Single-channel amplitude-time analyzer
Abstract Generated abstract
A compact single-channel analyzer for selecting pulses by amplitude or duration is described as a simpler alternative to versatile but bulky pulse-parameter analyzers. The instrument uses amplitude-time conversion followed by comparison of the converted pulse duration with stable reference pulses defining the discrimination threshold and channel width, with circuitry including a stabilized input amplifier, converter, Schmitt shaper, phantastron threshold generator, multivibrator channel-width generator, and coincidence circuit. The paper outlines the operating principle and gives design relations for the amplitude-time converter, including capacitor charging and constant-current discharge conditions. The completed portable instrument provides adjustable threshold timing, fixed channel widths, and about 1% measurement error, and was applied to photomultiplier amplitude-resolution studies and multivibrator duration-instability measurements.
Full Text
SINGLE-CHANNEL AMPLITUDE–TIME ANALYZER
L. M. Anan’ev, V. M. Sustavin, M. M. Shtein
(Presented by the scientific seminar of the Department of Industrial Electronics)
In radio-engineering practice and in physical experiments, various analyzers of pulse parameters are widely used. Many of the mass-produced analyzers are highly versatile and make it possible to solve a variety of problems in studying signal characteristics; however, for the most part they are complex and bulky devices.
For solving a certain range of special problems, simpler and, consequently, more reliable instruments may be used. One such instrument is the single-channel differential analyzer of pulse amplitudes and durations described below.
It is based on the principle of amplitude–time conversion and comparison, by duration, of the resulting pulse with the pulse of a reference generator. The operation of the analyzer is illustrated by the timing diagram (Fig. 1).
When investigating an amplitude spectrum or when measuring the amplitude of a pulse, the input signal (1) is converted into a rectangular pulse (2), whose duration is proportional to the amplitude of the input signal. Pulse (2) is differentiated and, by its leading edge (3a), triggers a generator that sets the discrimination threshold (4). The latter, by its trailing edge (5b), triggers a generator that determines the channel width. The trailing edge of the converted signal (3b) is fed to the coincidence circuit. Pulse (4) is adjusted in duration, thereby setting the discrimination threshold. From the diagram shown it is evident that the signal at the output of the coincidence circuit arises only in the case when the duration of the analyzed pulse \(T_{\mathrm{u}}\) satisfies the condition:
\[ T_{\mathrm{p}} < T_{\mathrm{u}} < T_{\mathrm{u}} + T_{\mathrm{k}}, \tag{1} \]
\(T_{\mathrm{p}}\) — duration of threshold discrimination;
\(T_{\mathrm{k}}\) — duration of the channel width.
The use of amplitude–time conversion of pulses for amplitude analysis makes it possible practically to eliminate the relative drift of the channel width [1].
The schematic diagram of the instrument is shown in Fig. 2. The analyzed pulses are fed to the input compensated divider (switch \(P_2\)) with positions \(1:1\), \(1:10\), \(1:100\).
Tube \(L_1\) is used for a phase-inverting stage, since the input amplifier is designed to amplify pulses of positive polarity only. As the input amplifier (\(L_2, L_3, L_4\)) a three-stage amplifying section with deep negative feedback is used to stabilize the gain.
Fig. 1. Timing diagrams of stabilizer operation
The positive-feedback circuit (\(R_{27}, C_{16}, D_1\)) transfers the output pulse from the cathode follower to the anode circuit of tube \(L_3\) in such a way that, during the action of the pulse, the screen and anode voltages increase by the amount of the pulse amplitude, thereby increasing the range of linearity of the amplitude characteristic of the amplifying section [1].
From the amplifier the signal is fed to the amplitude–time transformation circuit \(L_5\) and \(L_6\) [2].
The circuit operates as follows.
By the current of the cathode follower through diodes \(L_{5a}\) and \(L_{5b}\), capacitor \(C_{17}\) is charged to the amplitude value of the pulse. The capacitor is discharged by the anode current of tube \(L_6\) through resistor \(R_{31}\). The feedback voltage from resistor \(R_{31}\) is applied to the control grid of tube \(L_6\).
