JP3531510B2 - AGC device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for receiving a digital broadcast wave or the like, which uses a multi-carrier signal wave (hereinafter, referred to as an "OFDM wave") that has been subjected to wideband digital modulation. The present invention relates to an AGC apparatus having means for reducing an adverse effect caused by automatic adjustment of an AGC gain based on a temporary interference wave or a no-signal state when receiving. 2. Description of the Related Art In recent years, developments aimed at enhancing services, such as digitizing broadcast waves, improving sound quality, improving image quality, increasing the number of channels, and expanding to data broadcasting, have been actively conducted. A multicarrier multiplexing system called OFDM (orthogonal frequency division multiplexing communication system) is currently being considered as a domestic broadcasting system currently under study. This system transmits hundreds to thousands of carriers, each modulated by an input digital signal, collectively, and is known as a multiplexing system having high interference rejection capability and high frequency use efficiency. [0003] OFDM waves have a rectangular spectrum, and the band is spread. Therefore, even if there is an interfering wave, the occupation ratio of the interfering wave to the entire OFDM wave is reduced, and there is an advantage that the interference can be easily eliminated by error correction or the like. However, since the OFDM wave has a wide band, the probability of receiving an interference wave itself increases. Interfering waves occur due to various causes such as fading.Generally, there are continuous ones with fixed frequency like existing analog broadcast waves, and sudden ones with short time such as noise and spurious. is there. In general, a radio receiver for a broadcast wave or the like receives a very small input signal from an antenna, and has a large amplitude fluctuation during propagation such as fading or multipath. Therefore, in order to perform signal processing such as mixing with a local oscillator signal on the receiver side, it is necessary to make the received signal uniform in amplitude. Therefore, an AGC amplifier (AGC amplifier) that obtains a constant output amplitude by absorbing an input amplitude fluctuation is arranged in an RF unit (radio frequency receiving unit) of the receiver. The AGC amplifier has a constant response time constant. [0004] When the above-mentioned interference wave is superimposed on the AGC amplifier, the superimposed signal appears as an increase in amplitude. Therefore, the AGC amplifier of the receiver reduces the gain in order to suppress the output amplitude to within a specified value. To deal with. [0005] The interfering waves arriving at the broadcast wave receiver are roughly classified into those having fixed frequencies and continuous ones, and those having unspecified frequencies such as noise and spurious signals, and having a temporal characteristic. There is a short burst. When an abrupt interference wave shorter in time than the response time constant is superimposed on the AGC amplifier, not only the start of the gain control is delayed, but also after the interference wave disappears, the fixed time corresponding to the time constant is not reduced. The state where the gain is kept low continues, and the gain recovery is delayed. For this reason, there arises a problem that the effective amplitude for the original signal decreases and the time during which the sampling accuracy of the AD converter or the like decreases is prolonged. To cope with this, if the response time constant of the AGC amplifier is unnecessarily shortened, a new problem arises in that the response when an intermittent non-signal broadcast wave is received is delayed. Therefore, a constant response time constant has been determined by taking a compromise between the two. On the other hand, a constant response time constant is set in the AGC loop by adjusting the cutoff frequency of the low-pass filter (LPF) in the AGC loop. If this time constant is long, an input signal having a high frequency component cannot be controlled, causing saturation, or the amplitude is too small to cause a decrease in sampling accuracy. Conversely, if the time constant is too short, the above-mentioned problem is eliminated.However, when the input signal level temporarily decreases due to fading or the like and becomes close to no signal, the gain suddenly increases and the signal level increases. When is recovered, the gain is too large and saturates, and normal signal reception cannot be performed. For this reason, it is common practice to conduct an experiment using a fading simulator or the like and to set a time constant corresponding to a compromise between the two. However, in order to deal with fading with a faster response while reliably reducing unreceivable time, neither time constant is sufficient. [0010] Accordingly, the present invention provides an AGC amplifier having an AGC amplifier when the input signal level suddenly and temporarily increases or decreases considerably.
It is an object of the present invention to provide an AGC device that can more appropriately control a GC function. Further, the present invention provides an AG
Although the response time constant of the C amplifier is determined to be constant, as described above, even if the amplitude temporarily increases due to an interference wave shorter than the time constant of the AGC amplifier, the gain is temporarily reduced only by slightly reducing the gain of the AGC amplifier. AGC which can avoid recovery delay caused by the gain continuing to fall for a long time
It is intended to provide a device. The present invention further provides a measure against no signal.
An object of the present invention is to provide an AGC apparatus which can set a short response time constant of an AGC amplifier, but can prevent a sudden increase in gain when a normal state is restored from a no-signal state. [0013] In order to achieve the above object, the present invention provides a broadband digitally modulated multicarrier.
