JPS6345051Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a television receiver, and particularly aims to improve the delayed AGC operation of the television receiver.

Generally, the UHF tuners used in current television receivers have a higher gain than VHF tuners, which means that the IF (intermediate frequency) amplification circuit tends to saturate when receiving UHF. Furthermore, since a UHF tuner handles signals at a higher frequency than a VHF tuner, the tuner's noise figure characteristics deteriorate when receiving UHF. For these, against UHF tuners
Various problems occur when performing AGC operation. Therefore, first, these problems will be explained with reference to FIGS. 1 to 3.

Fig. 1 shows a schematic configuration of the main parts of a conventional television receiver, in which 1 is a VHF tuner, 2 is a UHF tuner, 3 and 4 are switches that are linked to channel switching, and 5 is a switch that controls the output of each of the tuners 1 and 2. This is a preamplifier for amplification, and is provided to compensate for the insertion loss of the IF filter 6, which is composed of a SAW (surface acoustic wave) element or the like.

In addition, 7 is an IC-based IF circuit, and 8 is an IF circuit.
9 is a video detection circuit, 10 is a video amplification circuit, 11 is an AGC detection circuit, 12 is an AGC delay circuit, and 13 and 14 are variable resistors and resistors for adjusting the amount of delay.

In such a receiver, one of the tuners 1 and 2 operates depending on the switching state of the switches 3 and 4, and the output of that tuner is sent to the preamplifier 5 and the IF.
The signal is introduced into the IF amplifier circuit 8 through the filter 6.
Then, until the receiving input level exceeds a certain constant value, the above IF is detected by the output of the AGC detection circuit 11.
Only the amplifier circuit 8 is gain controlled (IF/AGC),
When the above constant value is exceeded, the tuner will control the gain (tuner/AGC) by the output of the AGC delay circuit 12.
It is becoming more and more common.

Figure 2 specifically explains the tuner/AGC operation during VHF reception (when each switch is in the state shown in the figure), and 16 is the AGC characteristic curve.
A ₁ represents the noise area of VHF tuner 1 and B ₁
It represents the satiri region in the IF amplifier circuit 8.
That is, the operation starting point P and the slope of the characteristic curve 16 are set so that the tuner AGC operates within the range C ₁ sandwiched between the regions A ₁ and B ₁ . When receiving VHF, the range _C1 is relatively wide as shown in the figure, so it is relatively easy to set it as described above.

However, when receiving UHF (when each switch is in the opposite state as shown in the diagram), the noise figure characteristic of the UHF tuner is worse than that of the VHF tuner, as explained earlier, so the noise area of UHF tuner 2 is as shown in Figure 3. of
As shown in A ₂ , it will be closer to the right than in Figure 2.
Moreover, if the delay amount of the AGC delay circuit 12 is set to be exactly the same as the previous VHF reception during this UHF reception, the gain of the UHF tuner 2 is larger than that of the VHF tuner 1, so the UHF tuner
The AGC characteristic curve, as shown by curve 17 in FIG. 3, is shifted to the left side compared to FIG. 2 by the gain difference between the two tuners. Therefore, for these two reasons,
The above characteristic curve 17 is in the noise region as shown in Figure 3.
It has entered A ₂ and is normal Chuyuna AGC
This means that the operation cannot be realized.

Therefore, in the circuit shown in _FIG .
AGC is applied to the connection points 3 and 14.
The delay amount of the delay circuit 12 is increased, thereby moving the operation start point of the tuner/AGC from point P to point Q in FIG. 3. That is, in this case, the AGC characteristic curve becomes as shown in 18 in the same figure and escapes from the noise region _A2 .

However, the gain of UHF tuner 2 is VHF
If it is larger than the tuner 1, the IF amplifier circuit 8 becomes more susceptible to saturation as explained above, and in this case the saturation region is shifted to the left side as shown in FIG. 3, _B2 , compared to the case in FIG. 2.
The AGC characteristic curve 18 approaches the boundary line of the saturation region _B2 , and sometimes enters the saturation region _B2 . That is, when receiving UHF, the range _C2 in FIG. 3 becomes very narrow as shown in the figure, so considering the variations in the resistors 13 to 15 for adjusting the AGC delay amount,
There was a problem in that it was very difficult to set the AGC characteristic curve 18 so that it fell within the above range _C2 .

The present invention has been devised to solve this problem, and one embodiment of the invention will now be described with reference to FIGS. 4 and 5.

