JP2003198981A - Agc control type intermediate-frequency amplifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve reception performance even when the flatness of a frequency characteristic in an IF band is spoiled owing to variance among tuner components. <P>SOLUTION: The IF signal from a tuner 51 is divided into three frequency bands by SAW filters 1 to 3 having mutually different center frequencies. The output level of an IF AGC amplifier 4 is determined by making gain adjustment according to the signal level of a lower frequency band from the SAQ filter 1, a comparator 10 compares the DC voltage generated by detecting and smoothing its output by a smoothing and detecting circuit 7 with the DC voltage generated by detecting and smoothing the output from the AGC amplifier 5 by a smoothing circuit 8, and the gain of the IF AGC amplifier 5 is so adjusted that the difference between the both is eliminated. Further, a comparator 11 compares the DC voltages generated by the smoothing and detecting circuits 7 and 9 and the gain of an IF AGC amplifier 6 is adjusted. Consequently, the flatness of the frequency characteristic is improved to decrease the disorder and gradient of the frequency characteristic within the correctable range of an equalizer of a demodulator 14. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiver having a wide band IF reception range, and more particularly to an AGC control type intermediate frequency amplifier circuit provided in a cable modem of a CATV digital receiver.

[0002]

2. Description of the Related Art FIG. 7 shows a configuration of a QAM receiver for digital CATV as an example of a conventional receiver. First, the processing of the tuner 51 in this receiver will be described.

The input plug of the tuner 51 has about 30-
A QAM modulation signal having a dynamic range of 40 dB is input. This QAM-modulated signal has low-frequency components removed by the high-pass filter 51a, is then amplified by the first-stage amplifier 51b, and is divided into 2 to 3 bands (here, 3 bands) according to the frequency band.

Lowest frequency band (VHF Low)
Passes through the bandpass filter 51c, the RF AGC amplifier 51d, and the bandpass filter 51e. The intermediate frequency band (VHF High) is
Bandpass filter 51f and RF AGC amplifier 5
1g and the band pass filter 51h. Further, the highest frequency band (UHF) passes through the bandpass filter 51i, the RF AGC amplifier 51j, and the bandpass filter 51k. The frequency bands that have passed through the bandpass filters 51e, 51h, and 51k are mixed by the mixer 51l. The mixed signal from the mixer 51l is output from the tuner 51 via a SAW (Surface Acoustic Wave) filter 52.

Amplifier 51b and each bandpass filter 5
The switch 51m switches the connection between 1c, 51f, and 51i. The switch 51n switches the connection between the band pass filters 51e, 51h, 51k and the mixer 51l.

The QAM signal is converted by the mixer 51l into an IF (intermediate frequency) signal having a frequency lower than the pass band of the high-pass filter 51a and output from the tuner 51, and then a narrow band having a pass band of the reception channel signal band is obtained. The out-of-band signal is suppressed by the SAW filter 52 as a band pass filter. The signal from the SAW filter 52 is amplified to a constant level by the gain-adjusted IF AGC amplifier 53 regardless of the input level, and then further amplified by the IF amplifier 54. The signal that has passed through the IF amplifier 54 is input to the demodulator 56 in a state where only the low-frequency component passes through the low-pass filter 55 and noise is removed.

A digital demodulator 56 having an A / D converter converts an input signal to digital and demodulates it by digital processing. Further, the demodulator 56 is A /
Generate a DC signal by taking the difference between the signal level digitized by the D converter and the internal reference voltage, and converting the signal proportional to the difference by the D / A converter.
RF AGC amplifier control circuit (RF AGC control in the figure) 57 and IF AGC amplifier control circuit (I in the figure)
FAGC control) 58.

The IF AGC amplifier control circuit 58 converts a signal into a direct current and performs an appropriate level conversion. In general,
Regarding the input signal input to the tuner 51, it is important to increase the SNR, which is the ratio of the signal level and the noise level, in the weak input. Therefore, even if the input level changes, the RF AGC amplifiers 51d and 51g in the previous stage
The input level to the A / D converter in the demodulator 56 is controlled to be constant by maintaining the maximum gain without decreasing the gain at 51j and controlling the change of the input signal by the IF amplifier 53 in the subsequent stage. There is.

On the other hand, when the input signal becomes large, it is distorted by the amplifier and the mixer, so that IM distortion is likely to occur and demodulation performance is deteriorated. Therefore, even if the input level becomes high, the gain is lowered by the RF AGC amplifiers 51d, 51g, 51j so that a large signal is not applied to the amplifier and the mixer, and the input level to the A / D converter is made constant. It is controlled to become.

Further, the demodulator 56 controls the AGC start input level which is a switching level between the RF AGC amplifiers 51d, 51g, 51j and the IF AGC amplifier 53.

When the demodulator 56 applies the above DC signal, that is, the RF AGC control signal to the RF AGC amplifier control circuit 57, the RF AGC control signal is R
The input level to the FAGC amplifiers 51d, 51g, and 51j is controlled to be a constant level from the weak input to the AGC start input level. Similarly, from the demodulator 56 to I
The IF AGC control signal applied to the F AGC amplifier control circuit 58 controls the input level to be a constant level from the AGC start input level to the strong input.

However, the received signal level is set to both AGC.
Since it is controlled by an amplifier, the demodulator 56 uses an A called TOP (Take Over Point) set by an internal register.
The point at which the GC amplifier control signal level is switched is set.
Therefore, the AGC start input level may be different even for the same TOP due to variations in the control signal voltage of the AGC amplifier and the gain suppression level characteristic.

