Author Topic: New low-cost ($170) 100MHz Differential scope probe from Micsig  (Read 9496 times)

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Online BravoV

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #150 on: August 18, 2017, 04:56:15 PM »
Those isolated banana adapters look neat! I'm thinking about modifying my MicSig diff. probe with an isolated BNC plug so I can attach a 'normal' probe to it or just solder a piece of coax (with isolated BNC) to the DUT.

It a genuine old Fluke/Philips banana to bnc adapters, bought them while ago when they were cheap at the bay  :P (posted here -> Philips BNC/Banana converter, but nowdays Aliexpress also carries clones now, though not sure about quality compared to genuine Philips one (as its also convertible into non shrouded banana). 

Regarding isolated BNC plug, I bought this (attached below) few months ago at Aliexpress too, saw it at the 1st time, and instantly ordered it even I didn't need it, yeah, hoarder nerves kicked in.  :-DD

Online BravoV

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #151 on: August 18, 2017, 05:03:00 PM »
It can't be be coax.. It must be twisted pair.. Both cables are "equal" but different polarity. Same impedance, capacity.....
Also, coupled interference has to to be induced equally in both cables so amplifier can cancel it out.. If you put one to be on coax shield , it will get more coupling than one protected inside... You could do twisted pair with outside common shield..

Ok, noted, thanks.  :-+

Well, my intention or at least expecting that with banana plug instead of fixed wires, we can improve the probe and cabling, say like using shorter cables, twisted cheap wires soldered on the test points, or using dual coax cables with both shield say connected to ground earth and etc.

My point is, with banana plug instead of fixed wires, it opens up the flexibility and variations at probing the HV test points.

Online 2N3055

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #152 on: August 18, 2017, 05:08:12 PM »
....
My point is, with banana plug instead of fixed wires, it opens up the flexibility and variations at probing the HV test points.

That is correct. I agree.
 

Offline toli

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #153 on: August 18, 2017, 06:04:47 PM »
Just noticed these probes a couple of days ago on Aliexpress, and and a quick google search brought me here obviously  ;D

So what are the short term conclusion from these who own the probes? Do they seem to be reliable and worth the money? Or do they start coming apart/exhibit some sort of issues?

I understand there's a problem with the long leads and signals of ~40MHz and over. However, even with a 20MHz limit active on the scope, a 1300V differential probe for 160$ shipped is much cheaper than anything else you can get your hands on. Especially considering the fact it looks quite well designed and built.
Even with the X50 setting, and 130V max input, this can prove quite useful for genera use.

 

Online BravoV

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #154 on: August 18, 2017, 06:28:01 PM »
The negative pulse on the Gate waveform s is probably not real. It there because the differential probe does not have enough common mode rejection ratio, to reject the large dv/dt impressed by the switching.

Is there an easy way to verify and remake a customized test in order to bring out this unwanted artifacts "intentionally" ?

I mean like say do the test using this diff. probe at certain test points or simple custom circuit, and then compared the result again at the same test points vs ordinary passive scope's probes.  :-//

I guess the test points must has low enough (safer) voltage as we're going use common passive scope's probes.

Just a thought.
« Last Edit: August 18, 2017, 07:05:12 PM by BravoV »
 

Offline David Hess

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #155 on: August 19, 2017, 07:34:48 AM »
Just curious can we get "better" signal integrity at probing, if it is designed with standard female banana input like at DMMs ? Instead of two dangling long and loose wires.

No.

The common mode rejection relies on balanced input impedances.  A single ended probe will just create a greater mismatch in impedance.  Low voltage differential probes can be used this way but only because they have much higher input impedance and are connected directly to the circuit without leads.  They rely on moving the probe as close as possible to the test points for their performance which includes not having input attenuators limiting them to low voltages.

What will work is two identical probes connected differentially which is how the old style high voltage differential probes worked.  They used special probes however which could be calibrated to match each other.  The simple leads on a modern high voltage differential probe are an attempt to make sure they match.

Common high voltage differential probes do *not* have a 1 megohm input resistance designed to work with single ended passive probes so the compensation will be all screwed up.  A special probe adapter that provides a 1 megohm load to the passive probe would be required unless a x1 probe was used.

Would a x1 probe work?  It would add a lot of differential and common mode capacitance.  I am dubious but try it and do the test below.

Is there an easy way to verify and remake a customized test in order to bring out this unwanted artifacts "intentionally" ?

