JPH06105878B2 - AGC device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an AGC (automatic gain control circuit) device for a satellite broadcast receiver.

2. Description of the Related Art In satellite broadcast receivers, the target rating is set as the level range of the input signal is -61 dBm to -28 dBm (Reference: Satellite broadcast receiver No. 1 Target Rated Radio Engineering Association, 1983, 6). Monthly publication). For this reason, in a satellite broadcast receiver, AGC is generally used to keep the signal level to a demodulator that performs FM demodulation of a video signal constant.
Conventionally, the AGC function has adopted a forward AGC method that utilizes the fact that the gain decreases as the emitter current of the transistor increases.

Hereinafter, the conventional AGC as described above will be described with reference to the drawings.
The apparatus will be described. FIG. 5 (a) shows a conventional AGC device. In FIG. 5 (a), 1 is the first
Intermediate frequency (1st IF) signal input terminal, 2 wideband amplifier, 3 image filter. 4 is the second frequency converter (2n
d Mix), 5 is a second local oscillator, 6 is a variable gain amplifier,
Reference numeral 7 is a second intermediate frequency amplifier, 8 is a peak detector, and 9 is a DC amplifier.

The operation of the AGC device configured as above will be described below. FIG. 5 shows an example of the configuration of the satellite broadcast receiver, and elements not related to AGC are omitted. The 1st IF signal input to the input terminal 1 is amplified by the wideband amplifier 2 by about 15 dB, then the image filter 3 removes noise in the image band, and then input to the 2nd Mix 4. 2nd Mix 4 fulfills the channel selection function with the signal of the 2nd local oscillator 5, and the 1st IF signal requires 1
It plays the role of frequency conversion of only waves to 2nd IF signal. 2
The 2nd IF signal output from the nd Mix 4 is input to the variable gain amplifier 6. The variable gain amplifier 6 has a gain control terminal, and the gain can be varied by the input voltage to the terminal.

FIG. 5 (b) shows an example of the configuration of the variable gain amplifier 6, and if a transistor (for example, 2SC3077) specially designed for this application is used, a gain control of about 40 dB is achieved with one transistor stage. Is possible. In FIG. 5 (b), 10 is an input terminal, 11 is an output terminal, 11 and 18 are capacitances for a DC block, 15 is a capacitance for grounding the base, and resistors 12, 13, and 17 are for bias setting. Is.
By applying a control voltage VAGC to the gain control terminal 14, the emitter current of the transistor 16 can be controlled to change the amplification gain. In order to obtain a wider gain control range, the grounded-base amplifier is preferable to the grounded-emitter amplifier, and the forward AGC method in which the emitter current increases when the signal level is high is generally used in order to maintain good intermodulation characteristics. is there.

The 2nd IF signal output from the variable gain amplifier 6 has a constant signal level, is input to the second intermediate frequency amplifier 7, and is amplified to a level suitable for the peak detector 8.
The peak detector 8 outputs a DC voltage according to the peak value of the input signal. The output DC voltage is the DC amplifier 9
After being sufficiently amplified by, the signal is applied to the control terminal of the variable gain amplifier 6. The AGC closed loop constructed in this way is
It constitutes a negative feedback loop, and even if the level of the 2nd IF signal input to the variable gain amplifier 6 fluctuates, the control system works so that the output has a constant level.

However, in the above-mentioned configuration, when the input level of the 1st IF signal is large, there is a problem that intermodulation occurs in the second frequency converter 4, so that in recent years, the second frequency modulator 4 The development of an AGC device that inserts a variable attenuator in the previous stage was also desired.

In view of the above problems, the present invention does not pose a problem with the intermodulation characteristics of the second frequency converter when the input level of the 1st IF signal is high, and the NF when viewed from the 1st IF input terminal when the input level is low is AGC. Therefore, an AGC device that does not deteriorate is provided.

