Re: Open-hardware power rail oscilloscope probe
On Sun, Jul 30, 2023 at 05:57 PM, Patrick Coleman wrote:
You can find the schematic
yup... can't view the schematic with anything but your link (doesn't load anything in Altium 365, or directly on the Kicanvas Webpage.)
|
Re: Open-hardware power rail oscilloscope probe
There is no mouse.? There is only a touchpad.? Yes, you can zoom in, but it's a bit tricky.
I did look.? I'm not sure about the power supply, I'd be tempted to perhaps do it differently.? You can get a 5 volt 200 ma isolated converter (5v-5v) which might be better.? The grounds then don't matter.? If noise is a concern, then run the converter only when the batteries are being charged.
Harvey
toggle quoted message
Show quoted text
On 7/31/2023 11:02 AM, Roy Thistle wrote: On Sun, Jul 30, 2023 at 06:17 PM, Harvey White wrote:
Well, I tried looking at the schematic you supplied,
Are you using Kicanvas?
then... CTRL down... and use the wheel (on your mouse) to zoom in out.
wheel to scroll up and down
SHIFT down... and use the wheel to scroll left and right
|
Re: Open-hardware power rail oscilloscope probe
On Sun, Jul 30, 2023 at 06:17 PM, Harvey White wrote:
Well, I tried looking at the schematic you supplied,
Are you using Kicanvas? then... CTRL down... and use the wheel (on your mouse) to zoom in out. wheel to scroll up and down SHIFT down... and use the wheel to scroll left and right
|
Re: Keithley 261 Picoampere source improvement
Hello again,
I have made good progress on the "drastic change" option, i.e. replacing the original PCB with a homebrew version which is immune to the flaws we discussed in previous messages. In the next few days I will send out PCB Gerber files to JLCPCB, and given the shipping time from China to Europe, I expect to have the new design fitted in the instrument by the end of August.
Of course I will keep you posted !
Joel Setton
|
Re: Open-hardware power rail oscilloscope probe
Interesting project.?
For this function, I would use a Tek 7000 series mainframe withe
7A13 OR 7A22, although neither would go to 100 MHz.
Seems to me that if the Offset pot is towards either extreme end,
and the input V was high and of the same polarity, that the Vin +/-2V Absolute Maximum rating would be
exceeded. A pair of diodes from the - to the + input would protect
against that.? In addition, a pair of LED's (Red for 1.8V drop) on
the output of U1 could indicate an overload condition and turning
direction to correct.? An op amp with gain to the U1 output could
make this a much more sensitive indicator when the scope is not
visible while probing.
John??? KK6IL
On 7/30/2023 5:03 PM, Patrick Coleman
wrote:
toggle quoted message
Show quoted text
I've spent the last couple of weekends working on an
open-hardware power rail oscilloscope probe: connect it to a
low-impedance source via a 50¦¸ cable/termination and it
applies an adjustable offset, allowing you to view ripple on
relatively high voltage rails.
Commercial probes are quite expensive (~$4k), so I've taken
a first pass at a low-cost version based on an .
Currently it's just a schematic, but it's been a few years
since I went this deep on a low-noise design (and my maths is
terrible) so I'd appreciate any early review people here can
offer.
You can find the schematic and analysis at?.
Feedback appreciated on-list or via GitHub issue.
|
Kind regards
|
| Patrick Coleman |
|
| Technical Director |
| BLINKENLIGHT Ltd |
| web. |
|
Re: Open-hardware power rail oscilloscope probe
Hi,
Harvey, try pressing CTRL and use the mouse wheel to zoom. The offset pot needs to be in this position to be able to adjust the input to the first op-amp close to 0V.
Patrick,
I'm no expert in low noise, but only issue I see with this design, is that the lower frequency noise is divided down and then amplified again, so any noise in the probe power supply or introduced by the op-amp will be added to the noise you try to measure. The higher frequencies will bypass the op-amps though the capacitor. In this design, there is no amplification of the noise, so you will be limited by what your scope are able to resolve.
