High Frequency Oscilloscope Probes

high frequency oscilloscope

Hi all,

I'm aware that measuring voltages at high frequencies (~10 to 20 MHz) with a standard oscilloscope probe is not a good idea. I'm going to have to measure low voltages which could go as low as 10mV at frequenices around 10MHz-20MHz

The oscilloscope channels have a connection of 1 MegaOhm

Which probes do people recommend, i don't really want to have to build my own. Will I require an RF probe or an Active probe?

I don't know where to start


Regards

Cat

high frequency scope probe

Well,

It's really a matter of loading. Will your circuit under test be affected by ~10-15pF of capacitance and 1M ?

For some circuits, this is acceptable, and the easiest way to make the measurement is to just use an ordinary 1x probe. For other circuits (e.g. a crystal oscillator or an LC tank, etc...) the capacitance of the probe will change the operation of the circuit, and you need to take other measures. The normal thing to do is to go to a 10x or 100x probe, which cuts the probe capacitance by 10 or by 100, respectively. On the other hand, it also cuts the signal amplitude -- and you mentioned that you were looking at small signals. A FET probe might be an alternative in this case.

tektronix p2200 scope probe ground clip

I have an old Philips FET Application book and it has a nice DC to 100MHz FET unity-gain amplifier circuit described, especially suited to your problem. It has a 4-pF input capacitance and intended to drive 50 Ohm at the output. If you're interested, let me know...

high frequency oscilloscopes

I assume I wouldn't be able to use a 50 Ohm probe as my oscilloscope has an input impedance of 1 MegaOHm for both channels ports. Can somone please explain this to me?

Do I have to use a 1 Megaohm probe?


Regards

noise oscilloscope 1 megaohm

From the mentioned probe you would connect 50 ohm coax to the oscilloscope input with a T connector. One end would be connected to the oscilloscope input and the other to the 50 ohm termination resistor.

But measuring 20MHz signal, which is by the way relatively low frequency, 10X probe would do the job, if you don't have any special requirements. You didn't even mention what will you be measuring...

planar coil induced voltage oscilloscope

Apologies, I will be measuring the induced voltage across the terminals of a small planar coil.

Even when not measuring and just leaving the oscilloscope probe tip and ground not connected to anything there is still a waveform with a pk to pk voltage of greater than 150mV. So surely this probe I have isn't doing its job. Or am I wrong?

I have the Tektronix P2200 probe , see the link ...
**broken link removed**

Is this probe suitable?


Regards

10x and 100x probe tutorial

Hi Catalist,

The probe is OK, it's normal, but when you short the probe tip
you should get a straight line. Check and calibrate the probe with
the calibrator.
If you want to use an active probe with a 50 ohm output, you can
always terminate it with an external 50 ohm adapter like the
Tektronix 011-0049-01.

on1aag.

how to build a 50 ohm oscilloscope probe

When i connect the probe tip to the ground clip I don't get a straight line. I get a fair bit of noise about 16mV peak to peak. This is after it is calibrated.

This doesn't seem right.

Any ideas?

tds2014b 50 ohm setting

Hi Catalist,

What scope do you have there ?
Is the bandwith limit filter on or off ? Switch it on.
Is the X5 or X10 magnifier on or off ? Switch it off.
What does it look like if you switch the vertical input to GND ?

on1aag.

calculate frequency oscilloscope

Hi,

I have the Tektronix P2200 probe , see the link ...
h**p://www.iese.uiuc.edu/ge494/manual/Oscilloscope%20TDS1012%20Passive%20Probe.pdf

I get noise of around 1.44mV when set to 1X and BW limit is turned ON

Not sure what you mean when you say to set the vertical to ground.


Regards

tds2014 input capacitance

Even when you connect the ground clip to the probe tip you still have a loop antenna, so it still picks-up noise. To reduce the picked-up noise, remove the 4-inch (10 cm) ground clip-wire and replace it with a shorter one. Agilent has some probe accessories, look at (3):

"Not sure what you mean when you say to set the vertical to ground."

Switch the input of the scope's channel to GND instead of AC or DC. This should give you a hint of the internal scope's noise which you can not get rid of...

oscilloscope 1m 50 ohm input impedance

But you see, theres no point me using 1X as the capacitance is around 100pF

rf fet probe vpp circuit

The Agilent 1130A differential probe work well. If you don't have an Infinium then you'll need the amplifier and power supply also, which is nice becuase then you can use it with a spectrum analyzer.

high frequency oscilloscope probes

Catalyst look here once more:

high-frequency probes drive 50-ohm measurements

Which leads me to the question........

1. can we agree on what type of probe do I need for this? I can't afford a $3000+ differential probe.

2. Would an rf or active probe be better suited?

3. Can i use a 50 Ohm probe with the 1 MegaOhm channel input of the oscilloscope if I purchase this the Tektronix 011-0049-01 50 Ohm external adapter (see link) **broken link removed**


In reply to Borber:

Well, about building my own active probe, I've asked my supervisor and he doesn't think its a good idea.....so in that case......if it is agreed that an active probe would still be the best option for me.....which one should I buy?

Are there any recomendations for a probe that has minimal capacitance that will measure voltages accurately at frequencies between 10 MHz and 20 MHz?

Never realised this topic was so complex


Thanks again all

measuring 10mV signal with normal passive 10x or Borber's active probe (20dB which is again a 10x probe) gives you 1mV signal. Hmmm, what noise value did you measure on the scope, when short-circuited?

I think that without some amplification you'll have a hard time. Then again, what will be your dynamic range? Active probes usually don't measure several Vpp...

Active probe (low input capacitance necessary) with 0dB or 1x is what you should look for, I guess...

I agree it is dificult to use 10:1 probe to measure 10mV accurately. Analog oscilloscope noise is usually about 150-200µVpp on 1mV/div range and the measured signal is cca. 3mVpp. It is dificult to get 1% accurate result which oscilloscope normally enables.

OK,

Get a cup of coffee folks......

Lets just forget the amplitude being 10mV, that was an extreme example. I will most likely be measuring induced voltages anywhere from 100mVpp to 5 Vpp.

I'm more concerned about the amplitude being measured across the terminals actually matching the amplitude on the screen of the oscilloscope. If that makes sense. Just accurate readings.

------------------------------------------------------------------------


It may be better to give an example of the problem I'm having.

Right, I have a signal generator outputting a 1 V peak to peak sine wave at 13.56MHz. When I connect the oscilloscope probe directly to the BNC output connector on the signal generator itself the measured voltage on the oscilloscope is around 560mV (this is the maximum voltage attained after tuning with the compensation capacitor). As I decrease the frequency of the signal the voltage on the oscilloscope increases and eventually reaches the correct 1 Vpp reading. This is when the probe and oscilloscope are set to 1X.

When the probe and oscilloscope are set to 10X then I can tune the voltage to 1V at 13.56MHz with the probe's compensation with a screwdriver.... However.....when I change the pk-pk voltage on the generator to say.....10 Vpp, the reading on the oscilloscope reads as 10.6 Vpp........

Where has the extra 0.6 Vpp come from?

How can I trust these measurements at all?


I know i'm probably missing something so tedious here but this is driving me nuts, hehe. All I want are accurate voltage readings


Please, can someone shed some light on this

Regards

What you were doing with the probe compensation???????

The probe has a screw which alters its capacitance, it says in the manual it can be used. Should this not be touched?