Is 10x really the optimal attenuation for a passive oscilloscope probe? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: June 05, 2023, 11:22:05 am
content: As Dave explained in one of his videos, IIRC, 10x probes have 10x attenuation for a couple of reasons:-- To protect the scopes from high voltages (maybe?)-- But mostly as a consequence of the passive probe cable being a deliberately highly lossy transmission line, because a lossless transmission line can't be built with a 1 megaohm characteristic impedance, a 50 ohm lossless transmission line would hopeless load down the circuit under test, etc, etc.Meanwhile, oscilloscope manufacturers are pushing the limits of how low they can push the noise floor of their instruments, advertising ever lower voltages/division. But we're throwing away a factor of 10 most of the time!So anyway, there's clearly a compromise in probe design here between A) load on device under test, B) reflections resulting from impedance mismatches at the ends of the probe screwing up the signal, and C) loss of SNR from the attenuation of the signal. And we've long settled on the 10x passive probe we all know and love today.My question is this: is it really true that 10x is what 6-fingered aliens would conclude is the optimal compromise? Or are 10x probes a holdover from when scopes were analog and the attenuation had to be taken care of via mental arithmetic (for which dividing/multiplying by 10 is obviously easiest for us 10-fingered humans)?

author: tggzzz
author_rank: Super Contributor
timestamp: June 05, 2023, 12:32:47 pm
content: There are many many many different types of probes, each of which interacts with your circuit in different ways and has different characteristics. FFI see the references inhttps://entertaininghacks.wordpress.com/scope-probe-reference-material/For even more information, see Tektronix and HP literature from the 60s.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 05, 2023, 01:25:24 pm
content: normally i use x10 probes .... and i use 100x probes who give much higher impedance load .... but in very sensitive situations you may need active probes, but its another storySome x100 probes can go up to 2.5 kv on some models ...Normally an x10 probe have x1 or x10 selection, the important thing is to do the probe compensation adjustmentIE to have very square rising edges on the square wave output you have on the scope ...

author: Marco
author_rank: Super Contributor
timestamp: June 05, 2023, 02:57:50 pm
content: I think it's still the best compromise. That said, 200 MHz range active 1x probes should have been commodity items by now.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2023, 04:46:21 pm
content: Quote from: rs20 on June 05, 2023, 11:22:05 amProbes which include shunt resistors can do that, however most x10 and x1000 probes do not and rely on the oscilloscope's 1 megohm input termination, so usually the oscilloscope needs to be able to handle full voltage at the probe tip whether the probe is attenuating or not.QuoteThe transmission line is made lossy to suppress reflections. You can make a high impedance passive probe with 50 ohm cable, however it will be limited to accurately reproducing lower frequencies because of reflections in the cable.The main reason attenuating probes are used is to lower the capacitance at the probe tip.QuoteManufacturers like to advertise lower noise, however semiconductor physics has not changed. If they are using JFET or MOSFET input stages, then their input noise is no better than oscilloscopes from the 1970s that used JFETs. Usually noise is worse because they are using either a MOSFET input stage, or a higher frequency transistor so that the oscilloscope can be "upgraded" without changing the hardware.Modern (since the 1980s) oscilloscopes tend to use dual path input buffers with a second low frequency input path which itself has attenuation at its input to isolate its input capacitance and increase its input range, but this also increases input noise at low frequencies. Oscilloscopes are not designed to be low noise or they would not use this circuit topology.My favorite oscilloscope inputs are high voltage (1) differential which inherently have like 5 times the input noise because of differential operation and the bootstrapping circuits needed for a wide input range. They have visibly more noise unless bandwidth is limited to 5 MHz, but it is hardly ever a concern and the noise rejection of having a differential input often makes up for it anyway.Quote10x is not far from optimal. (2) 100x probes do not have significantly lower input capacitance. 5x probes used to be available but they were intended for trigger inputs on oscilloscopes and frequency counters where signal fidelity is less important than increased sensitivity, and even with a 5x probe on an oscilloscope, that would only double the sensitivity. If you want lower noise measurements, then there are other ways to achieve it.(1) High voltage differential in this case means +/-10 volts, which is provided by bootstrapping the input stage. Old oscilloscope inputs typically operate with input signal range of +/-250 millivolts. Modern cost reduced oscilloscopes operate with an input range 10 times higher without bootstrapping which has the disadvantages of requiring higher slew rates and creating more distortion, but it saves by having one less input attenuator.(2) If we used base-8 arithmetic, then our probes would be 8x for convenience. If we used base-12 arithmetic, then our probes would be 12x for convenience. There is no application where the difference in noise would matter between 8x, 10x, and 12x probes.

author: Hydron
author_rank: Super Contributor
timestamp: June 05, 2023, 09:14:57 pm
content: PMK make some 20x probes as a compromise to get lower input C than a 10x probe (5.6pF), but higher signal level than a 100x. They are unfortunately annoyingly expensive and hard to get hold of. Note that a few manufacturers rebadge PMK probes - Keysight carry the 20x one I believe.

author: Someone
author_rank: Super Contributor
timestamp: June 05, 2023, 10:38:48 pm
content: Passive probes, tradeoff of low capacitance vs high impedance at DC10x for a 1M ohm input is a good balance and from history/momentum, just as 50 ohm. The "ideal" is probably a little off from there but it's close enough not to matter or to break the backwards compatibility/practice.

author: tggzzz
author_rank: Super Contributor
timestamp: June 06, 2023, 10:16:37 pm
content: Quote from: Someone on June 05, 2023, 10:38:48 pmThe "at DC" is an important caveat.In practice the misnamed "high" impedance *10 probes arelowerimpedance than "low" impedance *10 Z0 resistive divider probes.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: June 06, 2023, 10:32:31 pm
content: A x1 probe with a high impedance signal is loaded down by 10pf input capacitance.A x10 probe has a capacitor across the potential divider to compensate and so doesn't have the same losses.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2023, 11:16:36 pm
content: Quote from: nigelwright7557 on June 06, 2023, 10:32:31 pmA x1 probe has more like 200 picofarads of input capacitance. It will depend on the length of the x1 probe's cable.

author: Someone
author_rank: Super Contributor
timestamp: June 07, 2023, 01:06:44 am
content: Quote from: tggzzz on June 06, 2023, 10:16:37 pmPointing out the need for accuracy, and then immediately leaving a comment about impedance at a non-specific frequencyIdentification of the probes termination impedances and ratios are left as an exercise for the reader...

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 07, 2023, 01:16:07 am
content: You can read the Tektronix Concept Series on the subjecthttps://w140.com/tekwiki/images/6/62/062-1146-00.pdf