In the demodulator 56, the RF AGC control signal is controlled so as to have a constant value from weak input to TOP, and thereafter, it is controlled so as to be proportional to the difference between the received signal level and the internal reference voltage. In addition, the IF AGC control signal is set to TO
Control is performed so that the value from P to the strong input is constant, and until then, control is performed so as to be proportional to the difference between the received signal level and the internal reference voltage. In the demodulator 56, the digital signal converted by the D / A converter is converted into a DC voltage by passing through a low-pass filter (not shown) and further amplified to an appropriate voltage by an operational amplifier (not shown), so that the RF AGC amplifier control circuit 57 and IF AG
It is given to the C amplifier control circuit 58.

As a result, the RF AGC amplifier 51d
51g and 51j control the input level so as to have a constant reception signal level from weak input to AGC start input level, and the IF AGC amplifier 53 has a constant reception signal level from AGC start input level to strong input. To control. By controlling both of them, the signal level input to the A / D converter becomes constant regardless of the tuner input signal level.

On the other hand, in the tuner 51, the local signal generator 5 is used for RF frequencies ranging from 50 MHz to 1 GHz.
The center frequency of 30 to 60 depends on 1o and the mixer 51l.
Frequency conversion is performed in the IF frequency band of MHz and 6 MHz. Since the input frequency is in a wide range, by switching the switches 51m and 51n according to the input frequency, the bandpass filters 51c and 51e, the bandpass filters 51f and 51h or the bandpass filters 51i and 51k can obtain the selectivity and the bandpass. The bandwidth is adjusted so that the frequency characteristic in the inside becomes flat.

The PLL 51q is a local signal generator 51.
Even if o is set to the reception frequency + IF frequency and the output of the mixer 51l changes the reception frequency in the range of 50 MHz to 1 GHz, a constant intermediate frequency, frequency band and gain are always secured, while the band pass filter 51c. ，
The center frequencies of 51e, 51f, 51h, 51i and 51k are adjusted to the reception frequency.

The band division and frequency characteristics of the tuner 51 are determined by the VHF band RF tuning circuit shown in FIG. Band pass filters 51c, 51e and AG
C amplifier 51d, band pass filters 51f, 51h
And AGC amplifier 51g, and band pass filters 51i and 51k and AGC amplifier 51j are a VHF low band RF tuning circuit and a VHF High ban, respectively.
It composes the d-band RF tuning circuit and the UHF band RF tuning circuit.

In this VHF band RF tuning circuit, the diodes D1, D2 and D3 are low band and high b of VHF.
It is provided to switch between and and. Further, the coils L1 and L2, the variable capacitance diode VC1, and the capacitor C1 constitute a single tuning circuit 61. The bandpass filters 51c and 51f are configured by the input stage circuits in the input VHF band RF tuning circuit including the single tuning circuit 61. Further, the coils L3 and L4, the capacitor C2, and the variable capacitance diode VC2 form a double tuning circuit 62, and the coil L
5, L6, the capacitor C3, and the variable capacitance diode VC3 form a double tuning circuit 63. Bandpass filters 51e and 51h are configured by an output stage circuit in the input VHF band RF tuning circuit including the double tuning circuits 62 and 63.

AGC amplifiers 51d, 51g and 51j are formed between the input stage and the output stage by a circuit including a transistor FET and a plurality of resistors. A bias voltage + B for giving to the transistor FET and an AGC voltage for gain control (voltage from the above-mentioned RF AGC amplifier control circuit 57) are given to this circuit.

When the band switching voltage Vb becomes high, the coil L1 is short-circuited by the current flowing through the diode D1 through the resistor R1. Therefore, the tuning frequency is
Hi determined by 1 / (L2 × (VC1 // C1)) ^1/2
It becomes a gh band. On the other hand, when the band switching voltage Vb becomes low, no current flows in the diode D1, so the tuning frequency becomes a low band determined by 1 / ((L2 + L1) × (VC1 // C1)) ^1/2 .

The double tuning circuits 62 and 63 also operate in the same manner as above. That is, when the band switching voltage Vb becomes high, the diode D passes through the resistors R2 and R3, respectively.
The current flows through the coils 2, 2 and D3, so that the coils L4, L
Since 6 is short-circuited, the tuning frequency becomes High band. On the other hand, when the band switching voltage Vb becomes low, the diode D
Since no current flows through 2, D3, the tuning frequency is in the low band
Becomes

When the tuning frequency is switched by dividing into a plurality of bands as described above, it is difficult to cover the entire VHF band from 50 MHz to 470 MHz with one variable capacitance diode, and the local oscillation frequency and the RF tuning frequency. This is because it is difficult to maintain a constant IF frequency difference with

Regarding the overall characteristics, the single tuning circuit 61 and
The double tuning circuits 62 and 63 determine the selectivity and the in-band flatness as shown in FIG. In the figure, the frequency characteristic a (indicated by the solid line in the figure) by the single tuning circuit 61.
The double tuning circuits 62 and 63 determine the frequency characteristic b (indicated by a chain line in the figure), and the frequency characteristic c is determined by adding the frequency characteristics a and b. Then, this frequency characteristic c is IF at the local oscillation frequency.
Is converted to.

FIG. 10 shows L3, L4, C2, VC2, L.
The frequency characteristic when the coupling degree of 5, L6, C3, and VC3 is changed is shown. In order to obtain optimum characteristics by adjusting the degree of coupling, adjustment is performed by widening the winding interval of the coils L3 and L4, which are air-core coils, or contracting them in the opposite direction, to secure the flatness and selectivity of the IF band. is doing. The frequency characteristic c shown in FIG. 9 described above is not maintained in a frequency range in which a wide frequency range shows a flat frequency characteristic, and the frequency characteristics e to g as shown in FIG. Fluctuates like. Therefore, the coil L1 is set for each tuner.
It is necessary to adjust the coupling degree of L6.

