[time-nuts] Impedance question

[Tom Holmes]

Belinda...

In general you want to have the impedances matched in order to keep the
waveforms clean. What that implies is that you have to know the source
impedance as well as the load impedance. Not all sources are 50 ohms, it
seems, so some homework is in order to be successful. 

Tom Holmes, N8ZM

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of SimBeej
Sent: Monday, September 07, 2020 8:04 PM
To: time-nuts at lists.febo.com
Subject: [time-nuts] Impedance question

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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