JPH0218586Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable frequency bandpass filter that can obtain desired characteristics in one band.

For example, the current transmission signal frequency bands for television broadcasting in Japan are low band channels 1 to 3 (90MHz to 108MHz) and channels 4 to 12 in the VHF band.
It is divided into two high-band channels (170MHz to 222MHz), and the minimum and maximum frequencies of both bands are more than twice as far apart. In addition, the bandwidth of one channel is set to 6MHz, and in order to avoid mutual influence between adjacent channels in actual broadcasting, at least one free channel is provided above and below.

Due to this relationship, the larger the amount of out-of-band attenuation between adjacent channels via an empty channel, the less likely they are to receive interference. Furthermore, it is desirable that the band characteristics of each channel be flat and have little loss over the entire 6 MHz band, and that the bandwidth and loss of all channels do not change with frequency.

Figure 1a is a circuit diagram showing a conventional variable frequency bandpass filter used for this purpose.
IN is the input terminal, OUT is the output terminal, 1, 1', 6
is a capacitor, 2, 2', 4, 4', 5 is a coil (inductance element), 3, 3' is a variable capacitance element consisting of a variable capacitance diode, a variable capacitor, etc., 8 is a resistor, DC is a DC power supply terminal, 14 is a This is a bandpass filter circuit section. Capacitors 1, 1', and 6 each operate as high frequency passing DC blocking capacitors, coils 2 and 2' operate as coupling coils, coil 5 operates as an electromagnetic coupling adjustment coil, and resistor 8 is a variable capacitor. It operates as a resistor for applying DC voltage to the elements 3 and 3'.

The coils 4 and 4' act as resonance coils and constitute a parallel resonance circuit (tuned circuit) together with the variable capacitance elements 3 and 3', and the tuned circuits are electromagnetically coupled to each other.

If the inductances of the coupling coils 2, 2' and resonance coils 4, 4' are constant in FIG. 1a, the bandwidth of the filter is determined by the inductance of the electromagnetic coupling adjustment coil 5, and generally this The larger the inductance, the wider the bandwidth.

For example, as shown in Figure 1c, 6MHz at 90MHz
When the resonant frequency is increased by determining the bandwidth of is about
It will spread to 30MHz.

On the other hand, as shown in Figure 1b, 6MHz at 220MHz
If the resonant frequency is decreased by determining the bandwidth of the variable capacitance elements 3 and 3' and decreasing the voltage applied to the variable capacitance elements 3 and 3', the bandwidth will gradually become narrower and the insertion loss will increase. It becomes like this.

Therefore, in the filter shown in Fig. 1a, both the bandwidth and the band loss change as the frequency changes, so the filter is limited in its application to the transmission of television signals as described above. It becomes practical only in a narrow range.

Therefore, in order to improve the above-mentioned drawbacks, the filter shown in FIG. 2a, which separates the target band into two bands, a low band and a high band, has been proposed and put into practical use. In the figure, 9 and 9' are resistances for applying DC voltage to the variable capacitance elements 3 and 3';
10', 16, 16' are high frequency passing DC blocking capacitors, 11, 11' are low band frequency correction capacitors, 12, 12' are resonance coils, 1
3 and 13' are switching diodes, 15 is a high-band electromagnetic coupling adjustment coil, and 5 is a low-band electromagnetic coupling adjustment coil.

If we now apply a positive voltage to the DC power supply terminal DC', this positive voltage will be applied to the coils 5, 12,
12' to the switching diode 13,
Each diode 13, 13' is grounded via the coil 15, since the diode 13' acts to forward bias the diode 13'. By this, capacitor 1
Since the connection points J and J' between the resonant coils 12 and 12' are at approximately ground potential, the bandpass filter circuit section 14 operates in the same manner as the configuration shown in FIG. Perform the following actions.

On the other hand, if a negative voltage is applied to the DC power supply terminal DC' or zero bias is applied, the switching diodes 13 and 13' act to be biased in the reverse direction. The connection points J and J' are insulated from each other. As a result, the resonance coils 4 and 12 and 4' and 12' are connected in series and the inductance increases to perform resonance operation, so that the bandpass filter circuit section 14 performs low band operation.

In other words, in the filter of Fig. 2a, the DC power supply terminal
The switching diodes 13 and 13' are controlled to be turned on and off in accordance with the bias voltage applied to DC', thereby switching between the high band and the low band.

However, in the filter of FIG. 2a, when the switching diodes 13 and 13' are forward biased and perform high-band operation, these diodes 13 and 13' form part of a resonant circuit. Therefore, the circuit loss increases due to the internal resistance, and as shown in FIG. 2b, there is a drawback that the bandpass loss increases in the high band.

For this purpose, a method has been considered in which Q-dump resistors are connected to the resonant coils 12, 12' that serve as low-band coils, thereby intentionally deteriorating the low-band characteristics and making the characteristics uniform over the entire band.

However, increasing the loss in this way will greatly reduce the desired performance of the entire circuit including the filter, if the filter is used after the high-frequency amplification stage, but if it is used before the high-frequency amplification stage. A problem arises. Furthermore, the number of parts required for the switching circuit for switching between the two bands increases, and in addition, there is a risk of malfunction because accurate control of the switching operation is difficult.

The present invention was developed in response to the above problems, and the object thereof is to provide a variable frequency bandpass filter having a configuration in which both the bandwidth and the band loss do not change depending on the frequency. .

