Butterworth Lowpass Filter  (LPF_BUTTER)

This element is used to provide filtering in the RF path. This system symbol is available on the System toolbar.

 

 

Models and Symbols

 

Name

Model

LPF_BUTTER

Alternate Models

LPF_BUTTER_C

Schematic Symbol

LPF

Alternate Schematic Symbols

None

NOTE: The alternate model "..._C" is a circuit model created from a synthesis process with real components. The circuit is not visible to the user. Simulation time for the circuit models is generally longer since there are more components.

 

Model Parameters

Parameter

Description

Units

Default Value

IL

Insertion Loss

dB

0.01

N

Filter Order

none

3

Fpass

Passband Edge Frequency

MHz

200

Apass

Attenuation at passband

dB

3.01

Amax

Max Attenuation in stopband

dB

100

Z1

Reference Impedance Port 1

Ohm

50

Z2

Reference Impedance Port 2

Ohm

50

TYPE

Input Stopband Impedance (0 = Short,  1 = Open)

none

1

 

The Butterworth filter characteristic is a maximally flat response in the passband generated by poles only. This results in no ripple in the passband, but a cutoff which is less sharp than some other filters. The insertion loss only affects the forward (S21) and backward (S12) transmission, but not the reflection coefficients (S11,S22). The input impedance in the stopband only affects the phase of the reflection coefficients. For additional details on filter types see the FILTER Synthesis Manual.

Note: The insertion loss and passband attenuation must be greater or equal to zero. The filter order must be an integer greater or equal to 1. The frequency of the passband edge must be positive.

DC Block - DC will only be blocked by the amount of attenuation at 0 Hz.

Maximum Attenuation in stopband - The maximum attenuation in the stopband is modelled with a resistor between the input and output port of the filter.  An equation is used to determine the value of the resistor from the given attenuation.  This parallel resistor will only be effective when the filter stopband impedance TYPE is set to open. For even filter orders the impedance looking into one port is high and the other low so the maximum attenuation in the stopband has no effect.