Why Is My Attenuator Resistor Value Negative?

[fran1942]

Hello, I am trying to work out the resistor values for an 'O' attenuator.
I am using the attached formula to work out resistor 3's value.
My values to insert into this formula are:

Input impedance (Z1) = 50 Ohm
Load impedance (Z2) = 600 Ohm
a =1.995

However when I substitute these figures I am getting a negative number result.
Obviously I can't have a negative Ohm resitor, so how do you interpret that ?

Thanks for any help

 

[gneill]

Your circuit diagram looks like that of a $\Pi$ attenuator, not an O-pad.

There's a minimum attenuation that is determined by the source and load impedances. Have you checked to see whether or not your "a" value meets this criterion?

 

[fran1942]

sorry, I attached the wrong image.
This is the O-pad attachment.

What does it mean when I get a negative value from one of the resistor formula calculations (as I am in this case).
Obviously I can't have negative Ohm resistors ?

 

[gneill]

sorry, I attached the wrong image.
This is the O-pad attachment.

What does it mean when I get a negative value from one of the resistor formula calculations (as I am in this case).
Obviously I can't have negative Ohm resistors ?

Usually it means that you can't match the impedances while achieving the desired attenuation. There's a minimum amount of attenuation incurred when matching impedances that are not close; if your desired attenuation is less than this then some other type of matching circuit would be necessary.