M and K

In radio we use a lot of gear that even obsessed audiophiles and stereo-enthusiasts would never have need for.  Among these arcane devices are filters. (Not that there aren't filters already built into some of your devices.) Electronic filters can be broken into many categories such as: analog or digital, passive or active, linear or non-linear, infinite impulse response (IIR) or finite impulse response (FIR). They have a hundred year history and they're often over looked.

The first electronic filters were passive and analog linear which makes sense. These were made with just the early resistors and capacitors, resistors and inductors. These are known as RC and RL single-pole filters respectively. They are still in use today and they are called RC or RL filters for resistor–capacitor and resistor–inductor. (Remember L is for Lenz). Today there are hundreds of electronic filters on the market. I'll get to a post about those early filters but today let's discuss M and K filters which were more or less contemporary to each other.  

Image impedance is connected to impedance matching. It is the impedances which will simultaneously terminate all the inputs and outputs of a network in such a way that the impedances are equal in both directions. This avoids internal reflection loss and optimizes power output. Constant K-filters are the first and simplest of this type. They consist of a ladder network of identical sections of passive components. Constant K filters were invented by George Campbell at AT&T. He first published his work in 1922 but probably invented it as early as 1917. He referred to it as a "wave" filter. The big improvement with a K filter was that that they could be designed for any desired degree of stop band rejection or roll off between pass bands. You can read that here.


Otto Julius Zobel built upon this work and invented the M-type filter in 1920. You'll note that this year is before Campbell published. Zobel and Campbell worked together at AT&T so he had access to Campbells work before the general public. The different between these two image impedance filters was that the M has a sharp filter response which roll off very rapidly at the cut-off frequency. AT&T used this to squeeze the maximum number of intelligible phone calls onto one wire. Zobel later created hybrid filters that combined the strengths of the K and M filters, with had both the fast transition of the M and good stop-band rejection of the K.