Means for Solving the Problem In order to solve this problem, an AGC device of the present invention uses one terminal of a first capacitance as an input, and the other terminal is grounded via a first resistor, and The cathode terminal of the first PIN diode is grounded, the anode terminal of the second PIN diode is connected to the anode terminal of the first PIN diode, and the cathode terminal of the second PIN diode is grounded via the second resistor. , And one terminal of the second capacitance is connected, the other terminal of the second capacitance is used as an output terminal, and one of the third resistors is connected to a connection point of the anode terminals of the first and second PIN diodes. A first variable attenuator which is connected to the other terminal of the third resistance, is connected to the other terminal of the third resistance to the attenuation control terminal via the fourth resistance, and is grounded via the third capacitance. The input signal through Input to the wave number converter and output the frequency-changed first
Via the amplifier, one terminal of the fourth capacitance is used as an input terminal, the other terminal is connected to the anode terminal of the third PIN diode, and the cathode terminal of the third PIN diode is connected to the fourth PIN diode. Connected to the anode terminal of the fourth PIN diode, the cathode terminal of the fourth PIN diode is grounded through the fifth resistor, and one terminal of the fifth capacitance is connected, and the other terminal of the fifth capacitance is connected to the output terminal. And connect a sixth resistor to the connection point between the fourth capacitance and the cathode terminal of the third PIN diode,
The other terminal is connected to the attenuation control terminal via the seventh resistor and is input to the second variable attenuator that is grounded via the sixth capacitance, and its output is amplified until it can be detected. Input to the second amplifier for amplification, the output is input to the peak detector to obtain a DC voltage output according to the input level, the DC voltage output is compared with the reference voltage, and the comparison output is the DC amplifier. And the amplifier output is connected to the gain control terminals of the first and second variable attenuators.

With this configuration, in the AGC device of the present invention, the first variable attenuator separates the DC bias between the input terminal and the PIN diode by the first and second capacitances, and the control terminal voltage is in the forward direction of the PIN diode. When it becomes higher than the voltage drop, a forward current flows through the PIN diode. This is P
This is because the IN diode is connected in parallel between the control terminal and ground. The internal resistance of the PIN diode changes due to the forward current, and the PIN diode exhibits almost pure resistance even for a high frequency signal. The higher the forward current, the lower the internal resistance. The first, second and third resistors and the two PIN diodes form a π-type attenuator circuit, and the amount of attenuation thereof decreases as the forward current increases.

Similarly to the first variable attenuator, the second variable attenuator separates the DC bias of the input / output terminal and the PIN diode by the fourth and fifth capacitances, but the two PIN diodes are connected in series between the control terminal and ground. It is located in. Therefore, when the control terminal voltage becomes higher than twice the forward drop voltage of the PIN diode, a forward current flows in the PIN diode. The second variable attenuator also constitutes a π-type attenuator circuit by the fourth and fifth resistors and the PIN diode, and the amount of attenuation decreases as the forward current increases.

The forward currents of the PIN diodes of the first and second variable attenuators are determined by the control terminal voltage, but if the control terminal voltage gradually increases from zero, the PIN diode of the first variable attenuator must first be forwarded. Current begins to flow in the direction, followed by the second. Therefore, the attenuation amount of each variable attenuator is different even with the same control terminal voltage, and the attenuation amount of the first variable attenuator decreases first. The fourth and seventh resistors serve to limit the current of each PIN diode.

The signal that has passed through the first and second variable attenuators is no longer controlled to a constant level, but this control signal is obtained by the peak detector that obtains a DC voltage output according to the input level of the signal and uses this as a reference. It is obtained by comparing with a voltage and sufficiently amplifying the comparison voltage through a DC amplifier. The AGC function is achieved by this negative feedback action.

By the way, the role of the first variable attenuator is 1s at the time of strong input.
This is to suppress the generation of interfering signals due to the intermodulation of the frequency conversion of the t IF stage.
It is desirable that the amount of attenuation is as small as possible so as not to deteriorate NF. The role of the second variable attenuator is mainly to have a sufficient variable range of gain so that the AGC function works over a wide level range of the input signal. This variable attenuator is 2nd
Since it is placed after the IF amplifier, the increase in attenuation does not affect the NF value seen directly from the input end. Therefore, at the time of weak input, the second variable attenuator in the 2nd IF band mainly operates,
A configuration in which the first variable attenuator operates only when the input signal level becomes high and a strong input is obtained is desirable from the viewpoint of NF and intermodulation characteristics.

In the AGC device of the present invention, the control terminal voltage value at which the operation of each variable attenuator starts is different by configuring the connection type between the control terminal of the PIN diode and the ground to be different in parallel and series. Thus, the intermodulation characteristic at the time of strong input is improved, and the NF deterioration amount at the time of weak input is small.

