Oscilloscope input

MrChips

Joined Oct 2, 2009
25,040
Not correct.
You increase the input impedance and create 10x attenuation by adding R1 = 9MΩ in series, not by adding capacitance.
The impedance of R1 is approximately 9MΩ but not quite. R1 and the probe cable both have capacitance and inductance.
The impedance of R1 increases with frequency.
The purpose of the capacitance C1 in parallel with R1 is to reduce the impedance with frequency. It adds back the high frequencies that were overly attenuated. This capacitor is a trimmer capacitor and it allows you to adjust the impedance at high frequencies. That is why it is called HF compensation adjustment.
 
OK, RG-174 is a rather thin flexible coax at 31pf/ft and you have 22 pf at the scope. It's likely that the reactance has to be 1/9 too. The cable has to divide the amplitude of all frequencies up to the bandwidth of the scope the same.

Sometimes the standard scope probe won't work.

The added bonus is that when you add a 10x probe, the input Z goes up to 10M.

A scope will generally have a 50 ohm or 1M|| 22pf or switchable.
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
58
Not correct.
You increase the input impedance and create 10x attenuation by adding R1 = 9MΩ in series, not by adding capacitance.
The impedance of R1 is approximately 9MΩ but not quite. R1 and the probe cable both have capacitance and inductance.
The impedance of R1 increases with frequency.
The purpose of the capacitance C1 in parallel with R1 is to reduce the impedance with frequency. It adds back the high frequencies that were overly attenuated. This capacitor is a trimmer capacitor and it allows you to adjust the impedance at high frequencies. That is why it is called HF compensation adjustment.
You are turning everything i thought i had learned this week upside down haha. Let me explain my theory and please correct me where i am wrong.

I thought that:
- First, a coaxial design is needed in order to not only allow for higher frequencies to pass (skin-effect of regular lead), but also to reduce the noise with the help of shielding, which at the same time allows for measuring lower voltages with more accuracy.

- Secondly, because of the large amount of coax-capacitance (which at this point makes low impedance as frequency increases), high loading effect occurs, and that is going to limit the bandwidth. This is why 1x attenuation is low bandwidth, because what i have described so far, is 1x probe design.

- Thirdly for attenuated designs, as you point out, the resistance network is going to determine attenuation ratio. Which means that when we add 9MΩ in series, we set the ratio at 10x because input resistance is 1MΩ. But so far this only goes for DC/LF, so for higher bandwidth you want to add a variable capacitor in series with coax-capacitance as well, so that total capacitance decreases. This means that impedance is increased for HF as well, minimizing loading effect, and allows for more bandwidth.

(I read that above a certain frequency, the resistance network does nothing, and attenuation is determined by the capacitance only.)

- Fourthly, because the 9MΩ and variable capacitor is added in parallel, they form a RC-network. But input resistance of 1MΩ and the coax-capacitance also form an RC-network. We end up having these two RC-networks in series that needs to have the same time-constant, i.e. compensation in order for a flat square. This is why you add a variable capacitor.

And this is why the input capacitance don't make sense to me.
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
58
OK, RG-174 is a rather thin flexible coax at 31pf/ft and you have 22 pf at the scope. It's likely that the reactance has to be 1/9 too. The cable has to divide the amplitude of all frequencies up to the bandwidth of the scope the same.

Sometimes the standard scope probe won't work.

The added bonus is that when you add a 10x probe, the input Z goes up to 10M.

A scope will generally have a 50 ohm or 1M|| 22pf or switchable.
I'm sorry but i don't follow. Are we just assuming that the input capacitance is there?
Do you mean that reactance of 1/9 ratio cannot be achieved without input capacitance in this case?
 

Deleted member 115935

Joined Dec 31, 1969
0
I'm sorry but i don't follow. Are we just assuming that the input capacitance is there?
Do you mean that reactance of 1/9 ratio cannot be achieved without input capacitance in this case?
Ok @ZimmerJ

You seem to be going round in circles,
re asking the same questions , etc.

How about a suggestion,

You seem to have split out lots of seperate questions,

How about starting a new post on one of the questiosn,
and when you have that, create another forum post on the next one,
 

MrChips

Joined Oct 2, 2009
25,040
- First, a coaxial design is needed in order to not only allow for higher frequencies to pass (skin-effect of regular lead), but also to reduce the noise with the help of shielding, which at the same time allows for measuring lower voltages with more accuracy.
No. Every piece of electrical conductor is a transmission line with a characteristic impedance.
RG-174 is a typical coax cable with 50Ω impedance.

At 1MHz the attenuation is 1.9dB for every 100ft. Hopefully your scope probe is not 100 feet long.
At 100Mz the attenuation is 0.8dB for 10-ft cable.
There is little attenuation over a 10-ft cable.

Coax cable is used because it has a well defined and consistent impedance over a very wide bandwidth.

https://catalog.belden.com/techdata/EN/8216_techdata.pdf
 
OK.. Now suppose the capacitor isn't in the scope. You just hung a cable that has about 20pf of capacitance in it just because of it's length. Make the effect of that capacitance go away so the probe divides by 10 at ALL frequencies? e.g. divide by 10 up to the bandwidth of the scope.
 

MrChips

Joined Oct 2, 2009
25,040
The capacitance in the cable would be more than 20pF.
Typical capacitance is about 20pF/ft.
Hence a 4-ft long scope probe cable would be about 80pF.
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
58
No. Every piece of electrical conductor is a transmission line with a characteristic impedance.
RG-174 is a typical coax cable with 50Ω impedance.

At 1MHz the attenuation is 1.9dB for every 100ft. Hopefully your scope probe is not 100 feet long.
At 100Mz the attenuation is 0.8dB for 10-ft cable.
There is little attenuation over a 10-ft cable.

Coax cable is used because it has a well defined and consistent impedance over a very wide bandwidth.

https://catalog.belden.com/techdata/EN/8216_techdata.pdf
It's starting to make sense in one hand. I guess i figured that there was an inner conductor, an outer conductor, and then a shield for interference. But now when i'm looking at most probe designs, i can actually only see an inner conductor and a shield (connected to GND), so any HF signal is getting passed to GND.

On the other, i'm now wondering how these HF signals can even get measured at all considering the impedance. I mean almost all of it must be passed to GND.

Anyway i have been throwing out all kinds of questions here and i thank you for the answers! I have more information to go on now.
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
58
OK.. Now suppose the capacitor isn't in the scope. You just hung a cable that has about 20pf of capacitance in it just because of it's length. Make the effect of that capacitance go away so the probe divides by 10 at ALL frequencies? e.g. divide by 10 up to the bandwidth of the scope.
I seem to have had the wrong idea of the whole thing here. I'll read about the probe cable-design some more and then come back here. Appreciate the answers!
 

MrChips

Joined Oct 2, 2009
25,040
A 5-ft length of coax cable is not just a 100pF capacitor.
It is primarily a transmission line.
The attenuation at 100MHz is about 0.4dB, i.e. voltage loss of about 5%.
Most of the 10x attenuation comes from the 9MΩ + 1MΩ resistance voltage divider.
The trimmer capacitor is there to allow you to adjust the HF attenuation so as to achieve the flattest frequency response across a now extended bandwidth.
 
Top