A disturbance detecting means for detecting the interference wave generated in the reception frequency band of the A signal wave, the by the interference wave detecting means
When an interference wave is detected, the gain is held for a predetermined time.
Control means for outputting a control signal to instruct, and the control means
When the control signal output from the
AGC amplifier having an input connected to the input of the output means
And a holding means for temporarily holding the gain . FIG. 1 shows a spectrum of a general OFDM wave WO. The horizontal axis is frequency f and the vertical axis is amplitude A. A plurality of digitally modulated digital signals are dispersed and multiplexed on hundreds to thousands of carriers. Carriers are set and arranged at frequency intervals that do not cause mutual interference, and line spectra appear randomly in short-time (micro) observations in symbol units, but since such line spectra appear continuously,
In a long-time (macro) observation, a wide-band rectangular spectrum WO is obtained as shown in FIG. When an interference wave NS such as artificial noise or spurious is superimposed on the received wave, it appears on the frequency f-axis as shown in FIG. When this interference wave NS is viewed on the time t-axis, as shown in FIG. 2, the temporally short interference wave NS is superimposed and appears as a temporary increase in amplitude. FIG. 3 is a block diagram of a general OFDM digital signal receiver. The broadcast wave received by the antenna 2 is input to an AGC (automatic gain adjustment) amplifier (AGC amplifier) 6 via an RF (radio frequency) amplifier 4.
The AGC amplifier 6 is used to ensure the subsequent signal processing.
It is provided to absorb fluctuations of the input signal and output a signal of a constant amplitude. The output signal of the AGC amplifier 6 is supplied to a mixer (MI) having a local oscillator (OSC) 9.
X), the signal is introduced to an IF (intermediate frequency) amplifier 10, where the frequency is set to an intermediate frequency, and further, the A / A
It is guided to a D converter 16 and digitized here. A feedback circuit 7 for gain adjustment is formed between the A / D converter 16 and the AGC amplifier 6. The output digital signal of the A / D converter 16 is FF
After being converted to the frequency domain with the help of T19, it is demodulated by the demodulator 18 and error-corrected by the error corrector (FEC) 20 to become a transport stream, which is output to an MPEG decoder or the like (not shown). Then, they are decoded into images and sounds and reproduced. FIG. 4 shows the input versus output characteristics of the AGC amplifier 6. This is because when the amplitude of the received signal fluctuates with time due to fading, multipath, or the like, FIG.
In order to make the output level as constant as possible even if the input level fluctuates, considering that the signal processing after the mixer 8 does not operate properly unless a signal of substantially constant amplitude is input, FIG. As shown in FIG. 4, an input signal having a certain value or more has such a characteristic that the output is saturated.
In order to obtain such characteristics, the receiver of FIG. 3 monitors the output data of the A / D converter 16 and realizes the magnitude of the signal by feeding it back to the preceding RF amplifier 6 as a gain control signal. FIG. 5 (a) shows a state in which an incoming wave in the form of a desired wave superimposed on an interfering wave is input to the AGC amplifier 6 of FIG. 3 and the amplitude increases transiently as a function of time t. It was done. It is assumed that this interference wave is generated in a pulse shape in a time shorter than the time constant of the AGC amplifier 6.
In this case, since the output of the AGC amplifier 6 is temporarily increased by the amount of the interference wave, the AGC amplifier 6 lowers the gain G by the operation of the feedback circuit 7 and tries to keep the output constant. The state is shown in FIG. As described above, since the interference wave is shorter than the time constant of the AGC amplifier 6, the AGC
The output of the amplifier 6 delays decreasing the gain with respect to the interference wave, and delays the recovery of the amplitude A and the gain G. for that reason,
Assuming that the amplitude at which the A / D converter 16 of FIG. 3 operates with the highest accuracy is Ae, the output of the AGC amplifier 6 of FIG. The time below Ae becomes longer, and the conversion accuracy decreases. Therefore, a slow amplitude fluctuation such as a normal fading (the period of the amplitude fluctuation is sufficiently longer than the time constant of the AGC amplifier 6) is dealt with by the conventional AGC input / output characteristic as shown in FIG. In the case of a short-time interference wave, the gain of the AGC amplifier is held for a certain period of time, so that the decrease in the gain can be prevented from continuing for a long time. FIG. 6 is a block diagram showing an AGC amplifier circuit 60 according to the present invention. The signal input from the antenna 2 is amplified through the RF amplifier 4 and input to the AGC amplifier circuit 60. On the other hand, the output signal of the RF amplifier 4 is AGC
In addition to being input to the amplifier 61, it is also input to an interference wave detector 62 realized by a band-pass filter or the like, where an interference wave that causes a relatively short-time amplitude fluctuation is detected. Relatively long amplitude fluctuations such as fading are not detected here. The interference signal detected by the interference detector 62 is supplied to a gain hold controller (GHC) 63.