FIG. 4 shows an embodiment of the television receiver of the present invention. Parts corresponding to those in FIG. It is characterized by That is, the connection of the variable resistor _{15 shown} in _{FIG .} In this configuration, the diodes and capacitors are connected in series, and the power supply _VB for the VHF tuner is applied to the connection point A between the diode and the capacitor via a resistor _R4 . Note that _C1 is also a signal bypass capacitor.

According to this configuration, when receiving VHF, the power supply _VB is applied to the point A, so the diode _D1 is on, and therefore the signal feedback resistance of the transistor Tr is only _R1 . Therefore, the preamplifier 5 operates with high gain. On the contrary
When receiving UHF, the above power supply V _B is not applied, so
Diode D ₁ is turned off and the signal feedback resistor is R ₁
+R ₂ , so the preamplifier 5 operates at a low gain.

By doing this, when receiving UHF, the IF
The amplifier circuit 8 becomes less saturated by the gain reduction of the preamplifier 5, and the noise figure characteristics of the UHF tuner 2 itself in this case do not change. Therefore, if the amplification degree of the preamplifier 5 at the time of UHF reception is set to be lower by the amount corresponding to the gain difference between the tuners 1 and 2, the UHF reception time in this case is shown in Fig. 5. In this case, the noise area _A3 is the third
While the same as in the figure, the Sachiri region
B ₃ is closer to the right than in Figure 3, and as a result,
The range _C3 between these two areas becomes wider.

Therefore, the UHF tuner 2 is adjusted so that the AGC characteristic curve 19 falls within the range _C3 shown in FIG.
AGC operation starting point (point R in Figure 5), that is, AGC
This means that the delay amount of the delay circuit 12 and the slope of the AGC characteristic can be set relatively easily. Moreover, if the amplification degree of the preamplifier 5 during UHF reception is set as described above, the AGC operation start point during VHF reception will also be the same as the above R point, and the noise region during VHF reception will be the second As shown in the figure, it is closer to the left than A ₃ in Figure 5, so the tuner when receiving VHF is
Naturally, the AGC characteristic curve 20 also falls within the range _C3 .

Note that the amplification degree of the preamplifier 5 during UHF reception does not necessarily have to be set as low as the gain difference between the tuners 1 and 2, as described above. That is,
For example, if the amplification degree of the preamplifier 5 during UHF reception is set to be slightly lower than the above gain difference, the characteristic curve 19 during UHF reception will shift slightly to the right compared to the case shown in FIG. However, it is clear that it can be done this way.

In addition, in the conventional example shown in FIG. 1, the amplification degree of the preamplifier 5 is set to
Although the S/N ratio is not selected to be too large at the cost of sacrificing the S/N ratio, in the embodiment shown in Fig. 4, the amplification degree of the preamplifier 5 is set separately for VHF reception and UHF reception. It is also possible to select a large amplification degree of the preamplifier 5 to improve the S/N ratio.

As mentioned above, according to the present invention, when receiving UHF and
The operable range of the tuner and AGC during VHF reception is expanded, and therefore the settings for each tuner and AGC operation can be easily and accurately performed.

Further, the circuit configuration is simple and can be realized at low cost.

[Brief explanation of the drawing]

Figure 1 shows the schematic configuration of the main parts of a conventional television receiver, and Figures 2 and 3 respectively show its VHF
FIG. 3 is a diagram schematically showing tuner/AGC characteristics during reception and UHF reception. Figure 4 shows an embodiment of the television receiver of the present invention, and Figure 5 shows its UHF
FIG. 3 is a diagram schematically showing tuner/AGC characteristics during reception. 1...VHF tuner, 2...UHF tuner,
5...Preamplifier, 6...IF filter, 8...
IF amplifier circuit, 9...Video detection circuit, 10...Video amplification circuit, 11...AGC detection circuit, 12...
AGC delay circuit.

Claims (1)

[Scope of utility model registration request] In a television receiver equipped with a VHF tuner and a UHF tuner with different gains, the output of one of the tuners is selectively guided to an intermediate frequency amplification circuit, and a delay AGC operation is performed on each tuner. , a television characterized in that a preamplifier for compensating for insertion loss of an intermediate frequency filter provided before the intermediate frequency amplifier circuit is provided with a circuit for switching the gain of the preamplifier between when a VHF tuner is operating and when a UHF tuner is operating. Jiyoung receiver.