Sure, this is easy.  Connect both leads of the differential probe to the same signal which has the large common mode voltage swing.  In an off-line switching power supply, this would usually be the emitter or source lead of the high side switch.  Since the leads are connected together, they see the same source impedance and zero volts between them.  Anything on the output represents the *best* possible results.  Real results when one lead is moved to the actual signal like the base or gate will be worse so if the best possible results are not good enough, the measurement will be corrupted.

When testing a low voltage active probe or a standard oscilloscope probe, the same test works.  Connect the probe tip to the same point as the ground lead.  Anything besides a flat trace that the oscilloscope shows represents the minimum error that a real signal will produce.

What level of common mode rejection is needed?  A typical off-line switching power supply operates with a peak-to-peak voltage of about 340 volts and a gate voltage of about 10 volts.  That comes out to about 31dB of common mode rejection for a completely corrupted measurement.  10% accuracy would require 51dB of common mode rejection.

An excellent high voltage differential probe can maintain 51dB (355:1) up to about 20 MHz or 17.5 nanoseconds which makes a 100 MHz specification ludicrous.
 

Online Hydron

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Re: New low-cost ($170) 100MHz Differential scope probe from Micsig
« Reply #156 on: Yesterday at 09:00:31 PM »
I wouldn't say that 51dB CMRR @ 20MHz (well above what any of this type of probe will manage) is essential for them being useful - many circuits will be at mains potential but not at a high frequency potential (e.g. most parts of a PFC boost converter are referenced to the most-negative of the two mains input connections - this will move around a few hundred volts but at 50-60Hz, not in the 100s of kHz).

I do agree that measure the high side Vgs voltage of a high frequency converter will be difficult/impossible for this type of probe - even if the switching frequency is not high, the harmonics from the edge will make a huge mess of any measurement unless high frequency CMRR is very good. For the times I've had to do such measurements I've used a single channel of a battery powered PC-based scope controlled/connected over WiFi, but even with this you need to watch out for oddities due to parasitic capacitance to ground and inductance of the probe coax.

I'm not aware of high voltage diff-probes that do >50dB CMRR at 20MHz other than Tek's IsoVu or some niche products designed for this job (e.g. https://cleverscope.com/news/cs448/ ), and these are both in the $10k USD range (in the case of the Tek, up to $24k per channel!). Do you have examples of what probes you are thinking of?

On a different note, I did some rough checks of different cable lengths with the Micsig probe (temporarily added banana jacks to the probe to allow different length leads to be tested):
- The only way I have to do this is with a cheap spectrum analyser + tracking gen (normalised with a direct cable connection before measurement) so take these with a grain of salt.
- Only a single ended signal could be used as the input, so the -ve lead of the probe was at ground potential
- The tests were done at a 50 ohm input impedance, so 42dB attenuation is expected in 50x mode (=20log((50/(50+75))*(1/50))).
- At low frequencies there are also some measurement artifacts.
- Original probes best match the "medium" length (they are ~58cm plus whatever tip is selected).

The results show that the length of wire makes a large difference to frequency response, and in fact is relied upon for getting anywhere close to the rated bandwidth. Personally I'd be inclined to keep the original leads intact and treat them as 50MHz probes (potentially even mod them to roll off above this!), as while greater bandwidth is potentially possible, controlling peaking would be a nightmare as it depends strongly on physical arrangement of the leads (and probably more stuff too, e.g. source impedance, which was fixed at 50R for this test).
 

Offline David Hess

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I'm not aware of high voltage diff-probes that do >50dB CMRR at 20MHz other than Tek's IsoVu or some niche products designed for this job (e.g. https://cleverscope.com/news/cs448/ ), and these are both in the $10k USD range (in the case of the Tek, up to $24k per channel!). Do you have examples of what probes you are thinking of?

The older style ones which used adjustable passive probes could do it or come close if they are calibrated against the source impedance.  I think LeCroy is the only one that makes these now in the form of their DA1855A with one of their fully adjustable differential probe pairs.  The difference is that the probe pair has 8 calibration adjustments for each side and it has to be calibrated against both the differential amplifier and the source.

An oscilloscope with an isolated input is more cost effective and performs better if an unbalanced input impedance and low common mode input impedance is acceptable.  And that just leads back to floating the oscilloscope or preferably floating the device under test if an oscilloscope with isolated inputs or an isolated probe is not available.

Thanks for mentioning Cleverscope.  They make some great looking products.
« Last Edit: Today at 02:28:14 AM by David Hess »
 


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