JP3050883B2 - Electronic tuner - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic tuner for CATV reception used in a television receiver, a video tape recorder, and the like.

2. Description of the Related Art A conventional electronic tuner of this type will be described with reference to a circuit diagram of FIG.

The television signal input to the tuner input terminal 1 is transmitted to the tuning circuit section A via the IF trap circuit 2 and the DC blocking capacitor 3.

The CATV reception range is divided into a high band and a low band, and the tuning circuit unit A supplies a tuning coil 5 for high band and a tap coil 10 for high band to these bands.
A low-band tuning coil 8 and a low-band tap coil 9 are provided, and a tuning circuit is constituted by these coils 5, 10, 8, 9 and a tuning varicap diode 17. Further, a coupling varicap diode 6 is connected in series to the high-band tuning coil 5, and a voltage is applied to a tuning voltage terminal 23 so that the resistor 4 is turned off.
A voltage is applied to the tuning variable diode 17 and the coupling variable diode 6 via the circuit, so that the tuning frequency can be varied. Reference numeral 25 denotes a resistor for zero bias to the anode side of the coupling varicap diode 6.

By applying a voltage to the high-band voltage terminal 22, the resistor 20 and the band-switching diode 12 are turned on.
Current flows through the resistor 16 and the band switching diode 12 is turned on to enable tuning in the high band, and by applying a voltage to the low band voltage terminal 15, the resistor 14 and the resistor The voltage generated by the division ratio of 16 is applied to the cathode of the band switching diode 12.
No voltage is applied to the anode, and the band-switching diode 2 is turned off to enable low-band tuning.

Further, in the tuning circuit section A, an image trap capacitor 24 is connected to a series circuit section of the high band tuning coil 5 and the coupling varicap diode 6.
Adjustable image trap circuit B for high band
Is configured. The image trap capacitor 24 is set to about 20 pF or less.

The television signal applied to the tuner input terminal 1 passes through the tuning circuit section A and the RF signal via the coupling capacitor 18.
Is applied to the gate 1 of the transistor 19 and amplified.

The variable amount of the frequency is analyzed with respect to the image trap circuit section B in the input circuit section of the tuner configured as described above. FIG. 3 shows an equivalent circuit of the image trap circuit section B.

L ₁ is the inductance of the tuning coil 5 for high band,
C _T represents the capacitance of the coupling varicap diode 6, and C ₁ represents the capacitance of the image trapping capacitor 24.

In this case, admittance ₁ and impedance ₁ of the equivalent circuit are as follows.

And than the impedance ₁ infinite anti-resonance point and the image tiger class tap Te summer when are configured, when the frequency of the image tiger class tap at that time and _{ω 1, ω 1 (C 1} + C T -ω 1 2 L 1 C ₁ C _T ) = 0 (3) From equation (3) Becomes

Then go in search of image Tiger class tap frequency ω ₁ in the actual circuit using Equation (4). Now take the case of North American CATV reception as an example.
~ ChJJ is received in the high band.

Suppose that chJJ is received at the tuning voltage BT = 23v, and the trap frequency at that time is exactly adjusted to the image frequency of chJJ.
In addition, a high N ratio (C _2V / C
_If a varicap diode 6 having a _25V = N ratio) is used, C
_T = 3 pF. If C ₁ = 4 pF to further improve the trap attenuation characteristics, L ₁ = 73.7 nH from the equation (4).

Therefore, Becomes The tuning voltage BT and the trap frequency are calculated using equation (5), and the relationship among the reception frequency, image frequency, and image trap frequency at that time is shown in FIG.

As can be seen, in the conventional method, the tuning frequency is varied to change the reception frequency from chC (133.25 MHz) to chJJ (355.25 MHz).
When covering z), the required image frequency at that time is 224.75MHz to 446.75MHz and a frequency ratio of 1.99 is required, whereas the change range of the image trap is
Only 1.46 can be realized with a frequency ratio of 309 to 446.75 MHz.

