Design of linear phase filters

The use of delay lines in producing specific delays is quite common in the design of feed forward amplifiers. In the past transmission lines have been used to introduce these delays. Whilst the transmission lines do have some good characteristics such as being very linear and being very stable over temperature extremes, they do have some drawbacks. Typically delays of 15 ns are required, this means 4.5 metres of transmission line for Er=1. Transmission lines of this length are both lossy and bulky. An alternative to the cable is a linear phase filter. Designed correctly it can exhibit the same linear phase response as a cable and have as good temperature stability, but at a fraction of the size and with lower insertion loss. This paper describes a method for designing such a filter. A typical specification is provided and step by step procedure is followed to design a filter to meet the specifications