JPS6133415B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-pass filter used for converting a pulse train signal into an analog signal in a pulse width conversion type D/A conversion circuit used in a channel selection circuit of a television receiver. This is related to the improvement of.

In the D/A conversion method recently used in the tuning circuit of television receivers, digital information corresponding to the tuning voltage is stored in about 13 bits of memory, and when selecting a channel, the 13 bits of information are is read out from memory and has a pulse width corresponding to the digital information to generate a pulse train signal, that is, a pulse train signal whose repetition period is constant and whose duty ratio is changed according to the digital information,
What is used is to apply the output to a low-pass filter and convert it into an analog signal to create a tuning voltage (B _T ) to be applied to the variable capacitance diode of the tuning circuit.

Conventionally, a low-pass filter used in such a tuning circuit has a configuration as shown in FIG. 1. Here, 1 is an input terminal for a pulse train signal whose pulse width has been converted, 2 is an inverting amplification transistor, and resistors 3, 4, 5, 6 and capacitors 7, 8, 9 constitute a multistage low-pass filter, and transistors 10, 11 The output signal is taken out at a high input impedance and fed back through the capacitor 12 to form an active filter configuration. 13 is an output terminal for output voltage converted into an analog voltage.

Here, the required performance as a low-pass filter for the D/A conversion circuit in the channel selection circuit is as follows: 1. The output ripple should be 1 mVp-p or less even when a pulse input with a low repetition rate (approximately 1 msec to 2 msec) is input. 2. The output voltage Covers a wide range from about 1V to about 30V, 3. Maintains temperature regardless of the output voltage.
Must be ±30mV or less (in the range of -20℃ to +60℃), 4. Conversion speed at both rise and fall.
It is necessary to satisfy these items such as 40msec or less.

If the first term is not satisfied, ripple-like flicker noise will appear on the screen when the reception channel is switched, making it unsightly. This is a receive band with a large rate of change of tuning voltage vs. frequency.
Significant when receiving UHF bands.

If the second term is not satisfied, it becomes impossible to receive channels close to the upper limit of the UHF band, or it becomes impossible to receive the lower limit, for example.

If the third term is not satisfied, the reception frequency deviates from the normal frequency as the temperature changes, making it impossible to receive an image or resulting in an image containing noise.

If the fourth item is not satisfied, the screen will flicker and become distracting when changing channels.

As mentioned above, if the four items are not satisfied, the performance of the television receiver will be greatly degraded. Therefore, it is necessary for a low-pass filter to satisfy four performance requirements.

However, in the conventional system as shown in FIG. 1, in order to satisfy the first and second items of the above-mentioned necessary items, the third and fourth items had to be sacrificed to some extent. For example, in the circuit example shown in Figure 1, if the first and second items are satisfied, the third term will be ±50mV (-20℃ to +60℃), and the fourth term will be
It had become around 150msec to 200msec.

Therefore, an object of the present invention is to provide a low-pass filter that can satisfy all of these necessary conditions, and one embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 2 shows a basic circuit diagram of one embodiment, in which terminal 14 is a supply terminal for a +33V power supply voltage, terminal 15 is an input terminal for a pulse train signal subjected to pulse width conversion (duty ratio conversion), Terminal 16 is an output terminal for the tuning voltage (B _T ) converted into an analog voltage.

In this device, the load of the transistor 17, which corresponds to the transistor in the conventional device shown in FIG. transistor 19 and transistor 2 to reduce temperature dependence.
0 constitute a phase compensation circuit, and the current values of the constant current circuits 21 and 22 of these loads are made almost equal to improve the temperature characteristics. Here, if resistors with a large resistance value are used instead of the constant current circuits 21 and 22, the temperature dependence of the resistance value is large and stability is poor, and the temperature characteristics of h _fe of the transistors 19 and 20 Since it is impossible to achieve balance, it is also impossible to improve the overall temperature characteristics. If you use a constant current circuit,
Its temperature dependence can be made very small,
Moreover, if it is a constant current circuit using transistors, it can be stabilized by balancing h _fe of the transistors 19 and 21. Furthermore, by controlling h _fe of the transistor 19, the temperature compensation characteristics are further improved.

On the other hand, in order to improve smoothing characteristics and reduce ripple, a feedback loop is provided in the multi-stage low-pass filter using a capacitor 23 to form an active filter circuit. moreover,
The resistor 25 and capacitor 26 constitute a low-pass filter to remove harmonic components, and also absorb electrostatic noise in the television receiver, thereby preventing the transistor 20 from being destroyed. .

