Strange asymmetric nV noise of Keithley 148 - Page 1
Forum section: electronics / general

author: yus
author_rank: Contributor
timestamp: May 15, 2023, 09:12:34 pm
content: I recently bought a Keithley 418 with tube (not FET) input. I bought it out of pure curiosity, I do not need it for living.I am a physicist by education (programmer by occupation) and I was curious if it can really measure voltages below 10nV.On arrival I had to:1) fix the power cable connection, fortunately I noticed an issue before a major AC 220V short arrived,2) hack the chopper+demodulator oscillator : to be able to set the required 94Hz, I adjusted R320 to a slightly lower value*. The oscillator runs stably within specified +-0.1Hz instability,3) hack the 94Hz amplifier-demodulator : to be able to set the required response time to about 0.25sec for 100nV step signal, I adjusted R169 to a slightly lower value*,(*) adjusted to a slightly lower value = to be about 25% lower. This was a quick hack to verify if the instrument is doing something reasonable at all settings.What good I have after the hacks above:1) The calibration appears to be very close to the specs even on the lowest 10 nV scale.2) 24 hour drift I estimate to be of about +-6nV max non cumulative (=around initial value at 0 time). It may probably be up to 2 times higher than a better new instrument or up to the specs, please see the picture of the expected long term drift from Keithley document The-measurement-of-nanovolts, attached pic 5. I do not want to actually run it for 24 hours due to the ugly annoying 2KHz noise of its power supply.What I do not understand and consider an issueis the "short-time" noise I see with the input shorted with a copper wire thermally isolated at the lowest 10nV scale. I would expect the "short time" noise to be up-down symmetric. Obviously it is not.Almost always the voltage slowly rises and then sharply drops 1-2 nV on average.Please see the attached pictures 1-2.This is not what the 1/f "flicker-noise" is supposed to look like.This is also not what I see on Fig 13.b , attached pic 5.Chopper+demodulator oscillator as well as amplifier-demodulator at first sight are not supposed to cause any up-down asymmetry even if they may function to some degree incorrectly. Since the max long term drift is below 6nV, no asymmetry is noticeable at 100nV scale or above.If the mechanics in the chopper is functioning somewhat incorrectly, say it is worn above specs, it would cause increased noise/drift, but also symmetric.All the transistors are probably old germanium ones, I cannot even find their specs anywhere, probably noisy and leaky, still no asymmetry is expected.On the pictures and supporting doc-s:5) Expected short-term and long-term drift of Keithley 148 from Keithley document The-measurement-of-nanovolts, by Julius Praglin. In the same document there is the actual amplifier+demodulator+power_supply schematics for the tube variant. Vertical scales indicated a=6nV, b=2nV, c=2nV.1-2) The actual "short-term" noise/drift on 10nV scale. Where after cleaning I can see occasional rare up-jumps and/or down-drift, encircled with green ellipses, otherwise it is always slow rises and sharp drops. Do not pay attention to the "needles" on the pictures, they are picked up from a pc-power-supply and a lamp power supply nearby. It will change nothing if I turn them off, already verified. 200mV per division vertical scale of the linear output corresponds to 2nV input. Horizontal scale is 100s per division.3) What was cleaned up physically in the nanovolt circuitry,4) What was cleaned up shown on schematics,6) Keithley 148 manual with FET input schematics, see:https://www.tek.com/en/manual/model-148-nanovoltmeter-instruction-manual-29029-rev7) Keithley document The-measurement-of-nanovolts, by Julius Praglin with Keithley 148 tube input schematics, will upload if requested.My suspects for the asymmetry were:* Oxides at the nV circuitry. So I cleaned what I suspected could be the most critical contacts: input contacts, chopper wire contacts outside + 1Ohm resistor in the feedback divider, see the green ellipses in pictures 3-4. This may have changed the "short-term" noise/drift a slight bit in that on rare occasions some minor up-jumps appeared as well as some minor slow movements down, green eclipses on pics 1-2.* The chopper innner contacts may be oxidised,* Maybe some strange "out of phase" asymmetry of the demodulator, though I cannot imagine how it may cause any up-down asymmetry.* Even if the oscillator were not stable, I would had not being able to imagine how it would have caused any up-down asymmetry.====My final guess is that to get a better idea I would have to completely disassemble and clean all the nV circuitry including the chopper inner contacts.The complete careful disassembly is necessary because bending some of the copper wires can easily break them, especially the short ones of the redfeedback 1Ohm divider resistor. This sounds like an 8 hour project. May be I find some day for it in the future.Why large jumps is an issue and slow drift is no issue for the accuracy:If no jumps then after zeroing,I would expect some drift of the order of the square root of time. Then the actual resolution would indeed be better than 1 nVas Keythly 148 manual says,if zeroing and measurement time is limited by say 1 minute. That iszero and measure something quickly enough before the instrument drifted too much. With jumps it is just possible to say that the measurement accuracy is +- the jump value, with the average jump of about less than 3nV. Buton the 10nV scale the 3nV random jump may be several times above say 1nv signal.Questions:==========* If you have Keithly 148, could you please upload a picture of its "short-term noise" with input shorted by thermo-isolated copper wire for at least 20-30 min at the corresponding scale ?* What is your idea of what can cause this type of up-down noise asymmetry ?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 16, 2023, 08:21:08 am
content: You have probably seen my old post:https://www.eevblog.com/forum/repair/keithley-148-nanovoltmeter-repair/msg1393448/#msg1393448From my experience nanovoltmeters are very troublesome to repair.Intermittent, oxidized copper to copper crimped contacts are common issue.The electromechanical chopper have ~2000h of service life so you may be facing wear out of the contacts.