The equivalent circuit of the tuning circuit of the oscillator circuit of the CATV electronic tuner is as shown in FIG. 4, and the basic configuration is different from that of FIG. 3, which is the equivalent circuit of the image trap circuit B. As can be seen from FIG. 8, there is a large shift in the low frequency channel.

In FIG. 4, C _T0 is the capacitance of the tuning horsepower guy flop diode 17 in the oscillator circuit component, L ₀ denotes the inductance of the high-band tuning coil 5 in the oscillator circuit.

That is, the trap frequency cannot be set to the image frequency according to each channel, and the image interference ratio cannot be sufficiently secured.

Problems to be Solved by the Invention In CATV reception, the number of channels is expected to increase more and more in the future, especially in North America CATV, which is the conventional UHF frequency band of 550 MHz.
It has also been used in CATV broadcasting. For this reason, also as a television receiver / video tape recorder, there are many combinations of beat interference from a channel that is higher in frequency than the reception channel, and it is particularly required to improve the performance of beat interference due to an image frequency.

Beat interference due to this image frequency is determined by the performance of the electronic tuner. Especially CATV
In the receiving electronic tuner, as shown in FIGS. 12 and 13, one band divided has a large number of channels, so that a fixed image trap is not effective enough and a tunable image trap is not available. In the conventional method, the variable range was narrow and limited.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an electronic tuner in which the above-mentioned problem is solved and the variable range of the image trap is wide and the setting can be performed accurately.

Means for Solving the Problems In order to achieve this object, the present invention provides an electronic tuner capable of receiving CATV, wherein an input circuit thereof comprises a tuning coil and a first coupling varicap diode connected in series. A first series-connected body, and a second series-connected body in which the first series-connected body and the tap coil are connected in series, and the second series-connected body and the second A tuning circuit is configured by connecting varicap diodes in parallel,
An image trap circuit having a third varicap diode having substantially the same characteristics as the second varicap diode is connected in parallel with the first series connection body;
A connection point between the series connection body and the tap coil is connected to a tuner input terminal, and a connection point between the first series connection body and the second varicap diode is set as an output of this input circuit, It is formed such that the same tuning voltage is applied to the first, second, and third varicap diodes.

Operation With this configuration, since the second series-connected body including the first coupling varicap diode and the second varicap diode for tuning are connected in parallel, the reception band is widened. In the image trap circuit, since the third varicap diode is provided in parallel with the first series connection body including the first varicap diode for coupling, the trap variable range is widened.

Further, since the third varicap diode used for the image trap circuit has substantially the same characteristics as the second varicap diode for tuning, the variable range of the image trap circuit is changed to the variable range of the tuning circuit. It is possible to make them substantially equal, and accurate follow-up becomes possible.

An embodiment of the present invention will be described below with reference to the drawings. The same components as those of the conventional example shown in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted.

FIG. 1 is a circuit diagram of an input circuit section of an electronic tuner showing one embodiment of the present invention. In the electronic tuner of the present invention, a series circuit portion of an image trapping barrier diode 26 and a putting capacitor 27 is newly provided in parallel with a series circuit of a high band tuning coil 5 and a coupling barrier diode 6. In addition, an image tracking correction capacitor 24 is connected in parallel to the image trap variable diode 26. A zero bias is applied to the anode sides of the varicap diodes 26 and 6 by the resistors 25 and 28, and the resistor 4
A voltage is applied to the cathode side of the varicap diodes 26, 6 via the, thereby varying the image trap frequency.

The variable amount of the image trap circuit section B is analyzed. FIG. 5 shows an equivalent circuit of the image trap circuit section B.

C _T1 is a varicap diode 26 for image trapping.
C ₂ is a capacitor for image tracking correction
24 capacity, C ₃ capacity of putting capacitor 27,
L ₂ is the inductance of the tuning coil 5 for high band, C
_T2 indicates the capacitance of the coupling varicap diode 6.

 FIG. 6 shows the equivalent circuit of FIG.

C _T ′ represents the equivalent capacitance of the image trapping varicap diode.

