KR100423398B1 - Auto gain control circuit for single conversion tuner of catv - Google Patents

Abstract

The present invention relates to an automatic gain control circuit for a tuner for a first-order conversion method that can obtain a linear characteristic required in a cable system applied to a tuner for receiving a cable-based broadcast implemented by a first-order conversion method. The automatic gain control circuit includes a gain control unit including a dual gate field effect transistor (FET) for attenuating and outputting a high frequency signal input to a first gate terminal according to a voltage applied to a second gate terminal; And overlapping and applying the automatic gain voltage to the second gate terminal of the field effect transistor and a voltage of a predetermined level applied through the nonlinear element, thereby applying an externally applied automatic gain voltage and a voltage applied to the field effect transistor. Alternatively, it comprises a maximum attenuation ratio adjusting section for adjusting the maximum attenuation ratio at the time of gain adjustment.

Description

AUTO GAIN CONTROL CIRCUIT FOR SINGLE CONVERSION TUNER OF CATV}

The present invention relates to an automatic gain control circuit provided in a tuner to adjust the gain of a received signal. More specifically, the attenuation characteristic according to the automatic gain control voltage (V _AGC ) appears linearly with the characteristic of the cable broadcasting signal. A primary conversion type cabletive is related to an automatic gain control circuit for a tuner.

As shown in FIG. 1, a first conversion tuner applied to a terrestrial broadcast receiver includes a shear amplifier 11 for amplifying a received terrestrial broadcast signal and a terrestrial broadcast passed through the shear amplifier 11. The first filter unit 12 and the first to third filters 121 to 123 for band-passing the signals of the UHF, VHF HIGH, and VHF LOW bands of the signals, respectively, and the first filter unit 12 of the first filter unit 12 First to third _AGC circuits 131 to 133 which receive the output signals of the first to third filters 121 to 123 and attenuate the corresponding terrestrial broadcast signals to the attenuation ratio according to the gain control signal V _AGC . Fourth to sixth output signals of the gain control unit 13 and the first to third AGC circuits 131 to 133 of the gain control unit 13 to remove noise of the terrestrial broadcast signal, respectively. A terrestrial broadcast signal input to the second filter unit 14 including the filters 141 to 143 and the outputs of the fourth to sixth filters 141 to 143 of the second filter unit 14, respectively. First to third oscillators 154 for providing a local oscillation signal of a frequency corresponding to a selected channel to each of the first to third mixers 151 to 153 and the first to third mixers 151 to 153 for frequency down. A mixing unit 15 comprising: a mixing unit 15, an intermediate frequency amplifier 16 amplifying and outputting the frequency conversion signal of the selected channel output from the mixing unit 15, and detecting the gain of the frequency converted channel signal The AGC detector 17 for applying the gain control voltage V _AGC to the first to third AGC circuits 131 to 133 of the gain control unit 13 and the channel signal selected according to the selected channel are band-passed and the frequency is down. The PLL unit 18 controls the first and second filter units 12 and 14 and the mixing unit 15 to be possible.

At this time, each of the first to third ACC circuits 131 to 133 provided in the gain control unit 13 uses a dual gate FET (field effect transistor) Q1 as shown in FIG. An amplifier circuit is configured, an RF signal is applied to the first gate G1, and the voltage of the second gate G2 is adjusted to V _{AGC so} that the gain of the RF signal is output to the adjusted drain D.

At this time, as 5V is generally used as the driving voltage, the AGC voltage (V _AGC ) may be used from 4V to 0.5V. Therefore, conventionally, as shown in FIG. 2, the FET Q1 provided as a 5V driving device is configured such that the source terminal S and the first gate terminal G1 operate as self-bias. The voltage of the second gate G2 is changed from 3V to 1,2V to have attenuation characteristic of 50 to 65dB.

However, in the conventional CTV receiver, since a double conversion method is usually used, a tuner is configured as shown in FIG. 3, and the AGC unit 31 does not use a field effect transistor. By using pin diode, it is composed of attenuation circuit and has automatic gain control function in all reception bands.

Since the electric field of the broadcasting signal input in the cable mode is generally in the range of about 30 dB to 0 dBmV ± 15 dB, a linear attenuation characteristic is required in the automatic gain control circuit in the range of 30 to 40 dB depending on the frequency band and AGC voltage. .

In the case of configuring such a cabtive tuner in a single conversion method, when the AGC unit is configured as shown in FIG. 2, there is a problem in that it is difficult to obtain the AGC characteristics required by the cabitive system.

The present invention has been proposed in order to solve the above-mentioned conventional problems, and an object thereof is to obtain a linear characteristic required by a calibrated system applied to a tuner for receiving a calibrated broadcast implemented by a first-order conversion scheme. The differential conversion method cable provides an automatic gain control circuit for the tuner.

1 is a configuration diagram of a general primary conversion tuner.

FIG. 2 is a circuit diagram showing an automatic gain control circuit in the primary conversion tuner shown in FIG.

3 is a block diagram of a secondary conversion tuner that is generally applied to the cable.

