Equipment advice - Page 1
Forum section: electronics / beginners

author: hallkbrdz
author_rank: Contributor
timestamp: February 12, 2024, 01:54:07 am
content: The time has come to buy an oscilloscope for myself. My last use of one being a Textronics tube unit in school in the 80s, so I'm not a "beginner" but my oscope knowledge is a bit dated (ha). After reading MANY threads, some here, and watching way too many videos, I am probably more undecided now that before. LOL. Maybe a Rigol MSO or DHO unit? I'll spend what is needed, but I don't want to go overboard paying for sensitivity I don't need.Here are the main things I need it to be able to do:1. Monitor single digital signal pulses (3.3v) on the output of the teensy 4.1 microcontroller I'm using for a coil driver project. It's currently running at the stock 600 Mhz, and the pulses I'll be outputting will be no less than about 50uS. Four digital outputs to a pair of coil drivers at a time.2. Then monitor the output from the two coil drivers to the MOSFET gates (full H-bridge). 12v above the coil voltage source for each output. Four outputs total - although only two active at a time!3. Then monitor the output / response of the two ends of the driven coil. Single pulses only, not PWM. No more than 150v (input is 120v) from ground, zener overshoot limited. Two of these with two of #2 at the same time would be optimum.1 and 2 are to confirm programming / circuits are driving the FETs as intended. 3 is to see how the coil actually reacts (versus what I think it will do based on simulations).My goal is to be able to verify the timing and output voltage to get the optimum pulse energy-wise (battery source) into the coils (four wired in parallel) to produce the exact amount of magnetic flux required for this POC project. In the future, it would also be nice to be able to decode/save CANbus traffic, but that it not a requirement for now.Thanks in advance,Bryan

author: tatel
author_rank: Frequent Contributor
timestamp: February 12, 2024, 02:46:10 am
content: If I understand correctly, you want to measure pulses at 20 KHz frequency / 150V max? 3.3-12V signals? If so, I would say almost any 4-channel oscilloscope would do it? Even a vintage one?Not really sure I'm not missing something, but I can't see the problem at first glance

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2024, 09:44:17 am
content: You should also consider the type of probe most suitable for your requirements, and factor that into your calculations.If you can connect all your probes' shields to a common ground/PME, then your life will be simpler: youmaybe able to use the scope's maths features to synthesise a differential voltage measurement - but be aware of the consequences of CMRR at high frequencies.If not, then you should consider either HV differential probes (verify suitable CMRR) or one of the few scopes with fully isolated inputs. The latter can also have interesting features, such as input BNC sockets where the part that fits into the slots on a BNC plug is made of plastic (not metal).

author: jonpaul
author_rank: Super Contributor
timestamp: February 12, 2024, 11:40:51 am
content: schematoc , photo of DUT?j

author: watchmaker
author_rank: Unknown
timestamp: February 12, 2024, 01:06:41 pm
content: Quote from: hallkbrdz on February 12, 2024, 01:54:07 amI read this as you describing your present needs, but not necessarily the only thing you will be doing with a scope. I would suggest you ask on the Test Equipment subforum. Several good brand-new scopes cost less than $500 that may interest you and can be upgraded with accessories. Plus, there is a discount offered by Saelig. I bought my Siglent 1104 ex for $350 and upgraded it to the 1204 with a python hack that is easily found (simply activates features that are already on the scope).

author: hallkbrdz
author_rank: Contributor
timestamp: February 12, 2024, 08:17:03 pm
content: Thanks for all the input.I agree that bandwidth wise any scope should work (for now) for this need. The "high voltage" 120v+ input could be resolved with the quite expensive HV diff probes for sure, although I was planning on just using a 4:1 resistor voltage divider (not wire wound) to make sure the snapback negative voltage never comes near a scope limit during those first few ns's where the protection diodes won't be fully conducting yet. Accuracy of values is less import to me for this exercise than just being able to observe the reaction of the coils working in close proximity to strong permanent magnets as fields are created and then are quickly collapsed.Bryan

author: tggzzz
author_rank: Super Contributor
timestamp: February 12, 2024, 08:26:45 pm
content: Quote from: hallkbrdz on February 12, 2024, 08:17:03 pmMake sure you understand why it may need to be differential as well as high voltage.

