JPH11205204A - Demoduiator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a narrow band inexpensively by securing necessary adjacent channel selectivity by using an intermediate frequency filter of a band that is narrower than a receiving band width and performing amplitude compensation so as to be a root square cosine characteristic by using an equalizer. SOLUTION: A signal that is received by a high frequency amplifier 3 is amplified and sent to a 1st mixer 4 and converted into a 1st intermediate frequency signal by a signal of a 1st local oscillator 5. After a 1st intermediate frequency filter 6 eliminates an unnecessary component from the signal, an AGC amplifier 7 amplifies it so that the level of an output signal may be constant. Next, a 2nd mixer 2 converts it into a 2nd intermediate frequency signal with a signal of a 2nd local oscillator 9. A 2nd intermediate frequency filter 10' eliminates an unnecessary component from the 2nd intermediate frequency signal. This filter characteristic is a filter characteristic of a narrow band rather than a root square cosine characteristic that is demanded as a receiving band limitation characteristic and secures a necessary adjacent channel selectivity characteristic. An amplitude delay equalizer 15 corrects amplitude deviation that takes place at that time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio demodulation system.

[0002]

2. Description of the Related Art In wireless communication, it is required to narrow the bandwidth to minimize interference with adjacent channels and not to degrade transmission quality from the viewpoint of effective use of frequency.

In order to satisfy the above requirements, the raised cosine characteristic is often used as a band limiting filter in a digital communication system. (PDC, PHS system, etc.
A raised cosine characteristic with a roll-off rate α = 0.5 is used. Also, in order to perform optimal signal transmission, a method is generally used in which 50% of the raised cosine characteristic is distributed to each of the transmission and reception, and the root raised cosine characteristic is used as the filter characteristic of each of the transmission and reception. As a method of realizing a filter having a root-square cosine characteristic for reception, a method of giving an intermediate frequency filter an approximate characteristic is often used because of its simple configuration.

[0004] A conventional demodulation system using an intermediate frequency filter having a root-square cosine characteristic will be described below with reference to FIG. The received signal received from the antenna terminal 1 is input to the high frequency amplifier 3 via the duplexer 2 and amplified. The duplexer 2 separates a signal input from the transmission unit from a reception signal. The amplified received signal is converted into a first intermediate frequency signal by a signal of a first local oscillator 5 in a first mixer 4. The converted first intermediate frequency signal is subjected to removal of unnecessary components by the first intermediate frequency filter 6, and then amplified by the AGC amplifier 7 so that the output signal level becomes constant. Next, the signal amplified to have this constant output is converted into a second intermediate frequency signal by the signal of the second local oscillator 9 in the second mixer 8. The second intermediate frequency signal is subjected to removal of unnecessary components by a second intermediate frequency filter 10 and waveform shaping by a root-square cosine characteristic, and then amplified by an intermediate frequency amplifier 11. The signal amplified by the intermediate frequency amplifier 11 is then converted by a third local oscillator 13 in a quadrature detector 12 into a baseband signal of an in-phase component I and a quadrature component Q. In the D converter 14-1, the Q component is A / D
Each is converted into a digital signal by the converter 14-2. The digitized I component signal and Q component signal are both sent to a detection circuit 16, and the detection circuit 16 reproduces data from the input I component signal and Q component signal. If the characteristic of the second intermediate frequency filter 10 in FIG. 2 is close to the ideal root cosine characteristic as shown in FIG. 3, the necessary adjacent channel selectivity is reduced as shown in FIG. Is secured. However, the frequency spacing is 25 kHz
As the bandwidth is narrowed down to z, 12.5 kHz, 6.25 kHz, ‥‥‥, the root raised cosine characteristic is realized only by the intermediate frequency filter and the adjacent channel selectivity is secured as in the above-described conventional example. Things get harder.

As shown in FIG. 6 (b), when the same intermediate frequency is used due to the relationship between the Q of the element constituting the filter and the fractional band, the steeper the narrower the band, the steeper the attenuation characteristics. Can not be. Contrary to this, the narrower the band is narrowed, the more abrupt attenuation characteristics are required to remove adjacent channel interference.

