Delayed sweep in an oscilloscope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The author of the article continues the topic of improving the accuracy of oscillographic measurements that he touched upon earlier. The simple device he recommends allows you to improve a homemade or simple industrial oscilloscope to a level that only oscilloscopes with a signal delay device or a digital sweep can provide.

In the vertical deflection channel of the oscilloscope, the time delay of the signal under study is carried out, which is necessary to observe its initial section. This is usually achieved by a delay line (DL). A radio amateur who decides to introduce a delay into his oscilloscope may experience difficulties: it is practically very difficult to independently calculate and manufacture a LZ with the necessary parameters.

It would be possible to use an LZ of industrial production, but on sale, as a rule, there are no suitable ones for a broadband oscilloscope. In particular, LZs with lumped parameters, despite their considerable diversity, are still unsuitable for operation in a wide band: they have a long rise time at the output [1]. LZ with distributed parameters, made of special delay cables, have better parameters [2], but they are too bulky. So, the LZ of the C1-79 broadband oscilloscope has dimensions of 160x180x30 mm and a weight of 600 g, which is usually a bit much for a small-sized amateur oscilloscope. In addition, it is also quite difficult to manufacture and configure such a LZ. True, for industrial models of oscilloscopes, microelectronics methods [1, 3] produce modern high-quality small-sized DLs, but it is impossible to purchase them in stores.

Yet the situation is not so hopeless. For periodically repeating signals used by radio amateurs when measuring parameters, with the help of a delayed sweep, the problem is completely solvable even without LZ.

Assume, for simplicity, that we are examining a sequence of pulses. You can delay not the pulse under study, but the time that this pulse triggers the sweep generator. The start time is chosen so that the beginning of the next pulse falls on the sweep section visible on the screen. By changing the duration of the trigger delay, it is possible to move the image of the studied signal on the oscilloscope screen and examine in detail any of its details. And since the duration of the linearly varying voltage pulses (LIN) can also be changed, this detail is examined, as it were, under a microscope with magnification, i.e., with a large stretch in time. No LZ will provide such an opportunity. Of course, this does not mean that a delayed sweep oscilloscope does not need it. It's better to install it. This will expand the capabilities of the oscilloscope. It is only desirable that the delay line can be turned off when it is not needed, since any LZ introduces distortions.

The delayed sweep device contains two single vibrators, the pulse duration of which can be changed independently of each other, an RS flip-flop, a Schmitt trigger (TS) and a LIN shaper. The schematic diagram of the sweep generator is relatively simple (Fig. 1). In the absence of synchronization pulses, the generator operates in a self-oscillating mode. After turning on the supply voltage at output 6 of the RS-flip-flop DD1.1, DD1.2, and hence at the input A of the one-shot DD2.1 (OB1), the log level is set. 1, at the output Q - log 0. At the output Q of the single vibrator DD2.2 (OB2), the log level also operates. 0. Consequently, the diodes VD2, VD3 and the switching transistor VT2 are closed, while the capacitor Cτ is charged by the current flowing through the resistor Rτ, i.e., the formation of LIN begins. When the voltage at the junction point of resistors R12 and R13 reaches the trigger level TSh DD1.3, DD1.4, it switches and a log appears at its output 11. 1, which is transmitted to the input B DD2.2. The OB is triggered, 1 appears at its output Q, the diode VD2 and the transistor VT2 open, the capacitor Cτ is discharged and the formation of LIN stops. TS returns to its original state. At the end of the OB2 pulse, the duration of which is ti = 0.45C7R8, the transistor VT2 closes and the formation of a new LIN pulse begins. The level difference from 1 to 0 at output 8 DD1.3, fed to input 5 of the RS flip-flop, cannot change its state and disrupt the self-oscillating process, since the log level has been set at input 4 since the power was turned on. 0.

(click to enlarge)

With the arrival of the synchronization pulse, since the moment of its arrival is random, two situations are possible.

