Linearly Varying Voltage Generator for Analog-to-Digital Converters
Abstract Generated abstract
This paper proposes a tube based linearly varying voltage generator intended for analog to digital converters and related pulse applications requiring highly linear, high amplitude ramp voltages. The circuit combines positive feedback with an independent compensation method that controls the resistance of the capacitor charging element through a nonlinear pentode circuit, allowing the fall in charging current to be offset. The authors derive conditions for selecting the compensation capacitance and estimate the resulting nonlinearity coefficient, then compare the design with a combined feedback generator under similar parameters. For a 100 microsecond, 120 V output pulse at 50 Hz, calculations predict very low nonlinearity and measurements indicate a value below 0.05 percent, while the design also permits reduced recovery time and adjustable overcompensation when needed.
Full Text
LINEARLY VARYING VOLTAGE GENERATOR FOR ANALOG-TO-DIGITAL CONVERTERS
V. P. BONDARENKO, V. A. BONDAR, G. I. PEREDELSKII
(Presented by the scientific seminar of the Department of Computer Engineering)
The use of LVV generators in analog-to-digital converters, as well as in measuring equipment, radar, and radio navigation in calibrated-delay devices, requires very high linearity of the output pulses. Among transistor generators of linearly increasing voltage, the most highly linear are considered to be generators with positive feedback (PFB), providing a gain in the PFB circuit greater than unity, and generators with nonlinear PFB. The latter provide the smallest nonlinearity coefficient: \(\xi\)—tenths of a percent [1]. Tube generators with PFB also provide \(\xi\) values of only tenths of a percent [1]. Hundredths of a percent can be obtained [2, 3] in generators with combined feedbacks, the development of which began comparatively recently. These generators are developed mainly on transistors, which limits the production of LVV pulses of increased voltage (more than 100 V) and reduces their thermal stability. In the present work, an LVV generator on tubes with PFB is proposed, in which independent linearization of the output-voltage pulse is also used. Such a combination of linearization principles in LVV generators makes it possible to obtain high linearity of the output pulses together with a high voltage-utilization factor and increased speed in comparison with tube generators with combined feedbacks (CFB).
The idea of constructing such LVV generators arose comparatively recently [4]. The physical meaning of its implementation consists in constructing a circuit for independent control of the resistance of the charging (discharging) element of the generator capacitor. The effectiveness of applying the new method of linearization is especially appreciable, in the indicated respect, in circuits of generators with PFB.
Figure 1 presents a circuit of such an LVV generator with PFB on tubes, by analogy with which a transistor circuit can be constructed. In such a generator, an increase in \(\xi\) due to the input resistance of the emitter follower can be compensated by the circuit of independent control of the resistance of the charging element.
The circuit in Fig. 1 differs from known circuits with PFB by the presence, in the charging circuit of the shaping capacitor, of a controlled nonlinear element—a pentode. A passive independent-control circuit is included in the discharge circuit.
In the initial state, the cathode current of the pentode flows through \(R_k\) and the discharge triode \(L_{1a}\). The capacitance \(C_k\) is charged up to the voltage drop across \(R_k\). With the arrival of the blocking pulse at \(L_{1a}\), the latter closes and the capacitor \(C\) begins to charge. The voltage at the cathodes of diodes \(L_{3a}\) and \(L_{3b}\) rises rapidly, and they close, after which the screen-grid current is closed through the capacitor \(C_e\), while the capacitor \(C_0\), in turn, becomes the supply source for the anode circuit of \(L_2\) for the entire time during which the LVV pulse is being formed. Next, the capacitance \(C_k\) discharges, so that the negative voltage on the pentode grid continuously decreases. It is necessary to choose the magnitude of the voltage change on \(C_k\) so that the corresponding change in the voltage on the pentode grid completely compensates the drop in the anode current charging the capacitance \(C\). This drop occurs because of the nonideality of the transfer coefficient of the positive-feedback circuit,
\[ \gamma = 1 - K + C/C_0 \]
and the finiteness of the internal negative feedback in the pentode itself,
\[ \beta \approx R_i/R_{\text{st.o}}. \]
The nonlinearity coefficient determining this drop can be found as
\[ \xi_0 = \gamma \varepsilon/\beta, \]
where \(\varepsilon = U_m/E\) is the voltage utilization factor.
Fig. 1. Circuit of a linearly varying voltage generator with positive feedback and an independent nonlinearity-compensation circuit
The value of the required voltage step \(\Delta U_k\) on the pentode grid can be determined as follows:
\[ \Delta U_k = \xi I_{a0}/S_0, \]
where \(I_{a0}\) is the initial charging current of \(C\).
When this condition is satisfied, which determines the choice of the value of \(C_k\) for a given \(R_k\), depending on \(I_{a0}\), the resulting value of the coefficient is found as
\[ \xi \simeq \xi_k \xi_0, \]
where \(\xi_k\) is the coefficient of nonlinearity of the voltage on \(C_k\) during the LVV pulse.
For the indicated circuit parameters and an LVV pulse duration \(T_i = 100\ \mu\text{s}\), at a repetition frequency \(f_p = 50\ \text{Hz}\) and an output-pulse amplitude of \(120\ \text{V}\), the calculated value is \(\xi = 6.34 \cdot 10^{-4}\%\). Measurement with an S1-19B oscilloscope by the method of N. Hartun [5] showed that, for the circuit under test, \(\xi < 0.05\%\). The calculated value of \(\xi\) for the circuit with combined feedbacks based on that shown in Fig. 1 (the capacitance \(C\) is connected in parallel with \(L_{1a}\), and \(C_k\) is absent), with the same parameters, is \(0.1\%\). The enormous margin in \(\xi\) for the circuit of such an LVV generator with positive feedback makes it possible, for some applications, to reduce considerably the recovery time of the circuit. From the generator circuit it is seen that \(T_z\) is determined by the values of the capacitances \(C_e\) and \(C_k\), i.e.,
\[ T_z \simeq 3\left[C R_{\text{st.1}} + R_k(C_k + C_e)\right], \]
where \(R_{\text{st.1}}\) is the resistance of the open triode \(L_{1a}\).
The parameters of the trigger pulses are: amplitude 30 V, duration \(T_{\mathrm{i}} = 100\ \mu\text{s}\), \(f_{\mathrm{p}} = 50\ \text{Hz}\).
Similar results can be obtained by eliminating the capacitor and connecting the screen grid directly to the anode of the pentode. This is possible for tubes that allow the same supply voltage on the indicated electrodes.
Under some special conditions it is necessary to obtain a voltage with a negative nonlinearity coefficient, which corresponds to overcompensation of the value of the charging current. In the present circuit this is easily obtained by reducing the value of \(C_k\). Thus, the present LIN generator with positive feedback is distinguished by very high linearity of the output voltage at an increased voltage-utilization coefficient. The recovery time in such circuits is no worse than in LIN generators with POS. The generator parameters remain sufficiently high, since they are designed for the use of large loads.
REFERENCES
-
L. M. Goldenberg. Theory and calculation of pulse devices using semiconductor devices. Moscow, “Svyaz,” 1969.
-
V. N. Yakovlev. Pulse generators using transistors. Gostekhizdat, USSR, 1963.
-
Marosi Gilbert. Get ultralinearity from ramps. “Electron. Design,” 14, No. 28, 1966.
-
Schneider H. D. FRG Patent, class 21 ab. 36/02 (HO3k), No. 1220887, 1967.
-
Harthun N. Linearitätsmeßmethode für Sägezahngeneratoren nach dem Prinzip der Urstromquelle. “J. E. R.,” Bd. 22 (1968), No. 3.