What's the input capacitance of an oscilloscope's 50 ohm input?

[TimFox]

If you want to know reactance, it's better to show a curve - reactance vs frequency, because if you write "2pf" it doesn't represent actual value... It can be 2 pF at 1 GHz and at the same time 2 nH at 2 GHz...

Get your basics straight. Reactance has unit of Ohm, not F or H. Capacitive or inductive reactance is still expressed in Ohms. Capacitance and inductance are circuit elements in equivalent circuit and are always present there, not turning on and off magically when crossing SRF. At high frequencies, parasitic capacitance does not turn into parasitic inductance, but both these parasitic elements are always there, it's just that the ratio of their impedances turns in favor of another.

When the system passes through SRF, the net reactance (for a series model) or susceptance (for a parallel model) changes sign.
Below SRF,  the admittance (reciprocal of impedance) has a capacitive component (positive susceptance) and above SRF the susceptance is negative (inductive).
Similarly, below SRF the reactance is negative (capacitive) and above SRF the reactance of the series model's impedance is positive (inductive).
(Note that for admittance, the susceptance polarity is the opposite of the polarity for the reactance for the series model.  Complex algebra.)
For real circuits, there will be further changes in polarity as the frequency is increased above the "first" SRF discussed here.

[Kleinstein]

It depends on the scope if there is a separate 50 ohm front end or just an switched in 50 ohm resistor,  possibly with a little series inductance - parasitic or intentional.
The input capacitance of the normal mode in the normal mode often is also not just some 20 pF to ground. It is not so uncommon to have some extra resistance in series for at least part of the capacitance.
The capacitance is to a part the actual input capacitance of the amplifier, but also parasitic capacitance from switches and cables and extra added capacitance at least to make sure the capactance does not change with the ranges. The compensated dividers for the higher ranges also need some capacitance.

[tggzzz]

The capacitance is to a part the actual input capacitance of the amplifier, but also parasitic capacitance from switches and cables and extra added capacitance at least to make sure the capactance does not change with the ranges.

Frequently scopes have an attenuator between the BNC socket and the Y amplifiers.

That attenuator has to "deal" with the amplifier input capacitance, and may reduce the capacitance visible at the BNC socket.

[Kleinstein]

The lowest range usually has very little attenuator between the input and the amplifier, especially in the 1 M mode.  Another part that is needed for a scope input is some series resistor (e.g. 50 K range with parallel capacitor) for protection purposes This may be part of an attenuator. Still the reducion if the input capacitance would be small and possibly more parasitic capacitance added.

The actual input capacitace is usually way smaller than 20 pF (more like 2-5 pF), but some added capacitance (maybe with series R) may be needed for stability also with an open input. The usual JFET source follower is more complicated (frequency dependent) than just a fixed input capacitance.

For the 50 Ohm mode a littel fixed attenuator may be there to improve on the SWR or in other words getting  closer to true 50 ohm input impedance.

[Wallace Gasiewicz]

Quote from: dietert1 on Today at 02:26:04 pm

For a 50 Ohm input the internal line from the scopes input connector to its amplifier input can be a 50 Ohm terminated line, with the 50 Ohm resistor sitting very close to the input amplifier. The amplifier input capacitance is small, like 1..3 pF.
In high impedance mode, this isn't possible. Instead the connection effectively behaves as a capacitive load that even gets an additional capacitor to make a high bandwidth compensated divider with an adjustable capacitor in the scope probe.
A 1 MOhm || 20 pF input with direct connection (no divider) is the worst of all. How bad it is depends on the type of cable. In my opinion those probe cables with a 1:1 - 1:10 switch are a trap. Scope probe cables are very, very special with center conductors the thickness of a hair. Still their characteristic impedance cannot reach more than some hundred ohms.

Regards, Dieter


My level of expertise is not near Dieter's.  But I have to strongly agree with him on the probes with the 1:1,  1:10 switch. Besides being dangerous to your equipment, they are really quite bad.

