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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Two analog frequency counters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology In the LF generator [1], the frequency of the output signal is set using the readings of a simple frequency meter with a dial indicator. Experience with such a generator has confirmed that it is possible to obtain sufficient frequency setting accuracy. However, in some cases, through parasitic connections, the frequency meter itself can introduce significant interference into the generator signal. After all, it can be recognized as "analogue" only with some assumptions, since high-order harmonics appear already in the input shaper of the "meander" and interference from a single vibrator is added. Therefore, most of the analog frequency meters and combinations of "analogue frequency meter with digital readout" or "digital with pointer instrument" can hardly be considered purely analog.
In an analog device with increased sensitivity, it is desirable to avoid pulse signals altogether. One of the simplest solutions is to measure the signal applied through the RC divider with an AC voltmeter. The analog frequency meter circuit is very simple (Fig. 1). The capacitance Xc of the capacitor decreases with increasing signal frequency: Xc = 1/ωС (ω= 2πF), and the voltage at the input of the voltmeter depends only on the frequency and voltage of the signal Uc. For a signal with a constant amplitude, the voltmeter reading will change in proportion to the change in its frequency. Usually, measures are always taken in the generator to stabilize the amplitude of the output signal, and there are no difficulties in determining its frequency.
On fig. 2 shows a diagram of a simple, purely analog frequency counter that does not add any interference (harmonics) to the generator signal [1]. Its feature, which is not always a disadvantage, is the frequency-dependent input impedance, which decreases in the high-frequency subrange from 20 kΩ at 10 kHz to 2 kΩ at 100 kHz. The voltmeter is made on the chip of a two-channel detector/rectifier K157DA1. The second channel is used as an output signal voltmeter in the generator. The DA1 chip provides an output voltage of at least 10 V, and the choice of microammeters is not difficult. Therefore, the diagram shows different types - those that were on sale. The variable resistor R1 and the DA2.1 chip with the OOS circuit correspond to R19 and DA5.1 of the output stage of the generator shown in the diagram in fig. 2 [1]. Its power supply is from a bipolar voltage source +/-17,5 V. In the simplest cases or small dimensions of the generator, you can get by with one microammeter, connecting it with a switch to the desired output to set the frequency or measure the output voltage of the generator. Voltmeter circuits are the same. Trimmer resistors R12 and R13 are used to compensate for the initial voltage at the output of the microcircuit and to linearize the initial section of the instrument scale. In the microammeter, the scale needs to be replaced, for which it is necessary to carefully open its case. The scale itself can be drawn very quickly using FrontDesigner 3.0. This Russified program is used in the design of front instrument panels. It belongs to the same series as the popular Layout (for PCB layout) and SPIan (for drawing circuits). For non-commercial use, it is distributed free of charge and is easy to find on the Internet. Of course, in terms of its capabilities, it is inferior to the CorelDRAW program, but it is incomparably easier and faster to master and work with it.
It turned out to be more convenient for the frequency meter to have a scale not of 100 divisions, but of 110, which greatly facilitates the fine tuning of the generator to a frequency of 1 kHz when measuring the harmonic coefficient using a millivoltmeter [2]. For an example in fig. 3 shows a sketch of the front panel with an analog frequency meter scale with automatic selection of the measurement limit. But if you need to use an analog frequency meter as a standalone device or build it, for example, into a voltmeter, then you will not be able to use the switch to select the frequency range of the generator. And since something is not always known in advance about the measured signal, it is desirable to have automatic selection of the measurement limit. On this occasion, only one article was found [3]. The frequency meter proposed there is not only complex in design, but can also create noticeable interference from pulsed signals.
If an RC divider is used in the automatic range switch, then here too it is possible to achieve significant simplification and eliminate the node with a pulse signal. The scheme of such a frequency meter is shown in Fig. 4. Here, the RC circuit must be designed to operate over a wider frequency range in order to confidently set the switching limits - "100 Hz", "1 kHz" and "10 kHz". From the output of the RC circuit, the signal is fed through a rectifier on the K157DA1 (DA1) chip to the comparators of the DA3 (LM324N) chip. The comparator thresholds are set using trimmers R30 (subrange up to 100 kHz), R32 (up to 10 kHz) and R33 (up to 1 kHz). At very low frequencies or at low signal levels, all comparators are off and the LEDs do not light up. With a signal with a frequency below 100 Hz and with a voltage of more than 50 ... 70 mV, the red HL4 LED (“100 Hz”) lights up. Supply voltage - +/-15 V.
