JPH0466405B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field of the Invention] The present invention relates to an electronic tuning circuit using a variable capacitance diode as a tuning element in a high frequency tuning circuit and a local oscillation circuit of the front end section of an AM radio receiver, and particularly relates to an AM radio receiver. It is suitable for an electrical tuning circuit of a stereo receiver, and can reduce tracking errors over a wide range. [Prior art] An example of an electronic tuning circuit for a general AM receiver is shown in the seventh example.
This will be explained based on the diagram. In the figure, 1 is a high frequency tuning circuit, and 2 is a local oscillation circuit. Variable voltage source 4
A tuning voltage V _T generated by a PLL synthesizer method or the like is supplied from a terminal 3 to a high frequency tuning circuit 1 and a local oscillation circuit 2 via resistors R ₁₀ and R ₁₁ , and is tuned to a desired frequency. The high frequency tuning circuit 1 has a trimmer capacitor C ₁₀ and a capacitor C _S10 connected in parallel to a tuning coil L ₁₀ , a capacitor _CP10 connected to their connection point, and a variable capacitance diode D ₁₀ connected to the other end. There is.
Capacitor C _P10 is a capacitor for blocking DC bias current. In addition, the local oscillation circuit 2 has a padding capacitor at one end of the oscillation coil _L11 .
C _P11 is connected, and the trimmer capacitor C11 and the capacitor are connected between the other end and the other end of the oscillation coil _L11 .
C _S11 and a variable capacitance diode D ₁₁ are connected in parallel. Generally, in an AM receiver, tracking adjustment is performed by adjusting the inductance of the trimmer capacitor and coil of the electronic tuning circuit after mounting the components of the electronic tuning circuit individually on a printed circuit board together with other amplifier circuits, etc. There is a problem in which errors occur logically. That is, in the superheterodyne receiver, its local oscillation circuit oscillates at a frequency higher than the receiving frequency by an intermediate frequency (450 KHz). For that purpose, MW band (522K
Hz to 1629KHz), the local oscillation circuit needs to change the oscillation frequency from 972KHz to 2079KHz. Comparing the frequency change ratios, the frequency variable ratio _A1 of the high frequency tuning circuit 1 is:
It is as follows. A ₁ =1629/522=3.12 (1) On the other hand, the frequency variable ratio A ₂ of the local oscillation circuit 2 is A ₂ =2079/972=2.14 (2). As is clear from equations (1) and (2), high frequency tuning circuit 1
It can be seen that the frequency change ratios of the local oscillation circuit 2 and the local oscillation circuit 2 are different, and the frequency change ratio of the local oscillation circuit 2 is smaller than that of the local oscillation circuit 2. On the other hand, the variable capacitance diodes used in the high frequency tuning circuit 1 and the local oscillation circuit 2 are usually of equal capacitance characteristics. Because of this, since the frequency variable ratio does not match,
It was difficult to reduce tracking errors. Usually 3, 600KHz, 1000KHz, and 1400KHz
Tracking is adjusted so that the tracking error is zero at the point. Furthermore, to explain based on FIGS. 6 and 7, the tracking error when the padding capacitor C _P11 is removed from the local oscillation circuit 2 in FIG. 7 is as follows.
The result will be a curve as shown in Figure 6A. If the local oscillation circuit 2 in FIG. 7 is composed of an oscillation coil _L11 , a variable capacitance diode _D11 , and a padding capacitor C _P11 , a tracking error as shown in FIG. 6B occurs. In a conventional electronic tuning circuit that combines these, only the low and high frequencies of the receiving frequency band are adjusted, so a tracking error as shown in FIG. 6C occurs. [Problems to be Solved by the Invention] In conventional AM receivers, tracking adjustment is usually performed after front-end electronic components are mounted on a printed circuit board. Therefore, due to the influence of uncertain elements such as stray capacitance, the tracking error may fall outside the control limits in the final adjustment process, and all components mounted on the printed circuit board may have to be discarded or It was necessary to replace the defective parts and perform tracking adjustment again. As mentioned above, there are many parts for tracking adjustment, and therefore there is a drawback that a large number of adjustment steps are required and the manufacturing cost is high. In addition, in conventional AM receivers, even if tracking adjustment is performed, a tracking error of 3KHz to 10KHz will theoretically occur, so tracking errors must be reduced over a wide band like AM stereo receivers. Conventional electronic tuning circuits have drawbacks such as poor stereo separation and distortion, and there is room for improvement. [Object of the Invention] The electronic tuning circuit of the AM receiver of the present invention was made to eliminate the above-mentioned drawbacks.
The purpose of the present invention is to provide an electronic tuning circuit that can reduce tracking errors over a frequency range of up to 1700 KHz. Another object of the present invention is to provide an electronic tuning circuit for an AM receiver in which the number of tracking adjustment points is reduced. Yet another object of the present invention is to provide an electronic tuning circuit for an AM receiver comprising an improved bias circuit. [Embodiments of the Invention] An electronic tuning circuit for an AM receiver according to the present invention will be explained based on FIGS. 1 to 6. In FIG. 1, a high frequency tuning circuit 1 has a capacitor C ₁ connected in parallel to tuning coils L ₁ and L ₁ each consisting of a double tuning circuit _.
