MKASIR/XG SVSrK\f FOR VIDEO 229 



TiiE Modulators 



The difficulties of precise measurement over a wide frequency band es- 

 sentially are concentrated in the modulator. With the precision to which 

 measurement must be made, effects ordinarily of small concern assume im- 

 portance. The following discussion is valid for any modulator, though the 

 specific example of the vacuum tube is used. 



It is the function of the modulator to convert linearly changes in ampli- 

 tude and phase from the input frequency F to the output frequency 31 kc. 

 The linear range of conversion is limited by overload at the high-level limit 

 and by noise at the low-level limit. 



Let the input x to 'a modulator consist of two frequencies Fi and F-i. 

 In the ideal square law modulator^ perfect linearity results between changes 

 in the input signal Fi and the output signal F2 — Fi . The output filter 

 rejects all frequencies but F2 — Fi . 



In actual tubes the plate current is 



(1) /p = Co + fll-V + fls-V- + flS-V + OiX -\- •■•. 



The effect of the term OiX^ and higher even-order terms is to contribute 

 output currents of frequency Fo — Fi which do not vary linearly with the 

 input. ^ In addition to this the effect of remodulation in plate, screen and 

 suppressor circuits is that the coefficients a-i , a^ etc. are not independent of 

 the input x and so contribute to the distortion. Further, in presence of 

 modulation of higher than second order, thed-c. term in even-order modula- 

 tion will cause distortion if cathode bias is used. Removal of d-c. degenera- 

 tion using fixed bias eliminates this effect. 



The high-level limit may be defined as the signal value for which the total 

 error due to overload equals the desired limits of modulator performance. 



The lowest input level into the modulator which may be tolerated, and 

 hence the lower limit of loss which can be measured, is determined by the 

 effective signal-to-noise ratio at the modulator output. If no amplification 

 exists preceding the modulator the input grid noise is usually limiting. The 

 signal-to-noise ratio of the signal Fi and a noise band centered on Fi is 

 unaffected by the modulation process as only the modulated portion of the 

 noise band passes through the output filter. Yet for a noise band centered 

 on the intermediate frequency F2 — Fi for which the output filter is trans- 

 parent the modulator acts as a straight amplifier; hence the effective signal- 

 to-noise ratio is degraded approximately by the ratio of amplifier gain to 

 conversion gain of the modulator. 



The low-level limit may be defined as the signal value for which the 

 error due to noise equals the desired modulator performance limit. For 

 example for a noise error of 0.01 db, a signal-to-noise ratio of 1000 to 1 or 

 60 db is required. 



