only a tiny tip of the noise-control iceberg. 

https://www.embedded.com/print/4014229

Have fun!

[eurofox]

I'm building a new LTZ1000 10V board and my intention is to mount it in a steel massive block completely hermetic, no humidity allowed 

It will be temperature compensated and all critical resistors at 2ppm.

The power supply will be a very low noise type.

I will post the results after finishing and aging for minimum 1 month.

eurofox