author: yus
author_rank: Contributor
timestamp: May 16, 2023, 03:58:00 pm
content: Quote from: doktor pyta on May 16, 2023, 08:21:08 amFirst of all thanks a lot for answering. Yes, I have read your post with lots of attention before deciding to buy this thing.If you still have data of that post, could you upload it please ?Later you posted the noise after replacing the electromechanical chopper. Unfortunately the vertical scale is 10nV so it isnot really possible to see much details. From that little that I could see on picturerecord ca. 13h sampled every 5 secthe long term drift is +-6nv and I do not see any jumps.Quote from: doktor pyta on May 16, 2023, 08:21:08 amKeithley 148 manual says that a worn electromechanical chopper will be seen as drift intens of nanovolts.I have the long-time drift of about +-6nV same as what you posted after the chopper replacement.Before replacement with a bad chopper it looks like you had about +-35nV long-time drift, that agrees to what Keithley manual says.from your noise picturerecord ca 22h sampled with 90sinterval it is difficult to guess if there are any jumps or not.Response time (10%-90% of signal) for Keithley 148 at the 10nV scale is about 5 seconds as can be seen from pic 5.c .It would be nice to have the noise data at 10nV scale with at least 5 sec sampling, 1sec would be even better.I also have thoughts on the claimed 2000h of service time for the electromechanical chopper. The cars from those years 1970, many still run well despite 50 years of service. So the milage can vary, I guess.Questions: If you still have Keithley 148, do you see any nV jumps up or down at 10nV scale? What is the "short-time" noise of your device ? Is it less than 2nV peak-to-peak over say 5 min time at 10nV scale as Keithley manual says (picture 5.b of my initial post) ?The noise I have, it looks to me as if there is some "nanovolt zener diode" there. And a random nanovolt thermal noise source is charging a capacitor throughthe "nanovolt zener diode" in forward direction. After reaching some critical voltage (that also fluctuates around some mean value of about 1.5nV) the breakdown occurs and the capacitor discharges. Then it continues like this again indefinitely.I checked the web and indeed it might be relevant somehow :https://www.sciencedirect.com/science/article/abs/pii/S0167577X13015899https://www.sciencedirect.com/science/article/abs/pii/S0925838810012284...Copper oxide (CuO) is a p-type metal oxide semiconductor with a narrow band gap (1.2 eV). It has received much attention because of its various applications in optoelectronic devices, gas sensors and catalysts...1.2V is not 1.5nV, but who knows. It might be thatsome unwanted / unwelcome HiTech science sneaked secretly into the nanovolt circuitry of this old box.Looks likecleaning is unavoidableIt may help or not, we'll see later.

