Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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You can improve the quality of speech reception by installing a low-pass filter (LPF) in the audio amplifier of your radio or transceiver. The modern elemental base makes it possible to manufacture compact active low-pass filters that can be built into almost any equipment.
Figure 1 shows a fourth-order active low-pass filter with a cutoff frequency of 3,3 kHz. It is assembled on a two-channel operational amplifier LM358AM. The filter gain is equal to one (0 dB). The filter provides 20 dB of out-of-band rejection at 5 kHz and 40 dB at 8 kHz.
If it is required to manufacture a low-pass filter with a transmission coefficient of more than one, then the filter circuit must be changed, as shown in Fig. 2.
The filter gain is determined by the values of the resistors R9 and R11. The table shows the calculated ratings of the filter elements for various filter gains (for resistors R8 and R10 with a resistance of 100 kOhm). When repeating the device, resistors R4 - R7 and capacitors C3, C4, C6, C7 should be selected with deviations from the required rating of no more than 3 ... 5%. In addition, it is desirable to use capacitors with the lowest possible TKE.
The filter is made by surface mounting on a printed circuit board of double-sided foil fiberglass (Fig. 3).
The second side of the board is left metallized and is used as a common wire. A sketch of the board with the elements located on it is shown in fig. 4. The board is designed for mounting a filter with any transfer coefficient. For a filter with a transfer coefficient of 0 dB, resistors R8 and R10, as well as capacitors C8 and C9, are not installed. Jumpers are installed instead of resistors R9 and R11. All capacitors are leadless K10-17 for surface mounting, resistors are P1-12.
The filter requires almost no adjustment. Almost any stabilized source with a voltage of 3 ... 12 V is suitable for its power supply. However, it should be noted that with a supply voltage of less than 9 V, it is necessary to select a resistor R1 to obtain the maximum undistorted output signal. For the LM358AM chip, with a supply voltage of 3 V, the resistance of the resistor R1 should be 62 kOhm. In this case, the maximum amplitude of the undistorted output signal (at a load of 1 kΩ) will be 0,7 V. At a supply voltage of 6 V, it will be 2,5 V, and at 9 V - 5 V. The current consumed by the filter from a 3 V power supply is - approximately 0,5 mA, and at a voltage of 12 V - 0,7 mA.
Other types of parts can be used in the device, for example, the K157UD2 microcircuit (with the corresponding correction circuits), resistors - MLT, S2-33, R1-4, capacitors - KLS, K10-17, KM, but this will entail an adjustment of the topology and board dimensions.
Author: I.Nechaev (UA3WIA)
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