Embodiment One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 (a) is a block diagram of an AGC device according to an embodiment of the present invention.

In FIG. 1 (a), 102 is a first variable attenuator, and its configuration example 103 shown in FIG. 1 (b) is 2nd having a gain of about 20 dB.
An IF amplifier and a second variable attenuator 104 are shown in FIG. 1 to 9 are equivalent to FIG. 5 of the configuration example in the conventional example. There are first and second variable attenuators that compose the AGC system, which are the 1st IF stage and the 2nd
It is placed at the IF stage.

The input level range of the 1st IF signal of the satellite broadcasting receiver is set as the target rating of -61dBm to -28dBm as described above, and the AGC range of the receiver is shown in Fig. 2 if it is sufficiently satisfied. The level diagram shown in is preferable. In Fig. 2, the operating range of the AGC is that the 1st IF signal level is -70d considering the mass production variation of the gain of each block of the receiver.
It was set to −20 dBm from Bm. Low input signal level, -70 dBm
From --50 dBm, only the gain of the 2nd IF stage changes,
At −50 dBm and above, the gain of the 1st IF stage also changes at the same time, so that the intermodulation characteristics do not deteriorate when the 1st IF stage has a strong input signal. In this figure, the variable gain range is 25 dB and 40 dB for the 1st IF stage and the 2nd IF stage, respectively.
Is assumed to have.

Fig. 1 (b) shows the variable attenuator 1 arranged in the 1st IF stage.
In the circuit diagram of 02, the capacitances 111 and 116 are variable attenuators 102.
The input / output terminals 110 and 107 are separated from the bias, and the impedance thereof can be ignored for a high frequency signal. The PIN diodes 113, 114 and the resistors 112, 118, 115 form a π-type attenuator circuit, and the attenuation amount of the variable attenuator 102 changes according to the change of the internal resistance of the PIN diodes 113, 114. The forward current (ID) flowing through the PIN diodes 113 and 114 is determined by the voltage (VAGC) applied to the control terminal 121. Here, the resistors 112, 115 and 118 compose a high frequency attenuator circuit, and the input / output impedance is 50Ω.
It is basically about 80Ω to 300Ω in order to match with, but since the resistor 120 can be made sufficiently larger than those, the forward current is limited by the resistor 120.

Similarly, FIG. 1 (c) is a circuit diagram of the variable attenuator 104 arranged in the 2nd IF stage, but the capacitances 131 and 135 are for separating the input / output terminals of the variable attenuator 104 from the bias, Its impedance can be ignored for high frequency signals. PIN diodes 132,133 and resistors 137,134
Constitutes a π-type attenuator circuit, and the attenuation amount of the variable attenuator 104 changes according to the change of the internal resistance of the PIN diodes 132 and 133. The forward current flowing through the PIN diodes 132 and 133 is determined by the voltage (VAGC) applied to the control terminal 140. Here, the resistors 137 and 134 form a high frequency attenuator circuit, and are basically required to match the input / output impedance to 50Ω. Although it is about 80Ω to 300Ω, the resistor 139 can be made sufficiently larger than those, so that the forward current is limited by the resistor 139.

Now, in the first and second variable attenuators 102 and 104 configured as shown in FIGS. 1 (b) and (c), two PIN diodes are used, respectively, and between the control terminals 120, 104 and the ground. Are connected in parallel or in series. The PIN diode is an element whose internal resistance changes when its forward current changes, and its characteristic diagram is shown in FIG. As the forward current increases, the internal resistance decreases.

Therefore, the relationship between the first and second variable attenuators, the attenuation amount, and the control voltage is as shown in FIG. 3 (b). Ie PIN
The variable attenuator 104 in which the diodes 132 and 133 are connected in series is
Since the forward current does not flow at a higher AGC control voltage (VAGC) than that of the variable attenuator 102 in which the PIN diodes 113 and 114 are connected in parallel, the amount of attenuation starts to increase from a higher VAGC as shown in FIG. 3 (b). The effect enables the gain control of the AGC as shown in FIG.

