10X oscilloscop probe transmission line cable

[liusupeng]

Hi,
what is special about the 10X oscilloscop probe transmission line cable with normal transmission line cables?

Added after 47 minutes:

does anybody have a practical lossy transmisson line model?

 

[keith1200rs]

As far as I know it's just ordinary (thin) coax.

Keith

 

[liusupeng]

i am just trying to simulate the 10X osciloscope probe which is composed of 1 meter long transmission line, 9Mohm res, 1 meg res, 90pf osc input cap and 10pF compensation cap.
If i use ideal transmission line, the output rings a lot. if i use a lossy one, i can get nice waveform

 

[karesz]

Hi,
As I know, in all cases by Tek, are probe cables mounted with resistor wires (some hundred Ohms?), exactly for dumping "your ringings"...
K.

 

[liusupeng]

according to wiki:
Lossy transmission line probe (modern 10X scope probe; hi-Z probe)
The most common design inserts a 9 megaohm resistor in series with the probe tip. The signal is then transmitted from the probe head to the oscilloscope over a special coaxial cable that is designed to minimize capacitance and ringing. The resistor serves to minimize the loading that the cable capacitance would impose on the DUT. In series with the normal 1 megohm input impedance of the oscilloscope, the 9 megohm resistor creates a 10x voltage divider so such probes are normally known as either low cap(acitance) probes or 10X probes.
it seems that the osciloscope probe cable is made lossy purposely to minimize ringing.

Added after 2 minutes:

Hi,
As I know, in all cases by Tek, are probe cables mounted with resistor wires (some hundred Ohms?), exactly for dumping "your ringings"...
K.

i think it is true. in my simulation, the total resistance of the 1 meter TL is set to 500ohm to minimize ringing. if lower than this, it rings a lot.

 

[karesz]

Yes & tnx!,
I remember on 600 Ohm, but dont tell more for you-for how long cable ...
Try to solder some probe cable (by the old ones) it will be inpossible. :-(
K.

 

[FvM]

Yes, it's a low capacitance (large D/d ratio), resistive wire coax cable. Exact simulation may be difficult, because the lossy line models of most simulators are representing the loss frequency characteristic (mainly set by skin effect losses) rather poor. I know the Aplac simulator as an exception, there may be others.

 

[keith1200rs]

I have successfuly designed and built active probes with standard 50 ohm coax to connect to the oscilloscope. I cannot remember if I have also done a passive one but I am away at the moment and won't be able to check until I get back.

Keith

 

[liusupeng]

I have successfuly designed and built active probes with standard 50 ohm coax to connect to the oscilloscope. I cannot remember if I have also done a passive one but I am away at the moment and won't be able to check until I get back.

Keith

Thanks Keith. I guess for active probes the impedance matching is taken care and that is why the probe cable need not be resistive to damp the ringing.

 

[keith1200rs]

I seem to remember all I had to do with the active probe was make sure my drive circuit was stable with the cable capacitance. I think the design was specific to cable length - a longer cable required a different resistance value in the compensation network.

Keith

 

[dick_freebird]

The line is fundamentally mismatched, being "terminated" with
megohms.

How about augmenting it with a shunt termination, AC-coupled?
That is, for 50-ohm coax, a 47-ohm resistor and a small, variable
(or select) capacitor that is large enough to quench the natural-
frequency ringing, but small enough not to degrade the signal
of interest? That becomes a tighter bind if you're looking at
high freqencies of course. But cheaper than an inline amp
if you're looking at megahertz.

 

[FvM]

I'm rather sure that the suggested AC coupled 50 ohm termination can't give sufficient performance for a high impedance probe. There are good reasons why oscilloscope probes are using resistive cables since about 50 years.

There's an interesting 1969 tektronix publication "Oscilloscope Probe Circuits" available as reprint on the internet, e.g. from It has a detailed explanation, why resistive cables are achieving considerable higher bandwidths than terminated lossless coaxial cables.

 

[liusupeng]

I'm rather sure that the suggested AC coupled 50 ohm termination can't give sufficient performance for a high impedance probe. There are good reasons why oscilloscope probes are using resistive cables since about 50 years.

There's an interesting 1969 tektronix publication "Oscilloscope Probe Circuits" available as reprint on the internet, e.g. from It has a detailed explanation, why resistive cables are achieving considerable higher bandwidths than terminated lossless coaxial cables.

Thanks a lot. it is good material.