[radiolistener]

Get your basics straight. Reactance has unit of Ohm, not F or H.

   
Ω is just a measurement unit, the same as F or H, you can convert from F or H to Ω and back, but it requires to take into account frequency.

At high frequencies, parasitic capacitance does not turn into parasitic inductance, but both these parasitic elements are always there, it's just that the ratio of their impedances turns in favor of another.

yes they are always here, but their total reactance at selected feeding point turns from capacitance to inductance and then back from inductance to capacitance when you sweep through frequency. The point where it crosses zero and changes from capacitance to inductance or from inductance to capacitance is known as resonant points.

For example capacitors turns into inductors above their resonant frequency. And inductors turns into capacitors above their resonant frequency... They still remains the same element, but working as opposite element at specific frequency ranges.

[TimFox]

Those are not quotes from me.  Look back at the original posts.
TimFox

[radiolistener]

fixed

[TimFox]

My admittance bridges (General Radio and Wayne-Kerr) treat inductance as a negative capacitance (at the measurement frequency).
The meaning is that the indicated negative capacitance value would resonate the inductance at the measurement frequency.
The Wayne Kerr units use ratio transformers, so that flipping the polarity of the transformer connections inverts the sign of the voltage applied to the circuit under test.
Therefore, both negative resistance (in interesting circuits) and negative capacitance (for an inductor) can be measured.

[tggzzz]

Get your basics straight. Reactance has unit of Ohm, not F or H.

   
Ω is just a measurement unit, the same as F or H, you can convert from F or H to Ω and back, but it requires to take into account frequency.

...and also to take in to account ±j

[TimFox]

Of course, if the capacitance is reasonably flat over a substantial capacitance range, as a quality capacitor below its SRF, then the reactance and susceptance (in ohms) varies greatly over that range.
In general, the impedance and admittance of a more complicated network (in real and imaginary ohms or Siemens, respectively) are defined at a given frequency, or graphed as they vary over a broad frequency range.

[EPAIII]

I stand corrected. But those are hardly common probes. In over 60 years of scope use I have never even seen one.

The point that those uncommon probes are needed should be strongly made when discussing the use of a probe with a 50 Ohm input. Otherwise the discussion will be misleading.

I think the best answer to this question is that you need to check the specs. of each individual scope. And none of them are perfect.

A 50 Ohm input is intended to be used with a 50 Ohm transmission line connected to it. It is not intended for use with a probe. You terminate the transmission line in the scope. And you must be careful with power levels. Depending on the power involved, an inline attenuator may be needed.

True, but misleading

"Resistive divider" "Z0" probes have a 50ohm transmission line and must be connected directly to a 50ohm input.

The HP10020A probe has a 1.5GHz bandwidth. Ditto the Keysight 10442B probe.
The Tektronix P6150A probe has, depending on option, up to a 9GHz bandwidth.
The Tektronix P6156A probe has a 3.5GHz bandwidth.
There are others.

Since they all have low tip capacitance (some 0.15pF, some <0.7pF), they all have a higher impedance than so-called high impedance *10 probes. That's rather important when probing high frequency signals.

[tggzzz]

I stand corrected. But those are hardly common probes. In over 60 years of scope use I have never even seen one.

The point that those uncommon probes are needed should be strongly made when discussing the use of a probe with a 50 Ohm input. Otherwise the discussion will be misleading.

I think the best answer to this question is that you need to check the specs. of each individual scope. And none of them are perfect.

A 50 Ohm input is intended to be used with a 50 Ohm transmission line connected to it. It is not intended for use with a probe. You terminate the transmission line in the scope. And you must be careful with power levels. Depending on the power involved, an inline attenuator may be needed.

True, but misleading

"Resistive divider" "Z0" probes have a 50ohm transmission line and must be connected directly to a 50ohm input.

The HP10020A probe has a 1.5GHz bandwidth. Ditto the Keysight 10442B probe.
The Tektronix P6150A probe has, depending on option, up to a 9GHz bandwidth.
The Tektronix P6156A probe has a 3.5GHz bandwidth.
There are others.