author: hallkbrdz
author_rank: Contributor
timestamp: February 13, 2024, 01:39:23 am
content: tggzzz - from what I understand one of the the main reasons for a differential probe is so that the measurement is not referenced to a common ground as usual. In this case that should not be a problem (driver circuit attached).A higher CMMR is another mentioned reason, but noise from what? The voltage pulse / noise from the FETs is the only thing I would think would be significant here, and that is rather what I am measuring. I'm curious as to what I would be missing here that could overwhelm that with this type of circuit? Coils are wired into the portion labeled LC1_OUT and RC1_OUT.Working operation here is to pulse opposite pairs of FETs depending on the magnetic direction of the coils required at around 1 TC. There are separate battery / cap sources for the 120vdc and 12vdc.

author: EEVblog
author_rank: Unknown
timestamp: February 13, 2024, 02:45:01 am
content: Quote from: hallkbrdz on February 12, 2024, 08:17:03 pm100:1 single ended scope probes are super cheap if you don't need differential.https://www.ebay.com.au/itm/404739311675I explain the need for high speed high voltage differential in this video, about 15min in

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2024, 03:06:22 am
content: 2 channel, 100 MHz BW, Siglent or Rigol. Will cover a lot in analog and digital and MCU land. I have a TDS1012 since 2008, not really needed more.

author: hallkbrdz
author_rank: Contributor
timestamp: February 13, 2024, 04:36:40 am
content: OK, I'm sold on a differential probe. And I agree that the optical probe is the only way to go for high voltage at 1 Mhz or other high frequencies. Great stuff and a great demonstration BTW.Now for the low frequency range (6-8 Khz in this case if my single pulse width was repeated), that seems a bit overkill, right? Standard differential probes like the HVP-70 shown seem like they should be adequate for this use without too much noise, right?

author: EEVblog
author_rank: Unknown
timestamp: February 13, 2024, 06:18:58 am
content: Quote from: dobsonr741 on February 13, 2024, 03:06:22 amNo major reason you wouldn't buy a 4CH scope these days. I wouldn't recommend a 2CH any more.

author: EEVblog
author_rank: Unknown
timestamp: February 13, 2024, 06:21:35 am
content: Quote from: hallkbrdz on February 13, 2024, 04:36:40 amCorrect. The HVP-70 is top shelf though, cheaper HV diff probes exit if you are on a budget. Go for a 10:1 / 100:1 range one though.

author: tggzzz
author_rank: Super Contributor
timestamp: February 13, 2024, 10:38:24 am
content: Quote from: hallkbrdz on February 13, 2024, 01:39:23 amNot "noise" per se.Connect the two (differential) inputs to the same signal. As the (common mode) signal changes in theory the differential is zero and so there display should be a flat line at 0. In practice imbalances mean it won't be, and that's reflected in the CMRR not being infinite. This effect occurs at all frequencies, but is more apparent at higher frequencies.This occurs in many circumstances, not just probes. It is the reason for the concept that a -1 opamp amplifier is "better" than a +1 opamp amplifier, since in inverting circuits the two inputs are always at 0V whereas in non-inverting circuits both inputs change with the input signal.Do also pay careful attention to a probe's maximum input voltage as a function of frequency. Get it grossly wrong and the probe melts

author: hallkbrdz
author_rank: Contributor
timestamp: February 13, 2024, 03:21:12 pm
content: I agree, 4 channel is what I'd choose for the small price difference.Based on your video and 6-8 Khz range I'll be in, I'll avoid the cheaper Micsig DP10007 unit with that horrible spike and go with two of the better HVP-70 units instead.https://youtu.be/Dez9KG6whb0?si=pJOgHcMgOYtRAusW&t=953Thanks,Bryan