JPH0338105A - Band pass filter circuit - Google Patents

Abstract

PURPOSE:To make the group delay characteristic in a signal pass band flat by adding a resistor in parallel with at least one parallel resonance circuit in a band pass filter consisting of at least one series resonance circuit and one parallel resonance circuit. CONSTITUTION:A band pass filter 5 consists of three series resonance circuits 78, 79, 80 connected in series with a signal line 71 and 3 parallel resonance circuits 3, 4, 89 inserted between the signal line 71 and ground. Inductors 72, 73, 74 and capacitors 75, 76, 77 in the series resonance circuits 78, 79, 80 are connected in series respectively and inductors 81,82, 83 and capacitors 84, 85, 86 in the parallel resonance circuits 3, 4, 89 are connected in parallel respectively. Resistors 1, 2 are added in parallel with two parallel resonance circuits 3, 4 with a high Q including rod or distributed constant type inductors 81,82 to decrease the load Q so as to be in matching with the load Q of the series resonance circuits 78, 79, 80.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is applicable to bandpass filter circuits that require flatness in group delay characteristics within the signal band, such as when band-limiting digitally modulated high-frequency signals. Regarding.

BACKGROUND OF THE INVENTION As shown in FIG. 4, when a frequency conversion circuit 50 performs band limiting of a digitally modulated high frequency signal, a band pass filter 51 is usually inserted after frequency conversion. Here, 52 is an RF amplifier, 53 is a mixer, 54 is a local oscillator, and 55 is an rF amplifier.

A specific example of the configuration of this bandpass filter 51 is shown in FIG. This example includes three series resonant circuits 78, 79, 80 connected in series to the signal line 71, and three parallel resonant circuits 87, 88, 80 inserted between the signal line 71 and the ground.
This is a sixth-order bandpass filter 70 consisting of 9 and 9. In each series resonant circuit 78.79.80 the inductor 72.7
3.74 and capacitor 78.79.80 are connected in series, and in each parallel resonant circuit 87.88.89, inductor 81.82.83 and capacitor 75.76.7
7 are connected in parallel one by one.

In order to reduce the fractional band (ratio of IF bandwidth to center frequency) of this bandpass filter 70, the series resonant circuit 7
The value of inductor 72.73J74 of 8.79.80 must be large, and conversely the value of inductor 81.82.83 of parallel resonant circuit 87.88.89 must be small. In one calculation example for providing a filter with characteristics of a center frequency of 140 MHz and a 3 dB bandwidth of 12 MHz, a series resonant circuit 78
．． The values of inductors 72, 73, and 74 of 79.80 are 0.3 μH, 2.5 μH, 0, 6 μII, respectively (the values of capacitors 75.7677 are approximately 5 pF, 0.
5pF, 2pF), parallel resonant circuit 87.88.89
The values of inductors 81, 82, and 83 are 9. On
H, 9, 2nH, 31, 4nH (capacitor 84°85
．． The values of 86 are approximately 145pF, 140pF, and 4, respectively.
0 pF ) in this calculation example, the parallel inductor 81.
When the inductor value is on the order of nll as in 82.83, a distributed constant inductor with a linear conductor is used in the -shell, and a series inductor 72.73.74
When the value is on the μH order, as in the case of FIG.

When limiting the band of a digitally modulated high-frequency signal, the phase distortion of the high-frequency signal will lead to errors in the demodulated digital signal, so the group delay characteristics should be flattened within the passband of the bandpass filter. There is a need.

However, in the case of the specific configuration example shown in FIG. 5, there are the following problems that impede this.

When using an inductor with a wire wound around a magnetic core,
Particularly in a high frequency band (several tens of MHz or higher), the internal resistance increases, resulting in a decrease in the Q value. On the other hand, since the inductor made of a linear conductor is a distributed constant type, the Q value decreases little as the frequency increases.Therefore, in the above example, the parallel resonant circuit 87.88.89 (using a distributed constant type inductor) An imbalance occurs between the Q value of the series resonant circuit 78.79.80 (using a wire-wound inductor), and the 6th
As shown in the figure, ripples (wavy non-uniformity) occur in the group delay characteristic curve 100 within the signal passband.

For example, when actually measuring a Bessel filter designed with the circuit of the above specific configuration/calculation example with the specification that the group delay characteristic is within 5 nsecp-1 at a bandwidth of 12 MHz, the group delay characteristic is as shown in Fig. 7. The ripple of curve 120 has a maximum value of 17 nsec pp, which is a very large value. (In addition, the attenuation characteristic curve 12 is shown in the same figure.
1 (center frequency 140 questions, 3dB bandwidth 12MHz
) are also shown. ) If such large non-uniformity occurs in the group delay characteristics within the passband, the signal passing through this bandpass filter will undergo phase modulation, causing waveform distortion and increasing the error rate of the demodulated digital signal. There was a serious problem.

