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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Narrow-band filter-detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony This article describes the use of the KT3170 DTMF receiver as a narrow-band detector of a single-tone sinusoidal signal in the audio frequency range up to 5 kHz. The device has high performance. In amateur radio and professional practice, it is often necessary to solve the problems of narrow-band filtering of low-frequency signals with their subsequent detection and digital processing to determine whether the signal belongs to a certain frequency or group of frequencies. An example of this is the DTMF signal receivers widely used in telephony (tone dialing) and radio communications (personal radio calling). Usually, to identify sinusoidal signals in telephony, telemechanics, analog filters (active or passive) are used, tuned to the desired frequencies. The selected signal is detected, fed to the comparator, from which the logical signal of the presence or absence of a tone of a given frequency is already removed. Such detectors are quite bulky and do not always meet the requirements for frequency stability with changes in temperature and supply voltage. With the advent of switched capacitor filter technology (SCT), the task of achieving high stable filter performance is greatly simplified. Many foreign companies produce various types of filters made using this technology. For example, MAXIM produces a wide range of integrated active bandpass and rejection filters, low and high pass filters with Chebyshev characteristics. Butterworth, Bessel, Gauss of different orders (from 2 to 9), for which you can program the center frequency / cutoff frequency from tenths of a hertz to 100 ... 200 kHz and the quality factor from 0,5 to 64 using jumpers or under the control of a microprocessor. Such versatility, of course, cannot but affect the price of these products. Their cost from domestic dealers is quite high, it is not always easy to purchase them, and the use of single-tone signals as a detector requires, as noted above, detection and further digital processing. In this case, it seems interesting to use the DTMF signal receiver KT3170 from SAMSUNG (analogous to MV8870 from GEC PLESSEY SEMICONDUCTOR), which has proven itself in telephony and radio systems. The domestic analogue of the KT3170 is the KR1008VZh18 microcircuit manufactured by the Minsk NPO Integral. This receiver allows 16 standard tone pairs to be decoded into a 4-bit code. Made in CMOS technology using switched capacitor bandpass filters, it has the following characteristics: type="disc">However, this receiver only decodes pairs of standard DTMF frequencies from the upper and lower frequency bands determined by the oscillator frequency (default is 3,58 MHz). and does not respond to single-tone signals. The principle of decoding a single-frequency signal is shown in the block diagram (Fig. 1).
Since the DTMF receiver decodes only pairs of frequencies, it is necessary to add an exemplary frequency F0 at its input to the single-tone signal under study with frequency Fc, which complements it to a standard pair. Consequently, a two-tone signal will be sent to the input of the DTMF receiver, which is decoded in the usual way. As a reference signal generator, it is convenient to use the TR5088 (TP5089) DTMF generator, which has a single-tone signal generation mode. Since the DTMF receiver and oscillator are synchronized from one internal crystal oscillator, standard pairs are formed automatically. Consider the schematic diagram of the device using the example of a fax signal detector (Fig. 2).
The detector must respond to the presence in the communication line of a signal with a frequency of 1100 ± 15 Hz with a duration of 0,5 s, which is transmitted by the calling fax machine when a connection for facsimile data transmission is established. The DTMF receiver DD2 is switched on according to the standard scheme. The operational amplifier built into the receiver chip is included as summing with a transfer coefficient equal to 1. The input impedance for the signal under study is determined by the resistance of the resistor R2 and is 100 kOhm. The clock frequency is stabilized by a ZQ1 quartz resonator. Clock pulses are fed to both the receiver DD2 and the generator DD1. Timing chain C5R5. connected to the ESO pin, serves to protect against possible interference, including speech, providing temporal filtering of the signal. With its help, the duration of the received signal is checked. Signals shorter than the specified duration are ignored. It also checks for a valid inter-character pause. In other words, the microcircuit will not accept DTMF signals shorter than the allowable duration and will not take into account the loss of a signal shorter than the allowable pause. With the ratings indicated on the diagram, this time is 80 ... 100 ms. The TP5088 chip from National Semiconductor is a DTMF signal generator controlled by a microcontroller. Its inputs DO - D3 (pins 9 - 12) are supplied with the binary equivalent of numbers, characters or letters (Table 1).