The IF output signal output from the tuner 51 has the selectivity secured by the SAW filter 52,
The signal is sent to the demodulator 56 at the optimum level and selectivity by the IF AGC amplifier 53 and the amplifier 54. In the demodulator 56, the IF output signal is converted into a digital signal by the A / D converter, its amplitude value is obtained, and the difference between the amplitude value and the reference amplitude value is obtained. The RF AGC control signal and the IF AGC control signal are generated so as to disappear. Further, an equalizer for correcting the frequency characteristic flat is prepared, and the waveform and amplitude disturbance due to reflection during transmission or the frequency characteristic disturbance of the reproducing system is corrected.

[0026]

Q transmitted by the transmitter
The AM modulated wave is received by the receiver in a deteriorated state because it is attenuated by the transmission cable, noise is added, reflection occurs on the transmission line, or it is directly added to the wave. In the receiver, noise is added by the NF (noise figure) of the amplification element, distortion of the active element, generation of spurious signals due to system clock harmonics, and disturbance of frequency characteristics, and input to the A / D converter of the demodulator 56. Since the signal deteriorates, the input signal cannot be demodulated accurately.

Among them, in the reproducing system, in particular, the tuner 5 is used for the purpose of preventing the deterioration of the performance due to the disturbance of the frequency characteristic.
In order to improve the flatness of the frequency characteristic of No. 1, many improvements and developments have been conventionally made in design and adjustment.
However, it is very difficult to always maintain a flat frequency characteristic of a 6 MHz bandwidth in a wide frequency band of 50 MHz to 1 GHz.

In order to maintain it, a difference in IF frequency is always provided between the oscillation frequency of the local oscillation circuit 51o shown in FIG. 7 and the center frequencies of the single tuning circuit 61 and the double tuning circuits 62 and 63 described above. Must be maintained. However, the variable capacitance diodes VC1 to VC3 of the single tuning circuit 61 and the double tuning circuits 62 and 63, and the local oscillation circuit 5
Since the voltage (tuning voltage Vt in FIG. 8) given to the variable capacitance diode of 1o is always equal, the difference is adjusted so as to be constant by the capacitance of the capacitor added to the variable capacitance diode, and the variation of parts is In order to absorb, it is necessary to adjust the inductor in the production process. For this reason, it is extremely difficult to flatten the waveform in the entire band, and it is usually difficult to keep the flatness in the IF band within 3 dB even with a well-designed tuner. On the other hand, when the QAM signal is demodulated, if the frequency characteristic is not flat, when the frequency is converted to the time axis, intersymbol interference occurs because the amplitude does not become zero for each symbol period.
As a result, the eye pattern is not sufficiently opened and the error rate is deteriorated, so that the performance of the receiver is deteriorated.

As described above, the disturbance of the frequency characteristic is caused by the reflection in the transmission line, the adjustment variation of the frequency characteristic in the tuner 51 of the receiver, etc. In order to correct this, the demodulator 56 outputs the IF output signal. After being digitized by the A / D converter, the frequency characteristics are corrected by the equalizer. In this equalizer, equalization is performed by a transversal filter including a delay circuit having a tap for each symbol period T of a part of the digital filter and a weighting circuit for weighting the output from each tap of the delay circuit. In order to improve this characteristic, a proposal has been made for its configuration. However, if the IF frequency characteristic has a deviation of 2 dB or more within the band, its demodulation performance is prominently deteriorated.

FIG. 11 shows an example of the relationship between the frequency characteristic flatness represented by the difference (deviation) between the maximum value and the minimum value in the IF band of 6 MHz and the error rate. From the figure, when the deviation is 2 dB or less, the error rate shows a constant value due to the effect of the equalizer, but when the deviation exceeds it,
It can be seen that the error rate increases because the correction by the equalizer does not work.

The present invention has been made in view of the above circumstances, and is input to the demodulator even when the flatness of the frequency characteristic in the IF frequency band is impaired due to the characteristic variation of the tuner component. It is an object of the present invention to improve reception performance by flattening the frequency characteristic of a signal and keeping the disturbance and slope of the frequency characteristic within the frequency correction range of the equalizer in the demodulator.

[0032]

In order to solve the above problems, an AGC control type intermediate frequency amplifier circuit of the present invention divides an intermediate frequency signal (IF signal) output from a tuner into a plurality of frequency bands. Frequency band dividing means, an AGC amplifier for amplifying the divided frequency band signals, a gain control means for controlling the gain of the AGC amplifier so that the outputs of the AGC amplifier are at the same level, and the gain control means And a synthesizing means for synthesizing the signals of the respective frequency bands amplified by.

In the above configuration, the IF signal from the tuner is divided into a plurality of frequency bands by the frequency dividing means, and the signals in the respective frequency bands are input to the corresponding AGC amplifier. The gain of each AGC amplifier is adjusted by the gain control means so that the respective outputs have the same level. In this state, the signal in each frequency band is amplified by the gain controlled by the AGC amplifier and further combined by the combining means to become one signal in the frequency band before division.

In this way, by dividing the IF band into a plurality of parts, amplifying each frequency band at an appropriate level, synthesizing and combining them into one band, the divided bands are Since it is narrower than the band, the gain monotonously decreases as the frequency increases, and in the case of the IF frequency characteristic, the deviation of the original bandwidth also decreases in accordance with the narrowed band. Then, by making the output levels of the respective AGC amplifiers the same, the deviations in the respective frequency bands also become equal,
It is possible to suppress the deviation of the signals whose combined frequency bands are restored to the equalized deviation. As a result, when the combined signal is demodulated by the demodulator, the disturbance or slope of the frequency characteristic falls within a range that can be corrected by the equalizer of the demodulator.