author: Whitefoot
author_rank: Contributor
timestamp: May 16, 2023, 08:38:24 pm
content: I'm not very familiar with the Keithley 148. I haven't touched it for decades. I pulled it off the shelf, turned it on and let it run over night. Then I scoped the output for a couple 25 minute runs. The 148 is set to 10nV full scale. The scope is set to 200mV/div (2nV/div at the 148 input) and 100S/div timebase. One trace shows the 148 without zeroing - the other trace shows it zeroed with the zero suppression controls.

author: yus
author_rank: Contributor
timestamp: May 16, 2023, 09:12:02 pm
content: Quote from: Whitefoot on May 16, 2023, 08:38:24 pmTo me it looks like yours have a tendency to jump up more, but it looks quieter and healthier than mine. Maybe they all jump asymmetrically to some extent ?It would be nice to see more of these from different K148 instruments. Anyway seems like for your K148 the drift is about +-2nV at 25min. Mine is more like +-4nV. And also it seems like if yours jumps up it mostly goes back rather quickly. Mine do not.Questions:Do you happen to know if your K148 has a tube of FET input ?Another question unrelated to noise, if the AC connected lamp lights up on your K148, what kind of lamp is it ?On mine, it was replaced with some kind of 12V lamp that though not broken, never lights up.

author: Whitefoot
author_rank: Contributor
timestamp: May 16, 2023, 09:47:13 pm
content: The input on mine is a tube. The AC lamp does not light up and I haven't looked to see why.

author: Whitefoot
author_rank: Contributor
timestamp: May 16, 2023, 10:08:54 pm
content: The lamp is an incandescent - I measure 14VAC at the socket.The lamp is a number 49.

author: Whitefoot
author_rank: Contributor
timestamp: May 17, 2023, 08:02:26 pm
content: I had another Keithley 148 on the shelf. I let it run over night and did an internal offset adjustment. I scoped the output for a 25 minute run. The 148 is set to 10nV full scale. The scope is set to 200mV/div (2nV/div at the 148 input) and 100S/div timebase. The trace shows the 148 without front panel zeroing.

author: Whitefoot
author_rank: Contributor
timestamp: May 17, 2023, 09:05:03 pm
content: The last post shows a lucky 25 minute run. The offset ran up to 20nV in the next 25 minute run, and now it's all over. That unit needs some work.

author: yus
author_rank: Contributor
timestamp: May 18, 2023, 08:44:07 am
content: Quote from: Whitefoot on May 17, 2023, 08:02:26 pmWhat I see on "a lucky run": Symmetric up-down jumps. The mean jump value +-2nV. Slower drift without jumps is more or less up-down symmetric. This is what I would expect for the noise to look like.Quote from: Whitefoot on May 17, 2023, 08:02:26 pmIf I remember it correctly, the K148 manual mentions the internal contact potential in the instrument itself of up to 30nV. So the front panel zeroing is unavoidable. It might have been that your second K148 was already zeroed? Another thing from the manual is that without a front short, K148 will go out of range immediately.The zeroing settings for my K148 at 10nV scale are : coarse=1, fine=5.92-5.96====For my K148. For the front short I use just a short bent thick piece of copper wire with rounded edges. I clean the socket and the short with just a piece of regular printing paper soaked with ethyl alcohol. There is enough micro abrasive in the paper already to clean soft metal like copper/gold. For thermal insulation I rab it with a piece of toilet paper and then I reinforce it with masking tape. good enough.Summary so far:=====(1) All three K148 posted have jumps in the nV noise. 2 of the 3 instruments have jump asymmetry to a various degree. Looks like the pattern of "jump asymmetry" is an inherent characteristic of a particular instrument.(2) If an assumption that "they all jump randomly in time" is true, then the best expected accuracy is the mean jump value. From what was posted here up to the date, "the mean jump value" =measurement precision about +-2nV at best on average (for a single measurement).

