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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The radio receiver is also a digital frequency meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Options for using familiar objects in their unusual functional purpose - isn't this the "aerobatics" of a skilled radio amateur? The broadcasting receiver of a serial sample, after very minor modifications, indicates the frequency of the measured voltage supplied to it ... Yes, even within the range of up to 800 MHz! This is not fantasy. Take the time to read an article familiar to many readers of our regular author. Maybe you want to repeat this design? Simple pocket miniature VHF-FM receivers ("MANBO" and similar) are now popular due to their low cost and satisfactory reception quality. As a rule, they are made on the basis of microcircuits TDA7000, TDA7088 (see Dahin M. Receivers with automatic tuning. - Radio, 2000, No. 6, pp. 33, 34). Recently, modifications of receivers began to be equipped with electronic clocks, and some of them with a digital scale. The last option - these are receivers under the brand name "Palito", "ESB", etc. - is of particular interest to radio amateurs, since the built-in electronic scale is nothing more than a frequency meter with a digital indication. After a simple modification, you can use this frequency meter in amateur radio practice without losing the basic properties of the receiver. It is not very accurate due to the fact that it has a four-decade indicator and displays hundreds, tens, units of megahertz and hundreds of kilohertz. It is difficult to make very accurate measurements with its help, but it is convenient to use it (especially in the VHF band) for conducting antenna measurements, searching for sources of radio emissions and detecting frequencies at which self-excitation of radio equipment occurs, as well as for tuning broadband filters, amplifiers, etc. Therefore, it can be a good addition to such devices as GIR, antennascope, signal generator, etc. Small dimensions, high efficiency (current consumption is only a few milliamps) and a large operating frequency range (up to 800 MHz!) make such a measuring device very attractive. Below, on the example of the receiver "ESV" (RS-218), the design of the receiver is considered and recommendations are given for its improvement and the obtained parameters are given. An enlarged block diagram of this radio receiver is shown in fig. 1. It consists of two boards, one of them is the board of the radio receiver itself (RPU) on the SC1088 (or TDA7088) chip, ultrasonic frequency on transistors and URF on two transistors. As an antenna in these receivers, a cord of headphones connected to the jack is used. On the second board there is a clock, elements of a digital scale (the frequency meter itself) and control buttons. The supply voltage is constantly supplied to the clock assembly, and when the receiver is turned off, the current time is displayed on the display. When the receiver is turned on with switch SA1, the supply voltage is supplied to the receiver and the frequency meter control bus. The local oscillator signal is amplified by the URF, fed to the frequency meter, and the tuning frequency is displayed on the indicator.
The receiver is built according to a superheterodyne circuit (lower tuning) with a low IF (70 kHz), and therefore, for the correct indication of the tuning frequency, the frequency meter readings are overestimated by 0,1 MHz, which must be taken into account when making measurements. Obviously, if a controlled signal is applied to the input of the frequency meter, then under certain conditions, its frequency will be displayed. First of all, for this, a small-sized high-frequency socket should be installed on the receiver housing. Suitable, for example, SMA, and it is best to place it closer to the input of the frequency counter. In addition, to turn on the frequency meter, you must install a small-sized switch (in the diagram, it is designated as SA2'). The placement of these elements in the receiver housing is shown in Fig. 2. The PD9-2 switch is installed (glued to the board) next to the volume control, for this jumpers J11, J14 and capacitor C11 (the numbering is given in accordance with the designation on the board) must be installed on the side of the printed conductors. The switch housing is connected to a common wire. The SMA jack is installed on the narrow side next to the J21 ribbon harness that goes from the receiver board to the clock (frequency counter) board. The central contact of the socket through a capacitor with a capacity of 500 ... 1000 pF is connected to the input of a frequency meter or URC, and the case is connected to a common wire.
The URC scheme is shown in fig. 3. Since it has two stages, there are three connection options: to the input of the first stage (point 1), to the input of the second (point 2) or to the input of the frequency meter (point 3). Obviously, the connection location will affect the operating frequency range and sensitivity of the frequency counter.
Studies have been carried out to determine these parameters. In this case, the local oscillator inductor must be short-circuited by a switch, and the capacitances of capacitors C4, C62, C63 should be increased to 10 pF. On the graphs of Fig. 4 shows the frequency dependences of the minimum input voltage (Uin), at which the frequency meter began to work stably, when a signal was applied to its various points in accordance with fig. 3. In any case, a signal voltage of more than 1 V should not be applied.
Using the above dependencies, you can choose the most suitable point. For example, when the measured signal is connected to the input of the first stage, the sensitivity in the frequency range up to 100 MHz is less than 1 mV. It should be noted that this sensitivity is excessive and leads to the fact that the frequency counter will be too sensitive to noise and interference. In addition, in this range, due to non-linear effects in the amplifier, distortion may occur and the frequency meter can indicate the frequency of the harmonic components of the signal. If the frequency meter does not respond to interference, then in the absence of a signal, the indicator will display 000,1 MHz. In the author's version of the frequency meter, point 3 was chosen for connection. In this case, an additional switch is connected between the battery plus (jumper J23) and the frequency meter control bus (see Fig. 1). To do this, you need to disconnect the red (or third from the top) wire in the J21 harness from the receiver board and connect it to the switch. This connection allows you to turn on the frequency counter when the receiver is turned off or turn it off when the receiver is turned on. The latter is convenient in that when receiving a radio station, the frequency meter can be turned off and the current time can be controlled. The appearance of the modified receiver is shown in Fig. 5.
The lower limit of the measured frequency is 0,5 ... 1 MHz, just where the error becomes too large. The upper limit depends on the supply voltage and for 2,5 V it is 600 MHz, for 3 V it is 700 MHz, and at 4 V it reaches 800 MHz. More voltage should not be applied. When the receiver is turned off, the current consumed by the frequency meter (together with the clock) depends on the measured frequency and varies from 0,3 mA in the absence of a signal to 0,7 mA at frequencies up to 50 MHz and up to 4 mA at a frequency of 600 MHz. Author: I. Nechaev, Kursk
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