In the above AGC control type intermediate frequency amplifier circuit, the frequency band dividing means is a plurality of band pass filters having a spectrum satisfying the Nyquist first condition and the total of the frequency band widths is the IF band. Preferably there is. A distortion-free condition can be obtained by using such a bandpass filter.

In the above AGC control type intermediate frequency amplifier circuit, the frequency band dividing means is a plurality of surface acoustic wave filters (SAW filters) having different center frequencies and each frequency band width is an IF band. Preferably there is. When such a SAW filter is used, it is not possible to obtain the distortion-free condition as in the above-mentioned bandpass filter, but it has a performance close to distortion-free.

In the above AGC control type intermediate frequency amplifier circuit, the gain control means detects the level of the output signal output from the plurality of AGC amplifiers and the detection level of the output signal from one AGC amplifier. As the reference level, the detection level and the detection level of the output signal from the AGC amplifier to be compared are used as the comparison level to compare the two levels and output the comparison level so that the comparison level becomes equal to the reference level. It is preferable to have a gain adjusting means for adjusting the gain of.

In this configuration, when the level of the output signal from one AGC amplifier is detected by the detecting means, the detected level is used as the reference level and the other AGC amplifiers are used.
The output signal of the amplifier is compared with the comparison level detected by the detection means. Therefore, it is not necessary to separately provide the reference level from outside.

In the above AGC control type intermediate frequency amplifier circuit, it is preferable that the frequency band dividing means divides the IF band into three. With this configuration, by setting the number of divisions to 3, the circuit configuration can be simplified and good flatness of the frequency characteristic can be obtained.

In the above AGC control type intermediate frequency amplifier circuit, it is preferable that the width of each divided frequency band is set to 2 MHz. This allows the IF bandwidth to be C
In the case of 6 MHz which is generally used in ATV, even if there is a deviation of 6 dB in that band, the deviation remains 2 dB in each of the three divided frequency bands and is compressed to 1/3 of the original. Become.

[0041]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to FIGS. 1 to 6.

The receiver according to the present embodiment is, for example, a QAM receiver for digital CATV, and is configured to divide the IF band into three and apply AGC to each. This receiver includes a tuner 51, a surface acoustic wave filter (hereinafter referred to as a SAW filter) 1 to 3, an AG.
C amplifiers 4 to 6, detection smoothing circuits 7 to 9, comparators 10 and 1
1, an IF amplifier 12, a low pass filter 13, a demodulator 14, an RF AGC control circuit 15 and an IF AGC control circuit 16.

Since the tuner 51 has almost the same structure as the tuner 51 in the above-mentioned conventional receiver (see FIG. 7), detailed description thereof will be omitted.

As is well known, the SAW filters 1 to 3 generate a surface wave by mechanical vibration occurring on the surface of the substrate when a signal is applied to the comb electrode on the input side, and transmit it to the electrode on the output side. Is a filter that obtains a desired band characteristic. SAW as frequency band dividing means
The center frequency of each of the filters 1 to 3 is 2 MH
The IF signal output from the tuner 51 is divided into three bands (bandwidth is 2 MHz) by setting to 42 MHz, 44 MHz, and 46 MHz that are shifted by z.

The IF AGC amplifiers 4 to 6 are I described later.
This is an amplifier that automatically controls the gain based on the IF AGC control signal from the FAGC control circuit 16. This IF
The AGC amplifiers 4 to 6 amplify the IF signal level to be constant in the level range from the gain control start input level to the strong input.

The detection and smoothing circuits 7 to 9 as detecting means are
IF of each band passed through IF AGC amplifiers 4 to 6 respectively
It is a circuit that detects the level of the IF signal by detecting the signal and then smoothing the detection output and outputting a DC voltage. In the detection and smoothing circuits 7 to 9, the IF signal is detected by diode detection, and smoothing is performed by using a capacitor.

The comparator 10 as a gain adjusting means compares the DC voltage (reference level) from the smoothing detection circuit 7 with the DC voltage (comparison level) from the smoothing detection circuit 8 so that there is no difference between them. And IF AGC according to the difference
A control signal is given to the IF AGC amplifier 5. Similarly, the comparator 11 as the gain adjusting means compares the DC voltage (reference level) from the smoothing detection circuit 7 with the DC voltage (comparison level) from the smoothing detection circuit 9 so that there is no difference between them. , IF AGC control signal according to the difference
It is given to the AGC amplifier 6. The comparators 10 and 11 are configured by a circuit including a differential amplifier that outputs a voltage according to the difference between the two DC voltages in order to perform the above operation.

The above detection rectification circuits 7 to 9 and the comparator 1
The gain adjusting means is constituted by 0 and 11.

The IF amplifier 12 as a synthesizing means is
The amplifier outputs all the amplified outputs of the respective bands from the AGC amplifiers 4 to 6 and amplifies the combined amplified output to a predetermined level.

SAW filters 1-3, IF A above
GC amplifiers 4-6, detection smoothing circuits 7-9, comparator 10,
An intermediate frequency amplification circuit is configured by 11 and the IF amplifier 12.

The low-pass filter 13 is the IF amplifier 1
The low-frequency component of the amplified output from 2 is passed and the demodulator 14
The noise component unnecessary for the processing in is removed.