Since they all have low tip capacitance (some 0.15pF, some <0.7pF), they all have a higher impedance than so-called high impedance *10 probes. That's rather important when probing high frequency signals.

There are many active probes designed to be used with 50ohm inputs. Have a look at the refs in
https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Apart from that...

    A: Because we read from top to bottom, left to right.
    Q: Why should I start my reply below the quoted text?

    A: Because it messes up the order in which people normally read text.
    Q: Why is top-posting such a bad thing?   

    A: The lost context.
    Q: What makes top-posted replies harder to read than bottom-posted?

    A: Yes.
    Q: Should I trim down the quoted part of an email to which I'm replying?

[bdunham7]

"good" is an adjective. I think I'm going to start a "Stamp Out Adjectives" society, with the motto "numbers not adjectives"

That's a nice curmudgeonly thought, one to which I'd retort that an SWR of 1.001 is a hundred times better than 1.1.  I don't see any reason to believe that a scope with a 1.5 SWR is necessarily going to have a worse (more distorted, less accurate, whatever) trace displayed than one with 1.1.  Perhaps it will and perhaps it won't.  I decided not to post a long reply here on this so I'll skip the pictures.  My 485 is a bit under the weather anyway (horizontal amp slew rate issues at the fastest 3 setting) and I'm done trying to tweak my NanoVNA given that so-so quality of the rest of the kit.  The short story is that of the 3 scopes, I can't really say which is the best for a fast edge pulse--they all have their issues.  So I might conclude that <1.5 is "good" and that other issues will become more important at this point.

Here's the TDR of a 485 and 2465 input. The TDR has a ~100ps risetime, i.e. better than ~10* faster than the scopes' response.
Vertical sensitivity is 50mρ/div (+100mρ=>60ohms, -100mρ=>40ohms).
Horizontal sensitivity is approx 10cm/div.
Division 1 marked ^ corresponds to the end of the cable.

Thanks for the TDR shots, I wish I had that capability--the NanoVNA 'TDR' isn't quite as nice.  However, is that timescale right---about 2.5ns/div?  I still have the same questions though--what is the effect on the displayed waveform?  If you display a fast edge, do you see corner distortions that correspond?

To try and answer the OP's question I'll just point out again that the VNA (and your TDR) do not show us anything like a 50-ohm resistor in parallel with a 18µF capacitor.  That would be drastically different and would have a much, much higher SWR.  I can't demonstrate on the NanoVNA at 400Mhz due to parasitics, but I can scale everything down by using a 22nF capacitor, a 50-ohm resistor and a 50-400kHz sweep.  I'd classify an SWR of 8.0 as "not good".

[tggzzz]

Thanks for the TDR shots, I wish I had that capability
...
If you display a fast edge, do you see corner distortions that correspond?

The distortions are not necessarily on the edges; the frequencies at which amplitude dips occur depend on the cable length (simple standing wave theory), and can be well below the scope's high frequency limit.

If those frequencies correspond to something in the signal being measured, then the signal will appear distorted.


And there's a working Tek 1502 for sale on fleabay ATM. Yup, I spend too much time enabling Test Equipment Addiction
Alternatively an SDR dongle plus noise diode can be used as a poor-man's TDR https://entertaininghacks.wordpress.com/2015/07/27/poor-mans-homebrew-tdr-with-4cm-resolution-part-1/

[dietert1]

The BNC connection of a scope is good up to about 2 GHz. And at 2 GHz the wavelength in a 50 Ohm cable is about 8 cm. For a scope one would require that the termination at the end of the internal transmission line from the BNC socket to the amplifier input sits no more than about 1 or 2 cm from the end of that transmission line - which is perfectly possible. Even easier for lower bandwidth, like 200 or 500 MHz: Just put the input amplifier close to the BNC socket.
From the Tektronix 485 schematics i seem to remember that they would route the transmission line through the amplifier microcircuits. So the amplifier input sits right on the transmission line, without any distance.

Regards, Dieter