I have been thinking about making a probe to measure power supply noise one day.
Not got around to it yet, and it might be some time before I do, but here are a few links of projects I have found, that I would use as basis:
Regards, Askild
On Mon, Jul 31, 2023 at 3:17?AM Harvey White < madyn@...> wrote: Well, I tried looking at the schematic you supplied, and I can't zoom in
on it.? I did look at the original design, and I have at least one
question.? The offset voltage is applied to the gain stage, which
suggests that any noise on that signal is then amplified.? Would it not
be better to apply the offset to the output stage instead?
Just leafing through his design, and ignoring most of it, when he gets
to comparing op-amps, the one with the least output swing has the
smallest noise figure.? Think you'd want that for the gain stage, and
perhaps carefully calculate the effect of a bit more gain in the output
(more swing but higher noise) stage?
I'm definitely not a low noise type, but those at least may (I hope) be
decent questions.
Harvey
On 7/30/2023 8:03 PM, Patrick Coleman wrote:
> I've spent the last couple of weekends working on an open-hardware
> power rail oscilloscope probe: connect it to a low-impedance source
> via a 50¦¸ cable/termination and it applies an adjustable offset,
> allowing you to view ripple on relatively high voltage rails.
>
> Commercial probes are quite expensive (~$4k), so I've taken a first
> pass at a low-cost version based on an existing public design
> <>.
> Currently it's just a schematic, but it's been a few years since I
> went this deep on a low-noise design (and my maths is terrible) so I'd
> appreciate any early review people here can offer.
>
> You can find the schematic and analysis at
> . Feedback appreciated
> on-list or via GitHub issue.
>
> Kind regards
> *Patrick Coleman*? ? ?
> Technical Director
> BLINKENLIGHT Ltd
> web. <>
>
> Sent from Front
>
|
Re: Open-hardware power rail oscilloscope probe
Hi, this is a pretty neato thing, if you commercialize your design I'd be interested in buying one! tx!
J
Patrick Coleman wrote:
toggle quoted message
Show quoted text
I've spent the last couple of weekends working on an open-hardware power rail oscilloscope probe: connect it to a low-impedance source via a 50¦¸ cable/termination and it applies an adjustable offset, allowing you to view ripple on relatively high voltage rails.
Commercial probes are quite expensive (~$4k), so I've taken a first pass at a low-cost version based on an existing public design <>. Currently it's just a schematic, but it's been a few years since I went this deep on a low-noise design (and my maths is terrible) so I'd appreciate any early review people here can offer.
You can find the schematic and analysis at . Feedback appreciated on-list or via GitHub issue.
Kind regards
*Patrick Coleman*
Technical Director
BLINKENLIGHT Ltd
web. <>
Sent from Front
|
Re: Open-hardware power rail oscilloscope probe
Well, I tried looking at the schematic you supplied, and I can't zoom in on it.? I did look at the original design, and I have at least one question.? The offset voltage is applied to the gain stage, which suggests that any noise on that signal is then amplified.? Would it not be better to apply the offset to the output stage instead?
Just leafing through his design, and ignoring most of it, when he gets to comparing op-amps, the one with the least output swing has the smallest noise figure.? Think you'd want that for the gain stage, and perhaps carefully calculate the effect of a bit more gain in the output (more swing but higher noise) stage?
I'm definitely not a low noise type, but those at least may (I hope) be decent questions.
Harvey
toggle quoted message
Show quoted text
On 7/30/2023 8:03 PM, Patrick Coleman wrote: I've spent the last couple of weekends working on an open-hardware power rail oscilloscope probe: connect it to a low-impedance source via a 50¦¸ cable/termination and it applies an adjustable offset, allowing you to view ripple on relatively high voltage rails.
Commercial probes are quite expensive (~$4k), so I've taken a first pass at a low-cost version based on an existing public design <>. Currently it's just a schematic, but it's been a few years since I went this deep on a low-noise design (and my maths is terrible) so I'd appreciate any early review people here can offer.
You can find the schematic and analysis at . Feedback appreciated on-list or via GitHub issue.
