60Hz Noise with Tektronix p6245 and 1103 Probe Supply - Page 1
Forum section: electronics / testgear

author: Tdeball
author_rank: Contributor
timestamp: January 03, 2025, 02:51:15 am
content: Hi,I am new to posting on the forum, but have been a reader for quite some time.I recently purchased a Tektronix p6245 and 1103 Probe Supply used from eBay. During my first few tests I realized there was 60Hz (and higher order AC) noise when using this probe. I then shorted the probe leads and the noise seems to be inherent to the probe/power supply. I tried both power supply channels and my lead accessories are as short as they can be to touch each other. My scope is the SDS2000X HD and the scope is in 10x mode, DC coupled, and 50 Ohm input impedance. I have a 12" high quality BNC cable connecting the scope and power supply. I do not see this noise on my passive probes.Does anyone know if this is this normal for this probe/power supply? If not, is this a probe or power supply issue?

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2025, 03:27:19 am
content: well you could load test the power supply and measure it for ripple, it would be severely bad if it can get through the amplifier.

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: January 03, 2025, 04:06:35 am
content: What was the probe connected to when you took the screenshot? If it gives such a high DC offset with the input shorted to ground then something is wrong. What waveform is on the offset pad on the power supply?

author: KrudyZ
author_rank: Frequent Contributor
timestamp: January 03, 2025, 04:31:20 am
content: The ripple is only 10 mV.However the signal itself sits at 2.5 V with a probe offset of -2.5V so it is actually at 5V unless Siglent handles / displays this differently from other brands.In other words the probe is pegged which would explain why you see the supply ripple. The amplifier is not closing its feedback loop.The 1103 supply feeds the signal straight through. It's just a coax extension inside.You are either missing the negative supply and or ground to the probe or the probe itself is shot.Edit:The P6245 supports +- 10V offset adjustments which is done using the knobs on the 1103, so if you're lucky you have that enabled and everything could be fine when you turn off the offset adjustment using its push button.

author: Tdeball
author_rank: Contributor
timestamp: January 03, 2025, 01:18:17 pm
content: Quote from: 44kgk1lkf6u on January 03, 2025, 04:06:35 amQuote from: KrudyZ on January 03, 2025, 04:31:20 amIn the image, the scope was connected to a 2.5v precision voltage reference dev board (LT6655). The offset button on the 1103 is turned off and the signal was shifted 2.5V with the scope offset functionality. I have attach a picture of the probe leads grounded together.

author: Tdeball
author_rank: Contributor
timestamp: January 03, 2025, 01:34:27 pm
content: Quote from: coppercone2 on January 03, 2025, 03:27:19 amI am not sure how to load the power supply, but I did probe each power rail vs the bnc ground and I did not see the same noise riding on top of the +-5V or +-15V supplies.

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: January 03, 2025, 03:10:50 pm
content: The power rails are shared between the channels. So one can connect the probe to the power supply, and then measure the pads for the other channel, in order to measure the power supply under load.The probe attenuates the signal by 10. You have the attenuation set correctly on your oscilloscope. So the 4 mV that appears on the screen is only 400 μV at the output of the probe. You may want to check whether or not the power supply has this much ripple. I expect the probe to have good enough power supply rejection at line frequency. So maybe still check the offset output as well. Maybe the switch for turning off the offset is rusty. To check for a small ripple, you can make the oscilloscope trigger on line, use averaging or high resolution, use the zoom function, and use a 1X probe.

author: Tdeball
author_rank: Contributor
timestamp: January 03, 2025, 10:20:13 pm
content: I have seen this waveform/noise several times in the past. I’m wondering if I have some weird EMI or ground noise in my lab.I will plug in the probe to one channel and measure the rails on the other channel in the next day or so. Thanks for the explanation of how to measure it under load.

author: tautech
author_rank: Super Contributor
timestamp: January 03, 2025, 11:04:59 pm
content: 1mV/div sensitivity will always show all sorts of nasties and when using a 10x input attenuation (probe) setting = 10mV/div we see in the screenshots.It can be a rabbit hole to diagnose.....

