GRID CURRENT MODULATION 129 



The influence of the input transformer upon input impedance 

 depends not only upon first order, but also upon higher order effects. 

 The first order effect is simply due to the transformer terminated in a 

 network having a linear current-voltage characteristic, which may be 

 calculated from the usual transformer theory. The higher order effect 

 is produced by the effect of the contributions to fundamental fre- 

 quencies caused by the flow of modulation currents, as discussed in 

 connection with Equations 13a and 4. For this reason the impedance 

 of the external grid circuit at other than input frequencies may have a 

 considerable effect on the input impedance. It has been found 

 possible to reduce the reflection from a resistance line to a small value 

 with suitable transformers. 



Outpiit Filters and Transformers 



The general effect of an output filter or retard coil in the plate 

 circuit with high impedance to all frequencies except the sideband is to 

 increase the output level for large inputs, since the opposing effect of 

 plate modulation is eliminated and the total load capacity of the tube 

 is employed solely in the amplification of the sideband. The output 

 transformer on account of its low ratio has very little effect in altering 

 the impedance-frequency characteristic of the output filter so that we 

 need not enter so thoroughly into the details as we did in the case of 

 input filters. 



Output Impedance 



The output impedance of a grid current modulator (looking from the 

 line into the output coil) is affected mostly by the transformer ratio and 

 the impedance to carrier in the plate circuit. If the impedance to the 

 carrier frequency is very high, as is usually the case, there will be very 

 little modulation with the carrier in the plate circuit, and neither the 

 carrier input current nor the external output impedance at signal 

 frequencies affects the output impedance appreciably. The reflection 

 may be made quite small over the frequency range without any great 

 difficulty. 



Gain- Frequency Characteristic 



The problem of obtaining a flat frequency-gain characteristic over 

 the voice range depends upon the attenuation of input and output 

 transformers, the attenuation of filters, and the impedance charac- 

 teristic of input and output coils when terminated by their respective 

 filters. The transformer attenuation is comparatively small and 

 affects the frequency characteristic mostly at frequencies below 200 

 cycles. The closer the carrier channels are spaced to each other or to 

 the voice band, the more difficult it becomes to obtain filters with suf- 

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