When the TE input (pin 2) is low, the DD1 chip is in micro-consumption mode and there is no signal at the TOUT output (pin 14). When the level at the TE input changes from low to high, the data at the inputs D0-D3 are stored in the microcircuit register, the internal generator is started (if it has its own timing circuit). In this case, the signal of the selected tone pair from the standard DTMF frequencies appears at the TOUT output and is present until a low level appears again at the TE input. Output TOUT - with an open emitter. Timing diagrams of generator operation and signal parameters are shown in fig. 3.
Capacitor C1, installed at the TE input, together with the internal resistor of the microcircuit, form a circuit for starting the generator when the supply voltage is applied. It is set if the tone decoder is used autonomously (without a microcomputer). The STE input (pin 3) controls the generation of one or a pair of tones. When it is connected to the positive terminal of the power supply, or not connected at all, a couple of tones are generated. In our case, this input is connected to a common wire to generate a single-tone signal. The signal at the GS input (pin 4) determines the generation of a single-tone signal from the upper or lower frequency group (Table 1). A low level at this input generates a signal with a frequency from the lower group, and a high (or disabled input) signal from the upper group. Now let's give a method for calculating the frequency of the master oscillator, which determines the frequency of generation of a single-tone signal and, as a result, the tuning frequency of the tone decoder. To do this, we determine the clock frequency division ratios, respectively, for each tone frequency of a standard DTMF signal using empirical formulas: k=Fn/Fg or k = Fv/Fn where Fn is the frequency from the lower group in hertz. Fв - frequency from the upper group in hertz. Fg is the frequency of the master oscillator in megahertz. The coefficients are calculated for standard DTMF frequencies, i.e., at a master oscillator frequency of 3,579545 MHz (3,58 MHz). The results of the calculation - in table. 2.
Next, for the desired tone decoder frequency of 1100 Hz, we determine the calculated frequency of the master oscillator Fr for each k using the formulas given above, and select a quartz resonator for a frequency as close as possible to the calculated one (Table 2, column 4). In this case, this is the frequency of the common resonator 4.608 MHz. Based on this, we calculate the frequencies using the same formula (Table 2, column 5). As can be seen from Table. 2, the original frequency of the tone decoder 1100 Hz (calculated 1097 Hz) corresponds to the frequency Ft0 from the lower group. Now if you choose any of the upper group as an auxiliary frequency, for example, FB1=1557 Hz. and use the truth table of the DTMF receiver and generator (see Table 1), you can determine the binary code. which must be applied to the input of the DTMF generator to receive a signal with a frequency of 1557 Hz, and the code read from the outputs of the DTMF receiver. corresponding to an input signal with a frequency of 1100 Hz. The generator will generate a signal with a frequency of 1557 Hz when a binary code is applied to its inputs, corresponding to all symbols whose tonal frequencies have a frequency of Fv1, namely: "1", "4". "7", In this case, of course, a high logic level must be applied to the GS input of the DDI chip. The diagram (see Fig. 2) shows the submission of the code corresponding to the number "1". The code at the output of the DTMF receiver will correspond to the number "7" (tone frequencies F&3 and F&1). It is quite obvious that up to four single-tone signals can be determined with one receiver. In our example, these are signals with frequencies of 899, 991, 1097 (our fax signal) and 1212 Hz. These four signals are identified by the code read from the DD2 outputs in the presence of a strobe signal at the DSO output (pin 15). which appears each time the receiver detects one of the specified frequencies. If it is known for certain that only one frequency can be present in the channel, it is permissible to use just the DSO output as the output of the tone decoder. It should be noted here that the digital signal processing algorithm provides protection against the reception of randomly matching signals, in particular speech, and also in the presence of more than two signal frequencies. This feature should be taken into account. For stand-alone devices, i.e. not controlled by a microcontroller or computer. the TP5089 chip can also be used as a generator. having inputs for connecting a 4x4 matrix keyboard. Closing the corresponding conclusions of the columns and rows between themselves or to a common wire, they achieve the generation of a single-tone signal of the required frequency. Options for constructing decoding nodes are shown in fig. four.
Since the data at the output of the receiver DD2 is entered into a latch and stored in it after the action of the DSO signal, the decoders must be gated by the DSO signal. The maximum frequency of the master oscillator, at which these microcircuits work stably, is 9-10 MHz. Therefore, the maximum frequency detected by the receiver lies within 4100 ..4560 Hz. Author: O.Potapenko, Rostov-on-Don
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