The RF AGC control circuit 15 converts the non-DC RF AGC control signal output from the D / A converter 14n of the demodulator 14 which will be described later into a direct current by using a low pass filter or the like, and sets its direct current voltage to an appropriate level. And the RF AGC voltage is output. The IF AGC control circuit 16 is also the D / A converter 14m of the demodulator 14.
The non-direct current IF AGC control signal output from is converted into direct current using a low pass filter or the like, the direct current voltage is amplified to an appropriate level, and the IF AGC voltage is output.

The demodulator 14 demodulates the IF signal based on the low-pass component from the low-pass filter 13, and at the same time, controls the RF AGC control signal and the IF AGC control signal supplied to the RF AGC control circuit 15 and the IF AGC control circuit 16, respectively. It is a circuit that generates a signal. less than,
The detailed configuration of the demodulator 14 will be described.

As shown in FIG. 2, the demodulator 14 includes an A / D converter (ADC in the figure) 14a and an amplitude detector (| X in the figure) in order to generate both AGC control signals.
| 14b, subtractor 14c, adder 14d, delay circuit (Z ^{−1 in the} figure) 14e, comparator 14f, selector 14
g, 14h, multipliers 14i, 14j, adders 14k, 1
4l and D / A converters 14m and 14n.

The A / D converter 14a converts the low frequency component from the low pass filter 13 into a digital signal with a sampling clock of a predetermined frequency. Amplitude detector 14
b is the amplitude (absolute value) of the digitized low-frequency component signal
Is detected by digital calculation. The subtractor 14c subtracts the reference data Ref, which is the reference amplitude level data, from the amplitude level data detected by the amplitude detector 14b, and outputs difference data between the two.

The delay circuit 14e is a circuit that delays the output of the adder 14d by the sampling period of the A / D converter 14a. The adder 14d is a circuit that adds the delay data from the delay circuit 14e to the difference data from the subtractor 14c. Thus, the adder 14d and the delay circuit 1
Reference numeral 4e constitutes an integrating circuit, which outputs the integrated output as a control signal.

The comparator 14f has a constant level comparison value Co.
Is compared with the value of the control signal from the integration circuit, and a switching signal of "1" is output when the control signal is larger than the comparison value Co, while "0" is output when the control signal is smaller than the comparison value Co. "Switch signal is output.

The selectors 14g and 14h are comparators 14f.
The input X2 is output when the switching signal from the comparator is "1", and the input X1 is output when the switching signal from the comparator 14f is "0". Specifically, the selector 14g outputs the comparison value Co as the input X2 when the switching signal is "1", and outputs the control signal as the input X1 when the switching signal is "0". The selector 14h outputs the control signal as the input X2 when the switching signal is "1", and outputs the comparison value Co as the input X1 when the switching signal is "0".

The multiplier 14i is a circuit for multiplying the control signal from the selector 14g or the comparison value Co by a predetermined coefficient μ1, and the coefficient μ1 sets the sensitivity dB / V of the IF AGC loop. The multiplier 14j is a circuit that multiplies the comparison value Co or the control signal from the selector 14h by a predetermined coefficient μ2, and the coefficient μ2 sets the sensitivity dB / V of the RF AGC loop. The adder 14k is a circuit that adds a predetermined correction value CP1 to the output from the multiplier 14i. Correction value C
P1 is set on the basis of the dynamic range so that the output from the multiplier 14i falls within the dynamic range of the D / A converter 14m. Adder 14l
Is a circuit for adding a predetermined correction value CP2 to the output from the multiplier 14j. The correction value CP2 is set based on the dynamic range so that the output from the multiplier 14j falls within the dynamic range of the D / A converter 14n.

By providing the above selectors 14g and 14h, the gain reduction is switched as follows. That is, for the RF AGC, the gain reduction is made constant from a low input level to the input signal level with a constant input signal level as a boundary, while the gain reduction is increased for input levels higher than the above-mentioned input signal level. (See Fig. 6). At the same time, with respect to the IF AGC, the gain reduction is increased from the low input level to the above-mentioned input signal level on the contrary to the RF AGC with a constant input signal level as a boundary, while the above-mentioned signal input level is increased. The gain reduction is made constant for larger inputs (see FIG. 6).

The D / A converter 14m is an adder 14
I by converting the addition output from k to analog
Output the FAGC control signal. D / A converter 1
4n outputs the RF AGC control signal by converting the addition output from the adder 14l into an analog.

Although not shown, the demodulator 14 corrects the frequency characteristic by an equalizer (not shown) after digitizing the low-frequency component from the low-pass filter 13 by the A / D converter 14a. This equalizer performs equalization by a transversal filter including a delay circuit having a tap for each symbol period T of a part of the digital filter and a weighting circuit for weighting the output from each tap of the delay circuit.

Next, the operation of the QAM receiver configured as above will be described.

From the tuner 51, the IF center frequency 4
An IF signal of 4 MHz and band 6 MHz is output.
The SAW filter 1 has a center frequency of 42M from the IF signal.
Hz, bandwidth 2MHz band (lower frequency band) is passed, SAW filter 2 from IF signal center frequency 44MHz, bandwidth 2MHz band (intermediate frequency band)
And the SAW filter 3 passes a band (upper frequency band) having a center frequency of 46 MHz and a bandwidth of 2 MHz from the IF signal. As a result, the IF signal is divided into three bands, and the total band of these bands becomes the IF band.

Each band component from the SAW filters 1 to 3 is an IF whose gain is adjusted according to the level of each band component.
It is input to each of the AGC amplifiers 4 to 6 and amplified to a constant level. The outputs from the IF AGC amplifiers 4 to 6 are detected by the detection and smoothing circuits 7 to 9, respectively, and the detected outputs are smoothed and output as a DC voltage. At this time, when the output level of the IF AGC amplifier 4 is determined, the output levels of the IF AGC amplifiers 5 and 6 are adjusted based on that.

