refence input impedance on counter

[TX5Z]

Howdy,

Curious about the reference input impedance on a couple of counters - 500 ohms and >100K ohms.   (HP 5316B and TTI TF930)

My reference output is 50 ohms.

1.  Should / could I use a 50 ohm terminator on the reference input?

2.  If I used a 50 ohm terminator, how would the input limits change? The 500 ohm input is 6Vrms max, if using a 50 ohm terminator it is now ____?

3.  In general, does using a 50 ohm terminator on 500 ohm or 100K ohm input give the same results as using a 50 ohm terminator on a 1M ohm input (like a scope)? 

I did search, and search and found much info on 50 ohm terminators - but couldn't find the above info.

MANY THANKS!

[wasedadoc]

Howdy,

Curious about the reference input impedance on a couple of counters - 500 ohms and >100K ohms.   (HP 5316B and TTI TF930)

My reference output is 50 ohms.

1.  Should / could I use a 50 ohm terminator on the reference input?

2.  If I used a 50 ohm terminator, how would the input limits change? The 500 ohm input is 6Vrms max, if using a 50 ohm terminator it is now ____?

3.  In general, does using a 50 ohm terminator on 500 ohm or 100K ohm input give the same results as using a 50 ohm terminator on a 1M ohm input (like a scope)? 

I did search, and search and found much info on 50 ohm terminators - but couldn't find the above info.

MANY THANKS!

You can use a term on the counter input but depending on the reference frequency and the cable length between ref gen and counter it may not be necessary.

Adding a term does not affect the input limits of the counter's ref input per se.  It does affect the amplitude fed into it.  It is almost halved. Basic voltage divider stuff.  Source contains 50 Ohm series resistor.  Load is 50 Ohm term in parallel with counter ref input impedance.

[bdunham7]

1.  Should / could I use a 50 ohm terminator on the reference input?

2.  If I used a 50 ohm terminator, how would the input limits change? The 500 ohm input is 6Vrms max, if using a 50 ohm terminator it is now ____?

3.  In general, does using a 50 ohm terminator on 500 ohm or 100K ohm input give the same results as using a 50 ohm terminator on a 1M ohm input (like a scope)? 

1.  No, not usually unless your cable from the reference to the counter was very long (like 5 meters or more) and even then probably not.

2.  The voltage limits wouldn't change at all since the 50R terminator is across the inputs.  However, you'd have to consider the limits of the terminator itself and the effect on the output of the reference.

3.  On the 100K input you'll end up with a total load of approximately 50R, but with the 500R input, you'll get ~45R combined, so not perfect.  It still probably won't matter at all.

I have an HP 5316B and I've never once worried about this issue.

[CaptDon]

The worst that could happen is reflections which if big enough in amplitude could make your counter think it had a 20MHz input when in fact you were feeding 10MHz from your frequency standard into your counter's reference input. And only about a .01% chance anyone has ever had that issue!!