Is transmitted to The GHC 63 is provided with a timer 64 for adjusting the hold time as described later. AG
The A / D converter 16 (see FIG. 3) is provided in the C amplifier 61.
Is fed back through the feedback circuit 7, converted into an analog signal through the latch 65 and the D / A converter 66, and given an appropriate time constant through a low-pass filter (LPF) 67. , AGC amplifier 61 are input to a gain control terminal. Upon receiving the interference wave detection signal from the interference wave detector 62, the GHC 63 sends a latch signal to the latch 65 for a fixed time (the time set by the timer 64) to temporarily hold the gain. FIG. 7A illustrates a gain adjusting operation of the AGC amplifier 61 when an interfering wave is superimposed on the arriving wave of the antenna 2 of FIG. As already mentioned, A
In the case of the conventional device, when an interfering wave arrives at time t1, the GC amplifier 61 reduces the gain G to suppress the amplitude as shown by the solid line. However, in the case of the apparatus of the present invention shown in FIG. 6, the interference wave is detected by the interference wave detector 62 at the time t2 which is delayed by the delay time td from the time t1 when the interference wave arrives, and the amplitude A increases. G decreases by Gd. At this time point t2, the GHC 63 sends a latch signal to the latch 65 for a fixed time, and holds a constant gain for a time th as shown by a broken line in FIG. As a result, the output of the AGC amplifier 61 changes the gain G to a small value G during the period of time th, as indicated by the broken line in FIG.
Since it is held constant in a state reduced by d, the input waveform of the interference wave is output as a substantially unchanged waveform. For this reason, a period exceeding the proper amplitude Ae occurs, and during this period, normal reception cannot be performed, but since the time is short, correction can be performed in post-processing. According to the present invention, as shown by the solid line in FIG. 7B, it is possible to avoid a situation where the time that is less than the amplitude Ae (the amplitude at which the A / D converter 16 can operate normally) continues for a long time. FIG. 8 shows the frequency characteristics of the LPF 67 provided in the gain feedback circuit 7. By setting the cutoff frequency fc, the time constant of the entire loop can be freely determined.
Thereby, dynamic characteristics such as a transient response of the AGC loop can be determined. This will be described with reference to FIG. Generally, an AGC amplifier 61 shown in FIG.
As shown in FIG. 9 (a), if the input signal has a certain amplitude variation, the AGC function of the AGC amplifier 61 causes the AGC function of FIG.
As shown in (b), an output signal having a substantially constant amplitude can be obtained. Here, if the time constant tc of the LPF 67 is increased (corresponding to lowering the cutoff frequency fc in FIG. 8), the response speed becomes slower, and the response signal becomes slower with respect to the input signal in FIG. ) Is output without being able to control a fine (high-frequency) signal component constantly. As a result, when high frequency is superimposed due to sudden noise or multipath, it is impossible to respond, and output distortion and A / D
This causes a decrease in the sampling accuracy of the converter 16. Therefore, when the time constant tc of the LPF 67 is reduced (the cutoff frequency fc in FIG. 8 is increased), the response speed is increased, and the response signal is increased with respect to the input signal in FIG. In response to a fine (high frequency component) signal component as shown in FIG. However. When the amplitude temporarily becomes extremely small due to fading or the like as in the Q part of FIG. 9D and the signal becomes a no-signal state, the signal responds so as to quickly recover the amplitude. Immediately thereafter, the amplifier gain G sharply increases, and the signal amplitude A
Is restored, the gain G becomes the maximum and FIG.
As shown in the figure, the signal temporarily increases, and the AGC
The loop works and the amplitude converges. In the case of FIG. 9E, the signal immediately after the no-signal is too large, the AGC amplifier 61 is saturated, and the A / D converter 16 at the subsequent stage cannot perform sampling properly, so that the signal cannot be received during that period. From the above, it can be understood that a problem is caused whether the time constant of the AGC amplifier 61 is large or small. For this reason, in a practical design, an eclectic time constant of the two was determined by experiments. However, in order to achieve a faster response and a reduction in the reception disabled time, the present invention employs a gain adjustment circuit as shown in FIG. Here, in order to generate a latch signal to be input to the latch 65, RF
The output signal of the amplifier 61 is rectified by a rectifier 71 and further passed through a low-pass filter (LPF) 72 to a comparator 7.