Kind regards
*Patrick Coleman*
Technical Director
BLINKENLIGHT Ltd
web. <>
Sent from Front
|
Open-hardware power rail oscilloscope probe
I've spent the last couple of weekends working on an open-hardware power rail oscilloscope probe: connect it to a low-impedance source via a 50¦¸ cable/termination and it applies an adjustable offset, allowing you to view ripple on relatively high voltage rails.
Commercial probes are quite expensive (~$4k), so I've taken a first pass at a low-cost version based on an . Currently it's just a schematic, but it's been a few years since I went this deep on a low-noise design (and my maths is terrible) so I'd appreciate any early review people here can offer.
You can find the schematic and analysis at?. Feedback appreciated on-list or via GitHub issue.
Kind regards |
| Patrick Coleman |
|
| Technical Director |
| BLINKENLIGHT Ltd |
| web. |
|
Re: Marconi Q meter exciter
Hi PeterS,
That's a terrific solution and I'm most impressed. I don't want to modify my TF1245A so won't go down that path, at least not yet! You mentioned in a previous post that you were not able to get adequate power transfer into the unmodified TF1245A and that's been my experience too, at least not without banswitching. I will try your earlier approach of feeding the sig gen into? a toroidal transformer and feeding it through the front panel directly. My signal generator only manages +13 dBm so I might still need an amplifier but the requirements are not so stringent.
73 de Morris VK3DOC
|
Re: Marconi Q meter exciter
TF1245A injection transformer update
The wet weather of the last few weeks persuaded me to bring forward my transformer injection experiments mentioned in post #1703.
The FT50-43 and 15mm copper pipe end cap is not a perfect combination as the capacitance betwen the toroid turns on the outside and the cap walls produces a resonant effect at about 110MHz, or slightly lower when the toroid is mechanically stabilised, resulting in the coupling increasing with frequency by about 0.5dB at 50MHz.? At some time I will try fabricating a slightly larger cup, perhaps 18mm internal diameter, in the knowledge that the stray capacitance will be lower and the hope that the HF coupling flatness can be improved.
For now, the existing transformer is adequate in conjunction with the HF compensation trimmer (C14*) in the "Q multiplier" meter circuit.
A 2:1 voltage divider comprising two 27 Ohm 0805 resistors across the transformer input winding provides the correct tapping point to preserve the non-linear "Q multiplier" meter calibration (0.5V RMS across the original injection resistor assembly, 1V RMS across the 50:1 transformer input).? The input impedance at the front panel is close to 50 Ohms (VSWR <1.1 at 50MHz) which will avoid any standing wave peculiarities in the generator lead regardless of length.
The TF1245A Input I (40kHz - 50MHz) can now be driven by any signal generator that can deliver ~14dBm into 50 Ohms, with acceptably accurate direct indication of Q and more precise measurement by delta-C or half power bandwidth methods (easy with a synthesised signal generator).? I am not looking for absolute values, but repeatability is important to me for comparisons.? I will probably build a dedicated exciter with ALC and vernier amplitude adjustment as I already have suitable prototypes from other projects.
The picture shows the transformer RF injection assembly installed in the TF1245A.
PeterS??? G8EZE
|
Re: Keithley 261 Picoampere source improvement
Be aware that using any switch or relay contacts in a ¡°dry¡± circuit (one with vanishingly small current) has a certain risk. Namely, there¡¯s insufficient wetting current to break through any amount of oxidation. Higher current, often at the mA level, normally will clear the oxidation via micro-arcing. A switch with a wiping action is much more immune to this problem, as are those with the right plating such as gold and various silver alloys. Cleaning is of course helpful. This may be a non-problem in your case but if you see odd behavior related to the switch, this could be the cause.
Gary NA6O
|
Re: Keithley 261 Picoampere source improvement
I just looked at a Bourns cermet trimpot list, and it looks like they are all around +/-100 ppm/C, which is pretty decent - not as good as WW, but probably OK for this in reasonable lab environment. So, maybe a multi-turn unit would provide good set-ability and resolution. I think 1T isn't enough, but I wouldn't go above 10T. There are a couple of 4T models - the 3339 and PV12 , which seem like a good compromise.