A variable capacitance diode D ₁ is connected to the connection point between the capacitor C 1 and _the capacitor C 1 , and a tuning voltage V _T is supplied to their commonly connected cathodes via a resistor R ₁ . In addition, the local oscillation circuit 2 has a padding capacitor C _P0 at one end of the local oscillation coil _L0 .
is connected, a variable capacitance diode D ₂ is connected between the other end and ground, and a variable capacitance diode D ₃ and a capacitor C _S2 are connected in parallel to the connection point of the variable capacitance diode D ₂ and padding capacitor C _P0 . , a capacitor C _S1 is connected in series with them, and the anode of the variable capacitance diode D ₃ is grounded via a bias resistor R ₃ . Variable voltage source 4
From there, a tuning voltage V _T is supplied to a high frequency tuning circuit 1 and a local oscillation circuit 2 via resistors R ₁ and R ₂ , respectively. Typically, the variable voltage source 4 supplies a predetermined tuning voltage V _T via a PLL synthesizer circuit based on a program signal. Also, in FIG _. 2 _, the high frequency tuning circuit ₁ is different from the case in FIG _. A capacitor C _P1 is connected to the connection point, and a variable capacitance diode is connected between the other end and ground.
D ₁ is connected. As shown in FIG. 2, the high frequency tuning circuit 1 is not limited to that shown in FIG. 1, and various types may be used. Furthermore, the local oscillation circuit 2 of the electronic tuning circuit according to the present invention has various modifications as described below. In the local oscillation circuit shown in FIGS. 1 and 2 above, the resistor R ₂ is connected at a high impedance position for high frequencies. If the impedance of the circuit becomes extremely high, abnormal oscillation may occur, so it is necessary to set the impedance to an appropriate level. In Figure 3, a padding capacitor C _P0 is connected to an oscillation coil L ₀ whose one end is grounded, and a variable capacitance diode D ₂ and a capacitor C _S1 are connected to the other end.
A variable capacitance diode D3 _and a capacitor C _S2 are connected in parallel between the other end of the capacitor C _S1 and the cathode of the variable capacitance diode _D2 , and the variable capacitance diode _D2 ,
The common connection point between D ₃ and capacitor C _S2 is the capacitor
It is AC-grounded via C ₂ , and a tuning voltage V _T is supplied to the connection point via resistor R ₂ . resistance _R3 ,
R ₄ is a bias resistor and a variable capacitance diode
Connected to the anodes of D ₂ and D ₃ , respectively, and resistors R ₃ and R ₄
The other end is grounded. The embodiment shown in FIG. 4 is different from the embodiment shown in FIG. 3 in that the padding capacitor C _P0 is connected to the ground side of the oscillation coil L ₀ , and the rest is the same as the embodiment shown in FIG. 3 . In the reference example shown in Fig. 5, the bypass capacitor _C2 of the embodiment shown in Fig. 3 is also used as the padding capacitor C _P0 , and the variable capacitance diode _{D2 is} Since it is grounded, the only bias resistor is _R4 .
Efforts are being made to reduce the number of parts. As shown in Figures 3 to 5, the bias resistance of the variable capacitance diode and the supply position of the tuning voltage V _T can take various forms depending on the receiver circuit, etc., and the balance may vary depending on the receiver. This has the advantage that it can be made into a highly integrated electronic tuning circuit. Furthermore, in the embodiments shown in FIGS. 3 to 5, the bias resistor of the variable capacitance diode is not connected in parallel to the oscillation coil, but is connected in series to the local oscillation circuit. Therefore, since no bias resistor is connected within the local oscillation circuit, stray capacitance generated between the terminals of the bias resistor is reduced, which is effective in reducing tracking errors. Now, the electronic tuning circuit of the AM receiver of the present invention is
This is an attempt to reduce tracking errors by dividing the receiving band into three major parts, and instead of making tracking adjustments at three specific points of the receiving frequency as in the past, the receiving frequency is mainly reduced by padding and With the capacitor C _P0 , the midrange is improved by the capacitor C _S1 , and the high range is mainly improved by the capacitor C _S2 and the variable capacitance diode _D3 . In other words, the specific three
Rather than setting the circuit constants of the electronic tuning circuit so that the tracking error is zero at each point, the circuit constants are set so that the receiving band is divided into three and the tracking error is reduced to almost zero over the entire receiving band. This is to set. The electronic tuning circuit of the present invention focuses on the fact that a capacitor connected in series with the oscillation coil of a local oscillation circuit has a greater influence on the reception frequency reduction than a capacitor connected in parallel. being done. Therefore, the capacitor C S2 connected in parallel with the variable capacitance diode D ₃ and _the capacitor C _S1 connected in series with these are combined into one composite capacitance C _S
Considering this, since the composite capacitor C _S is connected in parallel to the oscillation coil L ₀ , it has a larger effect in the high frequency range than the padding capacitor C _P0 connected in series, and the padding capacitor C _P0 has a large effect on low receiving frequencies. on the other hand,
Considering the combined capacitance C _S , the variable capacitance diode D ₃
A capacitor C _S2 is connected in parallel with the capacitor C S1, and a capacitor C _S1 is connected in series with the capacitor C S2. Therefore, the effect of the capacitor C _S1 is large in the middle range of the reception frequency, and the effect of the capacitor C _S2 is large in the high range. Circuit constants are determined based on this concept, and Tables 1 and 2 show examples of circuit constants for the embodiment shown in FIG.

