[time-nuts] Impedance question

[Richard (Rick) Karlquist]

I was the project manager of the HP5334B, which is similar to
many other models regarding the input.  IMHO, 1 megohm is ONLY
for scope probes.  For everything else, use 50 ohms.  Yes, an
advanced user (which doesn't sound like you) can play some
tricks with the 1 megohm input and a 50 ohm cable,  but you
have to know exactly what you are doing.

Rick Karlquist N6RK

On 9/7/2020 5:03 PM, SimBeej wrote:
> Dear fellow Time-Nuts,
> 
> I have a question about impedance matching (and I apologise in advance for
> my blatant ignorance on this matter, but I don't have a background in
> electronics and hardware, so am having to learn along the way).
> 
> When using frequency counters (in my case a 53230A and SR620) for making
> frequency or time interval measurements, should I be choosing 50 ohm for 1
> mega ohm as the input impedance (where the input to the counter might be
> either a 10 MHz signal or two 1 pps signals from a variety of oscillators,
> depending on whether I am doing frequency or time interval measurements)?
> Initially I thought it would be best to match the impedances (in which case
> I should be using 50 ohm), but now I am not so sure.
> 
> When I tried to read up on it, I found there is a lot of conflicting
> information out there. I trust the Agilent Application Note 200
> (Fundamentals of Electronic Counters)  and it says "for frequencies up to
> 10 MHz an input of 1 mega ohm is usually preferred". However, the same
> document also states that "the higher the impedance the more susceptible to
> noise and false counts the counter becomes". And could there possibly be a
> problem with reflections if the impedances aren't matched?
> 
> If anyone out there has a good handle on this sort of stuff and can provide
> me with some advice, it would be greatly appreciated.
> 
> Thanks.
> 
> Belinda
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