In order to divert the conventional method, the Q constituting the filter may be increased or the intermediate frequency may be increased to make the relation of the fractional band advantageous. However, there is a limit to the Q of the filter, and increasing the intermediate frequency is impractical in terms of cost and miniaturization.

Further, it is conceivable to make the attenuation characteristic of the intermediate frequency filter gentle and to secure a sufficient attenuation characteristic with the digital filter in the subsequent stage. However, as shown in FIG. 7, the signal level of the adjacent channel is higher than that of the desired signal as shown in FIG. If the level is high, the level of the desired signal is much lower than the level of the signal (interfering wave) of the adjacent channel remaining after the band limitation, so that the dynamic range of the A / D converter is almost taken into the level of the interfering wave. Therefore, there is a problem that an A / D converter having a sufficiently wide dynamic range is required, and the A / D converter becomes expensive.

[0008]

The above-mentioned prior art includes the following:
As the band becomes narrower, there are the following problems.

(1) Limit of the Q of the filter (2) Impracticality in terms of cost and miniaturization by increasing the intermediate frequency (3) A / D converter having a wide dynamic range is required SUMMARY OF THE INVENTION An object of the present invention is to provide a demodulator which eliminates the above-mentioned disadvantages, and which can be narrowed by using an inexpensive and small-sized intermediate frequency filter having a moderate attenuation characteristic.

[0010]

In order to achieve the above object, the present invention employs an intermediate frequency filter having a band narrower than a reception bandwidth (a bandwidth of a root-square cosine characteristic) to obtain a necessary adjacent channel selectivity. To secure. Further, the attenuation due to the use of the narrow band filter as the intermediate frequency filter is:
The amplitude is compensated so as to have the root raised cosine characteristic by using the equalizer. At the same time, a compensation function of the equalizer is provided with a group delay distortion compensation function to compensate for a group delay characteristic that is not a constant value.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. The received signal received from the antenna terminal 1 is input to the high-frequency amplifier 3 via the duplexer 2. The duplexer 2 separates a signal input from the transmission unit from a reception signal. The high-frequency amplifier 3 amplifies the transmitted signal. The amplified received signal is sent to the first mixer 4,
The first mixer 4 converts the input received signal into a first intermediate frequency signal based on the signal of the first local oscillator 5. The first intermediate frequency signal is subjected to removal of unnecessary components by the first intermediate frequency filter 6, and then amplified by the AGC amplifier 7 so that the output signal level becomes constant. Next, the signal amplified to be a constant signal is supplied to the second mixer 8 for the second signal.
Is converted to a second intermediate frequency signal by the signal of the local oscillator 9.

An unnecessary component of the second intermediate frequency signal is removed by a second intermediate frequency filter 10 '. As shown in FIG. 4, the filter characteristic has a narrower band than the root-square cosine characteristic required as the reception band limiting characteristic, and secures a necessary adjacent channel selectivity characteristic. Since the second intermediate frequency filter 10 'has a narrower band filter characteristic than the root raised cosine characteristic, the amplitude characteristic deviates from the root raised cosine characteristic. In addition, the second intermediate frequency filter 10 'has a narrow group delay characteristic within a necessary band because of a narrow band filter. The correction of this deviation is compensated by an amplitude delay equalizer 15 described later, and the overall characteristic becomes a root square cosine characteristic.

The output of the second intermediate frequency filter 10 'is amplified by an intermediate frequency amplifier 11 and input to a quadrature detector 12. In the quadrature detector 12, the input signal is converted by a third local oscillator 13 into a baseband signal of an in-phase component signal I and a quadrature component signal Q. Next, the in-phase component signal I is A
The quadrature component signal Q is supplied to the A / D converter 14-2.
And converted into digital signals. The digital baseband signals I and Q are connected to an amplitude delay equalizer 15.
And the amplitude delay equalizer 15 equalizes the amplitude and group delay distortion to secure the root-square cosine characteristic as the overall characteristic. Thereafter, the signal is input to the detection circuit 16, and the detection circuit 16 reproduces data from the input signal.