Ed
|
Re: Keithley 261 Picoampere source improvement
Joel, it seems weird that the 200 ohm pots are not wirewound - the parts list says R113-R119 are all 200R, 10%, 5W, "WWVar." Maybe they changed them in the later years of product life, but I'd find it hard to believe it would work right and meet specs without WW pots, especially with such a large adjustment range. It wouldn't have been the first time someone later "upgraded" an old design without knowing the full requirements. Or, maybe someone changed them for some reason later. Do they look like they are the original factory parts when built?
Maybe if you just put real WW pots in, it would work right, except for issues with the switch as you mentioned. Depending on the brand and type, you may be able to find a replacement switch wafer for just that one section, without disturbing the rest. An alternative is to keep the original switch part, but use it to instead actuate relays to handle the signals. Looks like eight small form A reed relays or such could handle it. I'd think that some new pots and added relays and coil circuitry would be simpler than a whole new regulator design. BTW the manual I've been using looks like a very old, possibly first printing. If you can find newer versions, maybe differences are shown.
Ed
|
Re: Keithley 261 Picoampere source improvement
Folks,
Thank you for your responses. I am, at the same time, grateful and impressed after seeing additional expertise !
Your various messages helped me better understand the weaknesses in the original design and construction of this unit.
- The reference voltage is set by a fixed 8mA current, flowing through a wafer switch and the (dreaded) 200-ohm potentiometers connected as variable resistors.
- The mechanical construction of the range switch is such that the rotating part of the wafer switch is sometimes slightly off-center, which has resulted in a lot of wear on the contact surfaces (from the date codes on various components, this instrument was manufactured circa 1980). Contact-cleaning chemicals have made no difference.
- The 200-ohm potentiometers are not wirewound types. They are "el-cheapo" types, with a carbon resistive track on a phenolic-paper substrate (the cardboard-like, brownish stuff). Using an ohmmeter, i.e. out of the original circuit, I found that resistance cannot be reliably set to any value, it always drifts up or down after setting.
Sooooo... I use this instrument a lot, so I decided to "bite the bullet" and go for the drastic change as briefly mentioned in Ed's message. I am designing a replacement for the voltage regulator which will use a feedback circuit in which the wafer switch will handle a minimal current (an op-amp bias current) and calibration will be set by modern cermet trimpots with near-zero wiper current. This will completely replace the original PCB.
I'll post a schematic once it's up and running !
Cheers,
Joel
|
Re: Keithley 261 Picoampere source improvement
Unfortunately, you're kind of stuck with it, without doing drastic changes. The range switch contacts should not be too much trouble (you should treat them with Deoxit if not already done). The real problem is in the 200 ohm range cal pots. It appears these are the single-turn, big old-school open-frame wirewound types. The problem with all wirewound pots is the discreteness of the setting versus rotation - the lower the R value, the bigger the resistance wire, and the coarser the setting resolution. 200 ohms is way down there in R for such things, so you should expect to clearly see the discreteness as it is set, but not big noise jumps.
If the pot is good, the wiper contact cleanly bridges maybe three or more turns (must be at least two turns at all positions) of the resistance element, and the resulting value is the interpolation of the adjacent values. So, the available values are more or less discrete, with slight variability, a certain amount of hysteresis, and good but not necessarily perfect monotonicity. If you are getting big jumps, then the pots need a good cleaning. You can try Deoxit on them too, but mostly they need to be exercised lots and lots of times (I usually claim up to 100 should do it, for almost any situation) through their entire rotation range to clean the finish on the wire and the wiper. You can look at the feedback signal on a scope to assess how well they're doing. If jumpiness won't go away, you may need to replace or modify the pot(s). Looking back to my statement about always bridging more than two turns, picture it as a string of individual resistors with make-before-break (MBB) switches selecting all the possible values. You should not get large jumps when all those MBB contacts are OK, even though they're carrying fairly high current. In fact, carrying at least some current is good for contacts (to keep from going "dry").
