JPH06303106A - High frequency tuning circuit for am receiver - Google Patents

Abstract

PURPOSE:To minimize a tracking error by providing a corrected capacitance to a capacitive element. CONSTITUTION:Since a varactor diode for a high frequency tuning and that for a local oscillation circuit are equal in the characteristic usually, the capacitance of a varactor diode 14 is decided. Thus, an inductance of a secondary coil 13 while an antenna 12 is not connected to a primary coil for a tuning transformer is decided. An inductance L0 is obtained based on an inductance of inductors L4, L3 and an inductance L2 of a primary coil 11 set in advance, and the inductance L0 is an inductance of the secondary coil 13. Thus, a tracking error is minimized by varying the capacitance of the capacitive element 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency tuning circuit for an AM receiver to which a loop antenna is connected.

[0002]

2. Description of the Related Art AM stereo broadcasting has started, but an AM receiver for receiving this broadcasting is required to have high sensitivity. However, it is difficult to increase the sensitivity of the receiver inside a house such as a housing complex constructed using reinforced concrete because the electric field strength is significantly reduced. In this case,
It is common practice to place the loop antenna in a place where the electric field strength is strong, such as near a window, to increase the sensitivity of the receiver. However, since the loop antenna is often separated from the receiving set, the length of the lead wire to the antenna becomes long. This leads to an increase in the inductance of the antenna by the length of the lead wire and a decrease in the self-resonant frequency of the entire antenna. Therefore, the inductance of the primary coil of the tuning transformer of the high-frequency tuning circuit to which the loop antenna with a long lead wire is connected is increased by the length of the lead wire, and the secondary coil of the tuning transformer coupled to the primary coil of the tuning transformer is increased. The side coil is also affected by the change in the inductance of the entire antenna.

A high frequency tuning circuit of a conventional AM receiver includes a primary coil of a tuning transformer, a secondary coil coupled to the primary coil, and a variable capacitance diode connected in parallel to the secondary coil. It has a capacitance element. The high frequency tuning circuit of the AM receiver uses a one-point adjustment method so that a predetermined intermediate frequency can be obtained at one point on the lower side of the reception frequency band.
Usually, the lead wire of the antenna is short, and the increase of the inductance of the lead wire and the influence of the self-resonant frequency of the entire antenna are small, so that the connection of the antenna to the primary coil of the tuning transformer is ignored.

Therefore, when a loop antenna having a lead wire length of, for example, 3 m or more is connected to the primary coil of the tuning transformer, the inductance of the secondary coil of the tuning transformer changes, and one-point adjustment is deviated, resulting in receiver sensitivity. As a result, the situation in which AM stereo broadcasting cannot be received often occurs.

[0005]

SUMMARY OF THE INVENTION The present invention is directed to a high frequency of an AM receiver which takes into consideration the change in the inductance of the secondary coil of the tuning transformer due to the change in the inductance of the antenna coil due to the length of the lead wire of the loop antenna. Provides a tuning circuit.

[0006]

A high frequency tuning circuit of an AM receiver according to the present invention comprises a loop antenna and a lead wire connected between the loop antenna and the primary coil when the primary coil is connected to the loop antenna. It is characterized in that the capacitance element has a capacitance value corrected so that tracking error is eliminated at one frequency below the reception frequency band. The inductance value of the secondary coil of the tuning transformer is set to the inductance value L _O obtained from the following equation. L _O = L _A + {(K ² L ₂ L _A ) / (L ₂ + L ₃ )} where L _A is the secondary coil of the tuning transformer when the loop antenna is not connected to the primary coil of the tuning transformer. Inductance, L ₂ is the inductance of the primary coil of the tuning transformer, L ₃ is the inductance of the entire loop antenna including the lead wire, and K is the coupling coefficient between the primary coil and the secondary coil of the tuning transformer.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a high frequency tuning circuit of an AM receiver of the present invention.
It will be described with reference to FIGS. 1 is a circuit diagram of a high frequency tuning circuit, FIG. 2 is a diagram showing a relationship between a frequency and a tracking error when an antenna is connected after performing a one-point adjustment by ignoring a change in the inductance of the entire antenna due to the frequency, FIG. Is a diagram showing the state of the inductance of the entire loop antenna which changes with frequency, and FIG. 4 is a diagram showing the relationship between tracking error and frequency in the high frequency tuning circuit of the AM receiver of the present invention. In FIG. 1, 11
Is a primary coil of the tuning transformer, 12 is a loop antenna, 13 is a secondary coil of the tuning transformer, 14 is a variable capacitance diode, and 15 is a capacitance element. The high frequency tuning circuit of the AM receiver of the present invention comprises a secondary coil 13 of the tuning transformer coupled to the primary coil 11 of the tuning transformer, a variable diode 14 and a capacitance element connected in parallel to the secondary coil 13 of the tuning transformer. It has 15. A loop antenna 12 is connected to the primary coil 11 of the tuning transformer.