It is an object of the present invention to solve such problems and provide a bandpass filter circuit with flattened group delay characteristics within a signal passband.

In order to achieve the above object of Section 2, the bandpass filter circuit of the present invention includes a series resonant circuit consisting of a series connection of an inductor and a capacitor inserted in series in a signal line, and a connection between the signal line and the ground. In a bandpass filter configured with at least 1 parallel resonant circuits each consisting of an inductor and a capacitor connected in parallel, a resistor is added in parallel to the inductor and capacitor of the at least 1 parallel resonant circuit. It is characterized by

By adding a resistor in parallel to a parallel resonant circuit with a high Q value, the Q value can be lowered to the same level as a series resonant circuit in a high frequency band. As a result, the Q values of the series resonant circuit and the parallel resonant circuit are balanced, and as shown in FIG. 2, the group delay characteristic curve 20 becomes flat within the signal pass band.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example in which the present invention is applied to a sixth-order bandpass filter similar to that in FIG. 5, and the same elements are given the same reference numerals.

In this embodiment, rod-shaped or distributed constant type inductors 81 and 8
By adding resistors 1 and 2 in parallel to two parallel resonant circuits 3 and 4 (corresponding to 87 and 88 in FIG. 5) with high Q values including 2, respectively, their load Q value is lowered,
It is made to match the load Q value of the series resonant circuit 78, 79, 80.

Regarding this bandpass filter circuit, the same specification conditions as above (center frequency 140MHz, 3dB bandwidth 12M11
2. In order to satisfy the group delay (within 5 nsec p-p at a bandwidth of 12 MHz), the values of each inductor and capacitor were made the same as above, and the value of the resistor 12 was optimized with a value of 100 to 300 Ω, as shown in Figure 3. As shown, the measured group delay characteristic 30 was within 2 nsec p-p, and compared to the conventional circuit, it was possible to achieve a significant reduction in ripple. At this time, the -1 attenuation characteristic 31 is 3 dB, which is the specification condition.
It satisfies the bandwidth of 12MHz.

In the above embodiment, the value of the resistor 1.2 was optimized through actual measurements, but these values can be roughly determined by taking into consideration the resistance of the wire-wound inductor used in the series resonant circuit. Further, the resistor 1.2 to be added is not limited to a fixed resistor, but may be a variable resistor or a combination of a fixed resistor and a variable resistor. Furthermore, the effect of the present invention can be obtained by adding a resistor to at least one parallel resonant circuit, but it is of course possible to obtain a flatter group delay characteristic by adding a resistor to all three parallel resonant circuits. becomes easier.

As explained above, according to the present invention, the group delay characteristic is flattened within the pass signal band of the band pass filter. Therefore, by using this bandpass filter to limit the band of a digitally modulated high frequency signal, it is possible to suppress the occurrence of phase distortion in the high frequency signal and prevent the mixing of errors into the digital signal. Further, in the present invention, since a resistor is simply added, it can be implemented easily and inexpensively, and the size of the filter does not become large.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a band-pass filter according to an embodiment of the present invention, FIG. 2 is a graph of a group delay characteristic curve of a band-pass filter according to the present invention, and FIG. 3 is a graph of attenuation characteristics of a band-pass filter according to an embodiment. A graph showing an experimental example of group delay characteristics,
Fig. 4 is a block diagram showing an example of the use of a band pass filter, Fig. 5 is a circuit diagram showing an example of the configuration of a conventional band pass filter, Fig. 6 is a graph of the group delay characteristic curve of the conventional example, and Fig. 7 is a block diagram showing an example of the use of a band pass filter. The figure is a graph showing an experimental example of the attenuation characteristics and group delay characteristics of the conventional bandpass filter. 5.70...Band pass filter 71...Signal line 3.4.87.88.89...Parallel resonant circuit 78.7
9.80...Series resonant circuit 72.73.74.81.82.83...Inductor 75.76.77.78.79.8o...Capacitor 1.2...Resistor applicant SHARP CO., LTD. Shizuo Sano Figure 3 Frequency Figure 3 Figure 1 Figure 5 Frequency ↑ Frequency (2MHz / diV,) -July 1989 l
1 day @

Claims (1)

[Claims] At least one series resonant circuit consisting of a series connection of an inductor and a capacitor inserted in series with the signal line, and at least one parallel resonant circuit consisting of a parallel connection of an inductor and a capacitor connected between the signal line and the ground. A band-pass filter configured with a flattened group delay characteristic within a signal passband, characterized in that a resistor is added in parallel to an inductor and a capacitor of at least one parallel resonant circuit. circuit.