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Old 23rd Mar 2017, 2:04 pm   #30
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Frequency counters

If you are doing this with a switchable x1/x10 scope probe set to x1 then I think you will see the level on the 1Meg //20pF scope drop dramatically as you increase the frequency from 1MHz to 100MHz. I'd expect to see it droop by a factor of about 20 if the sig gen source impedance is in the ballpark of 400 ohms.

The other thing worth mentioning is that a scope probe set to x1 isn't the same as a piece of regular 50R coax. The coax used in a scope probe is usually a special lossy/resistive type of coax and it won't have a characteristic impedance of 50R. So my suggested 50R termination test at the scope end won't work very well with a x1 scope probe. So be prepared for disappointment if you do get a 50R termination and try using it with a x1 scope probe rather than regular 50R coax
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Regards, Jeremy G0HZU
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