4 is a circuit diagram showing an automatic gain control circuit according to the present invention.

5 is a graph showing an operating state in the automatic gain control circuit according to the present invention.

* Description of the symbols for the main parts of the drawings *

Q40: Dual Gate Field Effect Transistor

C41 ~ C45: Capacitor

R41 to R45: resistance

L41: Coil

41: maximum damping ratio adjustment unit

The present invention provides a gain including a dual gate field effect transistor (FET) which attenuates and outputs a high frequency signal input to a first gate terminal in accordance with a voltage applied to a second gate terminal. Control unit; And

By applying the auto gain voltage to the second gate terminal of the field effect transistor and a predetermined level of voltage applied through the nonlinear element, the auto gain voltage applied from the outside and the voltage applied to the field effect transistor are different. An automatic gain control circuit including a maximum attenuation ratio adjustment section for adjusting a maximum attenuation ratio at the time of gain adjustment is provided.

FIG. 4 is a block diagram showing an automatic gain control circuit that can be applied to a tunable primary conversion type tuner according to the present invention, and includes a field effect transistor Q40 and a first gate G1 of the transistor Q40. 5V power is supplied through the resistor R45, and the cable broadcast signal RFin is input through the capacitor C42, and the second gate G2 of the transistor Q40 is connected to the capacitor C43. Grounded through), connected to resistor (R44), and resistor (R44) is grounded through resistor (R42) and simultaneously connected to 5V power supply terminal through resistor (R41) and through resistor (R43). The AGC voltage V _AGC is connected to be applied, the terminal of the resistor R43 to which the AGC voltage V _AGC is applied is configured to be grounded through a capacitor C41, and the source terminal of the transistor Q40 is capacitor Ground through C44, and the drain terminal of transistor Q40 is To be applied to the emitter (C45) to the output stage (RFout) power stage (5V) at the same time connected via a coil (L41) to be through the circuit.

The AGC voltage V _AGC is applied to the second gate G2 of the transistor Q40 through the resistors R43 and R44. At this time, since the resistors R41 and R42 are connected to the resistor R43, the voltage of the second gate G2 has the characteristics as shown in FIG. At this time, when the resistor R43 and the resistor R42 connected to the AGC voltage V _AGC input terminal are 15 mV, the resistors R41 and R42 are set to 150 mV and 1 mV, respectively. At this time, a voltage of 4 V is applied to the resistor R42 among the voltages of 5 V. Therefore, when the AGC voltage of 4 V is applied, as shown in the graph of FIG. The second gate voltage V _G2 is applied. On the contrary, when 1V is applied as the AGC voltage, a voltage slightly higher than the AGC voltage is applied. The attenuation ratio due to the second gate voltage V _{G2 at} this time was 50 to 60 dB when 1 V was applied as it is, whereas the attenuation ratio was reduced to 30 to 40 dB.

When 5V is set at the point "A" using the resistors R41 and R42, and the voltage at the point "B" is set using the resistor R43, the AGC voltage and the second gate voltage change differently. Thus, the characteristics as shown in FIG. 5 are obtained by adjusting the voltage at point B having the attenuation characteristic by the maximum AGC.

Therefore, the cable shift with less power variation of the signal transmitted through the cable can meet the attenuation characteristic of the system required up to 30 to 40dB and also obtain a linear gain control effect.

As described above, the AGC circuit is composed of a field effect transistor as in the AGC circuit of the conventional terrestrial broadcast receiver tuner without using a pin diode in constructing the cable broadcast tuner in the primary conversion method. In addition, there is an excellent effect that the cable can linearly provide the damping characteristics required by the system.

Claims (4)

A gain control unit including a dual gate field effect transistor (FET) for attenuating and outputting a high frequency signal input to the first gate terminal according to a voltage applied to the second gate terminal; And By applying the auto gain voltage to the second gate terminal of the field effect transistor and a predetermined level of voltage applied through the nonlinear element, the auto gain voltage applied from the outside and the voltage applied to the field effect transistor are different. An automatic gain control circuit for a primary conversion type cable tuner, comprising a maximum attenuation ratio adjustment unit for adjusting a maximum attenuation ratio during gain adjustment. The method of claim 1, wherein the maximum damping ratio adjustment unit The first resistor is connected between the second gate terminal of the field effect transistor and the automatic gain control voltage input terminal, and the second and third resistors are connected in series between the power supply terminal of the predetermined level and the ground, and the second and third resistors are connected in series. The primary gain type automatic gain control circuit for a tuner, characterized in that the contact is connected to the contact of the first resistor and the gate terminal. 3. The automatic gain control circuit according to claim 2, wherein the resistance ratio of the second resistor and the third resistor is determined such that a maximum automatic gain control voltage value is set at the contact point. The primary conversion method according to claim 2, wherein the maximum attenuation ratio adjusting unit uses a DC power supply of 5V, the first resistance is 15 kV, the second resistance is 150 kV, and the third resistor is 1 kV. Automatic gain control circuit for cable tuner tuner.