General-purpose electrometric set for scientific research
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V. V. Satarov, L. M. Garf, V. S. Grafov, V. A. Matrosov, and A. M. Shmidt
A distinctive feature of electrometric instruments is that since the currents and voltages measured by them approach the level of thermal noise, the components and devices used in their input stages are not mass produced but manufactured under highly sterile conditions and require application of high-vacuum techniques and precision testing methods. Such instruments should have the following limiting specifications: input resistance of at least i0 z6 ~ in the voltage measurement mode, current sensitivity at least i0 -z7 A, voltage sensitivity 10 -6 V, passband 0 to i00 kHz. In certain cases, the decisive parameters are accuracy, weight, size, and power consumption. However, it is impossible to design one general-purpose instrument which would combine all, or even some (e.g., maximum voltage sensitivity, high input resistance, and wide passband) such parameters at one and the same time. This is made clear by the following example. Assume that an instrument is to be designed with a sensitivity of 10 -6 V, accuracy 10%, input resistance I0 zs ~, a passband of 0-i00 kHz, and a measuring limit of 10 -4 V. Taken separately, none of these parameters is too high for modern measuring techniques: instruments are available having higher sensitivity, higher input resistance, and faster response. Nevertheless, such a combination of parameters cannot be realized in a single measuring instrument. In fact, according to [I], the energy limit of sensitivity is U2
c~?2Pt = ~ 2 ~ t. where y is the error, P is the required input power, t is the tifne constant, U is the voltage measurement limit, and R is the input resistance. For y = 0.i0, R = i0 Is ~, U = 0.i0 mV, and T = i/f = 10 -5 sec, we have c = 10 -28 J. At the same time, the limit of sensitivity of even the most sophisticated measuring instruments cannot be lower than c = 3.5 9 10 -2o J [i]. It is thus desirable to design a wide range of instruments which taken together would possess all the optimal characteristics. Workers of the All-Union Scientific-Research Institute of Scientific Instruments have designed a general-purpose set of electrometric instruments which is now being prepared for 9 ~' t! manufacture xn the I t Etalon plant in Voronezh. The set includes the following instruments: a high-sensitivity electrometer ITN-7, a wide-band electrometer EM-I, an attachment for measuring the impedance and specific resistance of solid dielectrics, and a low-current source IT-12. The ITN-7 electrometer is an amplifier in which the dc signal is converted into an ac signal by means of an EVP-8M vibrating capacitor. The voltage measurement limits of the electrometer from 3 9 10 -4 to 30 V are covered by ii ranges in 1-3-10 steps, the current force measurement range is 2 9 I0 -~6 to 3 9 I0 -~ A in the linear mode, i0 -~5 to 3 9 I0 -:~ A in the integration mode, and i0 -s2 to 10 -7 A in the logarithmic mode; the resistance measurement range is i0 e to 5 i0 s7 ~; using the PS-I attachment the range of measurement
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