Matching T Attenuator Calculator

Calculates the resistor values, attenuation, minimum attenuation, impedance, reflection coefficient, VSWR and return loss of an impedance matching T attenuator. This can be built into a FLEXI-BOX, and a transmission line (50 Ohm track) PCB is available which easily adapts for this circuit with two simple trimming operations. There is also a selection of 50 Ohm RF connectors available.

[Attenuator circuit]

1) Enter the required input and output impedance and the required attenuation then press "Calculate" below to obtain the ideal resistor values.

Input Impedance:	Ohms	Ideal Series In resistor:	Ohms
Output Impedance:	Ohms	Ideal Series Out resistor:	Ohms
Required Attenuation:	dB	Ideal Shunt resistor:	Ohms
		Minimum possible attenuation to achieve impedance match:	dB

2) Change the Preferred Resistor Values to those available then press "Calculate" below to obtain the resulting attenuator parameters.

Series In resistor:	Ohms	Forward Attenuation:	dB	Reverse Attenuation:	dB
Series Out resistor:	Ohms	Input Impedance:	Ohms	Output Impedance:	Ohms
Shunt resistor:	Ohms	Input VSWR:		Output VSWR:
		Input Reflection Coefficient:		Output Reflection Coefficient:
		Input Return Loss:	dB	Output Return Loss:	dB

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The required input and output impedance are used to calculate the minimum attenuation possible for correct impedance matching with a formula from RF Design Guide, Systems, Circuits and Equations by Peter Vizmuller Published by Artech House ISBN 0-89006-754-6

[Minimum loss possible to achieve matching formula]

Where Zin is the characteristic input system impedance

and Zout is the characteristic output system impedance

Peter Vizmuller also gives formulas for calculating the resistor values directly. This calculator uses formulas that I have derived and first finds the output voltage with a source EMF of 2 Volts and therefore an input PD of 1 Volt.

[Voltage out formula]

The individual ideal resistor values can now be found.

After selecting preferred values, simple series and parallel resistor calculations are used to find the attenuator input impedance, input voltage and output voltage. The source EMF is again assumed to be 2 Volts.

[Input impedance formula]

[Input Voltage formula]

[output voltage formula]

The remaining formulas for Attenuation, Voltage Reflection Coefficient and VSWR are available at:-

http://www.kathrein-scala.com/tech_bulletins/EngRef.pdf

This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX

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