High Frequency Oscilloscope Probes

[pisoiu]

The probe should be compensated in order to obtain a flat response over its designed bandwidth. Otherwise, what you see as 1Vpp@13,56Mhz may not be accurate. Usually all scopes have an output exactly for that purpose. The output generate rectangular signal, and you should change the variable capacitor from the probe (with a plastic screw driver) untill you see a nice rectangle, without overshoots and undershoots. Usually, all these operations are explained in detail in probe's manual. Remember, the compensation is when the probe is set to 10:1, not at 1:1.
The difference between the readings at 1V and 10V may come from your generator, it may be out of calibration.

/pisoiu

 

[rfmw]

Does your generator has 50 ohm output? If so, then you should terminate it with 50 ohm resistor first and measure the voltage with the 10x probe across this resistor.

 

[Catalyst]

In response to pisoiu:

I understand now, I'll recalibrate it to what it was before, suppose i was just getting a bit desperate, hehe. Well, before I messed around with the compensation the value at 10X was no where near 1 Vpp. So I'm back to where i started there.


In response to rfmw:

Yes, the signal generater does have a 50 Ohm output.......so your saying I should connect a 50 Ohm resistor across the output of the signal generator then measure the voltage across that? Is this all relating to impedance matching?


Regards

 

[pisoiu]

Probes tutorial.
https://zone.ni.com/devzone/conceptd.nsf/webmain/1034372750915B27862568CB004D6EA1

/pisoiu

 

[Catalyst]

Thanks pisoiu, understand it now, thanks for the link


In response to rfmw:

Yes, the signal generater does have a 50 Ohm output.......so your saying I should connect a 50 Ohm resistor across the output of the signal generator then measure the voltage across that? Is this all relating to impedance matching?


Regards

 

[pisoiu]

Yes, the output should be terminated in 50 ohm load at the other end of the transmission line. For example, if you connect the output of the generator directly to the BNC input of the scope (ideally would be a connection with a coax cable with two BNCs at the ends, not through the osc probe - the ground connection will form an impedance discontinuity), you should either set the scope input to have 50 ohm impedance instead of 1Mohm, on DC coupling (if the scope has this feature), or use a T connector with a 50 Ohm termination installed at the scope input. Without the impedance matching, the signal will be distorted and the amplitude will not be correct.

/pisoiu

 

[Catalyst]

Well, I have a BNC 50 Ohm terminator, Does it matter which branch it is connected to on the T connector? Is there any specific configuration?

I assume the probe connection would be the connected to the central socket of the T-connector, while one of the branches connects to the oscilloscope and the other branch connects to the 50 Ohm terminator. [/b]

 

[rfmw]

"Does it matter which branch it is connected to on the T connector? Is there any specific configuration?"

It doesn't matter how you connect all three branches of the T connector, what does matter is that 50 Ohm termination resistor should be placed as close as possible to the scope input BNC:


Signal generator -> ------- 50 ohm coax --------- <-T-> scope BNC (or 50 ohm term)
and the third one -> 50 ohm term (or scope BNC)

This is configuration is where your scope has 1MOhm input, not 50 Ohm. If your scope has 50 ohm input, then you would connect the coax directly to it...

 

[Catalyst]

---

 

[artem]

You dont control enough precision of your measurements. You are trying to get reason of 5 percent error . Taking tds2014 as a reference for DC it has accepted accuracy of 3 percent - this is 3 mV of your 5mV error (yes DC and AC not much related )
(**broken link removed**)
You should consult manual for the AC accuracy as it should be worse than DC. Then you dont have pure 50 Ohm impedance scope probe ( 8-14 pF does more than 2 percent influence for 50 Oh impedance at 13 MHz.) - it will impact as well, third reason could be signal generator output voltage accuracy . I dont mention cables and rest stuff as they should influence less if correctly choosen .

You can decrease influence of probe capasitance by using serially connected resistor divider let say 45 + 5 Ohm(attached to signal generator output) and take mearement from 5 Ohm resistor with as short wires as possible. Due to low resistance of output probe's reactive impedance influence will be reduced . But resistors have to have min inductance. Of course, got result has to be multiplied by division factor of resistive divider.

Then why do you need 5 percent accuracy over RFID measurements ?

 

[Catalyst]

In response to artem

When you say "Resistive Divider" do you mean a voltage divider? The 2 resistors in series?

 

[artem]

In response to artem

When you say "Resistive Divider" do you mean a voltage divider? The 2 resistors in series?

yes

 

[Catalyst]

I don't want to use a coax cable to connect from the generator to the scope. I'm trying to get an accurate reading with the probe.

I have recalibrated the probe now, not touching that compensation screw again
I have tried putting a 50 Ohm resistor across the generator (set a 1 Vpp) terminals and measured the voltage across it with the 10X setting. The scope reads 1.05 Vpp. I'm still not sure why there is an extra 0.05 Vpp.

If I increase the voltage to say 10 Vpp the scope reads 10.4Volts. An extra 0.4Vpp?!?!

Should I be worried about this extra voltage on the scope readings?


Regards