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Oscilloscope bandwidth and attenuation

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Elerion

Member
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!
 

JimB

Super Moderator
Most Helpful Member
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

Member
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

Super Moderator
Most Helpful Member
Sounds like a good result to me!

JimB
 

dknguyen

Well-Known Member
Most Helpful Member
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

Well-Known Member
Most Helpful Member
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.
 

JimB

Super Moderator
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