Oscilloscope bandwidth and attenuation

[Elerion]

Hi everyone.

I've got a 60 MHz scope.
Shouldn't I get -3dB attenuation measuring 60 MHz sinewaves?

• 20 MHz BW Limit ON:

If I turn the BW limit on, then, at 20 MHz, the sinewave is attenuated approx 75%, which is close to -3dB. At 60 MHz it reduces to 23% of the original amplitude. This is what I expected. But...

• With NO BW Limit:

If I feed a 5Vp-p sinewave at 1 MHz, and raise frequency up to 60 MHz, the amplitude still is 5 Vp-p. This is not what I expected.

Why?!

Thank you!

 

[dknguyen]

Attenuated to 75% or by 75%?

 

[Elerion]

to 75%.
From 5V to approx 3.7V

 

[JimB]

If I feed a 5Vp-p sinewave at 1 MHz, and raise frequency up to 60 MHz, the amplitude still is 5 Vp-p. This is not what I expected.

Why?!

Maybe the bandwidth of the scope is actually greater than the 60MHz quoted in the specification.

It is also possible that you are getting more voltage at the 'scope end of the cable than you are putting in from the generator.
No this is not magic or free energy madness.
Standing waves on un-terminated transmission lines can cause odd effects like this.

For now I would go with the simple explanation that the scope is better than specification.

JimB

 

[Elerion]

Standing waves on un-terminated transmission lines can cause odd effects like this.

You gave me the clue. Thanks.
I probed the 50 ohm coax with my 10x probe (directly).
At 60 MHz, wavelength is just 5 m, so VSWR is to be expected.

If I terminate the coax from the generator with a 47 ohm resistor, then measure voltage across it with my 10x probe, I do get -3dB amplitude for a 60 MHz sinewave.

 

[JimB]

Sounds like a good result to me!

JimB

 

[dknguyen]

You gave me the clue. Thanks.
I probed the 50 ohm coax with my 10x probe (directly).
At 60 MHz, wavelength is just 5 m, so VSWR is to be expected.

If I terminate the coax from the generator with a 47 ohm resistor, then measure voltage across it with my 10x probe, I do get -3dB amplitude for a 60 MHz sinewave.

Does your function generator have a high impedance input mode?

 

[unclejed613]

you should dig out the manual for the scope and see if that's normal. the manual for most scopes will have the calibration procedure included. if so check how the bandwidth measurement is done for the 20Mhz limit, and how it's done for full bandwidth. there might also be an adjustment for the 20Mhz bandwidth, and if the scope is out of calibration, it might need to be adjusted.

 

[Elerion]

Does your function generator have a high impedance input mode?

You mean high-Z output? No.
Inputs, just VCO and sync.

 

[dknguyen]

You mean high-Z output? No.
Inputs, just VCO and sync.

An output mode for a high impedance input.

 

[Elerion]

No, it doesn't. How would that help?

 

[JimB]

An output mode for a high impedance input.

How would that help?

Simple answer: It wouldn't.

What you need is what you did.
50 Ohm coax, terminated in 50 Ohms (ish).
No nasty reflections up and down the cable.
Job done.

JimB