Now, let us consider the inductance of the secondary coil 13 of the tuning transformer when the antenna 12 is connected to the primary coil 11 of the tuning transformer. When a load of impedance Z is connected to the primary coil 11 of the tuning transformer, the impedance Z ₂ on the primary coil 11 side of the tuning transformer is expressed by equation (1).

Z ₂ = jωL ₂ + Z (1)

However, L ₂ is the inductance of the primary coil 11 of the tuning transformer. Assuming that the mutual inductance between the primary coil 11 of the tuning transformer and the secondary coil 13 of the tuning transformer is M, the impedance Z _IN seen from the secondary coil 13 side of the tuning transformer to the primary coil 11 side of the tuning transformer is It is expressed by equation (2).

Z _IN = jωL _A + Ze (2)

However, L _A is the inductance of the secondary coil 13 of the tuning transformer when the antenna is not connected. (3) holds for Ze.

Ze = ω ² M ² / Z ₂ (3)

[0014] _{However, M = K × (L 2} × L A) is ^1/2. Here, assuming that the inductance of the entire loop antenna 12 is L ₃ , the equations (4) and (5) are established.

Z ₂ = jω (L ₂ + L ₃ ) (4)

Ze = K ² L _A L ₂ / (L ₂ + L ₃ ) ... (5)

Therefore, the expressions (2) to (6) are established.

Z _IN = jωL _A {1- (K ² L ₂ ) / (L ₂ + L ₃ )} (6)

In this way, the secondary coil 1 of the tuning transformer is
Impedance Z _IN when looking at the primary coil 11 side of the tuning transformer from the 3 side is displayed only by the inductance component, and the second term of the equation (6) is affected by connecting the antenna 12. is there. Therefore, if one point is adjusted in advance so that the inductance of the secondary coil 13 of the tuning transformer is matched with the inductance L _{0 obtained} by adding the second term, the influence of the connection of the antenna 12 is not exerted. Therefore, L ₀ is represented by the equation (7) obtained from the equation (6).

L _O = L _A + {(K ² L ₂ L _A ) / (L ₂ + L ₃ )} (7)

However, L _O is a substantial inductance value that changes depending on the length of the lead wire of the antenna 12, and L _A is the secondary coil of the tuning transformer when the antenna 12 is not connected to the primary coil 11 of the tuning transformer. 13, L ₂ is the inductance of the primary coil 11 of the tuning transformer, L ₃ is the inductance of the entire antenna 12 including the lead wire, and K is the primary coil 11 of the tuning transformer and the secondary coil 13 of the tuning transformer. It is the coupling coefficient. For the induction of each equation, refer to page 34 of "Electronic Circuit I" (written by Masamitsu Kawakami, Kyoritsu Shuppan Co., Ltd., published March 15, 1952).

Specific one-point adjustment of the high frequency tuning circuit of the AM receiver using the inductance value L _O is performed as follows. First, in a state where the antenna 12 is not connected to the primary coil 11 of the tuning transformer, one reception frequency, for example, 600 KHz, that causes the tracking error to be 0 is selected from the range of frequencies that can be received by the AM receiver. When the reception frequency is determined, the frequency of a local oscillation circuit (not shown) including a coil and a variable capacitance diode is determined. Further, since the inductance of the coil of the local oscillation circuit is determined in advance, the capacitance value of the variable capacitance diode is determined. Normally, the variable capacitance diodes of the high frequency tuning circuit and the local oscillation circuit have the same characteristics, so that the capacitance value of the variable capacitance diode 14 is determined.
Therefore, the inductance L _A of the secondary coil 13 of the tuning transformer when the antenna 12 is not connected to the primary coil of the tuning transformer of the high frequency tuning circuit is also determined. On the other hand,
The total inductance L ₃ including the lead wire of the connected antenna 12 is measured at 600 KHz. The inductance L _A , the inductance L _3, and the preset inductance L ₂ of the primary coil 11 of the tuning transformer are substituted into the equation (7) to obtain the inductance value L _O.
And the inductance value L _O becomes the inductance value of the secondary coil 13 of the tuning transformer. As a result, the tracking error at 600 KHz becomes zero.

