GRID CURRENT MODULATION 125 



necessary to consider how the required high impedance may be 

 obtained in practice. The input transformer must have an impedance 

 looking into the grid side which is high to all sideband frequencies, and 

 must at the same time transmit efihciently all signal input frequencies 

 A high impedance over the sideband range is best obtained by a 

 filter "^ on the low side of the input coil, care being taken to allow for the 

 effect of the transformer on the filter impedance. In order to deter- 

 mine the actual external grid impedance and to investigate the 

 modification of filter impedance by the input transformer, a high 

 impedance bridge was built in which precautions were taken to 

 prevent errors due to the high impedances involved (up to several 

 megohms). In each case only the end section of the filter adjacent to 

 the modulator was used, since this provided nearly the same impedance 

 as would be given by a complete filter. Low pass filters are used on 

 the input to the modulator and on the output of the demodulator, 

 while band pass filters are used on the output of the modulator and the 

 input of the demodulator. These are to be considered in turn. 



Low Pass Filters 

 The simplest type of low pass filter is the infinity type of section, the 

 impedance characteristics of which are shown in Fig. 6a. The filter 

 alone, as shown by the solid lines, has negligible reactance in the 

 transmission band (0-3 K. C.) and practically pure inductance in the 

 attenuated region. The input transformer resonates in the attenuated 

 region when terminated by this filter, as shown by the dotted lines, 

 because of the leakage inductance and distributed capacity of the 

 windings. The resonance peak is quite broad due to the comparatively 

 high a.c. resistance and so covers a considerable frequency range as is 

 shown by the ratio of Z_/(7?o + ZJ) in Fig. 6c. It may be made to 

 appear at higher frequencies by using a filter with a higher cutoff 

 frequency, and at a lower frequency by replacing the series inductance 

 by a parallel tuned circuit (an m-type section).^ A new type of filter 

 section developed for certain phases of this work and known as the 

 built-out type ^ has a particularly good impedance characteristic in the 

 attenuated region, as shown by the solid lines of Fig. 6b. But as shown 

 by the dotted lines the transformer impedance, when terminated in this 

 type of section, is very much modified by the coil constants. The 

 resulting efficiency as show by the ratio Z_/(i?o + ZJ) in Fig. 6c is not 

 as good as that of the infinity type section. 



^ For a general discussion of filter impedances and attenuations see Campbell, 

 Bell System Technical Journal, November, 1922; Zobel, January, 1923; Johnson ancl 

 Shea, January, 1925. 



* O. J. Zobel, Bell System Technical Journal, October, 1924. 



' Devised by T. E. Shea of Bell Telephone Laboratories. 



