JPH018023Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a tuning circuit for an electronic tuner, and particularly to a tuning circuit suitable for wideband reception in a band switching system.

Electronic tuners that use variable capacitance diodes as a tuning selection means are widely adopted, and when the receiving band is wide, band switching using high channel coils and low channel coils as tuning coils can solve the problem of variable capacitance ratio. method is adopted. However, in the tuning circuit of the conventional band switching type electronic tuner, there is a problem in that the circuit Q decreases when selecting a high channel band. for example,
Very wideband VHF like CATV channels
In a system that receives signals using a two-band switching method, when selecting a wide band on the high side, the bypass capacitor of the band switching circuit is included in the loop of the tuning circuit, lowering the Q of the resonant circuit, and thereby lowering the Q of the resonance circuit. This may cause oscillation to stop.
In particular, bypass capacitors that bypass the switching voltage supply resistor and allow high frequency components to pass through generally do not have a very high Q, and their resistance
Since Rs is large, it greatly affects the reduction in Q of the resonant circuit in the local oscillation stage, causing oscillation to stop.

Therefore, the present invention has been proposed in view of the above points, and its object is to provide an improved tuning circuit for an electronic tuner. In other words, in the tuning circuit of a band switching electronic tuner that is equipped with a band switching circuit of a switching diode and a bypass capacitor, the bypass capacitor used when selecting a high frequency band is excluded from the high frequency equivalent circuit when receiving a high frequency channel. This prevents the Q of the resonant circuit from decreasing.

According to the present invention, the tuning circuit includes a tuning coil in which a low-frequency coil and a high-frequency coil are connected in series, a tuning variable capacitance diode coupled to the tuning coil via a coupling capacitor, and band switching at the tuning coil connection point. It is equipped with a band switching circuit that connects a switching diode for tuning and grounds it at high frequency with a bypass capacitor, and connects the tuning variable capacitance diode with its anode connected to a tuning coil whose anode is DC grounded, and whose cathode is connected via a coupling capacitor. The switching diode is connected to a junction between the switching diode and the bypass capacitor. When receiving the Haiti channel band, the series circuit of the switching diode and the bypass capacitor short-circuits the low-frequency coil at high frequency, and substantially eliminates the influence of the bypass capacitor on the high-frequency circuit of the selected high-frequency channel. This prevents the Q from decreasing due to the bypass capacitor of the resonant circuit. Therefore, the use of the tuning circuit of the present invention prevents the oscillation of a specific channel in the local oscillator stage, for example, the lowest frequency channel of the high frequency band, from stopping and enables wideband reception, as well as when the circuit is configured with chip-shaped components. This is advantageous for low Q circuits such as. In addition, in the circuit of the present invention, since the variable capacitance diode is placed on the high frequency side, the coupling capacitor forms a bypass with respect to the resistor for supplying the tuning voltage, which is advantageous in preventing a drop in Q caused by this.

Hereinafter, embodiments of the present invention will be described in detail in comparison with conventional circuits.

1 to 3 show tuning circuits of a conventional electronic tuner, and FIG. 4 shows a high frequency equivalent circuit when receiving a high channel band. First, in FIG. 1, a tuning variable capacitance diode 1 is coupled to a high channel coil 3 and a low channel coil 4 connected in series via a coupling capacitor 2, and a switching diode 5 for band switching and a DC blocking/high frequency A passing capacitor 6 (hereinafter referred to as a bypass capacitor) is connected to a low channel coil 4.
It is inserted between each end of the and ground. Here, the switching diode 5 is connected to the connection point side of the high channel and low channel coils 3 and 4 to short-circuit the low channel coil 4 at high frequency when receiving the high channel band. Channel selection is performed by applying a tuning voltage from a tuning voltage supply terminal 7 to the variable capacitance diode 1 and a switching voltage from a band switching voltage supply terminal 8 to the switching diode 5, and an active circuit 9 coupled to this tuning circuit. For example, it controls a local oscillation circuit. Figure 2 is a modification of Figure 1 for applying a switching voltage across the switching diode, except that a second bypass capacitor 6' and a second switching voltage supply terminal 8' are added. It is similar to the figure and is indicated using the same reference numerals. In these conventional circuits, the variable capacitance diode 1 is not directly connected to the hot side of the tuning coils 3 and 4 in terms of high frequency, but must be connected via the coupling capacitor 2.
That is, it is not possible to configure a circuit in which the anode side of the tuning variable capacitance diode 1 is connected to the high channel coil of the tuning coil.

Figure 3 shows a conventional tuning circuit when the anode of the variable capacitance diode 1 is connected to the high-channel coil 3.Each function is the same as in Figure 1, and parts with the same function are designated by the same reference numerals. Indicated. In this case, a high frequency equivalent circuit when receiving the Haitian channel band is shown in FIG. 4, and the Q of the resonant circuit is determined by Q=ωL/Rs and Rs=r ₁ +r ₂ +r ₃ +r ₅ +r ₆ . In addition, in Figure 4,
r ₁ is the resistance of variable capacitance diode 1, r ₂ is the resistance of coupling capacitor 2, r ₃ is the resistance of high channel coil 3, r ₅ is the resistance of switching diode 5, and r ₆ is the resistance of bypass capacitor 6. , c ₁ ,
c ₂ , c ₅ , c ₆ and L ₃ respectively indicate the capacitance value and inductance value of the component indicated by the subscript.

