Multifetch .vi

[nyborn]

Hi:

I would like to ask how come I put a resistor on BNC cable. For example, 18kohm or 100ohm, it did not work. But I put 10ohm. It work. Why?

I am in a very urgent. I need a quick reponse. Thanks!

[Ayman K]

nyborn,

Unless you are clear about what you are doing, I can't really help you. You are talking about BNC cable and resistors but had no mention of how this is related to the multifetch.vi or the hardware you are using. It would be helpful if you could clarify your setup along with what you are doing in detail.

Ayman K

[nyborn]

Hi Ayman:

Attached is my setup.

I put 100ohm, it work. but I put R1: 18kohm. It did not work.

Could you please help me to explain this question? I could not figure it out why? I need to verify with you.

[nyborn]

Hi Ayman:

I still haven't hear from you. What is happening? Could you please help me out? I am in an urgent.

Thanks!

Regard,

John

[alipio]

Hi nyborn.

It looks like you are dealing with narrow pulses and coaxial cable.

If cable is RG58 or uquivalent, its Zo is 50 Ohms.

If cable is rg59 or equivalent, it is 75 ohms.

For other cables, see its specifications.

Energy transfer is maximun if losd resistor= cable impedance.

NI inpult also loads the cable, so its impedance is important, too.

In your case, 100 ohms is quite near impedance. so you can get a similar pulse an good level.

18 Kohms is near an open circuit, so you will get low level and probably a high diferenciated signal.

Please, study transmision lines theory, that will apply in this case, if I am right.

If the problem is different, please, attach a diagram of your circuit, including source,cable, load an ni digitizer.

Alipio 

[alipio]

Hi again.

Coupling is quite important, too.

AC 1 Mohm in parallel with typical load, will distort pulses( due to parallel capacitance). Use DC 1 Mohm in parallel with typical impedance.

or DC 50 ohms directly to coax cable. The digitizer will act as load in this case.

Hope it helps you

Alipio

[nyborn]

Hi Alipio:

Energy transfer is maximun if losd resistor= cable impedance.

What is losd? Is it Load?

study transmision lines theory can be found here.

I think it is. Is that what you meant?

 The path is below

National Instrument à NI SCOPE à Documentation à NI High Speed Digitizer Help à Impedance and Impedance Matching.

Does this path above apply what you said above? Could you please verify me?

[nyborn]

Coupling is quite important, too.

AC 1 Mohm in parallel with typical load, will distort pulses( due to parallel capacitance).

Attached is my graph? AC Couple is a high pass filter. 1Mohm in parallel with typical load is like my graph? Is that what you meant? 

Will distort pulse (due to parallel capacitance). Could you please tell the circuit digram? I did not quite get it here.

I always though AC couple can remove the offset. But what do you mean by 1 Mohm here. Could you pls describe a little bit? 

Use DC 1 Mohm in parallel with typical impedance.

or DC 50 ohms directly to coax cable. The digitizer will act as load in this case.

With the capacitor shorted, it would be “DC Coupled, is that meant the capaitor here is shorted ( I meant a wire) in a circuit that I attached. Is that right?

Then The digitzewr will act as load in this case? Am I get your meaning here?

Use 50 ohm mean the resistor I use should be 100 ohm, not 18kohm right? Then it will get the result, right?

[alipio]

Hi.
Sorry about my english.

1.- I meant LOAD
2.- Yes. Imedance matching is critical if working with narrow pulses and/or microwaves.
3.- See also NI522 input signal conditioning and AC DC coupling in same document.

About coupling, I was a little bit confused - Some red wine at lunch time -(actually C is in series).
If load is typical ( R1), you can use a 1 Mohm AC input coupling to remove dc.
If there is no load, you can select 50 Ohms. The digitizer will act as load.

There is a parameter known as cutoff frequency that applies in AC couplings.If there is pulse distortion, try DC coupling to be sure distortion is not due to coupling.

Things to try:
If transmission line is coaxial, and Vin are pulses and R1 is typical load, connect Vout to the NI card using 1 Mohm ac or dc and see the pulse shape. It shoul be correct.
Once loads and couplings introduce no distorion in your signal, go ahead and use C1 to implement a filter.

Obviously, I guess you are working with pulses, as it looks in your graphics.
If you do not get results, let me know type and values of signal, cable, C1 etc. and I will test the circuit using a Digital scope.

Hope it helps.
Alipio