32 BELL SYSTEM TECHNICAL JOURNAL 



Distortion Measurements 



One of the quantities of fundamental interest in studying class A 

 amplifiers is the amount of distortion to the applied signal generated in 

 the tube. The technique of making distortion measurements at audio 

 and carrier frequencies is well understood and presents no outstanding 

 problems. However we would not expect distortion measurements 

 made at low frequencies to have any significant application to ultra- 

 high-frequency operation. Since the input resistance of a tube at these 

 frequencies is obviously a function of the various voltages and currents 

 we should expect this input resistance to vary throughout the radio- 

 frequency cycle, that is, to be essentially nonlinear. The question of 

 whether or not this nonlinearity is of sufficient magnitude to cause 

 trouble can best be answered by making direct distortion measure- 

 ments at the ultra-high frequencies. After some consideration of the 

 various methods of measuring distortion we have chosen the two- 

 tone method as being the most promising. In this method two inde- 

 pendent frequencies suitably chosen in the transmission band of the 

 amplifier are fed into the amplifier and the amplitudes of these two 

 tones and such of their modulation products as are of interest are 

 measured in the output of the amplifier by means of a suitable voltage 

 analyzer. In the present case the "tones" are actually a pair of ultra- 

 high-frequency signals. The principal precaution which must be taken 

 in this method is to prevent the oscillators which supply the driving 

 frequencies from reacting on each other and producing distortion prod- 

 ucts ahead of the amplifier under test. In the present case we have 

 taken care of this requirement by using relatively high powered driving 

 oscillators, very well shielded, from which only very small amounts of 

 power are taken by means of very loosely coupled and electrostatically 

 screened coupling coils. The outputs of the two oscillators are still 

 further isolated from each other by connecting each across opposite 

 diagonals of a balanced capacity bridge and taking off the voltage to 

 drive the circuit under test across one arm of the bridge. A small 

 amount of the voltage developed in the output circuit of the amplifier 

 under test is picked up by a small coupling coil and fed into a voltage 

 analyzer by means of which the relative amplitudes of the testing fre- 

 quencies and their modulation products may be measured. This 

 voltage analyzer consists of a high gain superheterodyne receiver 

 having a rather sharply tuned, intermediate-frequency amplifier and an 

 extremely precise tuning arrangement on the beating oscillator. The 

 intermediate-frequency amplifier contains an attenuator which, in 

 conjunction with the second detector current meter, permits the rel- 

 ative amplitude of signals to be measured. 



