1214 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



signal is shunted by the capacity of the connecting cable. Since the volt- 

 age impressed on the grid of the first stage is approximately 



^2 = V^ 



Ci 



Ci + C2 ■ 



it is desirable to have Ci large compared to C2. In some types of investi- 

 gations cable whip may result in the introduction of spurious voltages 

 on the signal. These voltages are produced by capacitance changes and 

 static charges on the cable dielectric. To minimize this effect, the cable 

 may be shunted by a padding condenser at the expense of the voltage F2. 

 The National Bureau of Standards recently reported the development of 

 a low noise cable'^ that is reported to overcome the shortcomings of the 

 ordinary shielded cables. Cables with a sufficiently low noise figure have 

 also become available commercially. 



A timing and calibration voltage can be impressed across the 

 low valued resistor R for comparison with the disturbance signal. If 

 resonant frequency signals of the accelerometers are excited by the dis- 

 turbances being investigated, or the recorded frequency range is to be 

 limited, the signal is fed through low pass filters before being amplified. 

 This prevents possible overloading of the amplifier by the unwanted 

 frequencies, so that maximum gain can be realized for the desired signal. 



Fig. 7 — Schematic of accelerometer and associated recording circuit. 



A — Equivalent circuit of acceler- 

 ometer 

 B — Cathode follower stage 

 LP — Low pass filters 

 Ci — Internal capacity of acceler- 

 ometer 

 C2 — Cable capacity 

 C| — Padding condenser 



D — Voltage amplifier 



E — Recording instrument 



F — Audio oscillator 



— Voltmeter 



R — Calibrating resistor 



Vi — • Voltage source 



Vi — Voltage impressed on first stage 