3 to the comparison signal input terminal. The reference value Vt is input from the reference signal source 74 to the other reference signal input terminal of the comparator 73. The circuit elements 71, 72, and 73 detect the above-mentioned no-signal state, latch gain data according to the detection result, and hold the gain. As shown in FIG. 11A, a no-signal section Tf
Rectifier 71 outputs a signal shown in FIG. 3B according to the signal level. Since this rectified output still contains a lot of high frequency noise, LPF7
The signal is attenuated by 2 to obtain a signal as shown in FIG. This signal is compared with a reference value, that is, a threshold value Vt by a comparator 73. If the signal is equal to or smaller than Vt, a digital output SM as shown in FIG. If a no-signal state exists even temporarily, it can be detected as the digital output SM. The operation of the apparatus shown in FIG. 10 will be described with reference to FIG. First, when an input signal in the non-signal state SF temporarily as shown in FIG. 12A is input to the AGC amplifier 61, the non-signal state is detected by the comparator 73 in FIG. 10 and shown in FIG. As described above, the non-signal detection is performed for the time Th. The latch 65 receiving this detection output latches the gain in the digital data for the time Th and holds the gain. The solid line in FIG. Therefore, the output of the AGC amplifier 61 is
Since the gain G immediately before the absence of a signal is held as shown in FIG. 12B, the gain G does not increase abruptly even if the amplitude of the input signal is restored, and the gain G suddenly increases as shown by the solid line in FIG. The AGC loop operates with a signal having a continuous amplitude without increasing the signal amplitude, and normal amplitude control can be started. By the way, according to the configuration of the conventional AGC amplifier in which the amplifier gain is not adjusted by detecting no signal, the gain sharply increases as shown by a broken line in FIG. 12C, and the amplitude becomes excessive as shown by a broken line in FIG. Causes the amplifier to saturate. According to the present invention described above, when the input signal level suddenly or temporarily increases or decreases considerably, the AG
The AGC function of the C amplifier can be more appropriately controlled, and the recovery time for normal reception can be reduced. When an interference wave shorter than the time constant of the AGC amplifier is superimposed on the OFDM wave signal, by holding the gain of the AGC amplifier for a certain period of time, the time during which the amplitude is excessive can be shortened, and post-processing is performed. And can perform error correction and the like easily and quickly in a recovery process for normal reception of a signal. Further, by setting the time constant of the AGC amplifier to be short, the response to a high frequency input signal is improved, and the reception quality is improved. Even with such a setting, even when there is no signal, or when there is continuous input, holding the gain at the immediately preceding value prevents the output from becoming excessive after the signal level is restored, preventing reception. Time can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a spectrum of an OFDM wave and an interference wave. FIG. 2 is a waveform diagram showing a superimposed waveform of an OFDM wave and an interference wave. FIG. 3 is a block diagram of a general OFDM wave receiver. FIG. 4 is a diagram showing input / output characteristics of an AGC amplifier. 5 (a) and 5 (b) are diagrams showing output characteristics of the present invention and a conventional AGC amplifier. FIG. FIG. 6 is a block diagram showing a first embodiment of the AGC device according to the present invention. FIGS. 7A and 7B are a diagram showing an AGC gain characteristic according to the present invention and a conventional one, and a diagram showing an output characteristic of an AGC amplifier. FIG. 8 is a diagram showing frequency characteristics of a general LPF. FIGS. 9A to 9E are diagrams illustrating input / output characteristics of a conventional AGC amplifier. FIG. 10 is a block diagram showing a second embodiment of the AGC apparatus according to the present invention. FIGS. 11A to 11D are diagrams for explaining the operation characteristics of the no-signal detection device of the present invention. 12 (a) to 12 (d) are diagrams for explaining the operation characteristics of the AGC amplifier of FIG. 10; [Description of Signs] 2 Antenna 4 RF Amplifier (RF Amplifier) 6 AGC Amplifier (AGC Amplifier) 8 Mixer (MIX) 9 Local Oscillator (OSC) 10 IF Amplifier 12 Band Pass Filter (BPF) 14 IF Amplifier 16 A / D Converter 60 AGC device 61 AGC amplifier 62 Interference wave detector (interference wave detection means) 63 Gain hold control unit (gain hold control means) 64 Timer 65 Latch (hold means) 66 D / A converter 67 Low pass filter (LPF) 70 AGC Device 71 Rectifier 72 Low-pass filter (LPF) 73 Comparator (no-signal detection means)

Claims (1)

(57) [Claims] [Claim 1] Multi-carrier modulated by wideband digital modulation
A disturbance detecting means for detecting the interference wave generated in the reception frequency band of the A signal wave, and the interference wave is detected by the interference wave detecting means
Output a control signal instructing gain hold for a predetermined time.
A force control means and inputs the control signal outputted from said control means,
An input side connected to an input side of the interference wave detecting means;
Holding means for temporarily holding the gain of the AGC amplifier .