Measure PCB track impedance

[Scarr]

Hi all,

I'm doing a GPS project and it states in the PDF sheets that the RF track should be 50ohm impedance.

Now the PCB package I use can alegedly do this for me (Protel DXP) but how do I test / measure this? can it be done with a meter or scope?

Thanks

Steve

 

[Nigel Goodwin]

No, you can't measure in that way - essentially all you need to do is use the manufacturers recommended layout, there should be one in the datasheet?. The impedance of coaxial cable is set by the ratio of the inner conductor and the outer screen - presumably for a PCB track it will be the width of the track and it's seperation from the surrounding ground plane.

If your PCB package says it can do, I would tend to accept it!.

 

[JimB]

The impedance of a PCB track also depends on the dielectric constant of the board material.
The relationship of dielectric thickness, track width, track thickness and dielectric constant is mathematically complex. Probably best to just go with what the PCB design software gives. As long as the track is short and does not go snaking all over the board, you will never notice the difference.

JimB

 

[Scarr]

Thanks

Thanks guys,

Another question (a real newbie one) I want to measure the riplle on my scope but becuase it's 3.3v when I wind down the voltage the line goes off the top and I can't get it back on even with the adjustment?

How can I check the ripple?

Thx

Steve

 

[JimB]

There sould be a switch near the scope Y input which says (on my scope) DC Gnd AC. This sets the input coupling. To do what you are doing, set it to AC. A capacitor will block the DC and you will just be left with the AC and you can turn the sensitivty way up and see the ripple.

JimB

 

[stevez]

You might be able to measure the impedance with a reflectometer or by measuring reflected power with an experimental circuit board. All it would do is verify or allow you to adjust dimensions to suit the circuit board material you have on hand. If you terminate a transmission line with a non-inductive resistor that is equal to what you think the line impedance should be then you'd see little or no reflected power. You might do the reflected power measurement at frequencies that are more convenient. Some amateur radio operators in your area might be able to help with this.

You might also back your way into the PCB material characteristics by fabricating a capacitor from some material - to some exact dimensions - then measuring the capacitance. Knowing the characteristics would allow you to adjust, if required, the dimensions of the track.

All of the above is easier said than done unless you have a well equipped lab. The advice already offered may be sufficienct for your needs.