— wvw- 



c?z> C| 



(T-l) 



D ■=" 



(T-2) 

 \ f 



+ V 



b 



ATMOSPHERIC 



Fig. k. Capacity discharge curve. 



currents are functions of pressure; therefore, the 

 time required for the valley current to reach the 

 switching point during the discharge cycle varies 

 and produces shifts in frequency which are not 

 linear with pressure. In order to eliminate this 

 problem the charge and discharge characteristics 

 of the capacitor must be linearized. 



The charging or discharging current must be 

 constant in order to linearize the characteristics 

 of the charging and discharging time. The gen- 

 eral expression for capacitor voltage may be 

 written as 



1 

 Cl 



4 / idt 



(5) 



Fig. 3- Tunnel diode hybrid oscillator circuit. 



change of frequency with pressure. Referring to 

 Fig. 3j the characteristic times for the hybrid 

 oscillator circuit can be written as 



where e c is voltage across the capacitor, C-]_ is 



the capacitor and i is the charging or discharging 



current. However, if i is constant, Eqn. (5) 

 reduces to 



_ •(r 1+ r g )r 3 C 1 ^ (r^r^)! N ^ 

 r n +r~+r-, c \ w ' 



r 1+ r 2 +r 3 



kt 



(6) 



r 2 r 3 c 

 r +r 



d 5 x log„ 



l 2"3 



*t = tc+td 



h 



(3) 



00 



where t is charging time in seconds, t, is dis- 

 charging time in seconds, t t is the total time 

 per cycle and f is the frequency of square wave 

 output in cps . 



The capacitor discharge characteristics shown 

 in Fig. k result in a problem due to the non- 

 linearity of the curve. The peak and valley 



where k is i/C-j_ and t is the charging or dis- 

 charging time. 



The charging and discharging periods, there- 

 fore, can be made linear with time if i is held 

 constant . This is accomplished by making r-, very 

 large and E a high voltage, producing a constant 

 current source for charging Cq_. The transistor, 

 T-2, has the base current set with P-l to produce 

 a constant discharging current. The resistor, 

 re, is provided to prevent the voltage from being 

 applied directly to the base while the base drive 

 is being set. The value of r, is selected such 

 that the current reaches the peak current, I_, 

 before the capacitor is completely charged. The 

 transistor must be saturated when the peak vol- 

 tage of the tunnel diode is applied to the base. 

 Therefore, resistor r^ must have a value such 

 that the product of the collector current of 



191 