author: MarkL
author_rank: Unknown
timestamp: January 05, 2025, 05:45:55 pm
content: I have two 1103 power supplies. I never noticed, but they both have the same AC noise as you on the probe output. I tried several different P6245 probes, and a P6249 (4GHz single ended), each with about 1" of wire shorting ground to tip. Identical results in all cases.I'm seeing 2.3mVpp on one supply and 3.9mVpp on the other. This is with the scope at 10X vertical scale, which you would normally set for these probes.I also tried a TekProbe II adapter on my Keysight scope, which is a different probe power supply. The AC noise is not present in this case, so it's definitely coming from the 1103.I took the cover off the second supply and adjusted the voltages precisely according to the manual. No difference.I also tried disconnecting the ground going to the 1103 AC power input looking for a possible ground loop issue (As a TEST! Don't keep it this way!) No difference.I then tried shorting the offset output going to the probe to the 1103 chassis ground. This reduced the AC noise from 3.9mVpp to 1.2mVpp. So, a large part of the problem appears to be insufficient decoupling on the 1103 offset output. And this is with the 1103 offset switch set to OFF. It should be grounding the offset when set to off, but maybe there's too much resistance in that path.I didn't go any further than this since it's not really interfering with what I do. You can verify this on your unit and if you're seeing it too, perhaps the offset control can be improved.

author: Tdeball
author_rank: Contributor
timestamp: January 05, 2025, 08:43:44 pm
content: Quote from: MarkL on January 05, 2025, 05:45:55 pmThanks MarkL, much appreciated! It’s great to know I didn’t buy a defective one off eBay and to know this is an 1103 issue and not a p6245 issue. I regularly measure very small signals, which makes this setup unusable for me as it sits. I also do not have a scope with an active probe interface so I believe I am stuck with the 1103 if I want to use this probe. I will tinker around with the 1103 grounding and see if I can get the noise to go away. I’ll report back on my progress.

author: Tdeball
author_rank: Contributor
timestamp: January 06, 2025, 03:39:39 am
content: This seems to be an EMI issue with the transformer emitting within the box. I was concerned as soon as I opened the box and saw it. I crudely put a mu metal shield between the transformer and the main board and a piece on top and it greatly reduced the noise (pictures). Makes sense that the scope powered probe does not have the same noise.Now I need to decide if I want to build a custom shield, rip the AC to DC guts out of the box and run external power, or find something alternative all together. I could potentially make the box 2x as long and would only have to extend the transformer tap lines. I am sure more space between the board and transformer would do wonders.Does anyone know of an alternative power supply for the p6245?

author: tautech
author_rank: Super Contributor
timestamp: January 06, 2025, 04:05:27 am
content: Why is it every screenshot you have posted shows the trigger level outside the amplitude of the waveform ?

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: January 06, 2025, 04:59:14 am
content: What are your requirements? You are probing something that is fast enough to require an active probe. You already managed to reduce the line frequency noise to less than the high frequency noise. Yet you are still worrying about the line frequency noise.I looked at the pictures from Tek Wiki. It looks like they don't have a foil surrounding the transformer. So I would try wrapping the transformer in a foil to act as a shorted turn, like in the picture. Or adding a large capacitor at the closest convenient place to the output, for example the connector for the flex PCB.http://www.jestineyong.com/wp-content/uploads/2008/07/smps-transformer.jpgSiglent makes an adapter. I am surprised that Tau Tech did not point it out.https://www.siglenteu.com/wp-content/uploads/dlm_uploads/2024/04/TPA10_Datasheet_EN01B.pdf

author: Tdeball
author_rank: Contributor
timestamp: January 06, 2025, 02:16:30 pm
content: Quote from: tautech on January 06, 2025, 04:05:27 amOn the Siglent, I captured the waveform and then changed the vertical setting which automatically changes the trigger level (and the offset). This is a major gripe of mine with scopes. I wish they did not change multiple settings at the same time. If I want to change the offset, I will... On the Keysight, the trigger level is below in the signal in the images.Quote from: 44kgk1lkf6u on January 06, 2025, 04:59:14 amI have the SDS2000X HD which does not have the active probe interface. When I purchased it, I couldn't justify the SDS3000X HD just for the active probe interface.Quote from: 44kgk1lkf6u on January 03, 2025, 03:10:50 pmI do not think this is the case, the scopes that I have will scale the reading to display in proper units since I have the setting at 10:1. If this was the case, the 2.5V DC offset would read 25V. So the ~6mV noise is actually 6mV - not 600uV.Quote from: 44kgk1lkf6u on January 06, 2025, 04:59:14 amI am working on a high precision pulsed current source. A DAC, with a 1us setting time, provides a set-point on an op amp and then a feedback loop with a transistor and sense resistor drives the current proportionally to the DAC set point. I have filtering on the input and output sides of the op amp so I am nowhere near 1us - more like 100us settling time. What I care about is settling time to 99.9-99.99%, no ringing, etc. I care about the 1ish mV range of measurement so 6mV of AC noise is unacceptable. I went with this active probe because I was not getting mV range flat tops when calibrating/compensating my passive probes. I have not had good correlation to simulation and I have a wide range of currents and DUT resistances.We actually saw this same AC noise within our product's measurement system during the early design phase since we use a linear AC-DC supply. The solution was heavy shielding between the supply and measurement circuit.

