SIIORT-IIAri, CARRIKR TKLKGRAPII ()73 



The low pass filter between the detector and last stage serves to 

 remove carrier ripple and to decrease the effects of noise and other inter- 

 ference having demodulated frequency components greater than about 

 40 cps, which is slightly higher than the "dotting" frequency of 100- 

 word per minute signals. In order to prevent a change in the tuning of 

 (he discriminator when the reversing switch is operated, a balanced 

 low-pass filter structure without mutual inductance is employed. This 

 presents high and nearly equal impedances to ground from the positive 

 and negative sides of the detector circuit. 



Nearly all the \'oltage gain of the receiver appears ahead of the 

 tletector. Since the detector output voltage applied to the grids of the 

 beam power tetrodes is high enough to give an approximately square 

 signal wave shape in the loop, no intermediate stage of dc amplification 

 is needed following the detector. For unbiased signal reception, the de- 

 modulated signals should be centered on the grid characteristic of the 

 receiving tubes; that is, the marking and spacing voltages applied to 

 the grid circuit should be s^Tnmetrical about a potential a few volts 

 negative with respect to the receiving tube cathodes. To obvdate the need 

 for a voltage source negative with respect to the cathodes, the signals 

 are prebiased by unbalancing the detector so that the mean of the mark 

 and space output voltages from the low pass filter is about —5 volts. 

 Further adjustment of the mean signal value may be made by means of 

 the REC BIAS potentiometer to compensate for bias of signals received 

 from the line due to deviations in the mark and space frequencies from 

 their theoretical values or to other causes originating at the sending 

 terminal of the telegraph circuit as well as for bias due to discrepancies 

 in the discriminator network or to differences between mark and space 

 levels. 



These arrangements permit great freedom in the assignment of loop 

 battery voltages. The cathodes of the final stage may be fixed at — 130- 

 volt, — 48-volt or ground potential and the plates operated from ground, 

 -|-48-volt or +130-volt potential. The remainder of the circuit may be 

 powered by +130- volt battery for the plates and — 24-volt battery for 

 the heaters of the tubes, regardless of the loop conditions. 



By means of the reversing switch mentioned above, current maj^ be 

 caused to flow in the loop during the reception of the higher or the lower 

 frequency. Thus not only can various frequency allocations be accom- 

 modated, but the local circuit may be operated neutral (current for mark) 

 or inverse neutral (no current for mark). 



One tube is used in the final stage for 20 ma or 30 ma loop current, 

 and two for 60 ma loop current. 



