JPH05284182A - Reception filter circuit - Google Patents

Abstract

PURPOSE:To provide a reception filter circuit which secures a prescribed band width and the satisfactory definition and also has an improved S/N. CONSTITUTION:Frequency signal detecting circuits 2, 3 and 4 detect in parallel the frequency signals of different prescribed bands out of the signals supplied from an antenna. Then an adder circuit 5 adds together those frequency signals detected by the circuits 2-4. In such a constitution, the noise component generated by the frequency signal detecting circuit of the precedent stage is never amplified by the frequency signal detecting circuit of the next stage. Thus the influence of the noises is reduced and the S/N is improved in comparison with a reception filter circuit consisting of plural frequency signal detecting circuits connected in series. As a result, the gain of the general frequency characteristic can be set larger over a prescribed band width for a reception filter circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiving filter circuit, and more particularly to a receiving filter circuit suitable for use in, for example, an RF-ID card system.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing the configuration of a signal detection filter circuit in the medium wave band of an RF-ID card system.

In the figure, a signal detection filter circuit 51 is a resonance circuit using passive elements such as an inductor L and a capacitor C, or resonance circuits 52 and 53 including active elements such as an operational amplifier and an inductor L and a capacitor C. 54 is connected in cascade.

In the resonance circuits 52, 53 and 54 of the signal detection filter circuit 51, different tuning frequencies are set respectively, and 131 KHz to 135 KH as a whole.
It is possible to obtain flat frequency characteristics in the band up to z.

In this RF-ID card system, the RF
-The RF-ID card signal from the ID card is 131 kHz.
To 135 KHz in the FSK modulation signal band.

The signal detection filter circuit 51 detects the received signal from the antenna 55 from 131 KHz to 13 KHz.
RF-ID with FSK modulation signal band up to 5 KHz
The card signal is selected, amplified, and output to the detection or demodulation unit.

FIG. 4 is a frequency characteristic diagram when the tuning frequencies of the resonance circuits 52 and 53 are set to the center frequency fo of the FSK modulation signal band, and (a) and (b) of FIG. 4 are two-stage cascade connection. Of the frequency characteristics of each of the resonance circuits shown in FIG.
It is a comprehensive frequency characteristic chart of 2,53.

The signal detection filter circuit 51 is replaced with the resonance circuit 5.
As shown in (c) of FIG. 4, the characteristics of the configuration of the cascade connection of 2,53 are large in the gain in the central portion of the FSK modulation signal band, but this portion is greatly amplified,
Both ends of the FSK modulation signal band are attenuated.

FIG. 5 shows a signal detection filter circuit 5
1 is composed of the resonance circuits 52 and 53, and the resonance circuits 52 and 53
The tuning frequency of 13 is slightly shifted, from 131 KHz to 13
A flat characteristic is obtained over the FSK modulation signal band up to 5 KHz. (A) and (b) of FIG. 5 are frequency characteristic diagrams of two-stage cascade-connected resonant circuits, and (c) of FIG. FIG. 7 is a general frequency characteristic diagram including a circuit 53 and a resonance circuit 54.

According to this, the signal detection filter circuit 5
The characteristics after one pass are flat frequency characteristics over the entire FSK modulation signal band of the RF-ID card signal.

[0011]

In the conventional signal detection filter circuit, the tuning frequency of each resonance circuit is set to RF-I.
Center frequency f of FSK modulation signal band which is a D card signal
When set to o, F from 131 KHz to 135 KHz
The SK modulation signal bandwidth is not secured, the RF-ID card signal is attenuated at both ends of the FSK modulation signal band, and
When an active component such as an operational amplifier is used, the noise caused by these components is amplified by the resonance circuit in the subsequent stage, so the S / N ratio is not improved, and as a result, the level of the received signal must be increased. However, there is a problem that the RF-ID card must be brought close to the antenna.

Further, in the case where the tuning frequency of each of the resonance circuits 52 and 53 is slightly shifted to obtain a flat characteristic as a whole over the FSK modulation signal band, the frequency characteristic of the resonance circuit 52 of the first stage is The loss on the right side of the FSK modulation signal band is large, and the loss on the left side of the FSK modulation signal band is large in the frequency characteristic of the second-stage resonant circuit 53. Although the frequency characteristic of is a flat characteristic as a whole, it is a characteristic that a sufficient gain cannot be obtained and is attenuated, and the noise component included in the output of the former stage is amplified in the latter stage, and Q is sufficiently increased. However, there was a problem that the S / N ratio was not improved.

The present invention has been made to solve the above problems, and it is an object of the present invention to provide a reception filter circuit which secures a predetermined bandwidth and a sufficient sharpness and improves the S / N ratio. To aim.