The DC voltages from the detection and smoothing circuits 7 and 8 are both compared by the comparator 10. By performing feedback so that the difference between the two DC voltages due to the comparison is eliminated, the IF AGC voltage is output from the comparator 10 and the IF AGC voltage is output.
It is given to the GC amplifier 5. This allows IF AGC
The output level of the amplifier 5 becomes equal to the output level of the IF AGI amplifier 4. Further, the DC voltages from the detection smoothing circuits 7 and 9 are both compared by the comparator 11. The IF AGC voltage is output from the comparator 11 and fed to the IF AGC amplifier 6 by performing feedback so that the difference between the two DC voltages due to the comparison is eliminated. By such feedback control, the output level of the IF AGC amplifier 6 becomes equal to the output level of the IF AGC amplifier 4.

Therefore, it is possible to easily equalize the average levels of the divided frequency bands. in this case,
Although the output level of the lower frequency band is used as a reference, the same effect can be obtained by using the output level of the intermediate frequency band or the upper frequency band as a reference.

In the case of the IF frequency characteristic in which the gain monotonously decreases as the frequency becomes higher, 6d in the bandwidth of 6 MHz.
Even if there is a deviation of B, the deviation of each band is 2d because it is divided into three bands of 2MHz in the above configuration.
It becomes B. Therefore, as described above, by making the outputs (average levels) of the IF AGC amplifiers 4 to 6 equal, the deviation remains at 2 dB even if the IF bandwidth is 6 MHz. As a result, the deviation is compressed to ⅓ as compared with the case where the 6 MHz band is not divided, so that the disturbance and the inclination of the frequency characteristic fall within the range in which the equalizer is effective, and the error rate does not deteriorate.

In the present embodiment, the number of divisions is set to 3 in view of the performance, the simplicity of the circuit, and the cost merit, but the number of divisions may be 3 or more, or may be 2. Of course, the IF bandwidth is 7MHz with European specifications
Even at 8 MHz or 8 MHz, the same effect as above can be obtained.

The drop in frequency characteristic caused by reflection may reach 10 dB. This cannot be predicted by the design of the reproduction system, especially the tuner, and adjustments in the manufacturing process of the receiver, and could only be improved by correction by the equalizer. However, according to the method of the present embodiment, IFAG
Since the gain is increased by the C amplifiers 4 to 6, the characteristics are significantly improved.

The outputs from the IF AGC amplifiers 4 to 6 are
Further, after being amplified by the IF amplifier 12, only the low frequency component is extracted by the low pass filter 13 and input to the demodulator 14.

In the demodulator 14, the input low frequency component is A
The signal is converted into a digital signal by the / D converter 14a, and the amplitude of the low frequency component is detected by the amplitude detector 14b based on the digital signal. In the subtractor 14c, the difference between the amplitude data and the reference data Ref is calculated, and the integral value of the difference is obtained as a control signal by the integrating circuit including the adder 14d and the delay circuit 14e.

The comparator 14f compares the control signal with the comparison value Co. As a result of the comparison, when the control signal is larger than the comparison value Co, the selector 14g outputs the input X
The comparison value Co is output as 2, and the selector 1
From 4h, a control signal is output as the input X2. On the other hand, when the control signal is smaller than the comparison value Co, the selector 1
The control signal is output as an input X1 from 4g, and the comparison value C is input as an input X1 from the selector 14h.
o is output.

The control signal or the comparison value Co output from the selector 14g is multiplied by the coefficient μ1 in the multiplier 14i and further added with the correction value CP1 in the adder 14k, and then the analog signal is output by the D / A converter 14m. IF
It is converted into an AGC control signal. On the other hand, the selector 14h
The control signal or the comparison value Co output from the multiplier 1
The coefficient μ2 is multiplied by 4j, and the correction value CP2 is added by the adder 14l, and then the D / A converter 14n
Is converted into an analog RF AGC control signal.

The RF AGC amplifier control circuit 15 is supplied with the RF AGC control signal described above to generate a constant RF AGC voltage, and thereby the RF of the tuner 51.
Controls the gain of the AGC amplifier. On the other hand, IF AG
The C amplifier control circuit 16 receives the above IF AGC control signal to generate a constant IF AGC voltage and control the gain of the IF AGC amplifier 4.

Further, in the demodulator 14, the frequency characteristic of the low-pass component from the low-pass filter 13 converted into digital by the A / D converter 14a is corrected by the equalizer.

By the way, in the embodiment as a means for dividing the frequency, SAW filters 1 to 3 having different center frequencies are used.
However, instead of this, a bandpass filter satisfying the following condition may be used. Code speed fp =
In order to transmit a pulse without distortion by 1 / T, for the real part Yr (ω) of the frequency spectrum, Y1 (π / T−ω) = − Y1 (π / T + ω), 0 ≦ ω ≦ π / T Nyquist No. 1 that satisfies Yr (ω) = 1 + Y1 (ω), | ω | <π / T Yr (ω) = Y1 (ω), π / T ≦ | ω | ≦ 2π / T using the function Y1 Conditions (Nyquist first standard)
A distortion-free condition can be obtained by using a bandpass filter having a spectrum that satisfies (FIG. 3).

FIGS. 3A to 3C show the frequency domain, and FIGS. 3D to 3F are diagrams corresponding to FIGS. 3A to 3C, showing the time domain. There is.
The Nyquist first condition is no distortion if the amplitude is zero at intervals of the symbol period T, so that the Nyquist condition in the frequency domain in FIG.
Need not be rectangular as shown in FIG.
The frequency characteristic may be such that 1 (ω) decreases near π / T.