author: Whitefoot
author_rank: Contributor
timestamp: May 18, 2023, 03:44:03 pm
content: I think I made a mistake on the 2nd 148. It started out with a 50nV offset. I adjusted R301 which reduced the offset, but I think it might have changed gain. I hadn't read the manual very carefully. It's not yet clear to me if there is an internal offset adjustment.The lucky run seems to have been pure chance, because it soon started drifting all over. The normal appearance of the noise may be the result of a bad setting. I need to study the manual more.My short is also a bent copper wire, covered with a plastic cap.

author: yus
author_rank: Contributor
timestamp: May 18, 2023, 05:33:10 pm
content: Quote from: Whitefoot on May 18, 2023, 03:44:03 pmTo the best of my understanding there is NO internal offset adjustment. If offset cannot be adjusted for a properly shorted input you need to go through what I describe below.I read the part of the manual about how to make it work several times. What I understood:(0) I just removed the internal NiCd battery. I do not want to bother myself with this part until I see that the instrument works as it should.(1) check power supply voltages : manual 4-7 POWER SUPPLY CHECK OUT AND CALIBRATION:unregulated : -18 volts dc +-2 vdc, mine was 18.0Vthree regulated : -+12 and +1.2, mine were +11.60,-11.56, +1.55 vdc, left as is(2) adjust osc frequency and amplitude 4-8 OSCILLATOR CHECK OUT AND CALIBRATION.to 94.0+-0.1 Hz, mine is ok (adjusted with a hack, see my initial post)adjust osc amplitude to 15.4 volts peak-to-peak, mine could not adjust, the closest was 13.0 volts peak-to-peak, left as is.(3) 4-9 AMPLIFIER CHECK OUT AND CALIBRATIONadjust the amp response time to about 0.2-0.4sec for the 100nV step signal at 100nV scale with R109, adjusted with a hack to 0.25sec (see my initial post). This increased the ripple amplitude at the linear output (who knows what frequency the ripple is, maybe ?? 6Hz=50Hz*2-94Hz)to approx 0.1V/4~=0.025V. I decided that since I do not know if the rest works, I can deal with it later (0.1V/4 at the output = 1nV/4 at the input, hope I ever have this accuracy =))).(4) It is only now that it is time to adjust R301 and the position of the "degaussing coil". see4-3 MECHANICAL CHOPPER REPLACEMENTS and in there d. Degaussing coil adjustment procedure.Use two oscilloscope probes (A,B). Set oscilloscope to differential (+A-B). Observe the differential wave form between the emittersof transistors Q9 and Q10 (Test Points P and Q, Figure 25). Varying R301 and the coil position minimize the differential (+A-B).For Mine, the best coil position was where it was before: approx at the medium height, R301 at a min value, also where it was before(R301 at mine is at min probably because the osc amplitude is also below min). (WARNING !!!: use two 1/10 oscilloscope probes. Attach probe grounds to LO.Do not attach the probe ground to the emitters of Q9 or Q10. Looking at schematics I see that there might be a chance of destroying Q9 or Q10 this way by a too high base current, if the emitter is shorted to LO.)(IMPORTANT) That's it. It is better to stop here. As far as I understand K148 has to function somehow at its best at this point. It should be close to calibration if it was calibrated before. The reason is that no matter what is the actual amplifier gain it just compensates for the input voltage. So the actual gain influences just the amp response time, which is different for each scale. If the amp gain is not high enough it will not be possible to zero K148, it will just drift randomly in whatever direction it wants.I WOULD THINK MANY TIMES and BE EXTREMELY CAREFUL BEFORE REPLACING ANY SEMICONDUCTOR PART OR CHANGING ANY RESISTORS(resistors that set the corresponding GERMANIUM semiconductor DC biasing).The reason is that almost all semiconductors there are GERMANIUM ones. see: Redesign Germanium Transistor Circuits with Silicon Transistorshttp://www.hawestv.com/transistorize/germanium1.htm

author: Whitefoot
author_rank: Contributor
timestamp: May 18, 2023, 05:56:31 pm
content: I will try to go thru the adjustment procedure and check cal on my unit nr 2. I don't know how soon I'll do this.I took out the batteries and probably checked power supply voltages decades ago, and never touched either unit again until now.