It may be tempting to change the design in various ways, like upping the feedback resistance and such, but remember, this was all figured out for low R, low noise, and high stability with 1960s parts, and it seemed to have worked just fine. Once it's working "right," the main issue should be that each range reference voltage can't be "perfectly" set looking at our modern DMMs - they didn't worry too much about more than 3 digits or so for this one. If you really do want nearly continuous resolution, you could experiment with a cermet pot and fixed series R across each wirewound, to sort of get a fine tuning effect, but I wouldn't worry about it.The main thing is to get those original pots nice and clean and smooth. Good luck.
PS?? Another option, especially if it turns out that those pots are bad, might be to get say, a whole bunch of small 1 k WW pots, and put five paralleled to replace each 200 ohm pot. At the higher resistance, the pot resolution will be much better, and each pot will contribute only a portion of the net value, allowing for much finer tuning. But, the initial setup and tweaking to keep them all kind of even could get a little complicated. Say, set all at midpoint, then tweak each a little bit, all about the same amount, and so on, and iterate to a solution.
Ed
|
Re: Keithley 261 Picoampere source improvement
What kind of switches and wafers on the switches? What kind of pots? Multi turn Bourns or Clarostat? For this vintage of instruments I would expect open wafer switches with silver plated contacts - these need to be cleaned with contact cleaner and exercised though the entire range, leaving a very fine layer of the contact cleaner on the surface of the contacts. If the pots are sealed Bourns or Clarostat multi turn, note the position or measure the resistance, then rotate the pots end-to-end several (ie. 10x). This should clean the pots contacts. If they are not sealed pots use the contact cleaner on them also. I have found that this greatly reduces the noise/drift for a period of 1-5 yrs. You may also discover which switch or pot is your drift culprit by lightly touching the pot screw or switch wafer with a non-metallic adjuster and watching for a disturbance in your display.
On Sun, Jul 23, 2023 at 1:20 PM, Steven Greenfield AE7HD via groups.io <alienrelics@...> wrote: Maybe some newer tighter tolerance high ohm resistors and make the pots an even smaller change in the total resistance? Tight tolerance low PPM resistors won't be cheap, of course.
Steve Greenfield AE7HD WRWU703 CN87oa?? http://www.ae7hd.com
|
Re: Keithley 261 Picoampere source improvement
Maybe some newer tighter tolerance high ohm resistors and make the pots an even smaller change in the total resistance? Tight tolerance low PPM resistors won't be cheap, of course.
Steve Greenfield AE7HD WRWU703 CN87oa?? http://www.ae7hd.com
|
Re: Keithley 261 Picoampere source improvement
Try shorting the arm of the pot to the unused end.? That way, if the arm bounces, it only bounces to the maximum resistance and not a? complete open.
Harvey
toggle quoted message
Show quoted text
On 7/23/2023 8:33 AM, Joel Setton wrote: Hi Gary,
Quite a good point. Actually, the adjustment range of the pot is quite large (approximately +/- 15% of nominal output), because it has to match the wide tolerance of the high-megohm resistors. This in itself would be acceptable, but the root of the problem is that the pots are used as variable resistors (i.e. with only 2 terminals) and not as voltage dividers. As a result, any change in wiper resistance creates a large and unexpected change in output voltage. When such a pot is connected in series with an old wafer switch, the whole thing becomes utterly unreliable.
Joel
|
Re: Keithley 261 Picoampere source improvement
Hi Gary,
Quite a good point. Actually, the adjustment range of the pot is quite large (approximately +/- 15% of nominal output), because it has to match the wide tolerance of the high-megohm resistors. This in itself would be acceptable, but the root of the problem is that the pots are used as variable resistors (i.e. with only 2 terminals) and not as voltage dividers. As a result, any change in wiper resistance creates a large and unexpected change in output voltage. When such a pot is connected in series with an old wafer switch, the whole thing becomes utterly unreliable.
Joel
|
John Kolb