Let's assume that the synchronization pulse came during the formation of the LIN. It is inverted and amplified by the transistor VT1 and goes to the input 2 of the RS flip-flop, which switches, and at its pin 6 and at the input A DD2.1 the voltage level drops from the log. 1 to 0. The output Q DD2.1 is set to a single level voltage. This voltage through the diode VD3 opens the transistor VT2 and stops the formation of the LIN pulse. Clock pulses arriving later do not change the state of the active elements of the circuit, since they come to the same input 2 of the RS flip-flop. The countdown of the delay time for the start of the formation of LIN begins. The delay time is equal to the pulse duration at the output Q DD2.1, determined by the time constant (R6 + R7) C, where C - C4 - C6. The state of OV2 does not affect the base circuit of the transistor VT2 and does not load the 0V1 output, since it is separated from them by a closed diode VD2.

At the end of the delay pulse, transistor VT2 closes and the formation of LIN begins. When it ends, the TS is triggered, the pulse from its output 8 is fed to the input 5 of the RS flip-flop and returns it to its original state. The generator is ready to receive a new synchronization pulse. Diagrams of voltages at the points of the circuit for this case are shown in fig. 2. All voltages except Usync correspond to TTL levels.

In the case when the sync pulse arrives at the input of the generator at the moment of a pause between LIN pulses, OB1 is in the process of generating a pulse with a log level. 1 at output Q. A pulse from pin 6 RS of the flip-flop restarts OB1. Subsequent sync pulses cannot restart OB1 because its input is blocked by the RS flip-flop triggered on the first sync pulse. The pulse from the inverse output DD2.1 stops the action at the output Q DD2.2 of the pulse, which, through the diode VD2, kept the transistor VT2 open. But the transistor does not close, since a pulse came to it from the Q DD3 output through the VD2.1 diode a little earlier. With this pulse, the VD2 diode closes. Thus, the diodes VD2 and VD3 eliminate the influence of single vibrators on each other. Transistor VT2 continues to remain open, but from that moment on, the delay time for starting the LIN shaper is already counting, determined by the duration of the pulse at the output of OB1 after restarting. Then everything happens as in the first case. The work of the LIN shaper is not considered here.

The sweep delay range is divided into three subranges. When repeated, radio amateurs can choose them as they wish. On fig. Figure 3 shows the dependence of the delay time on the angle of rotation of the resistor R6 slider for the capacitance values ​​of capacitors C4 - C6 shown in the figure. Capacitor C3 is the sum of the capacitances of the chip and mounting. In this position of SA1 and the lower position of the slider of the resistor R6, the generator operates virtually without delay, since the duration of the OB1 pulse does not exceed a few hundredths of a microsecond. If this capacitance is not enough, you can add an external capacitor 5...10 pF.

On fig. 1, the sweep duration subband switch SA2 is not shown. It is performed similarly to the sweep time switch shown in [4, Fig. 2]. The main parameters of the generator and other data necessary for repeating the device are also given there.

Elements of the generator circuit are located on a printed circuit board with an MPH-14-1 connector. Switches SA1 and SA2 are placed outside the board. They are made using reed switches. A detailed description of the principles of operation and design of such switches is given in [5]. The types and values ​​of resistors and capacitors with tolerances are described in [4]. Variable resistor R6 - SPZ-9g with a functional characteristic of type B. KT316B transistors are interchangeable with KT316A or any other microwave transistors with a dissipation time of no more than 4 ns. It is permissible to replace the KT326B transistor with KT326A or KT363A, B, and the KP303A transistor with others of the KP303 series with a cut-off voltage of about 0,5 V. Instead of KD512A diodes, use KD513A or KD514A, and instead of KR1533 series microcircuits - MS series K155 and K555. The speed of the scanner in this case will decrease, but in most cases it will be sufficient; in this case, conventional high-frequency transistors and diodes are suitable.

When mounting microcircuits, it is recommended to connect free inputs to + Upit through a 1 kΩ resistor. Several inputs are connected to it [6].

Setting up the sweep generator is described in [4]. The LIN pulse amplitude should not be set to more than 5 V. When this value is exceeded, the LIN nonlinearity sharply increases, although this is not visually noticeable. To establish the linearity of the sweep "by eye" is the easiest, but not entirely logical, since the generator allows you to get a sweep with a nonlinearity not exceeding a few hundredths of a percent. To use this possibility, special methods for measuring non-linearity are needed. They are simple, but require a separate description [7].

