Checking Bridged Tee R values

[jeffbro]

There is no problem finding calculators to give the required resistor values for all kinds of pads and attenuators. They are everywhere.

But, after having to forego exact values and choose parts that are actually available, how does one calculate the resulting actual attenuation in a bridged tee? Trying to search that question just leads to more resistor calculators!

 

[jeffbro]

There is no problem finding calculators to give the required resistor values for all kinds of pads and attenuators. They are everywhere.

But, after having to forego exact values and choose parts that are actually available, how does one calculate the resulting actual attenuation in a bridged tee? Trying to search that question just leads to more resistor calculators!

Some progress. I found a derivation of the transfer function of a bridged tee attenuator:

Vout Z1Z4 + Z2Z3 + Z2Z4 + Z3Z4
--- = -----------------------------------------
Vin Z1Z2 + Z1Z4 +Z2Z3 + Z2Z4 + Z3Z4

where Z1 is the 'top' resistor, Z2 and Z3 are the series resistors, and Z4 is the shunt resistor.

Sadly it is incomplete for my purposes. When you plug in the values for a 6dB attenuator in a 600ohm system (all four R's = 600ohms) the result is a voltage ratio of .8, not .5 as expected. It turns out that the transfer function does not account for the 600ohm load on the attenuator. In a simulator, if you run the attenuator unloaded, a 1V input gives a .8V output. BUT if you terminate the attenuator with 600ohm then you get the expected .5V output.

Can someone with better math skills than mine modify the transfer function above to account for the terminating load?

 

[ruffrecords]

I would suggest it is probably a lot easier to simulate that it is to calculate.

Cheers

Ian

 

[Harpo]

Maybe my http://www.hausverwaltung-heger.de/al_leck_trick/Bridged-T_attenuator.xls can be of help.
The leftmost tab of this excel file is for your bridged-T attenuator. See related formulars (including Z-source/load) within the related cells.

 

[jeffbro]

Maybe my http://www.hausverwaltung-heger.de/al_leck_trick/Bridged-T_attenuator.xls can be of help.
The leftmost tab of this excel file is for your bridged-T attenuator. See related formulars (including Z-source/load) within the related cells.

Yes, thanks. I've have been using your wonderful spreadsheets since I discovered them years ago.

 

[emrr]

In practice the beauty of it is it's a 'closed system', so minor deviations don't result in significant changes. I have measured many different factory made T's, and no two types are exactly alike, most being made of precision wire wound resistances that don't drift in value. I have put closest available value R's back into dead spots of old T's with no measurable change in step value.

 

[abbey road d enfer]

I would suggest it is probably a lot easier to simulate that it is to calculate.

Cheers

Ian

+1. LTspice would answer that question in seconds.