Since the inductance L _O of the secondary coil 13 of the tuning transformer is fixed, the value of the capacitance element 15 is determined so that the tracking error at other frequencies falls within the allowable range.

However, the antenna 1 having a long lead wire is actually used.
When 2 is connected, the tracking error does not become 0 but becomes larger than the allowable range at frequencies other than 600 KHz as shown in FIG. This is the inductance L _{3 of the} antenna 12 including the lead wire in the equation (7).
Changes with frequency and the inductance value L _O when an antenna is connected does not take into consideration that it also changes with frequency. In particular, if the lead wire is long, the self-resonant frequency of the entire antenna 12 decreases and approaches the frequency of the reception band of the AM receiver, so the inductance L ₃
Varies greatly with frequency. FIG. 3 is a diagram showing an example of a change in the inductance of the antenna 12 connected with a lead wire having a length of 3 m or more depending on the frequency. However,
Since the one-point adjustment is performed while the inductance value L _O is fixed, the tracking error does not become zero at frequencies other than 600 KHz. However, the change in the inductance L ₃ of the entire antenna 12 depending on the frequency can be measured in advance according to the length of the lead wire.
An empirical formula can also be derived based on the measurement result. Therefore, at frequencies other than 600 KHz, by substituting the inductance L ₃ in the frequency (7), the inductance value L _O at that frequency is determined. Then, the capacitance value of the capacitance element 15 is changed based on the inductance value L _O obtained by substituting the changing inductance L ₃ .
Tracking errors can be minimized.

Then, even if the antenna 12 is actually connected to the high frequency tuning circuit of the AM receiver, the result of the one-point adjustment is shown in FIG.
Capacitance value that was determined so as not to shift like,
That is, the corrected capacitance value is applied to the capacitance element 15 of the high frequency tuning circuit of the AM receiver, so that the configuration shown in FIG. 4 can be obtained. Capacitance element 15
Is better to correct the capacitance value to a larger value than in the case where the change in the inductance L _O depending on the frequency is not taken into consideration.

[0026]

As described above, the high frequency tuning circuit of the AM receiver of the present invention changes in accordance with the change of the inductance of the primary coil of the tuning transformer when the loop antenna having the long lead wire is connected. One-point adjustment is possible by the capacitance value considering the substantial inductance value of the secondary coil of the tuning transformer. Therefore, even if an antenna with a long lead wire of 3 m or more is connected to the tuning transformer, one point adjustment may be misaligned because the capacitance element has a corrected capacitance value to reduce the tracking error due to the inductance of the antenna in advance. Not very practical.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a high frequency tuning circuit.

FIG. 2 is a diagram showing a relationship between a frequency and a tracking error when the antenna is connected after performing a one-point adjustment by ignoring a change in the inductance of the entire antenna due to the frequency.

FIG. 3 is a diagram showing a state of the inductance of the entire loop antenna which changes with frequency.

FIG. 4 is a diagram showing a relationship between a tracking error and a frequency in a state where one-point adjustment of the high frequency tuning circuit of the AM receiver of the present invention is completed.

[Explanation of symbols]

 11 Primary coil of tuning transformer 12 Antenna 13 Secondary coil of tuning transformer 14 Variable capacitance diode 15 Capacitance element

Claims (2)

[Claims] 1. A primary side coil of a tuning transformer to which a loop antenna is connected, a secondary side coil of a tuning transformer coupled to the primary side coil, a variable capacitance diode and a capacitance element respectively connected in parallel to the secondary side coil. Equipped A
In the high frequency tuning circuit of the M receiver, when the loop antenna and the lead wire connected between the loop antenna and the primary coil are connected to the primary coil, tracking is performed at one frequency below the reception frequency band. A high frequency tuning circuit of an AM receiver, wherein the capacitance element has a capacitance value corrected so as to eliminate an error. 2. The inductance value of the secondary coil of the tuning transformer is an inductance value L obtained from the following equation.
The high frequency tuning circuit of the AM receiver according to claim 1, which is set to _O. L _O = L _A + {(K ² L ₂ L _A ) / (L ₂ + L ₃ )} where L _A is the secondary coil of the tuning transformer when the loop antenna is not connected to the primary coil of the tuning transformer. Inductance, L ₂ is the inductance of the primary coil of the tuning transformer, L ₃ is the inductance of the entire loop antenna including the lead wire, and K is the coupling coefficient between the primary coil and the secondary coil of the tuning transformer.