On fig. Figure 5 shows a printed circuit board drawing for an analog automatic frequency meter. When wiring the PCB conductors, the Sprint Layout 3.0 program was used; many PCB manufacturers accept electronic drawings in this format.
The appearance of the analog frequency meter assembly is shown in the photo fig. 6. The manufactured device is set up as follows. Before tuning, it is better to unsolder one of the wires from the PA1 microammeter so as not to accidentally disable it. Trimmer resistor R28 must be set to the maximum resistance position. When adjusting, a signal from a generator with a voltage of 1 V is used. At a frequency of 100 kHz, a trimming resistor R12 sets a voltage of 8 V at output 10 of the DA2 detector. Then, at a frequency of 10 kHz, the operating threshold of the comparator DA3.1 is precisely set by resistor R30 so that the HL2 LED goes out and HL1 ("100 kHz") lights up. The type of LED does not matter. It is advisable to put the HL100 LED in the lowest frequency range ("4 Hz") in red, at a frequency of up to 1 kHz (HL3) - yellow, at a frequency of up to 10 kHz (HL2) - green. For the subrange of the highest frequencies (up to 100 kHz), you can set the blue HL1 LED. From the output of the comparator DA3.1, the control signal is fed to the electronic key VT3, which connects the resistors corresponding to the subrange in the RC divider (C11R13R14). Then, at frequencies of 1 kHz and 100 Hz, the operation thresholds of comparators DA3.2 (resistor R32) and DA3.3 (R33) are set. The DA3.4 comparator turns off the HL4 LED at very low input signal levels, as is done in the industrial INI C6-11. The threshold for its operation can be set by selecting the resistor R34. KT3102G work quite satisfactorily as electronic keys, but other silicon transistors can also be used. At the lowest frequency subrange, when all electronic switches are open, the resistance in the RC divider is determined by resistors R22, R23. At a frequency of 90 Hz, the trimmer resistor R23 sets the voltage at pin 12 of the DA2 chip to 2,5 V. When the comparator DA3.3 is triggered, the VT5 electronic key connects an additional circuit from R22, R23 in parallel with resistors R20, R21. Then, at a frequency of 900 Hz, the same voltage is set as at 90 Hz, with a tuning resistor R21. On the next subrange (up to 10 kHz), the trimming resistor R17 achieves the same voltage at a frequency of 9 kHz, and, finally, the same adjustment is carried out with a resistor R14 at a frequency of 90 kHz. Control frequencies are chosen below the maximum so that automatic range switching does not occur. Then the measuring head is connected and, with a signal with a frequency of 500 Hz, the readings of the device are accurately set with a tuning resistor R28. Their compliance is checked at a frequency of 200 Hz and, if necessary, a correction is made with a tuning resistor R18. Next, you need to check the accuracy of the scale in all ranges. A signal with a constant voltage must be applied to the input of the frequency meter "A" (to capacitors C10 and C11), since a change in voltage at the input of the frequency meter introduces a proportional error in its readings. Without automatic gain control, it will not be possible to do here. The voltmeter [2] already has a very good auto-regulator (ARUR) for automatic calibration of the non-linear distortion meter, in other cases it is necessary to install an inertial ARUR at the input. There is no need to achieve very small distortions introduced by the autoregulator or certain dynamic characteristics [4], but the level stabilization characteristic should be horizontal over the entire range of measured voltages. In the one shown in Fig. 4 of the low-frequency frequency meter circuit (up to 100 kHz), a simple auto-regulator at the input ensures the accuracy of the parameter readings sufficient for amateur radio practice at a signal voltage in the range of 0,1 ... 10 V.
It is very convenient to set up such devices using digital oscilloscopes, the advantages of which are known. HAMs used to be put off by high prices, but now there are relatively inexpensive digital storage oscilloscopes on the market. So, a two-channel oscilloscope PDS 5022S (up to 20 MHz) from Owon with a large color display (7,8 inches) or a similar ASK-2525 oscilloscope are cheaper than the well-known single-channel C1-94. Of course, these mentioned devices are not available to everyone, but with the help of such an oscilloscope, setting up some devices, such as a frequency counter, turns into a pleasure, especially since the reading of both the frequency and the signal amplitude is immediately visible. Four oscillograms can be saved and called up on a monitor as needed or recorded on a computer. Literature 1. Kuznetsov E. Low-frequency measuring generator with an analog frequency meter. - Radio, 2008, No. 1, p. 19-21.
Author: E. Kuznetsov, Moscow; Publication: radioradar.net
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Comments on the article: Michael A very helpful article. What is very valuable, there is a detailed description.
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