author: Tdeball
author_rank: Contributor
timestamp: January 06, 2025, 05:07:32 pm
content: I removed the back plate and added some distance between the transformer and AC/DC board. This did not improve the noise more than the shielding so I figured the AC/DC board or wires are also contributing to the EMI. I then removed the front panel of the box and added 8" jumper wires. This made the AC noise fall below the noise floor. It is not very practical but highlights the issue well.@MarkL, What are your noise measurements when powering the P6245 from a scope? What bandwidth is your measurement?

author: MarkL
author_rank: Unknown
timestamp: January 06, 2025, 10:11:54 pm
content: Quote from: Tdeball on January 06, 2025, 05:07:32 pmVery interesting that separating the front board affects the AC line noise so significantly.My measurements were made using line trigger and 8x averaging because I was looking specifically for AC line related noise. So I can only say the line noise was not there using the TekProbe adapter. The Keysight TekProbe adapter has a tiny DC/DC converter in it, and as I recall, has noise issues elsewhere in the spectrum.Also, this was done on a Keysight MSOX3104A, which is not great for such low-level measurements. I can measure the actual RMS and pk-pk noise on an R&S MXO4 which has a much better front end. I also have a TekProbe adapter for that. What BW would you like to see (1.5GHz max)?The fact that direct grounding of the offset significantly reduced the AC line noise in the 1103 makes me very suspicious that the noise is getting in through the offset input to the probe. While the amplifiers in the P6245 might have decent PSRR, it won't have any for the offset. Every 1mV of noise on the offset is going to show up as 10mV on the screen.The offset control is fairly simple, and not exactly "stiff", so it could be susceptible to all kinds of low frequency ingress when on. See below. Maybe a larger cap across C1 and C2 would work. I'm not sure how to explain why the switch is not effectively shorting the offset input to ground when it's off. Maybe poor grounding design? Lousy switch?

author: Tdeball
author_rank: Contributor
timestamp: January 06, 2025, 10:39:04 pm
content: Quote from: MarkL on January 06, 2025, 10:11:54 pmIf you don’t mind, can a you check the noise levels at 200MHz? Then I can compare apples to apples.I don’t intend on using the probe offset adjustment since it is basically just translating the waveform.My scope can do the same thing and I do not believe there is any dynamic range advantage to using it. When I turn on the offset though, there is a slightly larger amount of ac noise. I noticed it while playing around but did not get any screen grabs.I did have to desolder the power indicator light in order to move the front panel away from the AC to DC board.I don’t love the idea of an 8” longer box. I might make a power cable for +-15 and +-5v and then have the back of the box be a giant power brick.I am very happy with the flatness of the measurement of a pulse with this probe on my demo keysight hd304MSO. My SDS2204x HD has pretty poor dc stability comparatively. The top of a known good pulse seems to float up and down a mV or two over the course of 50 ms.

author: MarkL
author_rank: Unknown
timestamp: January 06, 2025, 11:58:18 pm
content: Quote from: Tdeball on January 06, 2025, 10:39:04 pmThe offset in the scope (when using the 1103) will move the 0V reference of the waveform mathematically, and will only display the waveform without distortion if it's within the limits of the dynamic range of the probe, which in this case is +/-8V. The probe offset input allows you to electrically move the 0V reference and place the +/-8V dynamic range where you need, within a +/-10V offset limit. So, using the probe offset input can be useful if you have a large signal with a large offset.The diagram from the P6245 manual says it better, below.Tektronix scopes with the TekProbe II interface will detect an active probe and will use the electrical offset input to move the 0V reference when you change the offset control (it's not just doing a numeric offset to the waveform points). The TekProbe adapters from other manufacturers probably do this too; I'd have to look at it again.If your signal is always within +/-8V, then it doesn't really matter since the P6245 can handle it.EDIT: And, yes, I noticed the increase in AC noise when the offset control is on too. Again, leading me to suspect poor grounding design.