Note that the part surrounded by the dashed line is the integrated circuit element 2.
Consisting of 7.

Next, further details of such a circuit will be explained with reference to FIG.

First, input terminal 15 has a constant repetition period.
(2 msec here), a pulse signal whose duty ratio changes is input, and the transistor 17
Then, invert and amplify this.

The load of this inverting amplifier circuit is the transistor group 28
and a resistor 29, and its current I ₂ is 1.2 mA here.

Here, this constant current circuit 18 will be explained. First, the resistor 30 is 33KΩ, and the resistor group 31 is each 400Ω, so that the total current I ₄ flowing through the transistor group 32 is about 1 mA. This is converted into a diode made up of transistor group 32.
The current is divided by 0.25 mA, and the intersection of the resistor 30 and these diodes is connected to the base of the transistor 22 and the base of the transistor 33. Here, each resistance of the resistance group 31 is 400Ω, and the current flowing through each of these resistance groups 31 is 0.25 mA, so the voltage across these resistance groups 31 is 0.1V. If the resistor 34 is set to 1KΩ, the current I ₅ of the transistor 35 will be 0.1 mA, and if the resistor 36 is set to 330Ω, the current I ₁ of the transistor 33 will be 0.3 mA. Here, if the base current of the transistor 37 is ignored and the resistor 38 is set to 1KΩ, the voltage across it will be 0.3V, and since the resistor group 29 is each set to 1KΩ, and each of the resistors The current flowing is
0.3mA, and the total current I ₂ is 1.2mA. Further, if the resistor 39 is set to 3.9KΩ, the current I ₃ of the transistor 40 becomes 0.08mA.

Now, the inverted output of the transistor 17 is the resistor 4
1 through a terminal 42 to a multi-stage CR type multi-stage low-pass filter 24 composed of resistors 43-46 and capacitors 47-49, and its output is input to a PNP transistor 5 through a terminal 50 through a resistor 51.
Supply to the base of 2. These resistors 41 and 5
1 is a protective resistor of about 100Ω to prevent electrostatic damage. The collector of the transistor 52 is connected to the base of the NPN transistor 53, and the intersection thereof is connected to ground through a resistor 54. A resistor 54 of 3.3KΩ is used. The emitter of the transistor 52 and the collector of the transistor 53 are connected, and the collector of the driving transistor 40 of the constant current circuit 21 is connected to the emitter of the transistor 52 and the collector of the transistor 53 . This constant current value _I3 is as explained above.
It is 0.08mA.

This output is input to the base of the NPN transistor 20 via a 1KΩ resistor 55 for preventing oscillation. The emitter of the transistor 20 is a constant current circuit 2
It is connected to the driving transistor 35 of No. 2,
The emitter follower circuit has a constant current load of 0.1mA constant current _I5 . Here, the base of the PNP transistor. Temperature characteristics between emitter and base of NPN transistor. It is configured to mutually compensate for the temperature characteristics between the emitters. The emitter output of this transistor 20 is outputted from a terminal 57 via a 100Ω protection resistor 56, and this output is smoothed one more step by a resistor 25 and a capacitor 26, and then outputted from an output terminal 16 as a tuning voltage _BT .

Furthermore, in order to further suppress ripple,
Active.
Configure filters.

In such a configuration, a waveform diagram of the pulse train signal inputted from the input terminal 15 is shown in FIG. The repetition period is constant at 2 msec as shown in the figure, and the pulse width t is changed according to the digital information, with the pulse width of the positive pulse being t. Figure 5 shows the relationship between the pulse width t and _the output voltage B _T .
The relationship is a straight line as shown in the figure.

Now, ideally, if the relationship between the current values I ₃ and I ₅ were made the same in the circuit described above, the temperature characteristics would be flat, but in reality, the emitter temperature characteristics would be flat. Since the gain of the transistor 19 in the first stage of the follower and the gain of the transistor 20 in the second stage are not constant, the temperature characteristic exhibits a characteristic as shown in A shown in FIG. 6. Note that the characteristics marked △ indicated by B in Figure 6 show the temperature characteristics of the power supply voltage +B at terminal 4, and a fixed voltage is supplied to this power supply, but it is Since this is the best data measured by placing the Zener diode in a constant temperature chamber, it can be seen that the temperature characteristics of the entire circuit are best when combined with the characteristics shown in B. In other words, since a circuit that uses the output of this low-pass filter device, such as a tuner, also uses the same power supply voltage +B, it is necessary to make the temperature characteristics of the output voltage of the low-pass filter device match the temperature characteristics of the power supply voltage +B. This is because when the temperature changes, the rate of change of both voltages becomes the same in the circuit used, and the influence of temperature changes can be substantially eliminated.

