[time-nuts] Impedance question

[Gary Chatters]

Someone else will have to give you the answers, but here are a couple of 
issues to consider:

How long will your cables be?

A wavelength at 10 MHz is 30 meters.  A quarter wave (7.5m) coax line 
sort of inverts the termination impedance.  So if you connect using a 
high impedance, your source will be almost a short circuit (milliohms). 
Nominally, I would assume that you are working on a bench with short 50 
ohm cables, so not much of a problem, but assuming can have unintended 
consequences.

What is the voltage level of your sources?

Your counters will have a limit as to how much power input they can 
tolerate.  P=E^2/R.  The voltage that the 50 ohm input can tolerate may 
be much lower that what the megohm impedance can tolerate.  My 5334A 
says 5V RMS max when load set to 50 ohms.  Again, don't assume, check.

Gary




On 9/7/20 8:03 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
> _______________________________________________