The above variable frequency is mixed with the Radio Frequency 

 (R.F.) voltage from the fixed oscillator. The purpose of mixing 

 these two frequencies is to form a beat or intermediate frequency 

 (I.F,). Since, the I.F. is the difference between the two R.F., 

 the I.F. will have a greater change in proportion to its resting 

 frequency in number of cycles per second than the R.F. from the 

 variable oscillator, making the gage more sensitive to frequency 

 changes. 



Some of the power of the variable oscillator will be absorbed 

 by the water and will vary with the distance of the probe from the 

 water causing the output to the mixer to vary in amplitude, as well 

 as in frequency. In order to prevent the effect of amplitude change 

 from showing in the output of the gage, the voltage from the fixed 

 oscillator is made smaller than the lowest value of voltage from the 

 variable oscillator. When beating two frequencies together the 

 resulting amplitude of the I.F. frequency can be no larger than the 

 smaller of the two frequencies. One method of lowering the signal 

 strength of the fixed oscillator is to drop it with a resistor 

 network; in the circuit diagram (Figure 2) following the R.F. voltage 

 path from pin 4 of the 6SN7 , it will be found that the voltage 

 from the fixed oscillator is divided by the two resistors 220K and 

 100K and the voltage is picked off from between the resistors, thus 

 lowering the signal strength on the grid of the mixer (6SA7). 



After the signal leaves the mixer tube it must pass two 455- 

 kilocycle I.F. transformers. The one in Figure 2 marked B is used 

 as a wave trap to shape the response curve of the output. The one- 

 megohm potentiometer is used to vary the effects of the trap. The 

 secondary of the B transformer will absorb some of the power in the 

 circuit and further shape the output to make the gage linear. The 

 transformer marked C is used to transfer the signal to the 6SK7 

 I.F. amplifier. The transformer C can also be tuned to make the output 

 from the gage more linear. In the following I.F. stage, consisting 

 of transformers D and E, the same action takes place, further amplifying 

 and shaping the gage output for linearity. 



The 6H6 detector tube is coupled to the output of the I.F. 

 amplifier by transfoimer E. The signal is rectified by the diode of 



6H6 , changing the alternating voltage to a direct voltage which 

 fluctuates as the distance between probe and water changes*. 



The output of the gage is fed into a Brush amplifier in order 

 to match impedance with the Brush recorder. A vacuum tube volt meter 

 is also placed permanently across the output of the gage so that the 

 voltage output of the gage can be kept at the level yielding best 

 linearity. 



Relationships affecting the linearity of the gage response are 

 changes in R.F. frequency of the variable oscillator due to changes 



35 



