BridgeD T Attenuator / Bridged-T Attenuator

Bridged-T Debilitating is another resistant debilitating design, a variation of the standard symmetrical T-pad Debilitator.

As the name suggests, the bridged-T debilitating has an additional resistance element that forms a bridged network between the two series resistances of the standard T-pad.

This additional resistant element allows the circuit to reduce the signal level with the necessary weakening without changing the characteristic impedance of the circuit, since the signal looks like a "bridge" on the T-pad network. In addition, the two series resistance of the original T-pad is always equal to input source and output load impedances. A "bridged-T debilitating" circuit, (T) is given below.

bridged t debilitating

Resistance R3 forms the bridge network through a standard T-pad debilitator. Two series of resistance R1 are selected to be equal to the weld/load line impedance. The biggest advantage of the Bridged-T debilitator over the T-pad network is that the bridged-T pad has a tendency to fit itself into the characteristic impedance of the transmission lines.

However, one disadvantage of the bridged-T debilitating circuit is that the debilitating input or welding impedance (ZS) requires it to be equal to the output or load impedance (ZL), and therefore cannot be used for impedance matching.

The design of the bridged T-debilitator is as simple as the standard T-pad debilitator. The value of the two series of resistances is equal to the characteristic impedance of the lines and therefore does not require calculation. Then, the equations given to calculate the parallel shunt resistance and additional bridging resistance of the bridged T debilitating circuit used for impedance compatibility in any desired weakening are given as follows:

bridged t debilitating

K is the impedance factor and Z is the welding/load impedance.

BridgeD T Debilitating Example

A bridged T debilitator is required to reduce the level of the 8Ω audio signal line by 4dB. Calculate the values of the necessary resistors.

bridged t debilitating

Then the resistors are equal to the R1 8Ω line impedance, the resistance is equal to R2 13.7Ω, and the bridging resistance is equal to R3 4.7Ω or the nearest preferred values.

As with the standard T-pad debilitator, as the amount of weakening required by the circuit increases, the serial bridge impedance value of R3 resistance increases, while the parallel shunt impedance value of R2 resistance decreases. This is characteristic of the T debilitating circuit with symmetric bridge used between equal impedances.

Variable Bridge-T Debilitating

When matching the characteristic impedance of the signal line, we found that a symmetrical bridgeD T attenuator can be designed to weaken a signal in a fixed amount. So far, we know that the bridged T debilitating circuit consists of four resistant elements, two of which match the characteristic impedance of the signal line and two of which we calculated for a certain amount of weakening.

However, by replacing two of the debilitating resistance elements with a potentiometer or a resistant switch, we can turn a fixed debilitating pad into a variable attenuator over a predetermined slimming range, as shown.

bridged t debilitating

For example, if we want a variable bridge-T debilitator above that will work on the 8Ω sound line with slimming that can be adjusted from -2dB to -20dB, we will need the following resistance values:

Resistance values at -2dB

bridged t debilitating

Resistance values at -20dB

bridged t debilitating

Then we can see that the maximum resistance required for a 2dB slimming is 31Ω and 72Ω at 20dB. Thus, we can replace fixed-value resistors with two pocinciometers of 100Ω each. One for VR1 and one for VR2.

However, instead of adjusting two different potentiometers one by one to find the required amount of weight loss, both potentiometers can be replaced with a single 100Ω "double adjustment" potentiometer, which is mechanically connected, so that each resistance varies inversely according to the value. the pocinciometer is set from 2dB to 20dB as shown below.

bridged t debilitating

With careful calibration of dual-set potentiometers, we can easily produce the fully adjustable bridgeD T debilitator in the range of 2dB to 20dB in our simple example. By changing the values of potentiometers to match the characteristic impedance of the signal line, theoretically any amount of variable weakening is possible for both VR1a and VR1b using the entire resistance range from zero to infinity, but in reality 30dB is the limit for the single-variable bridgeD T debilitator as the approximate resistance values shrink.

By taking this idea a step further, we can produce a gradual bridgeD T debilitating circuit by replacing potentiometers with fixed-value resistors and adjusted rotary switches, push switches or push button switches, and switching them at appropriate resistance. steps can be increased or reduced. For example, using our example of the 8Ω transmission line impedance above.

We can calculate individual bridge resistors and parallel shunt resistors for a weakening between 2dB and 20dB. But as before, to save time, we can produce tables for the serial bridge and parallel shunt impedance values required to create a T debilitating circuit with 8Ω, 50Ω or 75Ω switchable bridge. The calculated values of bridging resistance R2 and parallel shunt resistance R3 are given below.

Foam-T Debilitating Resistance Values

dB LossK value8Ω Impedance50Ω Impedance75Ω Impedance
R2R3R2R3R2R3
2.01.258930.9Ω2.1Ω193.1Ω12.9Ω289.7Ω19.4Ω
4.01.584913.7Ω4.7Ω85.5Ω29.2Ω128.2Ω43.9Ω
6.01.99538.0Ω8.0Ω50.2Ω49.8Ω75.4Ω74.6Ω
8.02.51195.3Ω12.1Ω33.1Ω75.6Ω49.6Ω113.4Ω
10.03.16233.7Ω17.3Ω23.1Ω108.1Ω34.7Ω162.2Ω
12.03.98112.7Ω23.8Ω16.8Ω149.1Ω25.2Ω223.6Ω
16.06.30961.5Ω42.5Ω9.4Ω265.5Ω14.1Ω398.2Ω
20.010.000.9Ω72.0Ω5.6Ω450.0Ω8.3Ω675.0Ω
bridged t debilitating
Interchangeable Bridge-T Debilitating

Therefore, the total resistance for bridging resistance set by VR1a at -10dB is equal to the sum of individual resistors, as given as follows:

5.2 + 4.1 + 3.3 + 2.6 + 2.1 = 17.3Ω

Similarly, the total resistance at -10dB for parallel shunt resistance set by VR1b will equal:

1.0 + 1.2 + 0.6 + 0.9 = 3.7Ω

Note that these resistance values of VR1a = 17.3Ω and VR1b = 3.7Ω both correspond to the -10dB weakening we calculated in the table above.

We found that the Bridged-T debilitator is a fully resistant fixed type symmetrical debilitator that can be used to provide a certain amount of debilitating loss when placed between equal impedances with the Bridged-T design, an improved version of the more common T-.

In some ways, we can also think of the bridged-T debilitator as a modified Pi-pad debilitator that we will examine in the next lesson. One of the main drawbacks of this type of circuit is that due to the bridging resistance, this type of debilitating circuit cannot be used for the pairing of unequal impedances.

The bridged-T debilitating design facilitates the calculation of the resistances required for the network, since the values of the two series of resistances are always equal to the characteristic impedance of the transmission line, which makes the debilitating symmetrical. Once the desired amount of weight loss is determined, the mathematics for calculating the remaining resistance values is quite simple.

In addition, this type of debilitating design ensures that the bridged-T pad is adjustable by replacing only two of the resistant elements for potentiometers or switched resistors, just as the standard T-pad debilitator will need three.

In the next lesson on debilitators, we will look at a different debilitating design called Pi-pad Debilitator, which uses only three resistant components to create a passive debilitating circuit, one in the series or even two in the parallel shunt line.