author: 44kgk1lkf6u
author_rank: Regular Contributor
timestamp: January 07, 2025, 02:08:52 am
content: Quote from: Tdeball on January 06, 2025, 02:16:30 pmI see. That is hard. According to Jim Williams, measuring 16-bit settling time is not a 1 week project.Did you verify that the probe itself settles to 100 ppm within 100 μs using averaging?Have you tried not using a probe? Connect the signal to be tested to a BNC connector on the board via a resistor. Connect it directly to the oscilloscope.You did do your homework and replaced all the bad capacitors right?

author: Tdeball
author_rank: Contributor
timestamp: January 07, 2025, 03:21:17 am
content: Quote from: 44kgk1lkf6u on January 07, 2025, 02:08:52 amI was able to validate the probe enough for my present task. The DAC set point is quite clean and predictable so I was able to measure a nice clean 40us settling time, then change a value in my RC filter to get 20us. Luckily for my testing, the higher currents are the ones in question which means the SNR with this probe’s new noise levels is not much of a factor.I think I’m happy enough with it. Ordered some m12 connectors and cable to turn the old box into a power brick (that can sit way the heck away from my workspace) and I’ll 3D print an enclosure for the front panel, which should take up less desk space too!Thank you all for your insight and help! This was an awesome first post for me!

author: MarkL
author_rank: Unknown
timestamp: January 07, 2025, 07:20:22 pm
content: Here is the P6245 on an R&S MXO4 with the R&S TekProbe adapter. The scope is BW limited to 200MHz and I've set up other acquisition options to be as close as possible to your screenshot a few posts ago.If I eyeball it to the bright area of the waveform (which is what I think you did), the pk-pk noise is 3.1mV, which is a little less than what you're showing, but it's certainly in the ballpark. I actually prefer to let the scope do the determination, and those numbers are 8.04mV pk-pk, and the AC RMS noise (sigma) is 711uV.The scope's noise with the P6245 disconnected (just a 50R terminator) is 3.84mV pk-pk and 339uV AC RMS, still using 10x vertical scale. And looking at this with an FFT, the R&S TekProbe adapter does NOT appear to have any additional switching noise like the Keysight one. I'm actually happy about that. I think the numbers above are about as good as it gets for the P6245.I know you found a solution for your 1103, but you've got me interested in digging into this some more. More in a bit...

author: coppercone2
author_rank: Super Contributor
timestamp: January 08, 2025, 06:50:06 am
content: It might be worth while to make a shield out of magnetic material if its working like that. Looks like a fair amount of work but then you can use the equipment you have, for not much cost, in the same form factor.There are alot of options for shielding materials, I recently bought hymu metal, iron sheet, and a amorphous foil. The seem quite affordable now.I think whatever 'pure iron sheet' is, it might be the best bet for cost, workability and magnetic properties. It holds to a strong magnet very strongly, compared to hymu etc.here is a RLC filter wrapped in ironit wraps well enough there is no point to fasten it further, I put a overlap of 1/4 turn

author: Tdeball
author_rank: Contributor
timestamp: January 08, 2025, 02:21:49 pm
content: I 3D printed a new face plate and enclosure to hold the 1103 electronics. I have some m12 connectors and 6ft cable on order. I'm happy to share the CAD files if anyone is interested.I'll probably have to adjust the voltage pots while measuring the rails in the little remote box once I get the longer cable.

