CROSSTALK AND NOISE FEATURES 



147 



when no crosstalk current flows in 3-7-8-4 due to / (the disturbing 

 current), the current /„. is 



Rs(R + Rs) + c^~Ls(L + Ls) 

 (R + R,r + o^KL + LsY 



J 



^(RsL - RLs) 



{R + i?.0- + 0,'iL + Ls)' _ 



I 



= La-jby, 



(2) 

 (3) 



where a and b are, respectively, the coefficients of the real and imaginary 

 parts of the expression. Hence, with a shunted coil the voltage induced 

 in the disturbed circuit is: 



e = — joomlw = — jwima — jmh)I. 



(4) 



The mutual impedance, Zm, equals jw{ma — jmb), or the effective 

 mutual inductance AI of the balancing coil may be written 



Af = Ah+jAh, 



(5) 



wherein Ala = ma and Alb = — mb. Assuming R, L, Ls and Rs to be 

 constant with respect to frequency of current and position of the cores, 

 it is seen from (2) and (5) that for any core setting, Ala and Afb are 

 functions of frequency only and their ratio at a given frequency is 

 theoretically constant throughout the operating range. 



To keep inductance L constant irrespective of the mutual inductance 

 settings, the length of the coil windings, the length of the magnetic 

 cores and their spacing with respect to the winding spacing are so 

 related that the change in inductance of one primary (or secondary) 



2 



3 — 

 4 



-WV-OKJ^ 



-VWvMiLr- 



— 5 

 .--6 

 .--7 



-—8 



Fig. 5 — Schematic of winding arrangement of trial balancing coil. 



winding caused by motion of its associated core is equal and opposite 

 to the change caused by the movement of the core associated with the 

 other primary (or secondary) winding. To keep R low over the type K 

 frequency range, cores of finely powdered molybdenum permalloy 

 pressed into a cylindrical form are used. 



