JPH0548966B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field of the Invention] The present invention relates to an electronic tuning circuit for an AM receiver that receives AM broadcasting by a tuning circuit using a variable capacitance diode. [Problems to be Solved by the Invention] As shown in FIG. 8, the electronic tuning circuit of the AM receiver includes an antenna tuning circuit 12, a high frequency amplification circuit 13, a local oscillation circuit 14, and a mixing circuit 15. is a variable voltage source that supplies a tuning voltage a to the high frequency amplification stage and an oscillation frequency control voltage b to the local oscillation circuit 14. Such an electronic tuning circuit includes tuning coils L11 to L
13, trimmer capacitors C11 to C13, variable capacitance diodes D11 to D13, stray capacitance C
21 to C23, and padding capacitor C3
1, trimmer capacitors C11 to C13,
Tracking adjustment is performed by components such as tuning coils L11 to L13 so that the receiving sensitivity is maximized at three points in the receiving frequency band. Therefore,
Conventional electronic tuning circuits have disadvantages in that many parts are used for tracking adjustment, requiring a large number of man-hours for adjustment, and resulting in high manufacturing costs. [Object of the Invention] The present invention was made in view of the above-mentioned problems, and its main purpose is to provide an electronic tuning circuit for an AM receiver that uses a variable capacitance diode to easily adjust tracking. . [Summary of the Invention] The present invention has been made to achieve the above-mentioned objects, and the electronic tuning circuit of the AM receiver of the present invention raises the oscillation frequency of the local oscillation circuit by an intermediate frequency higher than the tuning frequency. Alternatively, the oscillation frequency control voltage and the tuning voltage to the high frequency amplification stage are supplied by a PLL synthesizer method to set the frequency low.
The trimmer capacitor for tracking adjustment included in the local oscillation circuit of the electronic tuning circuit and the tuning circuit of the high-frequency amplification stage is removed, and the inductance and stray capacitance of the tuning coil of the high-frequency amplification stage and local oscillation circuit are removed. Tracking is achieved by adjusting only the inductance of the tuning coil of the high-frequency amplification stage to a predetermined frequency, with a tolerance range of within ±5% and a tolerance range of the capacity of the padding capacitor of the local oscillation circuit within ±2%. This is an electronic tuning circuit for an AM receiver that allows easy tracking adjustment. [Embodiment of the Invention] FIG. 1 is a diagram showing an embodiment of an electronic tuning circuit of an AM receiver according to the present invention. In the figure, 1
is an antenna, 2 is a high frequency amplification stage including an antenna tuning circuit, 4 is a mixing circuit, 5 is an intermediate frequency amplification circuit, 6 is a local oscillation circuit, and 7 is a PLL (phase locked
10 is a detection circuit, and 11 is a speaker. L1 and L2 are tuning coils, C1 and C2 are stray capacitances, C3 is padding capacitor, and D
1 and D2 are variable capacitance diodes. From the terminal 8, a program signal for receiving a desired broadcast wave is supplied to the PLL circuit 7. PLL
From the circuit 7, the tuning voltage a is supplied to the variable capacitance diode D1 of the high frequency amplification stage 2, and the oscillation frequency control voltage b is supplied to the variable capacitance diode D2 of the local oscillation circuit 6. This is a PLL synthesizer-type AM receiver in which the oscillation frequency is set to a predetermined value. The oscillation frequency is set based on the program signal to receive the desired broadcast wave, and the tuning voltage is also set, and the high frequency amplification stage 2 receives the desired frequency.
₁ is selected, supplied to the mixing circuit 4, and added to the oscillation frequency ₀ from the local oscillation circuit 6 ( ₁ - ₀ ).
Obtain the intermediate frequency of Of course, depending on the characteristics of the receiving frequency band, it may be set higher than the intermediate frequency. In such tracking adjustment of the electronic tuning circuit, the inductance of the tuning coil L1 of the high frequency amplification stage is set so that the receiving sensitivity is maximized approximately at the center of the receiving frequency band. It should be noted that although different types of receivers have different stray capacities, in today's advanced manufacturing technology, it can be determined that there is almost no variation between the same types of receivers. Furthermore, variable capacitance diodes with less variation in characteristics are used. FIG. 2 is a diagram showing another embodiment of the electronic tuning circuit of the AM receiver according to the present invention, in which 2 is a high-frequency amplification stage, in which the antenna tuning circuit ₃₁ and the high-frequency amplification stage are used to increase the selectivity. Circuit ₃ is formed of two stages. L21-L23 are tuning coils, C21-C
23 is a stray capacitance, D21 to D23 are variable capacitance diodes, and C24 is a padding capacitor. Regarding the electronic tuning circuit of the AM receiver shown in Figure 1,
This will be explained based on FIGS. 3 to 7 showing tracking error curves. FIG. 3 is a diagram showing the effect on the tracking error when the capacitance value of the padding capacitor of the local oscillation circuit is changed. FIG. 4 is a diagram showing a tracking error curve when the stray capacitance value of the local oscillation circuit is changed. FIG. 5 is a diagram showing a tracking error curve when the inductance of the tuning coil of the local oscillation circuit is changed. FIG. 6 is a diagram showing the situation in which the worst error occurs when the tolerance range of each element of the electronic tuning circuit is set to ±5%. FIG. 7 is a diagram showing a tracking error curve when the allowable range of the capacitance of the padding capacitor of the local oscillation circuit is set to ±2%. Table 1 shows the constants of the electronic tuning circuit for the tracking error curve shown in FIG. As shown in Table 1, the tuning coil L1 and stray capacitance C1 of the antenna tuning circuit, and the tuning coil L of the local oscillation circuit 6.
The values of 2 and stray capacitance C2 are set equal, and the capacitance of the padding capacitor is changed by −5% for B and +5% for C with respect to the capacitance of A.

