[time-nuts] Impedance question

[Bob kb8tq]

Hi

You actually have at least three components to your setup:

1) You have the *source* impedance of your signal

2) You have the impedance / length / loss / stability of the cable connecting the 
source to the load

3) You have the load impedance. 

In an ideal world, you would have cable that is truly 50 ohms at all frequencies. 
With that cable a 50 ohm termination at *either* end of the cable would absorb
all reflections. Either it is eliminated on hitting the load ( = load is matched) or
it is eliminated when it returns to the source ( = source is matched). Either way,
you have no ringing reflections. 

Indeed you often terminate *both* ends of the cable. That helps with cables that
may not *really* be 50 ohms. You get reflections reduced at both ends, so the
net is a bit better. 

====

In the case of a pure sine wave, things can be a bit “interesting”. Whatever the
impedances involved and whatever the cable impedance, it all works to a phase
shift and a loss. Loss is never a good thing. Phase shift (as long as it is perfectly
stable) is really not much of an issue. 

===

So what *is* your setup? 

PPS outputs may or may not be source matched. PPS outputs that are matched 
and running into a matched load result in logic levels that are 1/2 what you expected
them to be ( = you have a 2;1 attenuator). Normally PPS is matched at one end *or*
the other, but not both.

Cable length can matter. A 1 KM cable is a very different thing than a 1 M cable. 
Generally if the cable is < 1/10 the wavelength of the signal ( 10 MHz has a 30 M 
wavelength) the cable is better treaded as lumped elements than with transmission 
line equations. The typical cables normally used tend to fall into this ( < 3 M) range. 

Yes, this can go on and on …..

Lots of fun !!

Bob 

> On Sep 7, 2020, at 8:03 PM, SimBeej <t3acherbj at gmail.com> 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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