Measuring Oscilloscope Probe Capacitance

oscilloscope value probe capacitance

Hello,

I wish to measure circuits at high frequencues (~10-30 MHz), I was wondeirng if anyone could possibly outline a method of accurately measuring the input impedance & capacitance of an oscilloscope probe so i can have some confidence in the measurments I take.

How can i cancel out the impedance/capacitance effect on the circuit i am measuring?


Thanks for your time

Kind Regards

scope probe capacitance

I would use a hand held capacitance meter to measure the capacitance.

Things you can do:

1) live with it

2) sniff off a signal thru a 1000 ohm resistor

3) buy an expensive FET probe with lower capacitance

square wave value of probe capacitance

Just to Add...

The oscilloscope I have has an input impedance of 1 MegOhm and 20 pico farad, the probes i have can compensate the capacitance. Appareantly the probes can work upto 200MHz

I need to be able to measure AC signals between 10 MHz and 20 MHz accurately. I was wondering which method I can use to ensure I am getting good results.

I have a function generator set to 10MHz square wave, when i measure the result with the oscilloscope there is no sqaure wave at all, its practially a sine wive, which is very poor. I have tried using the probe trimmier but this does not help.

What can i do to ensure good high frequency measurement?

Which probe do people recommend I should use?


Sorry for the stupid questions, but if i don't ask, i don't learn

Regards

Cat

capacitance oscilloscope

What biff44 said is truth. Use capacitance meter, live probe connected to oscilloscope and measure it's capacitance. Measurement accuracy is good.

capacitance measurement oscilloscope

Take care: the problem could be on the signal generator!
If, for example, you are using a Function / Arbitrary Waveform Generator (aglient 33220A) with 20 MHz bandwidth, this is related to sine wave.
If you want to generate 10 MHz square wave, your generator should have at least 100 MHz BW.
I hope it can help
Mazz

measuring roll off on oscilloscope

What is meant by "sniff a signal through a 1000 OHm resistor" , how can the capacitance of the probe be measured like this?

measuring capacitance with oscilloscope

**broken link removed**

probe capacitance

If you see sinewave on the scope, when you actually have squarewave from the generator, sounds like you have too slow scope. What's its BW? 30MHz or so? Turn off the BW limitation in the menu...

"What is meant by "sniff a signal through a 1000 OHm resistor" , how can the capacitance of the probe be measured like this?"

Measure your signals through 1k resistor attached to the probe tip. This way you get rid of the probe's 10-15pF input capacitance. It's not a way to measure input cap...

what is probe capacitance

So its a 1K resistor in series with the probe tip so essentially the other end of the resistor is the new probe tip?

OR

Is the resistor connected across the probe tip and ground clip?


How does this get rid of the input capacitance of the probe?

Regards

probe and loading capacitance and resistance

"So its a 1K resistor in series with the probe tip so essentially the other end of the resistor is the new probe tip?" -> YES

lets say a 1k resistor has about 2-4pF capacitance, so two capacitors in series (one of the 1k res and the other of the probe) gives you a little higher capacitance than the resistors one...

oscilloscope probe capacitance

Combination of 1kΩ resistor in series and 15pF probe input capacitance gives about 10MHz -3dB frequency. Measurement accuracy will be poor.

oscillocope cutoff frequency

Borber can you elaborate a bit please......are you saying a 1K resistor in series with the probe wouldn't help?

How would it affect the accuracy?

Regards

oscilloscope input impedance capacitance

Simplified explanation is that you have now a lowpass filter in front of the probe. Filter consists of 1kΩ resistor and 15pF capacitor bypassed to ground after it. Cutoff frequency is about 10MHz. Probe gain at this frequency is -3dB. Without knowing the probe attenuation curve your measurements will be inaccurate.

verifying oscilloscope accuracy

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measure capacitor through oscilloscope

Hi

Where i put the AC capacitor in this diagram and how it to affects the value of capacitance probe's?

Thanks

What if a 500 Ohm resistor was used instead? The cut off frequency would be around 20MHz in that case.

Then again, the Impedance of the 15pF input capacitance of the probe would be around 700 Ohms at 13.5 MHz for example.

Would a 500 Ohm Resistor be any better?


Is there any other way I can minimise the input capacitance?


Regards

Catalyst said:

What if a 500 Ohm resistor was used instead? The cut off frequency would be around 20MHz in that case.

Then again, the Impedance of the 15pF input capacitance of the probe would be around 700 Ohms at 13.5 MHz for example.

Would a 500 Ohm Resistor be any better?


Is there any other way I can minimise the input capacitance?


Regards

Click to expand...

You can't with exist probe, very much of loading capacitance sitting in probes coax-cable, and if try shorten this to much in length, probe-tip capacitance going wrong value and not longer fit for 1:10 capacitive divide ratio and your loss of curve fidility.

If you try complete remove of cable with 20 pF input oscilloscope and builds probe directly on BNC-connectors (not so comfortable in practical work...) - you need tip-capacitance as 2.2 pF parallel over 9MOhm resistance (give 1.98 pF system loads) and you recive -3 dB limit around 78 MHz from 1 kOhm source - in theory...

Even if using expensive active FET-probe with around 1 pF load, you have still heavy loads on high frequency (1 pF = 1.6 kOhm on 100 MHz), and -3 dB roll off around 177 MHz on 1 kOhm source.

One (only?) way to measure high frequency with low loads is try/make transformer (on small toroids, matching network, directive coupler etc.) for example 10:1, if voltage from source transforms down ten times, loading impedances going up ten times and drawn power from source decrace to 1/100. But... It's very difficult to make transformer for very broad band frequency range.

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This is not for beginner and need lot of skill, work and time going to make tailor fit unit for small range of measure problem, verifying and make character and pin point out limits for this device (ex limited bandwith, unwanted resonance, needs DC-block capacitance and hunting parasites etc.).

RF-engineer spend lot of time to bulid controlled measure bridge and impedance transformer for many time only one measure point/measure problem in development process...