[Table] From the results, it can be seen that by changing the capacitance value of the padding capacitor C3 as shown in Table 1, the situation in which tracking errors occur as shown in FIG. 3 changes. That is, the error in the low receiving frequency region is larger than in the high frequency region. Also, tracking errors occur in all reception frequency bands. Next, the effect on tracking error when changing the stray capacitance of the local oscillator circuit 6 will be explained based on FIG. FIG. 3 is a diagram showing a case where the capacitance value is changed by about +6%.
This result shows that the tracking error in the high reception frequency region is extremely large.
Therefore, it is presumed that when the tracking error curve is at the upper limit, it deviates significantly from the allowable range. Therefore, it is necessary to set the value of the stray capacitance C2 within ±5%. Table 2 shows the constants of the main components of the electronic tuning circuit shown in FIG.

[Table] Figure 5 is a diagram to understand the influence on tracking error due to changes in the inductance of the tuning coil of the local oscillator circuit.When the inductance value is changed by ±2% with respect to the value of A. The curve diagrams are shown in B and C. This result shows that the tracking error fluctuates greatly. Table 3 shows the constants of the electronic tuning circuit at that time.

[Table] As is clear from these results, in order to simplify the tracking adjustment of the electronic tuning circuit, the minimum necessary conditions are the inductance, stray capacitance, and padding capacitor of the tuning coil in the electronic tuning circuit. This indicates that it is necessary to set the permissible range of the value within ±5%. This also eliminates the need to adjust these capacitances with trimmer capacitors. Based on the above results, the tolerance range of the inductance of the tuning coil of the electronic tuning circuit, the stray capacitance, and the capacitance value of the padding capacitor was set by ±5% each.
Fig. 6 shows the state in which a tracking error occurs when the tracking error is set to . This figure shows two types of combinations that produce the maximum error at the upper and lower limits of the tracking error. The constants of the electronic tuning circuit are shown in Tables 4 and 5.

【table】

[Table] The electronic tuning circuit with the constants shown in Table 4 is the tracking error curve of Figure 6, and that of Table 5 is the curve of Figure 6. In both cases, each capacitance is constant, and L1 , L2 are changing. The tracking adjustment of such an electronic tuning circuit is
At approximately the center of the receiving frequency band, the electronic tuning circuit shown in the drawing has a maximum receiving sensitivity of 1MHz.
The tuning coil of the high frequency amplification stage has been adjusted. From this result, the tuning coil L1 of the antenna tuning circuit
It can be seen that tracking adjustment is possible by adjusting . On the other hand, as is clear from Figure 3, since the influence of the padding capacitor of the local oscillation circuit affects the entire frequency band, an example in which the allowable range of the capacitance of the padding capacitor is set to ±2% is shown below. It is shown in FIG. Tables 6 and 7 show the constants of the electronic tuning circuit at that time.

【table】

〔Effect of the invention〕

As mentioned above, the electronic tuning circuit of the AM receiver of the present invention uses a padding capacitor of the local oscillator circuit with a tolerance range of ±2%, and tracking adjustment using a trimmer capacitor. It is possible to provide an electronic tuning circuit having practically acceptable reception sensitivity by simply adjusting the tuning coil of the high-frequency amplification stage. Moreover, since it is possible to remove the trimmer capacitor for tracking adjustment of the electronic tuning circuit, the number of parts of the electronic tuning circuit can be reduced. Therefore, the number of parts and the number of man-hours for tracking adjustment can be reduced, making it extremely effective in reducing manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of an electronic tuning circuit of an AM receiver according to the present invention. FIG. 2 is a diagram showing another embodiment of an electronic tuning circuit for an AM receiver according to the present invention. 3 to 7 are diagrams for explaining the electronic tuning circuit of the AM receiver according to the present invention. FIG. 8 is a diagram showing an electronic tuning circuit of a conventional AM receiver. 1...Antenna, 2...High frequency amplification stage, 3 ₁ ...
...Antenna tuning circuit, 3 ₂ ...High frequency amplification circuit,
4...Mixing circuit, 5...Intermediate amplifier circuit, 6...Local oscillation circuit, 7...PLL circuit, 8...Program signal input terminal, 10...Detection circuit, 11...Speaker, D1, D2, D21-D23...Variable capacitance diode, L1, L2, L21-L23...
Tuning coil, C1, C2, C21 to C23...stray capacitance, C3, C24...padding capacitor.

Claims (1)

[Claims] 1. The oscillation frequency control voltage for setting the oscillation frequency of the local oscillator circuit to be higher or lower than the tuning frequency by the intermediate frequency and the tuning voltage of the high frequency amplification stage are supplied by the PLL synthesizer method.
In the electronic tuning circuit of the AM receiver, the trimmer capacitor for tracking adjustment included in the local oscillation circuit and the tuning circuit of the high-frequency amplification stage that constitute the electronic tuning circuit is removed, and the capacitors in both circuits are all fixed capacitors. The high frequency An electronic tuning circuit for an AM receiver characterized in that tracking is achieved by adjusting only the inductance of a tuning coil in an amplification stage to a predetermined frequency.