It is. When the frequency for image tigers class tap and omega ₂ Equation _{(4) C 1 = C T} ' than, as L ₁ = L _2, Becomes Here, if C ₂ = 0 pF and C ₃ = 2.2 nF in FIG. 5, C _T ′ ≒ C _T1 . In FIG. 5, C _T1 and C _T2 are replaced by a varicap diode having the same characteristics. And C _T1 = C _T2 , and equation (7) becomes Becomes

Then go in search of image Tiger class tap for the frequency ω ₂ in the actual circuit using the equation (8). At this time, the same as the conventional example
Suppose that C to chJJ are received in one band, chJJ is received at the tuning voltage BT = 23 V, and the trap frequency at that time is exactly adjusted to the image frequency of chJJ. When a high-N ratio variable-capacitance diode used in the conventional example is used, C _T = 3 pF.
From this condition, L ₁ = 84.7 nH from equation (8). That is, Becomes Calculate the tuning voltage B _T and a trap frequency using the equation (9), also shows the relationship between the actual tuning frequency at that time in FIG. 9. Represents the relationship tuning voltage B _T of the image for a trap frequency when using simultaneously one embodiment of the present invention together with it represents the relationship tuning voltage B _T of the image frequency and the reception frequency to the ninth FIG. In other words, the tuning frequency is varied to change the reception frequency from chC (133.25 MHz) to ch.
JJ (355.25MHz), the image frequency is 224.7
While 5 MHz to 446.75 MHz and a frequency ratio of 1.99 are required, the change range of the image trap frequency is 120
The frequency ratio can be as wide as 3.72 at 446.75 MHz, which is about 2.55 times larger than the conventional frequency ratio of 1.46.

Further, as can be seen from FIG. 9, the change in the image trap frequency is large with respect to the change in the image frequency, and it is necessary to correct the change.

It will suppress the effectiveness of the image for a trap frequency omega ₂ of the second term of Equation (7) as the method. That is, the C ₃ in Fig. 5 and less than 2.2nF impart an effect as Patsuteingu so as to suppress the effect of the C _T1. Or by the case connexion is C ₂ so as to suppress effectiveness in high-frequency channel of C _T1 by adding a capacitor with a value of 0~30pF as C _2, C ₃
Determine the constant.

FIG. 10 shows the relationship between the reception frequency, the image frequency, and the image trap frequency after the correction is performed as described above. As can be seen from FIG. 10, the curve of the image trap frequency can be approximately approximated to the curve of the image frequency, and as a result, as shown in FIG. 11, the conventional method is applied to all channels during high band reception. More average 15 ~
The improvement can be as much as 20 dB, 80 dB or more can be secured, and a clean screen without image interference can be provided.

In the above embodiment, when receiving chC to chJJ as one band, the image frequency ratio of chJJ and chC is
Since it is necessary to be as large as 1.99, all of the tuning barrier diode 17, the coupling barrier diode 6, and the image trapping barrier diode 26 shown in FIG. ing.

When the band division of the CATV reception of the electronic tuner is as shown in FIG. 13, that is, when receiving chJ to chEEE as one band, the coupling varicap diode 6 in FIG. Bali Cap Diode 17
However, even if the normal N ratio is used, the reception range can be sufficiently covered. Since the image frequency ratio at this time is 1.80, the normal trap diode 26 for image trap
A ratio may be used. FIG. 15 shows the improvement of the image interference ratio in the case of this combination, and FIG.
Shown in the figure.

As described above, according to the present invention, in an electronic tuner capable of receiving CATV, an input circuit thereof includes a first series connection in which a tuning coil and a first coupling varicap diode are connected in series. And a second series connection in which the first series connection body and the tap coil are connected in series,
A tuning circuit is formed by connecting the second series-connected body and the second tuning varicap diode in parallel, and a third circuit having substantially the same characteristics as the second varicap diode is formed in parallel with the first series-connected body. An image trap circuit having a varicap diode is connected, a connection point between the first series-connected body and the tap coil is connected to a tuner input terminal, and the first series-connected body and the second
The connection point with the varicap diode is used as the output of the input circuit, and the same tuning voltage is applied to the first, second, and third varicap diodes. By configuring, the first for coupling
Since the second series-connected body including the varicap diode and the second varicap diode for tuning are connected in parallel, the reception band is widened. In the image trap circuit, since the third varicap diode is provided in parallel with the first series connection body including the first varicap diode for coupling, the trap variable range is widened.