[Table] (However, the capacitance of variable capacitance diodes D ₁ to D ₃ is
One that can be varied within the range of 447.4 pF to 28.0 pF is preferable. )

[Table] Now, the tracking error of the electronic tuning circuit of the present invention will be explained below. For example, in order to receive the MW band (522KHz to 1629KHz), the capacitance of variable capacitance diodes D ₁ to _{D 3} should be
A capacitance change of 30 to 380 pF is required, and the capacitance of the local oscillation circuit 2 at that time is determined, and the change ratio of the oscillation frequency is determined. Oscillation coil of local oscillation circuit 2
If the combined capacitance seen from between the terminals of L ₀ is C ₀ , it is expressed by the following formula. C ₀ = C _P0 (D ₂ + C _S ) / C _P0 + D ₂ + C _S ... (3) C _S = C _S1 (D ₃ + C _S2 ) / C _S1 + D ₃ + C _S2 ... (4) (However, D ₂ , _D3 indicates the capacitance of the variable capacitance diode, and C _S is the capacitance of the variable capacitance diode _D3 and the capacitor
It is the combined capacity of C _S1 and C _S2 . ) Now, the capacitance of variable capacitance diodes D ₂ and D ₃ is
Find the oscillation frequency f ₁ when the minimum and maximum values are 30 pF and 384 pF. The oscillation frequency f ₁ can be simply determined by the following equation. When the variable capacitance diodes D ₂ and D ₃ are 30pF,
The combined capacitance C _S is determined to be approximately 37.71 pF from equation (4).
Therefore, the capacitance C ₀ seen from between the terminals of the oscillation coil L ₀
is approximately 62.77pF from equation (3). Oscillation coil L ₀
Assuming that the stray capacitance of is approximately 5 pF, the oscillation frequency f _{1 nax} at this time is determined from equation (5) to be approximately 2184.5 KHz. On the other hand, the variable capacitance diodes D ₂ and D ₃
If it is 384pF, the combined capacitance C _S is approximately
It is calculated to be 92.62pF. Therefore, the oscillation coil L ₁
The capacitance C ₀ seen from between the terminals of is approximately
It becomes 305.48pF. The stray capacitance of the oscillation coil L ₀ is
If 5pF is assumed, the oscillation frequency f _{1 nax} is similarly determined from equation (5) to be approximately 1018.5KHz. Therefore,
The tuning frequency change ratio is as follows. f _{1 nax} / f _{1 nio} = 2184.5 / 1018.5≒2.14...
(6) As is clear from equation (6), the frequency change ratio of the local oscillator circuit due to the electronic tuning circuit according to the embodiment of FIG. This means that the tracking error is approximately zero. Furthermore, tracking errors in the electronic tuning circuit of the AM receiver of the present invention will be explained based on FIG. FIGS. 6-6 illustrate the state in which tracking errors occur when components having different circuit constants are used in the electronic tuning circuit of FIG. 1. Variable capacitance diode D ₂ of local oscillation circuit 2,
By applying the tuning voltage V _T to D ₃ , the oscillation frequency f ₀ changes, and if the reception frequency of the high frequency tuning circuit 1 is f ₁ , then the tracking error f _E can be calculated using the following relational expression: It can be expressed as f _E = f ₀ − f ₁ −450KHz ...(7) The circuit with the circuit constants shown in Table 1 and the tracking error f _E occur based on the relational expression in equation (7), and the tracking error f _E is illustrated in Figure 6. 6th
It is clear from the figure that the tracking error f _E of the electronic tuning circuit of the present invention is reduced over the entire receiving frequency band compared to the conventional one. The influence of errors in component constants of the high frequency tuning circuit 1 and the local oscillation circuit 2 will be explained based on Tables 3 and 4. The circuit constants in Tables 3 and 4 are different from those in Table 1.
Badging capacitor C _P0 and oscillation coil L ₀ are set to a tolerance range of ±1%, and capacitors C _{S1 and} C _S2 are set to a tolerance range of ±2%. The circuit constants are set to ±2%. Furthermore, Table 3 is for two-point adjustment, and Table 4 is for one-point adjustment. The second circuit constant in Table 3 is that the oscillation coil L ₁ and tuning capacitor C ₁ of high frequency tuning circuit 1 are 600KHz.
The tracking error is adjusted to zero at two points at 1400KHz, and the tracking error curve is the 6th one.
Shown in Figure D. Further, in the case of the circuit constant E in Table 3, a tracking error curve like E in FIG. 6 is obtained. Also, by adjusting only the oscillation coil _L1 of the high frequency tuning circuit 1 at the 600KHz tracking adjustment point, and setting the circuit constants as shown in G in Table 4, the tracking error as shown in G in Figure 6 can be avoided. If the circuit constants are set as shown in Table 4, the sixth
This results in a tracking error curve as shown in the figure.