In this embodiment, the first variable attenuator 102 is arranged in front of the 2nd Mix for the purpose of suppressing the intermodulation generated in the 2nd Mix 4 and minimizing the deterioration of the NF value seen from the 1st IF input terminal 1. However, as shown in the block diagram of FIG. 4, the first stage of the 1st IF amplifier (wideband amplifier), or 1st
Since the IF amplifier is generally configured as a transistor two-stage amplifier, it can be arranged between the stages. 1st I
When it is placed in front of the F amplifier 2, not only the 2nd mix but also the intermodulation at the time of strong input generated in the 1st IF amplifier 2 is suppressed. However, the NF value becomes a little worse, so it is necessary to design in accordance with the specifications, considering the effects of both. Further, it is needless to say that the polarity of the PIN diode may be a polarity other than that of the embodiment, as long as the forward current can be made to flow by the control terminal voltage.

Effects of the Invention As described above, according to the present invention, the first variable attenuator formed by connecting the PIN diode in parallel between the control terminal and the ground is arranged in the 1st IF stage, and the PIN diode is connected in series in the 2nd IF stage. The second variable attenuator is arranged to form an AGC system, and the second variable attenuator operates at a weak signal input level to reduce NF deterioration, and at a strong input level, the first variable attenuator. Can be operated to prevent the deterioration of the intermodulation characteristics, so that its practical effect is great.

[Brief description of drawings]

FIG. 1 (a) is a block diagram of an AGC device in one embodiment of the present invention, and FIGS. 1 (b) and (c) are circuit diagrams of first and second variable attenuators in one embodiment. The figure shows the gain distribution of the AGC system in this embodiment, and FIG. 3 (a) shows the PIN.
FIG. 3B is a characteristic diagram of the internal resistance of the diode and the forward current, FIG. 3B is a characteristic diagram of the attenuation amount of the variable attenuator and the control terminal voltage in the embodiment, and FIG. 4 is a block diagram in the other embodiment of the present invention. FIG. 5 (a) shows the AGC in the conventional example of the present invention.
FIG. 5B is a block diagram of the device and is a circuit diagram of the variable gain amplifier. 1 …… 1st IF input terminal, 2 …… Broadband amplifier, 3 ……
Image filter, 4 ... 2nd Mix, 5 ... Second local oscillator, 7 ... Second intermediate frequency amplifier, 8 ... Peak detector, 9 ... DC amplifier, 102 ... First variable attenuator, 103
...... Second intermediate frequency amplifier, 104 ...... Second variable attenuator.

Claims (1)

[Claims] Claim: What is claimed is: 1. One terminal of the first capacitance is used as an input terminal, and the other terminal is grounded via a first resistor and connected to the cathode terminal of the first PIN diode. The second PIN on the anode terminal of the PIN diode
Connect the anode terminal of the diode, ground the cathode terminal of the second PIN diode through the second resistor, and connect it to one terminal of the second capacitance and output the other terminal of the second capacitance. First terminal
And one terminal of the third resistor is connected to the connection point of the anode terminals of the second PIN diode, and the other terminal of the third resistor is connected to the attenuation control terminal via the fourth resistor. And a first variable attenuator which is grounded via a third capacitance, a frequency converter which frequency-converts an input signal input through the first variable attenuator, and a frequency-converted output signal. And a second variable attenuator for inputting the signal through the first amplifier, the second variable attenuator having one terminal of the fourth capacitance as an input terminal and the other terminal as a third terminal. Connected to the anode terminal of the PIN diode of, the cathode terminal of the third PIN diode is connected to the anode terminal of the fourth PIN diode, and the cathode terminal of the fourth PIN diode is grounded via the fifth resistor. Fifth Cap Connected to one terminal of Shitansu, the other terminal of the fifth capacitance and the output terminal, a sixth resistor connected to the connection point between the cathode terminal of the fourth capacitance and the third PIN diode, the sixth
The other terminal of the resistor is connected to the attenuation control terminal via the seventh resistor and is grounded via the sixth capacitance until the output signal of the second variable attenuator can be detected. A second amplifier that amplifies and a detector that peak-detects the output signal of the second amplifier are provided, a DC voltage output corresponding to the input level is obtained from this peak detector, and the DC voltage output is output to the DC amplifier. An AGC device which is amplified via the DC power amplifier and supplies the output signal of the DC amplifier to the gain control terminals of the first and second variable attenuators.