A little about improving the operation of the sweep generator. Despite the good sweep linearity, it cannot be called a high-precision device, since the amplitude and duration of the LIN pulses depend on temperature. By itself, the LIN driver is very stable due to the use of a source follower with tracking feedback on transistors VT3 and VT4. Due to partial compensation of the instability of the field-effect and bipolar transistors and deep feedback, the parameters of this follower depend very little on temperature [8]. With thermally stable elements Ct and Rt, the angle of inclination of the LIN practically does not change.

The temperature dependence of LIN is explained by a change in the threshold of operation of the TS. The dependence of the threshold on temperature is non-linear, as with semiconductor thermistors, which makes it relatively easy to achieve good thermal compensation. The diagram of the corrective circuit is shown in fig. 4. The placement of thermistors near the microcircuit case reduced the instability of the amplitude and duration of the LIN pulses from temperature by more than 10 times, in the temperature range of 20 ... 50 ° C it does not exceed 0,7%. In the correction circuit, the MMT-1 resistor is used, which has a resistance of 20 Ohm at T=1660°C. Resistors R4 and R5 - C2-29 with a power of 0,125 W with a deviation from the nominal value of not more than + 0,25%.

After the correction is introduced, the LIN amplitude increases by 0,8 V, but there is no need to strive to restore the previous amplitude: this can lead to a violation of thermal correction. It is easier to change the gain of the horizontal deflection amplifier.

Unlike dual-sweep oscilloscopes, which have two LIN generators and two types of synchronization, the delayed-sweep block contains only one synchronized LIN generator. This generator is easier to work with. In addition to the normal manipulation of the oscilloscope's controls, most often only the "Sweep Delay" knob (R6) and, in rare cases, the sub-range select switch (SA1) have to be used.

Most measurements made by a dual sweep oscilloscope can be made with an instrument equipped with the proposed delayed sweep. The exception is the "B highlight. A" mode: in this position of the "Sweep type" switch, the area that is to be examined with magnification is highlighted. But the procedure here is quite complicated, and there is no special need for illumination, since the desired area can be found without it.

The fundamental similarity between the two devices under consideration is that the sweep synchronization is carried out not by the signal that is visible on the screen, but by another. This makes it possible to view pulse edges and signals whose amplitude is not sufficient to trigger triggering.

It is hardly advisable to use the generator in a simple cheap oscilloscope, since its high accuracy is not realized in this case. Of course, this is a matter of taste and user capabilities, but it is better to complement them with a good accurate oscilloscope that does not have a delayed sweep. It can also be made as a separate self-powered unit. Then the output of the generator is connected to the input "X" of the oscilloscope. The generator is synchronized both by an external signal and by clock pulses from one of the vertical deflection channels, the outputs of which are available in each oscilloscope. You can also use the sawtooth voltage output of the oscilloscope for this. Then, in the console, you will have to install a synchronization type switch and a voltage divider, if necessary.

Literature

1. Baev E. F., Burylin E. I. Miniature electrical delay lines. - M.: Sov. Radio, 1977.
2. Blyudin E.K., Bodnar E.M. et al. Portable oscilloscopes. - M.: Sov. Radio, 1978.
3. Vishnevsky VN et al. Portable oscilloscopes with digital measurements of signal parameters. - M.: Energoatom from dates, 1991.
4. Dorofeea M. Oscilloscope sweep generator. - Radio, 1996, No. 11, p. 32-34.
5. Khazarov K. I. Switches with magnetically controlled contacts. - M.: Energy, 1978.
6. Digital and analog integrated circuits. Directory. Ed. S. V. Yakubovsky. - M.: Radio and communication, 1990.
7. Dorofeev M. Measuring sweep nonlinearity. - Radio, 1998, No. 7, p. 28.
8. Sevin L. Field-effect transistors. - M.: Mir, 1968.

Author: M.Dorofeev, Moscow

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