author: yus
author_rank: Contributor
timestamp: May 18, 2023, 06:14:16 pm
content: Quote from: Whitefoot on May 18, 2023, 05:56:31 pmThe important part is not try to calibrate it until it is absolutely clear that it is out of cal. You need just to make sure that the amp gain is high enoughso that it is possible to zero it and the response time is ok*, but not too high to avoid too much of the "ripple oscillations" at the linear output.====*the response time is ok: the amp response time should be about 0.2-0.4sec for the 100nV step signal at 100nV scale and also see img5 picture c of my initial post

author: Whitefoot
author_rank: Contributor
timestamp: May 19, 2023, 02:26:16 am
content: I found that both of my 148's have power supply problems.In unit nr 1 the -18V is about -11.4V.In unit nr 2 the -18V is about -10V.I didn't go any further yet. I don't know when or if I will try to fix this.

author: yus
author_rank: Contributor
timestamp: May 19, 2023, 09:57:06 am
content: Quote from: Whitefoot on May 19, 2023, 02:26:16 am* on the Rear Panel check the position of line switch S220 (117 volt or 234 volt),* remove both lamps on the front panel, disconnect the internal batteries,* check again the -18V, Test Point A, Figure 20 (K148 manual, !!!be careful!!!: Test Point A is at the bottom of the rotary switch, and above are 2 red wires, they are at the line voltage),* check -+12 and +1.2 power supply voltages, Test Points B,C,D, Figure 21 (K148 manual).What are these?

author: Whitefoot
author_rank: Contributor
timestamp: May 19, 2023, 05:46:44 pm
content: The line switches are correct.Both lamps are burned out.They don't have batteries.I'm going to put them back on the shelf. I have too many unfinished projects laying around.

author: Whitefoot
author_rank: Contributor
timestamp: May 29, 2023, 12:38:17 am
content: Before putting my K148s back on the shelf, I checked the regulated PS voltages.Unit nr 1:+12V -- +11.8V-12V -- -12.0V1.2V -- +1.17VUnit nr 2:+12V -- +12.00V-12V -- -12.05V1.2V -- +1.21VThese are all within spec. It appears that maybe the -18V is only important for the battery charging circuit. Neither unit has batteries.For a quick functional check I placed a 10ohm resistor across the inputs. I connected a battery at 6.2V thru a 1.0Mohm resistor in series with the 10ohm resistor. This gives roughly 6.2uA current thru the 10ohm, and both 148s measure 62uV, so they look to be basically functional. I haven't yet tried anything more with the low ranges - that gets time consuming.

author: Whitefoot
author_rank: Contributor
timestamp: June 04, 2023, 11:58:44 pm
content: Doing some testing with Keithley 148 unit nr 1. I haven't yet tried to go thru the internal adjustment procedure.For these runs I placed a 0.2ohm resistor across the input. I connected a battery at 6.8V thru two 100Mohm resistors in series with the 0.2ohm resistor. This gives roughly 6.8nV at the 148 input. The 148 is on the 10nV range. 6.8nV is at 3.4 divisions above center on the scope. Each scope vertical division is 2nV at the 148 input.This gets time consuming. The voltage drifted off scale for a number of runs. I'm sure this is mostly due to temperature. To get good nV readings a temperature controlled chamber would be necessary.The 1st 25 minute run shows 0V input.The 2nd 25 minute run shows 0V input a bit later - I have no idea why it's cleaner.The 3rd 25 minute trace shows a typical run, except that it stayed on scale.The 4th 5 minute trace shows a rare clean run, with the rise and fall times when connecting and disconnecting the battery.

author: Whitefoot
author_rank: Contributor
timestamp: June 12, 2023, 02:27:29 am
content: For nanovolt measurements, with quick-and-dirty input connections and lack of temperature control, I conclude that It's only possible to record clean signals by chance. The two clean traces of my last post probably show what the K148 can do when conditions are good.