However, when such a conventional circuit is used in the local oscillation stage, the Q of the resonant circuit becomes low and the circuit loss becomes large, causing the oscillation of a specific channel in the high frequency band to stop. For example, like Chuyuna for CATV
When the VHF band is configured with a two-band reception system, oscillation stops at the lowest frequency channel in the higher band. In the specific US channel system, the low frequency band is assigned to channels 2 to 6, and the high frequency band is assigned to channels A to I, channel 7.
In some cases, the three bands of channels 13 to 13, and mid, high, and super of channels J to W are assigned collectively.In this case, the high frequency band is from the center frequency of the A channel (123.5MHz) to the center of the W channel. A very wide range of frequencies up to 297.5MHz must be swept without band switching. In this case, the frequency ratio (max/min) is
297.5/123.5=2.41, and the ratio of the tuning voltage of the variable capacitance diode to the tuning frequency must satisfy the following relationship.

Here, load capacitance Cl=2pF, stray capacitance Cs=
2pF, coupling capacitance Cc = 2200pF, and minimum capacitance of variable capacitance diode Cmin = 5pF, n is approximately
10, and the ISV-50 type variable capacitance diode uses a tuning voltage range of 1 to 30V.

Under these conditions, in order to generate a local oscillation signal (341MHz) in the W channel, the inductance L of the high-frequency tuning coil has a very small value of about 20nH. Using this inductance L, A
When tuning up to the channel, the reactance component becomes very small when tuning the local oscillation signal (167MHz) in the A channel, and the influence of the resistor Rs mentioned above increases. For example, when Rs≒2Ω, the circuit Q becomes Q=ωL/Rs≒10. This value is extremely small compared to circuits which are said to normally require 20 or more, and the circuit loss becomes large, causing the local oscillation circuit to stop oscillating.

The present invention eliminates the above-mentioned defects, and substantially eliminates the resistance of the bypass capacitor that causes a decrease in the Q of the resonant circuit from the resonant circuit, increasing the Q of the circuit and causing the local oscillation to stop. Provides stable tuner characteristics. FIG. 5 is a specific circuit diagram of the present invention, which shows a tuning coil consisting of a series-connected low-frequency coil 11 and a high-frequency coil 12 that are connected to the tuning coil via a coupling capacitor 13. a variable capacitance diode 16 to which a tuning voltage is applied from a tuning voltage supply terminal 14 via a resistor 15 so as to select a desired channel;
and a switching diode 18 whose one end is connected to the connection point 22 of the tuning coil and whose other end is grounded via a bypass capacitor 17. A switching voltage is applied to the switching diode 18 from a switching voltage supply terminal 19 via a resistor 20, and conducts when receiving a high frequency band to short-circuit the low frequency coil 11 in terms of high frequency.
Here, a bypass capacitor 17 that forms a bypass for the resistor 20 allows high frequency components to pass through. 22 is an active circuit to which the tuning circuit is coupled, and a second variable capacitance diode 2 is connected to this coupling means.
4 is used to configure a part of a CATV tuner, for example, a local oscillation stage.

The feature of the present invention in the above configuration is that the anode of the variable capacitance diode 16 is connected to the high frequency coil 12 which is hot at high frequencies, and its cathode is connected to the coupling point 21 of the switching diode 18 and the bypass capacitor 17 via the coupling capacitor 13. It is connected to. In this circuit configuration, when the switching diode 18 is made conductive to select the high frequency band, the high frequency equivalent circuit is
As shown in the figure, the capacity of bypass capacitor 17
C ₁₇ and resistance r ₁₇ are excluded from the high frequency circuit loop. That is, compared to the high frequency equivalent circuit of the conventional circuit shown in FIG. 4, the resistance and capacitance of the bypass capacitor 17 can be ignored. This means that the Q of the resonant circuit is greatly increased by eliminating the relatively large resistance _r17 of the bypass capacitor 17. Typically, bypass capacitors have a resistance component r ₁₇ due to their high dielectric constant.
is relatively large, and since this resistance r ₁₇ can be ignored, oscillation is possible even at a low tuning voltage, thereby preventing the oscillation of a particular channel from stopping. In experiments, by using the tuning circuit of this invention, the tuning voltage was
It can oscillate even at 0.2V, and by expanding the tuning voltage variable range compared to conventional models, we have achieved a wider band.

[Brief explanation of the drawing]

1 to 3 are conventional tuning circuit diagrams, FIG. 4 is an equivalent circuit diagram when receiving the high channel band of FIG. 3, FIG. 5 is a tuning circuit diagram of an electronic tuner according to the present invention, and FIG. This figure is an equivalent circuit diagram at the time of high frequency band reception in FIG. 5. 11...Low frequency coil, 12...High frequency coil, 13...Coupling capacitor, 14...Tuning voltage supply terminal, 16...Variable capacitance diode, 17...
...Bypass capacitor, 18...Switching diode, 19...Switching voltage supply terminal,
21... Connection point, 22... Connection point.

Claims (1)

[Scope of utility model registration request] A tuning coil consisting of a low-frequency coil and a high-frequency coil connected in series that are DC grounded, a variable capacitance diode that is coupled to the tuning coil via a coupling capacitor to select a desired channel, and one end connected to the tuning coil. The band switching electronic tuner is equipped with a switching diode that selects a band by short-circuiting the low-frequency coil connected to a connection point of the variable capacitor and having the other end grounded via a bypass capacitor at a high frequency, the variable capacitance diode has its anode side connected to the high-frequency coil, its cathode side connected to one end of the coupling capacitor, and the other end of the coupling capacitor connected to the connection point of the switching diode and the bypass capacitor, and the A tuning circuit for an electronic tuner, characterized in that a bypass capacitor can be removed at high frequency from the tuning circuit when selecting a high frequency band.