Re: Now: 50 Ohm attenuator Re: What use for a 640 Ohm 1x Probe?
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Well for 100 W into 50 Ohms you will have 70.7V rms or 100 V peak.? If the 'scope can handle that, you don't need an attenuator.? If it can't, you can cobble an attenuator from a pair of resistors that will present a resistance of much higher than 50 Ohms, say at least 5000 Ohms.? So a 4500 Ohm resistor in series with a 500 Ohm resistor will do it for 10:1 but of course they have to be able to dissipate one Watt.
To complicate matters, if the coaxial line to the 'scope is an appreciable fraction of a wavelength (say 10%, which is 1 meter on 10 meters) you will have to deal with the SWR of that section.? So maybe a 100:1 attenuator would be better, using 4950 Ohms and 50 Ohms, with the 50 Ohm unit located right at the oscilloscope input terminal.
If you do that, you must be concerned about the stray capacitance across the 4950
Ohm resistor and, if necessary, compensate for it with a larger capacitance across the 'scope line input.
Generally these refinements won't be necessary for HF but anything higher and it becomes important, gradually.
Bob
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--- On Wed, 2/20/13, Cliff White wrote:
From: Cliff White
Subject: Now: 50 Ohm attenuator [TekScopes] Re: What use for a 640 Ohm 1x Probe?
To: TekScopes@...
Date: Wednesday, February 20, 2013, 7:14 AM
?
Ok, that's what I thought. What I'm
really trying to do is like this: 100W transmitter into a dummy
load, with a tee in that line going to the scope. Would 10x be
enough? Or should I aim for 100x?
On 02/19/2013 10:50 PM, Bob Albert wrote:
You don't generally
need impedance matching.? The 'scope input won't load a 50
Ohm source much.? I use a 50 Ohm termination without
attenuation and the high impedance of the oscilloscope has
negligible effect.
If you are handling substantial power you will need an
attenuator; the books tell you what the parameters should
be.? For 20 dB attenuation you need 45 Ohms in series and
5 Ohms across the 'scope input.? And of course the 45 Ohm
resistor has to handle the power.
If you are using a 50 Ohm cable, it needs to be in a 50
Ohm circuit, so the attenuator components should be right
at the 'scope.
Bob
--- On Tue, 2/19/13, Cliff White
wrote:
From: Cliff White
Subject: Re: [TekScopes] Re: What use for a 640 Ohm 1x
Probe?
To: TekScopes@...
Date: Tuesday, February 19, 2013, 8:05 PM
?
So, I've
had the idea of building a 50 ohm fixed 10x
attenuator to use inline with a 50 ohm cable.
What kind of impedance matching should I use for
the 1meg ohm on the scope?
On 02/19/2013 07:26 PM, Don Black wrote:
It should be 9 Meg ohms.
Then 90% of the signal is dropped across the
probes 9 Meg and 10% across the scope's 1 Meg
input impedance, giving 10:1 ratio.
The compensating capacitors across them are
adjusted for the same division at high
frequencies to maintain the flat response,
that's hat you're setting when you adjust for
flat square wave with the trimmer.
Don Black.
On 20-Feb-13 12:18 PM, David wrote:
?
On Wed, 20 Feb 2013 00:24:09 -0000,
"Philip" ndpmcintosh@...>
wrote:
>The publication on scope probes
mentioned earlier is good and I am working
my way through it. I already had it in my
document collection and it was on my
reading list.
>
>If I ohm out a 10x 10Mohm probe in the
same way, I get about 10 MOhms. I'll keep
reading though...
I get almost exactly 9.00 MOhms on each of
several different x10
probes within reach.
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