JPS6113409B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a passband selection circuit, in which a plurality of filters each having independent passband frequency characteristics and a switching diode are connected in series and connected in parallel to each other for a common connection. A main tuning circuit consisting of a tuning coil and a capacitor connected in parallel or series is connected to the common connection output terminal, and an auxiliary circuit is connected to the connection point between each filter and the switching diode. It has a configuration in which a reactance element is inserted, and in particular, in a passband selection circuit using a fixed filter such as a ceramic filter, mechanical filter, or surface acoustic wave filter, it is effective for suppressing spurious signals outside the selected specific passband. It is an object of the present invention to provide a passband selection circuit which has a passband frequency characteristic that provides good attenuation outside the passband.

Conventionally, as shown in FIG. 1, a pass band selection circuit is known which uses two ceramic filters as fixed filters and is configured to be switchable between two different pass frequencies. In the figure, 1 is a common input terminal, 2A, 2B are ceramic filters each having different passband frequency characteristics, 3A, 3B are switching diodes connected in series to each ceramic filter, 4A, 4B, and 5. 6 is a fixed resistor for passing an operating current to each switching diode, 6 is each switching diode 3A,
3B is selectively connected to the voltage source 7 through the fixed resistors 4A and 4B to make it conductive; 8 is a DC blocking capacitor; 9
is a common output terminal.

Now, as shown in FIG. 1, the changeover switch 6 is closed to the fixed resistor 4A side and opened to the fixed resistor 4B side, so the switching diode 3A is "on" and the switching diode 3B is "off", and the ceramic A signal is transmitted from the common input terminal 1 to the common output terminal 9 through the filter 2A. Therefore, its transmission characteristics are determined by the passing center frequency of ceramic filter 2A.
_A is mainly transmitted, and the ceramic filter 2
The pass center frequency _B of B and other frequencies exhibit pass band frequency characteristics at the pass center frequency _A where almost no transmission is made.

However, each of the ceramic filters 2A, 2
Since B has a unique out-of-band pass characteristic, it is not possible to obtain good attenuation outside the pass band.Furthermore, even when the switching diode 3B is in the "off" state, its minute capacitance makes it difficult for the ceramic filter to 2B is formed, and a frequency pass characteristic appears also at the pass center frequency _B , which has a further negative effect on the amount of attenuation outside the pass band, and this aspect is illustrated in Fig. 2. This is clearly shown in the transmission characteristic diagram showing the amount of attenuation with respect to the pass frequency. S _1A and S _2A in the figure represent spurious waves possessed by the ceramic filter 2A, which has a pass frequency characteristic of the pass center frequency _A , that is, waveforms that appear due to the unique out-of-band pass characteristic. Even when the frequency is switched from _A to _B , for the same reason as mentioned above, if good attenuation outside the passband cannot be obtained, the target signal frequency cannot be isolated purely on the output side, which will cause problems from a performance standpoint. However, there were some drawbacks that were extremely inconvenient.

Furthermore, the conventionally known passband selection circuit as described above is used in a television receiver that receives television signals of different systems having different audio intermediate frequencies (SIF) using a common receiver. However, in such a case, different audio intermediate frequency selection tuning circuits in the demodulator of the television receiver include a video signal or a carrier color signal (chroma signal) in addition to the main audio intermediate frequency signal. ), which tends to cause spurious buzz or noise in the demodulated audio signal. Therefore, the selective tuning circuit is switched to tune to another audio intermediate frequency that is out of tune with the corresponding audio intermediate frequency. Also, due to the above-mentioned unique out-of-band pass characteristic of the ceramic filter, the demodulated audio signal of the corresponding audio intermediate frequency is mixed in, which prevents the system switching in the television receiver from being performed normally. The problem was that it was difficult to determine whether something was present or not.

In addition, in order to suppress the spurious, that is, the inherent out-of-band pass characteristic of the ceramic filter, a resonant reactance circuit is connected in cascade to each of the ceramic filters, and the resonant frequency is made to match the pass center frequency of the ceramic filter. However, in such a conventional circuit, a reactance tuning circuit is further added to the selection tuning circuit using a plurality of ceramic filters, and the ceramic filter The same number of reactance tuning circuits are required corresponding to the number of the reactance tuning circuits, which not only complicates the overall circuit configuration and causes an increase in cost, but also has the passing center frequencies _A and _B close to each other. In the case of a ceramic filter, even the insertion of the additional reactance tuning circuit has little effect on suppressing spurious signals for the ceramic filter.