Therefore, if the current I ₃ is set to about 80% of the current I ₅ and the gain of the transistor 19 is controlled by the resistor 54 to make the resistor 54, for example, 3.3KΩ, the temperature characteristics at that time will be as shown in C in Figure 6. The characteristic is as shown by the mark, and can be made to match the power supply temperature characteristic B of the terminal 14. Here, increase the resistance 54
When it is set to 5.6KΩ, the characteristics shown by D are obtained, which indicates that the correction at high temperatures is too much. This is because the h _fe of the Darlington connection 19 of transistors 52 and 53 becomes extremely large at high temperatures, so it is necessary to reduce this gain by reducing the resistance 54 to some extent. By doing so, ideal temperature characteristics can be obtained.

Next, Figure 7 shows this low-pass. This shows the response speed characteristics of the filter. The broken line shows the response characteristic of the conventional device shown in FIG. 1, and the solid line shows the response characteristic of the present device shown in FIG. As is clear from this characteristic diagram, both the rise and fall are within 40 msec. In order to speed up the rise response, in this device, the constant current I ₂ of the constant current circuit 18 of the load of the transistor 17 is
It is approximately 15 times larger than the power supply _I3 , and has a constant current capability of 1.2mA. Also,
As the transistor 17, a transistor whose saturation voltage is lower than that of other transistors is used.

As described above, according to the present device, it is possible to configure a low-pass filter that satisfies all of the above-mentioned essential items.

Moreover, as shown in Figures 2 and 3, the 6-pin IC
It is also possible to

As detailed above, according to the present invention, it is particularly suitable for use in a pulse width conversion type D/A conversion circuit.
Furthermore, it is possible to obtain a useful low-pass filter device that can be used for various other smoothing purposes.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional low-pass filter device, FIGS. 2 and 3 are circuit diagrams of a low-pass filter device according to an embodiment of the present invention, and FIGS. 4, 5, 6, and 7. 2 is a waveform diagram and a characteristic diagram for explaining its operation. 15...Input terminal, 16...Output terminal, 17,1
9, 20... Transistor, 18, 21, 22... Constant current circuit, 23... Capacitor, 24... Filter,
25...Resistor, 26...Capacitor.

Claims (1)

[Claims] 1. A pulse signal whose duty ratio changes is inverted and amplified by adding it to the base of an NPN transistor whose emitter is grounded and a first constant current load is connected as a collector load, and the first NPN transistor The collector output of is applied via a multi-stage low-pass filter to the base of a PNP transistor whose collector is grounded and a second constant current load is connected as an emitter load to form a first emitter follower circuit. Emitsuta output,
Connect the collector to the power supply and use the third emitter load.
In addition to the base of the NPN transistor, which is connected to a constant current load to form the second emitter follower circuit, the NPN transistor of the second emitter follower circuit is
The emitter output of the NPN type transistor of the second emitter follower circuit is fed back to the multi-stage low-pass filter via a capacitor, and the emitter output of the NPN type transistor of the second emitter follower circuit is output as an analog output via another low-pass filter. A low-pass filter device characterized in that the current value of the second constant current load is approximately equal to or slightly smaller than the current value of the third constant current load. 2 PNP constituting the first emitter follower circuit
As a type transistor, a PNP type first transistor and an NPN type second transistor are used in a Darlington connection.
The base of the transistor is grounded via a resistor, and the current value of the first constant current load of the first emitter follower circuit is made smaller than the current value of the second constant current load of the second emitter follower circuit. A low-pass filter device according to claim 1, characterized in that: 3 The constant current value of the first constant current load of the NPN type transistor that inverts and amplifies the pulse signal is set to a value sufficiently larger, for example, about 15 times, than the constant current value of the second constant current load of the first emitter follower circuit. Claim 1 or 2 characterized in that
The low-pass filter device described in Section 1.