SU77444A3 - Intermediate frequency amplifier - Google Patents

Description

An intermediate frequency amplifier is proposed for a superager-heterodyne receiving receiver with adjustable selectivity. A distinctive feature of the proposed amplifier is the use of a three-loop filter, the third link of which is used to obtain a sharper selectivity curve, which has a wide range.

FIG. 1 shows the basic scheme of a radio receiver with an intermediate frequency amplifier; in fig. 2 and 3 show the performance of the amplifier; in fig. 4 shows a modification of a part of the circuit of FIG. one.

Box 1 (in FIG. 1) denotes a radio receiver that includes a conventional high frequency detector, a tuning system, and a tuning element 2, working by hand or in another way. The tuning system supplies the intermediate frequency signals to the intermediate amplifier 3, connected via circuit 4 to a second intermediate frequency amplifier 5, which in turn is connected via an output transformer 6 to the second detector 7.

The voltage of the detector-rectified signals, which occurs at the output impedance 8, is supplied through resistors 9, 10 and 11 to the corresponding low-frequency amplifier (ULF) to the automatic gain control circuit 12 (AGC) and the tuning indicator lamp 13. The latter is known 6E5 electron tube in which the DC voltage removed

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from the output impedance 8 and varying in accordance with the signals, adjusts the potential of the control element 14, which lies in the path of the electrons emitted by the cathode 15, to the fluorescent screen 16.

The signals supplied to the intermediate frequency amplifier are controlled by a circuit 4 providing the amplifier with a given selectivity characteristic, and are adjusted to create different degrees of selectivity with any number of stages, and for each of the stages a sharp peak can be obtained to provide a sharp tuning.

A communication circuit 4, providing an acute frequency response or said peak for tuning purposes, despite the selectivity setting for normal signal reception, contains a transformer with three windings 17, 18 and 19 parallel to each other, thereby providing electromagnetic links between these windings. The inductance of the windings is controlled by the sliding magnetic cores 20. Each winding is tuned to the same intermediate frequency by tuning capacitors 21. The resistance 22 is normally connected in series with the resonant circuit, but may be shorted by a special switch 23.

A second winding 18 is connected in series with the fourth winding 24 and the switch 25, creating an additional electromagnetic coupling between the circuit containing the winding IS and the first winding 17, the degree of communication being controlled by the switch 25.

In this position of the switch 25, the winding 24 is disconnected from the circuit of the second winding 18, and in the other two positions of the switch 25 two additional communication stages are created, including part or all of the winding 24 in series with the winding 18 and capacitor 21, respectively, with which the selectivity decreases communication circuits.

The winding 17 and the capacitor 21 represent the resonant anode output circuit of the first amplifier 3. The winding 17 is the primary winding of the transformer. The winding 18 and the tuning capacitor, together with the winding 24, when it is connected to the circuit, represents an intermediate, secondary or tuned coupling circuit serving to transfer energy from the input winding 17 to the output 19, included in the grid circuit of the second amplifier 5.

A transformer device of the specified type is known as a three-circuit communication transformer. The sequence has an input, intermediate and output (grid) resonant circuits and an additional variable connection between the intermediate and input circuits and the resistance in the output circuit, adapted to change the selectivity characteristics of the circuit.

The characteristic of selectivity with resistance in the output circuit is shown in FIG. 2 curves, 1 and 1and for three selectivity levels provided by switch 25. Curve I corresponds to the case when the switch is in the position shown in the drawing. It should be noted that, with the exception of the case of curve I, the sensitivity for the remaining selectivity steps has a planar vertex, due to which precise adjustment is impossible.

It is usually necessary to increase the selectivity in order to obtain frequency characteristics similar to those in curve I, so that

The tuning indicator lamp could accurately indicate the true resonance of the received signal, without having reduced communication by means of switch 25. By varying the attenuation in the loop with the help of resistances, you can obtain the desired reception characteristic with a high degree of accuracy.

FIG. 3 shows that curves II and 1P can be made with a sharp peak corresponding to curve 1 having a sharp peak 4. Thus, the tuning curves become peaked, as shown in FIG. 3, for three selectivity levels represented by curves I, II and III. As a result, a sharp adjustment of the lamp indicator is achieved at resonance without changing the intermediate circuit selectivity characteristic by switch 25.

Using the appropriate adventure switch 23 to setting item 2, as shown by dotted line 25 in FIG. 1, you can move the setting position. Circuit 4 is usually grounded by switch 23 at point 27 and resistance 22 is short-circuited by wire 28 through grounding points 27 and 29. For the time of adjustment, the circuit that short-circuits resistance 22 is instantly disconnected to provide a sharp tuning curve, and the overall selectivity remains unchanged, whatever position the switch 25 would be.

The link circuit 4 shown here may optionally be connected between other amplifier stages. In this example, it is shown to simplify the drawing and description.

FIG. 4 shows a modification of the case shown in FIG. 1. Here, the switch 30 according to the number of contacts or operating positions corresponds to the switch 25 and is controlled last for the advent of the wire 28 to earth when translating the switch 25 to the position corresponding to the highest degree of selectivity. This switch corresponds to the switch 23 and makes the operation of the switch 23 unnecessary: The above is possible because curve I in FIG. 2 is slightly wider than the same curve in FIG. 3, therefore, the same degree of reproduction accuracy is ensured.

Although only three selectivity levels are provided here, additional contacts and corresponding connections can be added to winding 24 to increase the number of selectivity levels.

With a large number of selectivity steps, it is desirable to use the modification of a part of the circuit shown in FIG. 4, to avoid the need to work with the switch 23 when manually adjusting to remote stations.

The subject of the invention.

An intermediate frequency amplifier for superagerterodyne receivers with variable selectivity, characterized in that the third link of the three-loop filter is used to obtain a sharp peak in the middle of the selectivity curve, which has a wide range.

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