The present invention is intended to eliminate the above-mentioned drawbacks, and will be described in detail with reference to the drawings.

FIG. 3 shows a pass band selection circuit which is an embodiment of the present invention, and in this drawing, the same parts as in FIG. 1 are designated by the same reference numerals. In the figure, 10 is a main tuning circuit consisting of a coil and a capacitor for tuning the passing center frequency, and 11A and 11B are auxiliary reactances connected between the connection points of ceramic filters 2A and 2B and fixed resistors 4A and 4B, respectively, and the ground. It is a capacitor that forms an element.

Now, as shown in FIG. 3, when the changeover switch 6 is switched to the fixed resistance 4A side and closed, the switching diode 3A is turned "on" and the auxiliary reactance element 11A is switched on, and the changeover switch 6 is switched to the fixed resistance side. When switched to the 4B side and closed, the switching diode 3B is turned "on" and the auxiliary reactance elements 11B are respectively inserted in parallel to the main tuning circuit 10, resulting in different passages of the ceramic filters 2A and 2B. A reactance tuned circuit with a resonant frequency matched to the center frequencies _A , _B is formed in each case.

Now, each of the auxiliary reactance elements 11A, 1
If 1B is composed of a capacitance element, the capacitances of which are C _A and C _B , and the capacitance of the main tuning circuit 10 is C ₁₀ , the following relational expression holds true.

C _B + C ₁₀ / C _A + C ₁₀ = 2 _A / _B Therefore, as is clear from the above equation, if _A < _B , then C _A > C _B. (1) Each auxiliary reactance element 11A, 11B When both are capacitance elements, C _A > C _B.

Furthermore, considering special cases that satisfy the above conditions: (2) When C _B =0.

The auxiliary reactance element 11B is not connected,
The auxiliary reactance element 11A is connected as a capacitance element.

(3) _CB : negative, _CA : positive.

The auxiliary reactance element 11B is connected as an inductance element L _B , and the auxiliary reactance element 11A is connected as a capacitance element.

(4) C _B : Negative, when C _A =0.

The auxiliary reactance element 11B is connected as an inductance element L _B , and the auxiliary reactance element 11A is not connected.

(5) When both C _B and C _A are negative.

The auxiliary reactance elements 11A and 11B are both connected as inductance elements _LA and _LB. (However, in each case, L _A >L _B ), the desired pass characteristics can be obtained.
When connecting the inductance elements _LA and _LB , it is necessary to take measures such as connecting a DC blocking capacitor in series so as not to change the balance of the switching current.

In the above formula, if the capacitances C _A and C _B are negative, each is expressed by the following formula.

Here, ω _A =2π _A , ω _B =2π _B , L′ _A ,
Both L′ _B indicate the inductance of the inductance elements L _A and L _B.

The transmission characteristics of the pass band selection circuit shown in FIG. 3 are clearly shown in FIG.

That is, by the tuning circuit formed by the main tuning circuit 10 and the auxiliary reactance element 11A added thereto, the spurious of the ceramic filter 2A that occurs at a frequency sufficiently distant from the passing center frequency _A is suppressed, and the spurious response of the ceramic filter 2A is suppressed. In this case, the passage center frequency _B of the ceramic filter 2B near the passage center frequency A caused by the capacitive leak impedance due to the cut-off switching diode _3B is suppressed by the auxiliary reactance element 11B. The attenuation amount of the ceramic filter 2A against the spurious and the attenuation amount of the other cut-off ceramic filter 2B against the passing center frequency _B can be made extremely large compared to the conventional example shown in FIG.

Even when the switching diode 3B is turned on and the pass center frequency _B becomes the pass band, extremely good characteristics can be obtained for the same reason.

