CVI.IHKATION AND TKSTING 



175 



A SCOPE SENSITIVITY-LOW 



o 

 o 



200 400 600 



RECEIVER INPUT IN (IM 



Figure 74. Typical receiver characteristics. 



600 



1000 



in this work is the lack of absolute measures of power 

 output, but much may be done with echo boxes and 

 field intensity meters. Reference is made to service 

 publications and instruction manuals for further 

 details. 



Signal Measurements 



Several methods are used for recording signal 

 strengths, and these determine the type of receiver 

 calibration required. Estimation of signal-to-noise 

 ratios by means of scales on the face of the scope 

 requires some means of specifying the gain setting. 

 The means used, such as height of noise, position of 

 gain dial, and so forth, should be calibrated with a 

 signal generator so that there is an assurance of 

 adequate sensitivity and a way of checking the 

 measurements. The saturation line on the scope is 

 assigned a height of 10, and the signal and noise 

 heights are read in proportion. Ratios in excess of 

 10 are usually read as 10+. This method requires 

 considerable skill on the operator's part and is 

 limited in scope. In Figure 74 is shown a calibration 

 curve on a typical "square law" receiver. In the circle 

 is represented a signal on an A scope which would 

 commonly be read as a signal-to-noise [S/N] ratio 

 of 8. Actually the ratio of receiver inputs correspond- 

 ing to the signal and noise heights is 8.5/3.25 or 2.6. 



A considerable improvement over the above 

 method may be obtained as follows. An index line 

 is drawn on the face of the A scope about an inch 



from the baseline. To measure a signal it is brought 

 to the index line by adjustment of the gain control, 

 and the gain control voltage is recorded. The gain 

 voltage required to bring the noise to the index line 

 is also noted occasionally during the test. A calibra- 

 tion curve is made using a pip signal generator or a 

 modulated signal generator connected to the receiver 

 input. The gain voltage required to bring the signal 

 to the index line is measured for various inputs. 

 Gain voltage readings on the test target and noise 

 are converted by means of the curve to equivalent 

 input voltages. Test data may be conveniently 

 plotted as decibels above noise after this conversion. 

 It should be noted that the calibration depends upon 

 the type and percentage of modulation. 



A third method involves calibration of the gain 

 control dial by comparison of permanent echoes. 

 Three lines are drawn on the scope face such as 3^, 

 1, and \}/2 in. from the baseline. The position of the 

 gain dial with the noise at J-i in. is marked db. A 

 permanent echo is selected which comes to the 1-in. 

 line at this setting. The gain dial is then turned to 

 bring this echo to the 3^-in. mark, and this position 

 of the dial is marked 6 db. Another echo is then 

 selected which is 1 in. high, and it is brought down 

 to % m - by further adjustment of the gain dial. This 

 position is marked 12 db. In this manner the gain 

 dial may be calibrated over the full range of adjust- 

 ment. It may be necessary to change the series 

 resistor on the gain potentiometer to spread the 



