1424 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



The search coil impulse is the same in each case, and for small errors, 

 the instrument will rise the same amount for either condition followed 

 by equal decays. The two maximum values Mi and M2 are not equal but 

 are related by: 



Ti 



Ml 

 M, 



K'+S) 



(15) 



The ratio of the two instnunent readings thus is a function of the ratio 

 of the two instrument decay time constants. For the case of Ti = 4 

 sec, T2 = 2 sec, the limiting error is 33 per cent. This shows that 

 without a damping resistor, the error can be many times the 3^ per 

 cent instrument error, but that the timing for switching the meter 

 damping is not critical. That is, small time gaps with the damper off 

 will introduce an insignificant error. In a sense, the omission of the 

 damper is equivalent to using an electronic integrator with finite in- 

 ternal gain. 



Vacuum Tube Filter Circuit^ 



The above discussion brings out the fact that visible motion of th 

 instrument pointer without a filter occurs, increasing as the cycle time i^ 

 lengthened. From equation (12) and using 7" = 0.1 sec, and Ti = 4 sec^ 

 the amplitude would be about 2}4 per cent of full scale. Reading the' 

 center of the vibration is easy for cycle times T less than 0.1 second 

 and for such measurements the filter is not used. Many measurements, 

 however, require a slow pulse rate and even a sluggish instrument will 

 follow the pulses to such an extent that an accurate reading is impossible. 



Fig. 6 is a schematic of a vacuum tube circuit which serves as a very 

 efficient low pass filter. This filter must fulfill a variety of very stringent 

 requirements, chief of which may be fisted : 



(a) Low loss to direct current. 



(b) High loss, probably exceeding 30 db to frequencies above one cycle 

 per second. 



(c) A constant resistance input. 



(d) AbiUty to suppress peaks of relatively high voltage of the order of 

 several hundred or thousand times the dc voltage being measured. 



