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Why are transformer based oscilloscope probes not common?

It should be possible to put a miniature 10:1 signal transformer at the probe tip, and connect it straight to the 50ohm input of the oscilloscope to create a 10X differential probe.

I would imagine some advantages:

  • Poor mans differential probe.
  • High impedance at high frequencies.
  • DC isolation allows high voltage probing.
  • Less expensive than active probes.
  • Functional over a wider temperature range, allowing measurements inside environmental chambers.

And a couple of disadvantages:

  • Bandwidth limited to approximately 1~500MHz.
  • Only usable on DC balanced signals, such as Ethernet, PCI-Express, and similar. Probably useful for some RF measurements as well.

What am I missing?

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    \$\begingroup\$ Frequency response, for one. \$\endgroup\$
    – DKNguyen
    Jan 30, 2020 at 23:23
  • \$\begingroup\$ How would you measure a DC signal? Or an AC signal with a DC offset? \$\endgroup\$
    – Aaron
    Jan 30, 2020 at 23:35
  • \$\begingroup\$ Yes, as stated, it would only work with DC balanced signals. And it would have a minimun frequency limit. It is NOT a general purpose probe. \$\endgroup\$ Jan 30, 2020 at 23:39
  • \$\begingroup\$ Scope inputs are often 1MΩ, not 50Ω, so an ordinary 10X probe has 10MΩ DC resistance \$\endgroup\$
    – Jasen
    Jan 31, 2020 at 4:30
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    \$\begingroup\$ All scopes, but the cheapest, have a 50ohm input setting. \$\endgroup\$ Jan 31, 2020 at 7:56

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A transformer has a very limited bandwidth.

It has a low frequency limit given by the primary inductance, and the V.s product of the core.

It has a high frequency limit given by the leakage inductance, and the self-capacitance.

As a result of this limit, there are many different transformers available, with different core sizes and permeabilities, and different winding turns, operating over different bandwidths. Most of the wider band RF ones are designed to operate at the very low impedance level of 50 ohms, and are 1:1. Bandwidth rapidly shrinks as transformatio ratios depart from 1:1. You can get low frequency transformers that cover several dacades, for microphones and for 100v speaker lines, but they tend to be big and heavy.

When you pick up an oscilloscope probe in anger (a) you don't want it to load the circuit so much that it changes operation and (b) you're often debugging, so you want to see anything that's going on that shouldn't be there.

(a) The typical impedance of useful low frequency probes, 1Mohm, 10Mohm, with a few pF to 30pF shunt C is way higher than you can get with transformers. Even at RF and microwave where you need lower C but can tolerate higher loading, so you might use 500ohms to 10kohms in series with a 50ohm scope input coax, these impedances are way higher than transformers that will operate there.

(b) With a resistive divider scope probe, you can see reasonably flat from DC to tens, or even hundreds of MHz. With one connection, you can see unexpected DC shifts, or instability at 100MHz, either of which may be a clue as to why your DC regulator, audio amplifier, 10MHz clock, or whatever, are not working properly.

For very specific applications, let's say monitoring ethernet, then the appropriate sized transformer can make a useful probe. However it will only tell you what's happening in the bandwidth you think you want to know about.

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  • \$\begingroup\$ With a 10:1 transformer, the loading would be 5000ohms, right? Plus whatever parasitic capacitance there is in the transformer. Way better than series resistors, as long as the parasitics can be kept low. \$\endgroup\$ Jan 31, 2020 at 7:52
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    \$\begingroup\$ @TimmyBrolin Put a link to the transformer you are thinking about in your next comment, an actual, available, commercial transformer, and then we can compare that with actual available 10:1 probes, and actual available resistors, and have a meaningful comparison of bandwidth and impedance levels. \$\endgroup\$
    – Neil_UK
    Jan 31, 2020 at 8:13
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    \$\begingroup\$ @TimmyBrolin If OP can't source real life transformers, here are some coilcraft.com/pdfs/pwb.pdf. PWB1010 (1:1, 3.5KHz-125MHz) and PWB-16 (1:16, 50kHz-80MHz) may be of interest \$\endgroup\$
    – DKNguyen
    Jan 31, 2020 at 16:00
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    \$\begingroup\$ @a concerned citizen Yes, 5 kOhms at high frequencies is very good. You need to remember that a 0.32 pF active probe is 5 kOhms at 100MHz. That 1 MOhm or 10 MOhm rating of your probe is only valid at DC. \$\endgroup\$ Jan 31, 2020 at 16:51
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    \$\begingroup\$ @Neil_UK passive 10X probes and transmission line probes with series resistors are good at what they do (they are great at DC). But neither of them is suitable for probing a differential signal such as Ethernet. It may very well be that it is a geometry problem. But since there are 16:1 impedance RF transformers.. That is not too far off from a 100:1 impedance (10:1 turns ratio) transformer.. \$\endgroup\$ Jan 31, 2020 at 17:12

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