JPS6247371B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic frequency control (AFC) device that automatically controls the deviation of the local frequency of a tuner in a television video device, and particularly to a television receiver using an electronic tuner. in,
The aim is to enable smooth channel selection while the AFC function remains in operation.

Recently, the use of ICs in television receivers has progressed, and a single IC has come to perform many functions at the same time. The AFC part is also as shown in Figure 1.
Generally, they are grouped together as one IC in the VIF section of the receiver. In Figure 2, the tuner output IF
A video signal is obtained by amplifying the signal with a VIF amplifier 1 and detecting it with a video detector 2. With the introduction of integrated circuits, a synchronous detection method has been adopted as the video detection method. This is a carrier amplifier 4 having a tuning circuit 3 that tunes the IF amplified signal to the video carrier frequency connected to pins a and b in Figure 2.
The video signal is obtained by amplifying the video carrier frequency component and synchronously detecting the signal and the original IF signal. The AFC detector 5 uses the output of the carrier amplifier 4. The AFC circuit shown in FIG. 3 is generally used. This will be explained. This detection method also
This is a phase detection method using multiplicative detection used in video detectors. The reference signal is transmitted from transistors 6 and 7 of the carrier amplifier section for video detection shown in Figure 3.
The phase-shifted signal is taken out from the intersection C of the collector resistors 8, 9, 10, and 11, and is applied via the capacitance between pins a and b, which are the tuning coil connection pins of the synchronous detection circuit, and pins d and e. Then, the phase-shifted signal is transmitted by the series-parallel resonant circuit 12 connected between pins d and e.
A phase is established between the signal and the reference signal, and phase detection is performed to detect changes in this phase due to variations in the carrier frequency, that is, variations in the local frequency of the tuner. The reference signal is transistor 13,1
4, 15, 16, and the phase shift signal is applied to the emitters of those transistors 13, 14, 15, 16,
Phase detection is performed by multiplication between base and emitters 3, 14, 15, and 16. Phase detection output is from transistors 17, 18, 19, 20, 21, 2
A constant current type differential amplifier including transistors 23, 24, and 25 is operated by a current mirror circuit according to No. 2. That is, the combined collector current of transistors 13 and 16 becomes approximately the base current of transistor 22, and the combined collector current of transistors 14 and 15 becomes approximately the base current of transistor 21. However, resistance 25,2
6, 27, and 28 have the same value. Collector current flows through the transistors 21 and 22 due to this base current, but since the transistors 23, 24, and 25 form a constant current circuit, the transistors 21, 22
A current equal to the difference between the two collector currents flows into or out of the AFC output terminal pin f, obtaining an AFC output voltage. This AFC output voltage is returned to the tuner.
Forms an AFC closed loop.

I will give some additional explanation regarding this constant current type differential amplifier. First, when the local frequency is normal, the bases of transistors 13, 14, 15, and 16
With respect to the reference signal at the emitter, the phase shift signal is 90
The phase difference is maintained by the series-parallel resonant circuit 12 so that the phase difference is .degree., and the average value of the collector output current of the phase detection transistor is the same as when there is no signal, and therefore the base currents of the transistors 21 and 22 are also equal. , the collector currents are also equal, so there is no current flowing in or out from pin f. Therefore, at this time, the AFC output voltage is Vcc×R ₃₀ /R ₂₉ +R ₃₀ . However, _R29 ,
R ₃₀ is the value of resistors 29 and 30. Next, assume that the local frequency shifts to a lower value, the base current of transistor 21 increases, and the base current of transistor 22 decreases, so that the collector current of transistor 21 becomes I ₁ and the collector current of transistor 22 becomes I ₂ . Then, due to the constant current circuit of transistors 23, 24, and 25, only I ₂ flows through transistor 24, so the difference current of I ₁ - I ₂ flows out from pin f, and the AFC output voltage becomes (I ₁ - I ₂ )×R ₂₉・R ₃₀ /R ₂₉ +R ₃₀ +R _3
0 Vcc/R ₂₉ +R ₃₀ , which is a higher voltage than in the local normal state. This voltage is determined by the value R ₂₇ of the resistor 27 being R ₂₉ · R ₃₀ /R ₂₉ +R
Since it is generally sufficiently smaller than the value of ₃₀ , the saturation value is approximately Vcc.
On the other hand, when the local frequency is high, using the same idea,
A current of I ₂ −I ₁ flows into the transistor 24 from pin f. Since the value R ₃₁ of the resistor 31 is generally sufficiently smaller than R ₂₉ ·R ₃₀ /R ₂₉ +R ₃₀ , the AFC output voltage becomes approximately 0 potential. In this circuit
Figure 7 shows the characteristics of the AFC output with respect to local frequency changes. As is clear from the above explanation, when a conventional constant current differential amplifier is used as an amplification circuit for AFC detection output, the AFC output has a wide range of change from 0 to Vcc, so the AFC detection output has a very wide range. It had a retention range. For example, Vcc
12 volts, tuner control sensitivity 2.5MHz/
If the center voltage of V, AFC is 6 volts,
The retention range is 2.5 x 6 = 15 MHz, and if you tune with AFC on, the local frequency will move to a point 3 MHz higher than the adjacent channel, and it will be drawn into the audio carrier of the adjacent channel. It turns out. As before,
Each time you select a channel, turn on the AFC function on pin g in Figure 3.
All you have to do is connect an AFC switch circuit to turn it off, but if you use an electronic tuner that selects channels continuously using the volume, the AFC is always on, causing the above-mentioned inconvenience. As shown in FIG. 4, the resistor 30 in FIG. 3 is replaced with resistors 32 and 33.
A possible method is to divide the signal into two and use the intersection output, but this will result in a shift in the AFC center voltage and a decrease in detection sensitivity. As shown in FIG. 5, a slicing circuit consisting of a resistor 34 and diodes 35 and 36 is provided at the AFC output pin f, and as shown in FIG. 8, the AFC output voltage V ₁ or more and V ₂ or less are sliced. can also be considered. In this case, the detection sensitivity does not decrease, and the holding range can be freely selected depending on the values of V ₁ and V ₂ , but it has the disadvantage of requiring the use of expensive components such as diodes.