To achieve such an object, the present invention has first and second tuning circuits that are electromagnetically coupled, and these tuning circuits include a variable capacitance element, a resonant coil, and a common electromagnetic coupling adjustment coil. A frequency variable type in which a parallel resonant circuit is constructed, and a desired band characteristic can be obtained in one band by capacitively coupling the first tuning circuit and the second tuning circuit through a capacitive element. A bandpass filter is provided.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3a is a circuit diagram showing a variable frequency bandpass filter according to an embodiment of the present invention. The same parts as in FIG. By doing so, the first tuning circuit and the second tuning circuit are capacitively coupled.

When parallel tuned circuits consisting of variable capacitance elements 3, 3' and resonance coils 4, 4' are electromagnetically coupled and further capacitively coupled to each other as described above, the result is as shown in Figure 3b. Band characteristics can be obtained.

Compared to the conventional characteristic shown in Figure 1c, this band characteristic is a desirable characteristic in which the broadening of the bandwidth is suppressed, especially in the high band, and the level of loss does not change and remains suppressed to a constant state. .

That is, by capacitively coupling electromagnetically coupled double-tuned circuits to each other as in the embodiment of the present invention, the bandwidth, especially in the high band, can be narrowed, and the degree of narrowing can be determined by selecting the capacitance value of the capacitive element 7. The capacitance can be adjusted by adjusting the capacitance, and generally a small capacitance value is sufficient.

For example, this capacitance can be formed by a stray capacitance existing between wiring patterns formed on a wiring board or a stray capacitance existing between lead wires of a component. In short, it is also possible to utilize minute capacitance existing between conductors on the wiring board.

On the other hand, the bandwidth in the low band can be adjusted by selecting the inductance value of the electromagnetic coupling adjustment coil 5, and can be kept constant.

Furthermore, in order to connect the high-impedance bandpass filter circuit section 14 to the low-impedance input circuit or output circuit, or both, it is necessary to match them. For this reason, in the embodiment of the present invention, coils 2 and 2' are particularly connected between the input terminal IN and the output terminal OUT and the bandpass filter circuit section 14 as coupling elements.

In general, it is conceivable to use not only a coil but also a capacitor as the above-mentioned coupling element, but since the filter targeted in this invention is of a frequency variable type, the impedance of the resonant circuit changes depending on the frequency. changes, so it is necessary to take this point into consideration when selecting a specific element.

As is well known, the impedance of a parallel resonant circuit changes to a higher value when the inductance of the coil is kept constant and the capacitance of the capacitor is made variable to shift the frequency higher. On the other hand, the impedance of a single capacitor decreases as the frequency increases, whereas the impedance of the coil increases as the frequency increases. Therefore, for example, if a capacitor is selected as the coupling element, if the frequency of the filter is shifted to a lower side, the impedance changes of the capacitor and the coil will contradict each other, resulting in coarse coupling and increased loss, which is not preferable.

If a coil is selected in this respect, and its inductance is set to an appropriate value, it is possible to minimize changes in loss with respect to changes in frequency. This is the reason why a coil was selected in this invention.

Of course, if the input circuit, the output circuit, or both have high impedance, the coupling element is not required.

FIG. 4a shows a circuit diagram according to another embodiment of the present invention, in which another capacitive element 7' is connected between the high frequency passing DC blocking capacitors 1 and 1' in parallel with the capacitive element 7. This shows an example of connection. By providing the capacitive element 7' in this manner and adjusting the capacitance appropriately, the band characteristic becomes steeper in the rise of each characteristic curve as shown in FIG. 4b compared to FIG. Performance can be improved. Therefore, it can be operated as a more practical filter.

As is clear from the above description, the present invention has first and second tuning circuits that are electromagnetically coupled, and these tuning circuits include a variable capacitance element, a resonant coil, and a common electromagnetic coupling adjustment coil. This constitutes a parallel resonant circuit, and the first tuning circuit and the second tuning circuit are capacitively coupled via a capacitive element, so that a desired band characteristic can be obtained in one band. Become.

As a result, it is possible to suppress changes in the bandwidth and band loss in one band in the prior art, and it is also possible to improve problems such as a decrease in performance, an increase in the number of parts, and occurrence of malfunction in two bands in the prior art. Therefore, remarkable effects can be obtained when applied to signal transmission in television broadcasting.
However, it is possible to obtain similar effects by applying not only to television broadcasting but also to other fields of signal transmission that require a wide band.

In addition, the present invention is effective when applied not only to wide bands but also to narrow bands. For example, if it is limited to only high bands, the performance as a filter will be even more faithful, so it will be effective when applied to narrow bands. Even if a direct channel exists, the interference can be dealt with.

[Brief explanation of the drawing]

Figures 1a and 2a are circuit diagrams showing conventional examples, and Figures 1b, c and 2b are band characteristics obtained by the circuits in Figures 1a and 2a, respectively. FIGS. 3a and 4a are both circuit diagrams showing an embodiment of the present invention, and FIGS. 3b and 4
FIG. b is a characteristic diagram showing the band characteristics obtained by the circuits of FIGS. 3a and 4a, respectively. 1, 1', 6, 10, 10', 11, 11', 1
6, 16'... Capacitor, 7, 7'... Capacitive element, 2, 2', 4, 4', 5, 12, 12', 15
...Coil (inductance element), 3,3'...
Variable capacitance element, 8, 9, 9'...Resistance, 13, 1
3'... Switching diode, 14... Band pass filter circuit section, IN... Input terminal, OUT... Output terminal, DC, DC'... Direct current power supply terminal.

Claims (1)

[Scope of utility model registration request] The frequency variable band type has first and second tuned circuits that are electromagnetically coupled, and these tuned circuits constitute a parallel resonant circuit by a variable capacitance element, a resonant coil, and a common electromagnetic coupling adjustment coil. A variable frequency bandpass filter, characterized in that the first tuning circuit and the second tuning circuit are capacitively coupled via a capacitive element.