Here, the frequency characteristic of FIG. 3C is a frequency characteristic obtained by combining the frequency characteristics of FIGS. 3A and 3B. That is, in the division of the upper, middle, and lower three frequency bands, there is a portion that overlaps with the adjacent divided frequency bands, and at this portion, the frequency with an amplitude of 1/2 (-3 dB) is adjacent. It only needs to match the frequency of the band.

Although the SAW filters 1 to 3 do not always satisfy this condition, they are bandpass filters which are approximately distortion-free and easily obtained.

Even if divided into three frequency bands, 2 MH
The frequency spectrum of the bandwidth of z may be shaped into an ideal rectangular shape, but this is not realistic. Also in this case, if the above Nyquist first condition is satisfied, the same effect as the ideal spectrum without distortion can be obtained. However, 3 dB
If a SAW filter having a falling bandwidth of 2 MHz is used, almost the same effect can be obtained.

In this embodiment, the A / D converter 14 of the demodulator 14 is used as the control means for applying AGC.
Although the analog method of rectifying and detecting the received signal has been described before a, the same effect can be obtained even with a digital method configuration as in the next embodiment.

Digital type Q as another embodiment
In the AM receiver, as shown in FIG.
IF amplifiers 21 to 23 are provided at the next stages of the C amplifiers 4 to 6, respectively.
And low pass filters 24 to 26 are arranged corresponding to the next stages of the IF amplifiers 21 to 23, respectively. Further, IF AGC amplifier control circuits 27 to 29 are provided for the IF AGC amplifiers 4 to 6, respectively.

Further, in the demodulator 30, as shown in FIG. 5, for each of the outputs from the low pass filters 24 to 26, the A / D converter 14a, the amplitude detector 14b, the subtractor 14c and the adder described above are added. 14d, delay circuit 14e, comparator 14f, selector 14g, multiplier 14
i, the adder 14k and the D / A converter 14m are I
It is provided to generate the FAGC control signal.
In the demodulation circuit 30, the selector 14 described above is used.
h, a multiplier 14j, an adder 14l and a D / A converter 14n are provided to generate the RF AGC control signal.

The coefficient μ1 multiplied by the multiplier 14j
.Mu.3 correspond to the lower, middle, and upper frequency bands, respectively. The correction values CP1 to CP3 added by the adder 14l also correspond to the lower, middle, and upper frequency bands, respectively.

In the demodulator 30 configured as described above, when the signal input level reaches the comparison value Co of an arbitrary magnitude, the selector 14g outputs the constant comparison value Co based on the comparison result by the comparator 14f. Is output. IF A
The GC control signal outputs the output of the selector 14g by μ (coefficient μ1
.About..mu.3) times, and the correction value CP (CP1 to CP3) is corrected to a value having an offset.

As shown in FIG. 6, IF1 AGC, IF
2 AGC and IF3 AGC represent the IF AGC control signal level for each of the bands (upper, middle, and lower frequency bands) obtained by dividing the IF band into three, and R
F AGC represents the RF AGC control signal level. IF1 AGC, IF2 AGC, IF3 AGC
Since the IF input signal levels corresponding to the respective signals are different from each other, a difference signal representing the difference between the amplitude data and the reference data Ref and each IF A generated based on this difference signal.
Different GC control signals, different IF AGC
A control signal is obtained. Then, as a result of controlling the gains of the IF AGC amplifiers 4 to 6 by the IF AGC voltages output from the IF AGC control circuits 17 to 29 based on the IF AGC control signals, the IF AGC amplifiers 4 to 6 Equal levels of IF output are obtained.

As described above, in the QAM receiver of this embodiment, the demodulator 30 serves as a gain control means.

Further, if the RF signal input level becomes large, the IF AGC voltage is lowered, the gains of the IF AGC amplifiers 4 to 6 are suppressed, and the IF AGC amplifier output is made constant. On the other hand, the RF AGC control signal has a constant input signal level (input level) from a low input level to an arbitrary comparison value Co, and when the input signal level becomes higher than that, the gain of the RF AGC amplifiers 4 to 6 is increased. Suppress.

As described above, according to the AGC control based on each control signal generated by the demodulator 30, the level difference between the frequency bands divided in the IF band is absorbed, and
At low input levels, RF gain is maximized and NF is minimized, while RF gain is suppressed to reduce distortion at medium and high input levels.

[0091]

As described above, the AGC control type intermediate frequency amplifier circuit of the present invention is divided by the frequency band dividing means for dividing the intermediate frequency signal (IF signal) output from the tuner into a plurality of frequency bands. AGC amplifiers for amplifying the signals in the respective frequency bands, gain control means for controlling the gain of the AGC amplifiers so that the outputs of the AGC amplifiers are at the same level, and of the frequency bands amplified by the gain control means. And a synthesizing means for synthesizing signals.

In this way, by dividing the IF band into a plurality of parts, amplifying each frequency band at an appropriate level, synthesizing and combining them into one band, the divided bands are Since the frequency band is narrower than the band, in the case of the IF frequency characteristic in which the gain monotonously decreases as the frequency increases, the deviation of the original bandwidth also decreases in accordance with the narrowed band. By setting the output levels of the AGC amplifiers to be the same, the deviations in the respective frequency bands are also equalized, and the deviations of the combined and restored frequency bands of the signals can be suppressed to equalized deviations.

