274 BELL SYSTEM TECHNICAL JOURNAL 



gain control. In the following discussion those portions of the terminals 

 beyond the second frequency converter will be given major attention. 



Experimental Transmitting and Receiving Arrangements 



For the laboratory transmission studies described in the following sec- 

 tions the transmitter and receiver were located nearby and connected to- 

 gether by means of an amplitude modulator and associated with various 

 sources of noise designed to simulate quantitatively and under controlled 

 conditions the variations which would be encountered in the actual medium. 



Throughout the tests 7.42 unit start-stop signals were used unless other- 

 wise stated, and the speed was 60 words per minute (23 dots per second). 

 Their peak distortion and bias were measured on a cathode-ray tube tele- 

 graph distortion measuring set. 



An exciter of the type shown in Fig. 2 was used as a source of signals. 

 A frequency of 6.4 mc. was employed, with the radio receiver connected to 

 the exciter output through an amphtude modulator. This modulator was 

 an electronic circuit permitting amphtude modulation of a frequency-shift 

 signal to produce unequal mark and space amphtudes. This modulator 

 was also used to amphtude-modulate a single frequency for the AM portions 

 of the measurements. 



A temperature-limited diode together with a two-stage tuned ampUfier 

 was used as a source of thermal noise centered around 6.4 mc. A polar 

 relay driven by 60-cycle a-c and arranged to produce sharp polar impulses 

 from the discharge of small capacitances connected to its contacts was used 

 as a source of impulse noise. The noise level was adjusted by an attenuator 

 and mixed with the 6.4 mc. carrier of the exciter. A minimum amount of 

 wave shaping was used, so that the modulation may be considered as having 

 been essentially square-wave. 



Receiving Arrangements 



The experimental data submitted in the following discussion was obtained 

 from reception through a laboratory setup essentially like that shown in 

 Fig. 5. The radio receiver proper was a commercial type of H.F. super- 

 heterodyne. The output of the second frequency converter was in the audio- 

 frequency range, which enabled the use of various band-pass filters at "b" 

 of the type used in voice-frequency telegraph systems. The amplitude 

 limiter was effective over an imput range of —60 dbm* to above -f-20 dbm. 

 The pass-band characteristics of the radio receiver and of the several band- 

 pass filters used in position "b" are shown in Figs. 6 and 7 respectively. 

 Unless otherwise stated, the frequency shift signals were centered about 



* The symbol dbm signifies "db referred to one milliwatt". 