By the way, among the cases (1) to (5) above, (2),
In (4), since the auxiliary reactance element is not added to one side, the attenuation amount for the pass characteristic by the cut-off ceramic filter is not improved only in that pass band, so it is usually a particular problem. (2), unless there is no
It is clear that it is desirable not to use the method (4), but to exclusively use the methods (1), (3), and (5).

Furthermore, although the case in which two ceramic filters are used to switch between two different passband characteristics is shown in FIG. 3, in the case of three or more ceramic filters, it is possible to simply connect and arrange multiple ceramic filters in parallel. As a result, only one main tuning circuit is required, and good passband characteristics can be obtained while reducing the number of parts as much as possible compared to the conventional example.

FIG. 5 shows another embodiment of the present invention, in which the main tuning circuit is of a series resonance type instead of the parallel resonance type of FIG. 3, and the same parts as in FIG. They are indicated using the same reference numerals, and their operational functions are the same as those in FIG. 3, so a detailed explanation will be omitted. However, in the circuit of FIG. 5, the main tuning circuit 10
Auxiliary reactance element 11A or 1 added to
Although the amount of spurious attenuation due to the ceramic filter 2A or 2B conductive by each tuned circuit formed with the cut-off ceramic filter 2B or 2A is improved, the attenuation amount for the pass characteristics of the cut-off ceramic filter 2B or 2A cannot be improved that much. .

In each of the embodiments shown in FIGS. 3 and 5 described above, a circuit is shown in which 1 is used as a common input terminal and 9 is used as a common output terminal, but even if these are reversed, the same function can be achieved. It goes without saying that this can be achieved, and it can be easily understood that various switching means can be applied to the switching diode.

As detailed above, according to the present invention, the input terminal sides of a plurality of filters each having independent passband frequency characteristics are connected as a common input terminal, and the output terminal sides are connected via switching diodes connected in series. In a passband selection circuit using a plurality of filters, each having a common connection output terminal that is commonly connected to each other, and a circuit means for electronically arbitrarily switching each of the switching diodes, the common connection output terminal has a tuning coil. and a main tuning circuit consisting of a parallel or series connection of capacitors, and auxiliary reactance elements are inserted at the connection points between each of the filters and the switching diode. By adding an extremely simple circuit configuration to a passband selection circuit using filters, spurious signals outside the passband, which are other than the pass center frequency of each filter, can be effectively suppressed, and spurious signals outside the passband can be effectively suppressed. In particular, when television signals of different systems having different audio intermediate frequencies are received by a common television receiver, the demodulation of the television receiver is improved. Since extremely accurate switching demodulated audio signals can be obtained from different audio intermediate frequency selection tuning circuits in the television receiver, system switching in the television receiver can be clearly distinguished. This provides superior effects compared to selection circuits.

[Brief explanation of the drawing]

FIG. 1 shows an example of a conventional passband selection circuit.
Fig. 2 is a transmission characteristic diagram thereof, Fig. 3 is a passband selection circuit showing one embodiment of the present invention, Fig. 4 is a transmission characteristic diagram thereof, and Fig. 5 is a passband selection circuit showing another embodiment of the present invention. This is a selection circuit. 1... Common input terminal, 2A, 2B... Ceramic filter, 3A, 3B... Switching diode, 4A, 4B... Fixed resistor, 6... Changeover switch, 9... Common output terminal, 10... Main tuning Circuit, 11A, 11B...Auxiliary reactance element.

Claims (1)

[Claims] 1 The input terminal sides of a plurality of filters each having independent passband frequency characteristics are used as common connection input terminals, and the output terminal sides are used as common connection output terminals that are commonly connected to each other via series-connected switching diodes. , in a pass band selection circuit using a plurality of filters equipped with circuit means for electronically arbitrarily switching each switching diode, the common connection output terminal has a main tuning circuit consisting of a tuning coil and a capacitor connected in parallel or series; The circuit is connected, and an auxiliary reactance element is inserted at the connection point between each of the filters and the switching diode, and in the transmission path where the switching diode is conductive, the main tuning circuit and the auxiliary reactance element are connected to each other. resonates at the center frequency of the selected filter,
On the other hand, in a transmission line where the switching diode is interrupted, the auxiliary reactance element acts as a bypass for spurious signals outside the selected passband.