By: Don McClatchie

          Low pass filters are useful to reduce noise for analog RF signals and as anti-aliasing for analog to digital equipment designs. When used for analog noise reduction designs the cut-off frequency is set just above the highest useful frequency in the program content. The same is true for analog to digital low pass filter designs when used as an anti-aliasing filters. This table will give you an easy way to build balanced and un-balanced low pass filters and you will be able to see the relationship between the capacitor and inductor values.

          The un-balanced table below shows input and output resistance values for the R, C, and L components. This table has component data for 5th order (5 active element) filters with a 0.1dB in band ripple. Use inductors with a high Q or (low internal resistance) for maximum rejection of out of band signals. When using an RF generator, the R1 resistor is not used and should be Zero because it is built into the generator output impedance. Also, the termination resistance R2 is also not used when the signal is used with any measurement device that has an internal or external 50 Ohm impedance. This table is used for RF filters with the input and output impedance of 50 Ohms. As long as the input and output impedance is 50 Ohms you can use this table for any of the cut off frequencies listed.

Most RF test equipment have 50 Ohm output and input impedance.

          Use the table below to fill in the values C1, L2, C3, L4, and C5 for the Un-balanced LPF filter schematic above. R1 and R2 are both 50 Ohms or zero if the test equipment has the source and termination resistance build in. You can mount this circuit in a metal box with two BNC video connectors and have a handy inline filter to limit the bandwidth of any RF signal to remove interference caused by out of band noise or signals.

          You will notice some extra values for L7, L9, C6, C8 and C10 used for balanced 100 Ohm LPF filters in the next schematic below. Each side of this filter is 50 Ohms making the balanced filter input and output 100 Ohm impedance. You will also notice that all the L inductors are equal in value.