【table】

〔Effect of the invention〕

As shown in FIG. 6, the electronic tuning circuit of the AM receiver of the present invention can reduce the tracking error to approximately zero in the reception band from 500 KHz to 1700 KHz. Therefore, when used in an AM stereo receiver, it is possible to provide an excellent electronic tuning circuit that can improve stereo separation and eliminate problems with distortion. In addition, the number of tracking adjustment points can be reduced from three in the past to two or one, which is an excellent feature that can reduce adjustment man-hours and reduce manufacturing costs. . Furthermore, the electronic tuning circuit of the AM receiver of the present invention can be formed without using a commonly used trimmer capacitor connected in parallel to the oscillation coil, as shown in Tables 1 and 2.
By configuring the high frequency tuning circuit and the local oscillation circuit using circuit constants such as the above, it is possible to reduce the tracking error to almost zero over a high frequency range. That is, in the electronic tuning circuit of the AM receiver of the present invention, substantially no adjustment is possible.
In addition, since a chip capacitor can be used without using a trimmer capacitor, it is possible to form an electronic tuning circuit in a small size, and it is inexpensive.
It has the side effect of providing an electrical tuning circuit for an AM receiver. In addition, since the bias resistor is not connected in parallel to the oscillation coil of the local oscillation circuit, but is connected in series with the local oscillation circuit, the generation of stray capacitance is reduced, making tracking adjustment easier. It has the following effect. Furthermore, since the bias resistors are each connected to the variable capacitance diode of the local oscillation circuit and grounded, by selecting individual resistance values, it is possible to set the high frequency impedance to obtain stable oscillation over the entire region.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of an electronic tuning circuit, FIG. 2 is a circuit diagram showing another example of an electronic tuning circuit, and FIGS. 3 to 4 are implementations of a local oscillation circuit according to the present invention. A circuit diagram showing an example, FIG. 5 is a circuit diagram of a reference example, FIG. 6 is a diagram for explaining a comparison between the present invention and a conventional electronic tuning circuit, and FIG. 7 is an example of a conventional electronic tuning circuit. FIG. 1: High frequency tuning circuit, 2: Local oscillation circuit, 3:
Terminal to which tuning voltage is supplied, 4: variable voltage source.

Claims (1)

[Claims] 1 Consists of a high frequency tuning circuit including a variable capacitance diode, an oscillation coil, and a local oscillation circuit including a capacitive element group connected to the oscillation coil via a padding capacitor connected in series.
In the electronic tuning circuit of the AM receiver, the capacitive element group is connected in parallel to the oscillation coil.
a variable capacitance diode, a second variable capacitance diode whose cathode is commonly connected to the cathode of the first variable capacitance diode, and a first fixed capacitance element connected between the anodes of the first and second variable capacitance diodes. and a second fixed capacitance element connected in parallel with the second variable capacitance diode, the first
and a fixed electrode whose one end is connected to the anode of the second variable capacitance diode and grounded via a bias resistor whose other end is grounded, and whose one end is connected to the cathode of the first and second variable capacitance diodes. It is grounded via a capacitive element, and the same voltage as the tuning voltage applied to the variable capacitance diode of the high frequency tuning circuit is applied to the cathodes of the first and second variable capacitance diodes via a resistor. It is characterized by consisting of a local oscillation circuit.
AM receiver electronic tuning circuit.