As shown in FIG. 4, the characteristics of the intermediate frequency filter have a narrower band than that of the ideal root raised cosine filter. Further, as shown in FIG. 5, the group delay characteristic is shifted from the ideal characteristic within the required band due to the narrow band.

In the above embodiment, the amplitude delay equalizer is used.
Although 15 is arranged after the quadrature demodulator 12, it can be arranged anywhere, for example, before the second intermediate filter 10 'to correct the narrowed band in advance.

It is apparent that a duplexer is not required for a transmitter and a receiver separate from each other.

As a result, a sufficient adjacent channel selectivity characteristic can be ensured by the intermediate frequency filter, and the total reception band characteristic is changed to the root square cosine in order to compensate for the amplitude distortion and the delay distortion by the equalizer. Characteristics, and transmission characteristics free of intersymbol interference can be ensured.

FIG. 8 shows a second embodiment of the present invention, which is a configuration example in the case of a digital radio. Although the codes are the same as those in FIG. 1, there is no duplexer 2, A / D converters 14-1 and 14-2 and demodulator 16, and 12 'is an equalizer, and all signals are digitally processed. Here, the amplitude / delay distortion generated in the signal band due to the filter characteristics of the first intermediate frequency filter 6 and the second intermediate frequency filter 10 'is reduced by the filter of the first intermediate frequency filter 6 and the second intermediate frequency filter 10'. The filter coefficient having the inverse characteristic to the characteristic is set in the equalizer 15 'in advance, and the equalization processing is performed to compensate for the amplitude / delay distortion.

In the example shown in FIG. 8, the signal level received from the antenna terminal 1 is affected by fading or the like, and is not constant but changes every moment. Therefore, the input level of the first intermediate frequency filter 6 fluctuates, and the attenuation characteristic changes due to the deviation of the impedance matching of the first intermediate frequency filter 6. For this reason, the compensation effect of the equalizer 15 is reduced, and the amount of C / N deterioration of the equalizer is increased.

Therefore, a third embodiment of the present invention will be described with reference to FIG. 9 as an improvement of FIG. In FIG. 9, reference numerals used are the same as those in FIG.
Is an equalization filter coefficient ROM. In addition to the output of the first mixer 4 being input to the first intermediate frequency filter 6, the output is also input to the level detector 100, and the output of the level detector 100 is input to the equalization filter coefficient ROM 110. The output of the equalization filter coefficient ROM 110 is input to an equalizer 12 '. The operation in FIG. 9 is the same as that in FIG. 8, except that a part of the output signal from the first mixer 4 is split and input to the level detector 100, whereby the level of the input signal is Is "Hi" or "Lo". Next, the result is sent to the equalization filter coefficient ROM 110, and the equalization filter coefficient ROM 110 outputs an equalization filter coefficient corresponding to the output signal level of the level detector 100 to the equalizer 15 '.
The equalizer 15 'performs an equalization process using the output signal of the quadrature demodulator 12 and the coefficient of the equalization filter coefficient ROM 110, and outputs a demodulated signal in which amplitude distortion and delay distortion have been compensated as reproduction data.

FIG. 10 is an example of a circuit diagram of the level detector 100, in which 21 is a level detector input terminal, 22 and 23 are diodes,
24 is a capacitor, 25 is a resistor, 26 is a detection circuit section, 27 is a comparator, 28 is a reference voltage, 29 is a latch circuit, 30 is a level detector output terminal, and 31 is a ground. In FIG.
The signal input to the level detector input terminal 21 is a diode
Detection is performed by a detection circuit section 26 composed of 22, 23, a capacitor 24 and a resistor 25, and the average input level is sent from the detection circuit section 26 to a comparator 27. The comparator 27 compares the transmitted average input level with the reference voltage 28 and outputs a selection signal indicating whether the equalization coefficient is a high input level equalization coefficient or a low input level equalization coefficient. The output result of the comparator 27 is input to and held in a latch circuit 29, and is sent through a level detector output terminal 30. The high input level equalization coefficient or the low input level equalization coefficient of the equalization filter coefficient ROM is transmitted. Selection signal.

