# Scope attenuation question

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#### Chris Wilson

##### Member
I have my probe set to X10. that then feeds through a x20 attenuator on the scope input. Let's say I have a 100V signal at the probe. Is the scope seeing 100V divided by 10 (10V), then 10V divided by 20 (0.5V)? or is it seeing 100V divided by 30 (3.33V)? Thanks, I am having a mental block about this!

#### JLNY

##### Active Member
By a 20x attenuator, do you perhaps mean a 20dB attenuator? That would be dividing by a factor of 100. That said, an attenuator is only rated for a specific matched line impedance. e.g. a 50 Ohm attenuator won't work correctly on a scope input with a 1MOhm input impedance.

Edit: oops, my RF background betrays me. There are 1M matched attenuators with linear ratings specifically made for scopes. So yes, a 10x scope with a 20x attenuator would be dividing by a factor of 200.

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#### crutschow

##### Well-Known Member
Attenuations multiply, so you would be reducing the 100V by a factor of 200, giving 0.5V.
Look at it this way.
The first attenuator doesn't care (or know) what the second attenuator does so it reduces the voltage by a factor of 10.
The second attenuator doesn't care (or know) what the first attenuator did so it reduces the output by a further factor of 20, for a total reduction factor of 200.

#### JimB

##### Super Moderator
my RF background betrays me. There are 1M matched attenuators with linear ratings specifically made for scopes.
My RF background betrays me also, I have never seen a high impedance attenuator which is intended for use with a 'scope.

JimB

#### Mikebits

##### Well-Known Member
By a 20x attenuator, do you perhaps mean a 20dB attenuator? That would be dividing by a factor of 100. That said, an attenuator is only rated for a specific matched line impedance. e.g. a 50 Ohm attenuator won't work correctly on a scope input with a 1MOhm input impedance.

Edit: oops, my RF background betrays me. There are 1M matched attenuators with linear ratings specifically made for scopes. So yes, a 10x scope with a 20x attenuator would be dividing by a factor of 200.
Most scopes these days have a 50 ohm input impedance setting.

#### JLNY

##### Active Member
My RF background betrays me also, I have never seen a high impedance attenuator which is intended for use with a 'scope.

JimB
After posting my original reply, I ran a quick search on "oscilloscope attenuator," and found things like this:

Not exactly the high frequency RF attenuators you and I were thinking of, but it makes sense now that I think about it. Actually, I'm surprised how low the bandwidth is on these, even for a 1M impedance match.

#### Chris Wilson

##### Member
Thanks for the fast replies! Yes, it's a Picoscope attenuator, similar to this one, they are probably all re-badged generic Chinese clones:

Now you have explained how neither attenuation "knows nor cares" about the other I can see how it is a combined attenuation of a factor of 200, thanks for the help.

I do normally use it for automotive diagnostics, but am currently looking at high voltage spikes on the drains of a push pull Class D FET RF amp on 136kHz. I am having issues with the end of a TX session blowing one or more FET's and I am seeing a series of descending level voltage spikes looking at the drains, but that's for another thread if I can't figure it out myself

#### Nigel Goodwin

##### Super Moderator
Most scopes these days have a 50 ohm input impedance setting.
I don't quite know where you got 'most' from, I've never seen one that did

Perhaps specialised scopes for RF use do?.

#### JLNY

##### Active Member
LOL, I like that link of yours.

#### spec

##### Well-Known Member
I don't quite know where you got 'most' from, I've never seen one that did

Perhaps specialised scopes for RF use do?.
Some scopes have a 1M/10p input and a 50 Ohm input.

spec

#### audioguru

##### Well-Known Member
By a 20x attenuator, do you perhaps mean a 20dB attenuator? That would be dividing by a factor of 100.
No. An oscilloscope measures voltage, not power.
-20dB is a voltage reduction of only 10 times. It is a power reduction of 100 times since when you reduce the voltage then the current and power are also reduced.

#### Mikebits

##### Well-Known Member
I don't quite know where you got 'most' from, I've never seen one that did

Perhaps specialised scopes for RF use do?.
Whens the last time you shopped for scopes? Even my low end scope has a 50 ohm setting. It is useful when connecting 50 ohm coaxial cables. Besides, aren't scope BW in the realm of RF these days?

#### Nigel Goodwin

##### Super Moderator
Whens the last time you shopped for scopes?
Last year!

My concern was your assertion that 'most' scopes have that, when in fact very few do.

#### ChrisP58

##### Well-Known Member
One thing to remember here is that a typical 10X scope probe is not a complete 10 to 1 divider. It is only a 9 Meg series resistor. It depends on the 1 Meg to ground impedance of the scope input to complete the divider.

For any downstream attenuation devices to be accurate when inserted between a scope probe and the scope they must:
1) have and input impedance of 1 Meg, and
2) be characterized to work into the 1 Meg input impedance of the scope.

#### spec

##### Well-Known Member
One thing to remember here is that a typical 10X scope probe is not a complete 10 to 1 divider. It is only a 9 Meg series resistor. It depends on the 1 Meg to ground impedance of the scope input to complete the divider.

For any downstream attenuation devices to be accurate when inserted between a scope probe and the scope they must:
1) have and input impedance of 1 Meg, and
2) be characterized to work into the 1 Meg input impedance of the scope.
That is true but it is the capacitance that causes the problem when connecting attenuators, including the normal 10:1 probes, to scope high impedance inputs.

A good scope has a consistent input resistance and capacitance of 1M Ohm in parallel with 20p - a surprisingly large capacitance - and that is why 10:1 probes have frequency response adjusters to compensate for the scopes input capacitance. The probes output capacitance, mainly caused by the probe's lead, is another variable.

On a good scope, the input capacitance will be constant for different Y gains, with frequency, and with signal amplitude variations (setting up a scopes input capacitance is part of the calibration procedure on a good scope).

On a not so good scope this is not the case and while this may not be a problem when measuring a low impedance signal without a 10:1 probe, it can introduce errors when a probe is used to measure high frequency signals.

Of course, if you are measuring low frequencies the capacitances have little effect.

It is quite interesting to see the difference in display between a scopes 50 Ohm input and the 1 M Ohm input, not to mention the difference when a 10:1 probe is fittted to the scope.

spec

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