Further, since the third varicap diode used in the image trap circuit has substantially the same characteristics as the second varicap diode for tuning, the variable range of the image trap circuit is reduced to the variable range of the tuning range. It can be made substantially equal, accurate follow-up becomes possible, and the effect is significant.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of an input circuit of an electronic tuner showing one embodiment of the present invention, FIG. 2 is a circuit diagram of an input circuit of a conventional electronic tuner, and FIG. 2 is an equivalent circuit diagram of the image trap circuit portion B of FIG. 2, FIG. 4 is an equivalent circuit diagram of a tuning circuit of the oscillator portion of the electronic tuner, FIG. 5 is an equivalent circuit diagram of the image trap circuit portion B of FIG. FIG. 6 is an equivalent circuit diagram of FIG. 5, FIG. 7 is a characteristic diagram showing a tuning voltage and a capacitance of a high N ratio varicap diode, FIG. 8 is a characteristic diagram showing a conventional image trap frequency variable range, and FIG. FIG. 9 is a characteristic diagram showing an image trap frequency variable range (state before correction) according to an embodiment of the present invention, and FIG. 10 is a frequency variable range (image after correction) of the image trap according to one embodiment of the present invention. FIG. 11 is a characteristic diagram showing And FIG. 12 is a characteristic diagram showing an image interference ratio of an electronic tuner according to the present invention. FIG. 12 is a diagram showing a band division method in an electronic tuner receiving two bands of North American CATV, and FIG. 13 is receiving three bands of North American CATV. FIG. 14 is a diagram showing a band division method in an electronic tuner. FIG. 14 is a characteristic diagram showing a conventional image trap frequency variable range in reception in a CATV3 band.
The figure is a characteristic diagram showing the frequency variable range of the image trap in the present invention in the reception in the CATV3 band. A: Tuning circuit section, B: Image trap circuit section
5: tuning coil for high band, 6: variable diode for coupling, 8: tuning coil for low band, 9:
… Tap coil for low band, 10… Tap coil for high band, 17… Baricap diode for tuning, 23…
… Tuning voltage terminal, 26… Variable diode for image trapping, 27… Putting capacitor.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-61-37638 (JP, A) JP-A-1-162430 (JP, A) JP-A-2-41519 (JP, U) (58) Survey Field (Int.Cl. ⁷ , DB name) H03J 3/18 H03J 5/24 H04B 1/18

Claims (2)

(57) [Claims] In an electronic tuner capable of receiving CATV,
The input circuit includes a first series connection body in which a tuning coil and a first coupling varicap diode are connected in series;
A second series connection body in which the first series connection body and the tap coil are connected in series is provided, and a tuning circuit is provided by connecting the second series connection body and the second tuning varicap diode in parallel. And a third capacitor having substantially the same characteristics as the second varicap diode in parallel with the first series-connected body.
An image trap circuit having a varicap diode is connected, a connection point between the first series-connected body and the tap coil is connected to a tuner input terminal, and the first series-connected body and the second An electronic tuner formed such that a connection point with the varicap diode is used as an output of the input circuit, and the same tuning voltage is applied to the first, second, and third varicap diodes. 2. The tuning coil is formed by a series connection of a high band tuning coil and a low band tuning coil, and the tap coil is formed by a series connection of a high band tap coil and a low band tap coil. Band switching is performed by simultaneously short-circuiting or opening the low band tuning coil and the low band tap coil, and an image trap circuit is formed in parallel with the high band tuning coil. 2. The electronic tuner according to claim 1, wherein a putting capacitor is connected in series with the varicap diode, and an image tracking correction capacitor is connected in parallel with the varicap diode.