As a result, when the combined signal is demodulated by the demodulator, the disturbance or inclination of the frequency characteristic falls within a range that can be corrected by the equalizer of the demodulator. Therefore, the gain frequency characteristic of the tuner is greatly improved, and the frequency characteristic in which both the SN characteristic and the distortion characteristic are stable can be obtained, so that the receiving performance of the receiver including the present AGC control type intermediate frequency amplifier circuit can be improved. Has the effect.

In the above AGC control type intermediate frequency amplifier circuit, the frequency band dividing means is a plurality of band pass filters having a spectrum satisfying the Nyquist first condition and the total of the frequency band widths is the IF band. As a result, the distortion-free condition can be obtained. Therefore, there is an effect that the performance of the receiver can be further improved.

In the above AGC control type intermediate frequency amplifier circuit, the frequency band dividing means is a plurality of surface acoustic wave filters (SAW filters) having different center frequencies and each frequency band width is the IF band. Due to this, it is not possible to obtain a distortion-free condition like the above-mentioned bandpass filter, but it has a performance close to distortion-free. Therefore, it is possible to provide a receiver with high performance.

In the above AGC control type intermediate frequency amplifier circuit, the gain control means detects the level of the output signal output from the plurality of AGC amplifiers and the detection level of the output signal from one AGC amplifier. As the reference level, the detection level and the detection level of the output signal from the AGC amplifier to be compared are used as the comparison level to compare the two levels and output the comparison level so that the comparison level becomes equal to the reference level. And a gain adjusting means for adjusting the gain. In this way, since the level of the output signal from one AGC amplifier is used as the reference level, it is not necessary to separately give the reference level from the outside. Therefore, there is an effect that the configuration of the receiver can be simplified.

In the above AGC control type intermediate frequency amplifier circuit, the frequency band dividing means divides the IF band into three parts, whereby the circuit structure can be simplified and good frequency characteristic flatness is obtained. be able to. Therefore, it is possible to provide a high-performance receiver with good cost performance.

In the above AGC control type intermediate frequency amplifier circuit, since the width of each divided frequency band is set to 2 MHz, when the IF bandwidth is 6 MHz which is generally used in CATV, that band is set. Even if there is a deviation of 6 dB, the deviation remains 2 dB in each of the three divided frequency bands and is compressed to 1/3 of the original. Therefore, it is possible to provide an intermediate frequency amplifier circuit suitable for CATV.

[Brief description of drawings]

FIG. 1 is a CATV QAM according to an embodiment of the present invention.
It is a block diagram which shows the structure of a receiver.

FIG. 2 is a block diagram showing a configuration of a demodulator in the QAM receiver.

3A to 3C are diagrams showing the Nyquist first condition in the frequency domain, and FIGS. 3D to 3F are diagrams showing the Nyquist first condition in the time domain.

FIG. 4 is a block diagram showing the configuration of a QAM receiver according to another embodiment of the present invention.

5 is a block diagram showing a configuration of a demodulator in the QAM receiver of FIG.

6 is a graph showing switching of a control signal level with respect to an input signal level in the QAM receiver of FIG.

FIG. 7 is a block diagram showing a configuration of a conventional CATV QAM receiver.

FIG. 8 is a VH of a tuner in the QAM receiver of FIG.
It is a circuit diagram which shows the structure of an F band filter.

FIG. 9 is a graph showing frequency characteristics of the RF bandpass filter of the tuner.

FIG. 10 is a graph showing adjustment frequency characteristics of a double-tuned circuit in the RF bandpass filter.

FIG. 11 is a graph showing the relationship between frequency characteristic deviation and error rate.

[Explanation of symbols]

1 to 3 SAW filter (frequency band dividing means) 4 to 6 IF AGC amplifier (AGC amplifier) 7 to 9 detection smoothing circuit (detection means, gain control means) 10, 11 comparator (gain adjustment means, gain control means) 12 IF amplifier (combining means) 14 demodulator 15 RF AGC control circuit 16 IF AGC control circuit 30 demodulator (gain control means) 51 tuner

Claims (6)

[Claims] 1. A frequency band dividing means for dividing an intermediate frequency signal output from a tuner into a plurality of frequency bands, and an AGC for amplifying each of the divided frequency band signals.
An amplifier, a gain control means for controlling the gain of the AGC amplifier so that the outputs of the AGC amplifier are at the same level, and a synthesizing means for synthesizing the signals of the respective frequency bands amplified by the gain control means. AG characterized by
C control intermediate frequency amplifier circuit. 2. The AGC control type intermediate frequency amplifier circuit according to claim 1, wherein the frequency band dividing means is a plurality of band pass filters having a spectrum satisfying a Nyquist first condition. 3. The AGC according to claim 1, wherein the frequency band dividing means is a plurality of surface acoustic wave filters having different center frequencies and the sum of the frequency band widths is an IF band. Controlled intermediate frequency amplifier circuit. 4. The gain control means detects the level of the output signal output from a plurality of AGC amplifiers, and compares the detected level of the output signal from one AGC amplifier with the detection level as a reference level. Using the detection level of the output signal from the target AGC amplifier as the comparison level,
The AGC according to claim 1, further comprising a gain adjusting unit that compares the two levels and adjusts the gain of the AGC amplifier that outputs the comparison level so that the comparison level becomes equal to the reference level. Controlled intermediate frequency amplifier circuit. 5. The frequency band dividing means divides the IF band into three.
The AGC control type intermediate frequency amplifier circuit according to claim 1, wherein the AGC control type intermediate frequency amplifier circuit is divided. 6. The AG according to claim 5, wherein the width of each divided frequency band is set to 2 MHz.
C control intermediate frequency amplifier circuit.