Owing to the presence of deep negative feedback, the discharge current of the capacitor is a constant quantity. This ensures linearity of the characteristic of the amplitude–time converter.
The pulses of the converter, taken from the screen grid, have a gently sloping trailing edge and therefore are applied to a shaping device constructed according to the Schmitt-discriminator circuit.
The shaped rectangular pulse is differentiated and, through the transition amplifier \(L_8\), triggers with its leading edge
Fig. 2. Circuit diagram of the analyzer.
the phantastron generator \(L_9\) and \(L_{10}\), the pulse duration of which determines the discrimination threshold.
The phantastron, in addition to the high stability of the pulses generated (instability less than 0.5% [3, 4]), which is especially important for reducing measurement error, has wide limits for adjustment of the pulse duration.
The duration of the phantastron pulse is adjusted in steps by means of capacitors \(C_{26}\) and \(C_{27}\), and smoothly by resistance \(R_{50}\), within the range \(20 \div 3000\ \mu\text{s}\). The indicating instrument \(V\) is calibrated in volts and microseconds.
The trailing edge of the phantastron pulse triggers the channel-width generator \(L_{11}\), assembled according to a multivibrator circuit with a positive grid. The channel width is determined by the value of the timing capacitance and has three fixed values: 5, 30, and \(100\ \mu\text{s}\).
The channel-width pulse is fed to the coincidence circuit \(D_2\) and \(D_3\); the differentiated trailing edge of the signal under investigation is also applied there. When condition (1) is satisfied, the signal from the coincidence circuit causes the thyratron \(L_{12}\) and the SB-1M counter to operate.
When pulses are analyzed by duration, the input signal, through switch \(P_1\), is applied directly to the time-selection circuit.
The calculation of amplitude-time converter circuits is covered insufficiently fully in the literature; therefore the method for finding the parameters of the converter circuit is of definite interest.
The capacitance of capacitor \(C_{17}\) is chosen from the condition that during a time equal to the duration of the leading edge of the input pulse it must become fully charged.
\[ \tau_{\phi} \geq 5(R_{\text{out}} + 2R_{\text{for}})C, \tag{2} \]
where \(R_{\text{out}}\) is the output resistance of the cathode follower,
\(R_{\text{for}}\) is the forward resistance of the diode.
By the end of charging, the voltage on the capacitance will become equal to the sum of the voltages at the cathode of tube \(L_4\) in the initial mode, \(U_k\), and the pulse amplitude \(U_m\).
During discharge of the capacitance we have:
\[ U_c = U_k + U_m - \frac{1}{C}\int_{0}^{t} i_c\,dt . \tag{3} \]
At the same time, the discharge current is equal to the current flowing through resistance \(R_{31}\); consequently,
\[ i_c = \frac{E_a + |U_g|}{R}, \tag{4} \]
where \(U_g\) is the potential of the right-hand plate of the capacitor, equal to the bias on the control grid of tube \(L_6\) in the operating mode.
Since for pentodes \(|U_g| \ll E_a\), the discharge current is a constant quantity.
Solving [3] and [4] jointly, and assuming that the discharge of the capacitance ends when the voltage on it again becomes equal to \(U_k\), we find:
\[ R = \frac{(E_a + |U_g|)t_u}{U_m C}. \tag{5} \]
For a known amplitude of the input pulse and a specified conversion time, the value of resistance \(R_{31}\) is determined. The coeffici-
The conversion coefficient can be changed by adjusting the voltage \(E_a\) with potentiometer \(R_{30}\).
In design, the analyzer is made as a portable instrument powered from the AC mains. The controls are mounted on the front panel: an indicating instrument and an electromechanical pulse counter, SB-1M. The dimensions of the instrument are \(510 \times 260 \times 300\); the power consumption is 80 W.
The analyzer was used to investigate the amplitude resolution of photomultipliers when operating from a pulsed light source [5] and to determine the instability of the pulse duration of multivibrators used in delay devices. The measurement error, due mainly to the instability of the conversion of amplitude into duration and to the instability of the discrimination-threshold pulse, is about 1%.
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