Incidentally, also in the above-described embodiment, the delay equalizer 15 '
Is arranged at the subsequent stage of the quadrature demodulator 12. However, the arrangement may be arbitrary, and it is obvious that the arrangement of the level detector 100 and the equalization filter coefficient ROM 110 changes with the arrangement.

[0023]

According to the present invention, the intermediate frequency filter does not need to have a receiving band characteristic such as a root-square cosine characteristic, and it is possible to sufficiently secure adjacent channel selectivity even in the case of a narrow band demodulation method.

Further, as a second effect of the present invention, the compensation characteristic of the equalizer is provided with a group delay distortion compensation function, so that a group delay characteristic that is not a constant value can be compensated.

Further, as in a third embodiment of the present invention,
It is possible to realize a demodulator that can cope with a change in input level due to the influence of fading or the like.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a block diagram showing one embodiment of the prior art.

FIG. 3 is a diagram showing filter characteristics according to the related art.

FIG. 4 is a diagram illustrating an example of a filter characteristic according to the present invention.

FIG. 5 is a diagram showing an example of characteristics of the equalizer of the present invention.

FIG. 6 is a view for explaining adjacent channel selectivity characteristics when a band is narrowed;

FIG. 7 is a diagram illustrating a case where an interference wave level of an adjacent channel is higher than a desired wave signal.

FIG. 8 is a block diagram showing a second embodiment of the present invention.

FIG. 9 is a block diagram showing a third embodiment of the present invention.

FIG. 10 is a circuit diagram showing an example of a level detection circuit used in the present invention.

[Explanation of symbols]

1: Antenna terminal, 2: Duplexer, 3: High frequency amplifier,
4: first mixer, 5: first local oscillator, 6: first intermediate frequency filter, 7: AGC amplifier, 8: second mixer, 9: second local oscillator, 10, 10 ': second Intermediate frequency filter, 11: Intermediate frequency amplifier, 12: Quadrature demodulator, 14-1: A / D converter, 14-2: A / D converter,
15: Amplitude delay equalizer, 15 ': Equalizer, 16: Detection circuit, 21: Level detector input terminal, 22, 23: Diode, 24: Capacitor, 25: Resistor, 26: Detection circuit, 27 : Comparator, 28: Reference voltage, 29: Latch circuit, 30: Level detector output terminal, 31: Ground,
100: Level detector 1, 110: Equalization filter coefficient RO
M,

Claims (6)

[Claims] 1. A wireless device, comprising: a first amplifier for amplifying a received signal; a first intermediate frequency filter for removing unnecessary components of a signal amplified by the first amplifier; An AGC amplifier for amplifying the first intermediate frequency signal from which unnecessary frequency components have been removed by the intermediate frequency filter to a constant output signal level, and a second intermediate frequency filter for removing unnecessary components of the signal amplified by the AGC amplifier; ,
A second amplifier for amplifying a second intermediate frequency signal from which unnecessary frequency components have been removed by the second intermediate frequency filter, a quadrature detector for quadrature detecting the signal amplified by the second amplifier, An equalizing means for performing an equalization process on a signal orthogonally detected by the quadrature detector;
Wherein the intermediate frequency filter is a filter having a band narrower than a reception band characteristic. 2. The demodulator according to claim 1, wherein said equalizer has an equalizing function for correcting delay distortion. 3. The demodulator according to claim 2, wherein the overall characteristic is an optimum reception band or a band restriction characteristic required for reception. 4. An apparatus according to claim 2, further comprising means for detecting a level of the input signal, and means for storing an equalization coefficient corresponding to the input level, wherein the equalization coefficient is stored in accordance with the input level. A demodulator characterized by changing. 5. The demodulator according to claim 4, further comprising a half-wave rectifier circuit, a comparator circuit, and a latch circuit, wherein the equalizer coefficient is changed according to an input level. 6. The invention according to claim 4, wherein the means for storing the equalization coefficient outputs an equalization filter coefficient corresponding to the level of the input signal based on an output signal of the means for detecting the level of the input signal. A demodulator characterized by changing the equalization coefficient.