[0014]

A reception filter circuit according to the present invention includes a plurality of frequency signal detection circuits for detecting frequency signals in different bands in parallel from a reception signal supplied from an antenna, and the plurality of frequency signal detection circuits. And an adder circuit for adding the frequency detection signals detected in parallel by the frequency signal detection circuit.

[0015]

In the reception filter circuit of the present invention, a plurality of frequency signal detecting circuits having predetermined tuning frequencies are set to detect predetermined frequency signals in parallel from the reception signal received by the antenna, and the plurality of frequency signals are detected. Since the frequency signals detected by the detection circuit are added by the addition circuit, a predetermined bandwidth and sufficient sharpness can be secured and the S / N ratio can be improved.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a receiving unit including the receiving filter circuit of this embodiment.

The reception filter circuit 1 has a tuning frequency of 13
From the first band-pass filter 2 set to 5 KHz, the second band-pass filter 3 whose tuning frequency is set to 133 KHz, the third band-pass filter 4 whose tuning frequency is set to 131 KHz, and the addition circuit 5 It is configured

The first band-pass filter 2, the second band-pass filter 3 and the third band-pass filter 4 are connected in parallel, respectively, and the weak RF received by the antenna is received.
An ID card signal is provided respectively.

The first band-pass filter 2, the second band-pass filter 3 and the third band-pass filter 4 are band-pass filters composed of an operational amplifier, a resistor R and a capacitor C. Is.

The adder circuit 5 is a circuit for adding the outputs of the first bandpass filter 2, the second bandpass filter 3 and the third bandpass filter 4 in an analog manner.

The low pass filter 6 has a cutoff frequency of 13
The filter is set to 5 KHz and blocks a sneak into the detection unit 7 of a 150 KHz transmission signal supplied to the antenna from a transmission unit (not shown).

In this embodiment, since the first bandpass filter 2, the second bandpass filter 3 and the third bandpass filter 4 are bandpass filters composed of biquad circuits, Q can be increased. And as a result,
The frequency characteristic of the reception filter circuit 1 including the adder circuit 5 has a slope having a large slope as shown in FIG.

In FIG. 2, 2a is a frequency characteristic of the first bandpass filter 2, 3a is a frequency characteristic of the second bandpass filter 3, and 4a is a frequency characteristic of the third bandpass filter 4. ..

As described above, in this embodiment, from 131 KHz to 13 out of the received signals received by the antenna.
Since the frequency signal in the band up to 5 KHz passes through the first bandpass filter 2, the second bandpass filter 3 or the third bandpass filter 4 only once, a plurality of bandpass filters are provided. Noise components generated by the respective bandpass filters are not amplified by the bandpass filters in the subsequent stages, so that the influence of noise is reduced and the S / N ratio is reduced. improves.

Further, since the influence of noise is reduced and the S / N ratio is improved, the overall gain of the reception filter circuit 1 can be set large.

[0026]

As described above, according to the present invention, the received signal supplied from the antenna is supplied in parallel to each of the plurality of frequency signal detection circuits, and the received signal is supplied depending on the pass band of each frequency signal detection circuit. The extracted frequency components are respectively extracted, and the sum of the extracted frequency components is detected as the received signal.
Compared to a reception filter circuit that is configured by connecting multiple frequency signal detection circuits in series, the noise component generated in the frequency signal detection circuit in the previous stage is not amplified in the frequency signal detection circuit in the subsequent stage, and the influence of noise is reduced. Reduced, S / N
The ratio is improved.

Further, since the influence of noise is reduced and the S / N ratio is improved, there is an effect that the gain of the overall frequency characteristic of the reception filter circuit can be set large.

[Brief description of drawings]

FIG. 1 is a block diagram showing a reception filter circuit according to an embodiment of the present invention.

FIG. 2 is a frequency characteristic diagram showing overall characteristics of the reception filter circuit according to the embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a conventional signal detection filter circuit.

FIG. 4 is a total frequency characteristic diagram of each resonance circuit and these circuits.

FIG. 5 is a frequency characteristic diagram of each resonant circuit when the tuning frequency of the resonant circuit is slightly shifted, and a comprehensive frequency characteristic diagram based on them.

[Explanation of symbols]

 1 Reception Filter Circuit 2 1st Bandpass Filter 3 2nd Bandpass Filter 4 3rd Bandpass Filter 5 Adder Circuit

Claims (1)

[Claims] 1. A plurality of frequency signal detection circuits for respectively detecting in parallel a frequency signal of a different predetermined band from a reception signal supplied from an antenna, and a plurality of frequency signal detection circuits for detecting the frequency signals in parallel. A receiving filter circuit including an adding circuit that adds the generated frequency detection signals.