author: MarkL
author_rank: Unknown
timestamp: January 10, 2025, 01:49:35 am
content: A few additional things on the noise issue...I was able to prove the noise is getting in through the P6245 offset input. The source of the noise is magnetic leakage from the transformer, as you had thought. I tried a few things to mitigate it, but I think your idea to distance the transformer is probably the best.Here's the detail if you're interested. I've included photos of probe points followed by scope captures. The scope vertical inputs are all using a scale factor 1x to make comparisons easier. The yellow trace is the 1103 output from the P6245 into 50 ohms, BW limited to 200MHz. The orange trace is the output from a LeCroy DA1855A differential amplifier, BW limited to 100kHz. The two black probes in the photos are the + and - input to the DA1855A.(Sorry for the bottom posted photos - in-line photos still have issues in this forum last time I checked.)First up is the waveform from the TekProbe connector ground to the offset input to the P6245. This is what the probe sees. The 1103 offset is set to OFF, which means the offset should be grounded (verified: 0.096 ohms to front panel ground using kelvin measurement). The orange trace shows there is still a waveform on the offset, even though it's grounded. It's almost identical to the waveform on the output of the P6245, with the the offset being slightly larger in magnitude. The P6245 has an offset gain adjustment internally, so the offset input is not necessarily going to be the exact same magnitude as the probe output. It's certainly close enough to say this is how the noise is getting into the probe.p6245_offset_input_probe.jpgp6245_offset_input_scope.pngIn the 1103, the small PCB at the front panel is grounded through a ring behind the upper offset control. Creating a ground from the TekProbe connector to the ground connection pin on the PCB helps a lot, but still does not get rid of all the noise.alternate_ground_probe.jpgalternate_ground_scope.pngModifying the probe by grounding the offset input inside the probe gets rid of the AC noise, but this may not be an acceptable solution. It does, however, serve as further validation that the noise is getting in through the offset input.modified_p6245_photo.jpgmodified_p6245_scope.pngFor curiosity's sake, I measured the induced voltages across the chassis and the front panel. (Sorry I do not have a probe photo of the front panel, but it is similar to the chassis probing.) It appears the chassis makes a great half-turn external secondary for the power transformer. And the ends of that are bolted to the front panel, adding to the issue of grounds on the front panel being at different potentials. (Unbolting the front panel from the chassis reduces the voltage across the front panel and improves the noise on the P6245 output, but does not get rid of it.)across_chassis_probe.jpgacross_chassis_scope.pngacross_front_panel_scope.png

author: MarkL
author_rank: Unknown
timestamp: January 10, 2025, 02:03:58 am
content: The other 1103 I have I believe is older, and is a little different internally. It has a different front panel PCB which is further from the transformer, and it's also in a different orientation which may make a difference in picking up stray EM fields from the transformer. I did not experiment with this unit as much, but the noise is clearly less severe.older_1103_photo.jpgolder_1103_scope.pngI decided to do the "alternate ground" from the previous post on the older 1103. But this time, I did it as a permanent mod and with a very short wire. The factory ground was disconnected. The results were very good.older_1103_with_ground_mod_photo_anno.jpgolder_1103_with_ground_mod_scope.pngResults were so good, I went back and did the permanent ground mod on the 1103 from the previous post. While there is still a little bit of noise, results were pretty good here too.1103_with_ground_mod_photo_anno.jpg1103_with_ground_mod_scope.pngThe noise still remaining are the tiny spikes that were more prominent before, and are related to the crest of the incoming sine wave as the capacitors charge up each cycle. After swapping transformers between the two units, I found that the tiny spikes followed the transformer, which I didn't expect. The transformer manufacturer is different between the units.All this was interesting and I had a chance to improve low signal performance of my two 1103 units. I don't think it's quite as perfect as remoting the transformer like you did, but I didn't really have a need to go that far. The ground mods were really easy.EDIT: Fix minor typo.

author: Tdeball
author_rank: Contributor
timestamp: January 10, 2025, 07:22:32 pm
content: Great work MarkL!Your solution packs much more bang for the buck compared to the effort of adding another enclosure. I plan to add that grounding wire to mine for peace of mind. I am happy how mine turned out, but this is probably not required for the average user.I think you’re spot on with the metal chassis is acting as an additional half turn of the transformer. I wonder if electrically isolating the front panel from the metal chassis would help.Now that you mention it, when I was playing around with shielding, I think I had better performance when my shielding was protecting the powered BNC connector compared to protecting the front panel circuit board.Thanks again!

author: MarkL
author_rank: Unknown
timestamp: January 10, 2025, 09:00:42 pm
content: The ground jumper approach might also come in handy for some who don't have a 3D printer (like me!).My first thought was also, "Aha! The chassis is forcing current through the front panel and ruining it as a ground!" But alas, unbolting the front panel did not fix the issue, although it did reduce it by about half, as I recall. Being a dead end I didn't post any screen captures of it (although it was mentioned).You're welcome. Sometimes I get caught up trying to figure out these weird little design problems. Thanks for pointing it out. I would not have expected such an issue from the Tek designers.