The present invention eliminates the above-mentioned drawbacks, allows smooth tuning while maintaining the AFC function in a continuously variable electronic tuner tuning method without reducing detection sensitivity, and further,
It is an object of the present invention to provide an automatic frequency control device that can be realized by simple means and at low cost.

An embodiment of the present invention is shown in FIG. 6 and will be described. The same parts as in FIGS. 3 to 5 are given the same numbers and the explanation will be omitted. The features of the present invention are
The point is that a resistor 37 of an appropriate value is inserted between the AFC output pin f and the intersection of the load resistors 29 and 30. To facilitate this explanation, we put practical values into these resistances and, as shown in Figure 9, R ₂₉ =
33KΩ R ₃₀ = 39KΩ, R ₃₇ = 31KΩ. This can be equivalently replaced with FIG. First, when the local frequency matches, as mentioned above, there is no current flowing in or out from pin f in Figure 6, so it is equivalently considered that there is an open between resistor 37 and pin f.
The AFC output voltage is Vcc×R ₃₀ /R ₂₉ +R ₃₀ , that is, the first
In Figure 0, it is 6.5 volts. Next, if the local frequency shifts to a certain extent lower and the collector current I ₁ of transistor 21 becomes larger than the collector current I ₂ of transistor 22 as described above, the current at I ₁ - _{I 2} will be lower than pin f. It flows out, passes through the resistor 37 in FIG. 10, and further flows into the 6.5 volt power supply line through the combined resistance (load resistance) 38 (R ₃₈ ) of the resistors 29 and 30. Therefore, a voltage of (I ₁ - I ₂ )×R ₃₈ +6.5 is generated at the AFC output point. In other words, AFC for local frequency changes
The change in output voltage, that is, the detection sensitivity, is exactly the same as the conventional one. Next, the difference current of (I ₁ - I ₂ ) becomes even larger, and (I ₁ - I ₂ )=Vcc-6.5/R ₃₇ +R ₃₈ = 12 _v
-6.5 _v /31KΩ+17.8KΩ=0.11mA, the pin f, that is, the collector voltage of the transistor 21 reaches Vcc, and there is no further increase in current. That is, the AFC output voltage is (12 _v −6.5 _v ) ×
It saturates at 17.8KΩ/31KΩ+17.8KΩ+6.5≒8.5 volts.

With the same idea, when the local shifts to the higher side,
6.5×31KΩ/31KΩ+17.8KΩ≒4.1V
It does not fall below the AFC output voltage, and has the same detection sensitivity as the conventional one until it drops to 4.1 volts. AFC at this time
The characteristics are shown in FIG. Practically speaking, the value of the resistor 37 is preferably about 1/3 to 3 times the value of the load resistor 30 shown in FIG.

As described above, according to the present invention, all that is required is to insert the third resistor of an appropriate value between the output terminal of the constant current type differential amplifier circuit and the intersection of the first and second load resistors. The amplitude of the AFC output can be reduced without deteriorating the detection sensitivity and without changing the AFC center voltage, and the AFC holding range can be narrowed. As a result, even in the continuously variable channel selection method, smooth channel selection operations can be performed while the AFC operation remains active. Furthermore, if the AFC function can be turned off each time the channel is selected using a button, the retention range can be maintained by simply shortening or reducing the resistor value of the present invention using the same IC. A wide range of characteristics can be easily reproduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional television receiver, Fig. 2 is a block diagram of the VIFIC of the same receiver, and Fig. 3 is a circuit diagram of a part of the same IC.
4 and 5 are circuit diagrams partially modified from the circuit in FIG. 3, FIG. 6 is a circuit diagram of an automatic frequency control device according to an embodiment of the present invention, and FIGS. 7, 8, and 11.
9 is a characteristic diagram for explaining the present invention, FIG. 9 is a partial circuit diagram of the circuit of FIG. 6, and FIG. 10 is an equivalent circuit diagram of the circuit of FIG. 9. 13, 14, 15, 16, 17, 18, 19,
20, 6, 7, 21, 22, 23, 24, 25...
...transistor, 3...tuned circuit, 12...series-parallel resonant circuit, f...output terminal pin, 29, 30,
37...Resistance.

Claims (1)

[Claims] 1. A detector for detecting a shift in the local frequency of the tuner and a constant current differential amplifier circuit for amplifying the detection output of this detector are provided, and a detector is provided between the power supply terminal and the ground potential point. First and second load resistors connected in series are inserted between the output terminal of the constant current differential amplifier circuit and the intersection of the first and second load resistors.
An automatic frequency control device characterized by inserting a third resistor to narrow the AFC holding range. 2. The automatic frequency control device according to claim 1, wherein the value of the third resistor is set in a range of 1/3 to 3 times the value of the first and second load resistors. . 3 The detector and constant current differential amplifier circuit are configured within the IC element, and the first and second load resistances and third resistance are not included within the IC element, allowing the AFC output amplitude characteristics to be freely selected. An automatic frequency control device according to claim 1, characterized in that the automatic frequency control device is configured as follows.