JP2679324B2 - Data reception filter circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a baseband filter circuit for data reception, and more particularly to a baseband filter circuit whose characteristics can be easily switched and which is suitable for receiving asynchronous data.

[Conventional technology]

Conventionally, a baseband filter for data reception is designed so that the Nyquist condition is satisfied and the influence of intersymbol interference is reduced over the entire transmission path. Further, particularly when the data receiving terminal is a small-sized portable carrying device such as a radio selective calling receiver, a large-scale and complicated circuit is not used as this type of baseband filter, and one operational amplifier and a resistor are used. A simple active filter consisting of a capacitor and a capacitor is often used.

[Problems to be Solved by the Invention] Since the electrical characteristics (frequency characteristics) of the conventional baseband filter for data reception as described above are fixed by hardware and cannot be easily changed,
There were the following disadvantages.

(1) When receiving data transmitted in asynchronous (burst mode) as shown in FIG. 2 by the FM modulation method, the characteristics of the receiver are FM characteristics, so S / N as shown in FIG. It becomes a characteristic. From FIG. 3, it can be seen that the demodulation output level when transmission is stopped (carrier off) is different from the demodulation output level during signal transmission. Therefore, the level diagram in the baseband after demodulation will be different, and some ingenuity will be required for signal processing.

(2) In particular, when the baseband level diagram optimized for the demodulation output level during signal transmission is configured in (1), noise is not detected when transmission is stopped and data output is all 1 (or all 0). There is a possibility of being in a state, and in this case, it becomes impossible to distinguish between the transmission stop and the all 1 (or all 0) data.

An object of the present invention is to provide a data receiving filter circuit that solves the above problems.

[Means for solving the problem]

To achieve the above object, a data reception filter circuit according to the present invention is a data reception baseband filter circuit, in which a preamble signal detection circuit for synchronizing bits of asynchronous data and a pass band by an output of the preamble signal detection circuit. And a filter for changing the characteristic. Further, in the data receiving filter circuit according to the present invention, the filter changes characteristics by changing in-band ripple.
The filter is a third-order Sallen-key type filter composed of at least one operational amplifier, at least three resistors, and three capacitors, and switches the AC-grounded capacitance of its input stage. It has a switch for switching.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

 FIG. 1 is a block diagram of an embodiment according to the present invention.

In the figure, an FM-modulated signal is added to an input terminal 101 and demodulated by a demodulation circuit 102. The demodulated signal includes an operational amplifier 109, resistors 103, 104 and 105, capacitors 106, 107 and 108.
The high-frequency component is removed by the low-pass filter configured by, and converted into 1/0 data by the comparison circuit 110. If the output of the comparison circuit 110 is a preamble signal of a predetermined bit rate, it is detected by the preamble signal detection circuit 111, and this output controls the switch 112. The switch 112 is in an OFF state until the preamble signal is detected, and is turned on when the preamble signal is detected. When the switch 112 is on, the normal third-order LPF is obtained, but when the switch 112 is off, the ripple in the band increases and the gain near the cutoff frequency increases. FIG. 4 is a third-order Bessel filter having a cutoff frequency of 300 Hz and shows the difference in characteristics when the switch 112 is turned on and off.

With the above configuration, the passband gain is high from the noise state at the time of transmission stop to the preamble signal detection, and it becomes a filter that makes it easy to detect noise in the baseband comparison circuit 110 and later. The filter has stable delay characteristics and little intersymbol interference. Since the preamble signal is a 1/0 regulated signal and has a single frequency component, there is no problem of intersymbol interference even with characteristics close to those of a bandpass filter as shown in FIG.

By switching the switch 112, the group delay characteristic of the filter changes abruptly, but since the amount of this change is known in advance, it is possible to prevent the error rate from deteriorating by performing correction in the signal processing unit 113.

〔The invention's effect〕

As described above, the present invention changes the in-band gain of the baseband filter by detecting the preamble signal to change the in-band gain, thereby demodulating the output level in the noise state when the transmission is stopped and the signal in the transmission state. The difference between the demodulation output levels can be reduced, and the baseband processing including the noise state can be facilitated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of an asynchronous signal, FIG. 3 is an S / N characteristic diagram of an FM receiver, and FIG. 4 is an implementation of the present invention. It is a figure which shows the frequency characteristic of the filter which concerns on an example. 101 …… input terminal, 102 …… demodulation circuit 103,104,105 …… resistor, 106,107,108 …… capacitance 109 …… operational amplifier, 110 …… comparator circuit 111 …… preamble signal detection circuit, 112 …… switch 113 …… signal processing unit

Claims (3)

(57) [Claims] 1. A baseband filter circuit for data reception, comprising: a preamble signal detection circuit for bit-synchronizing asynchronous data; and a filter for changing a pass band characteristic by an output of the preamble signal detection circuit. Data reception filter circuit. 2. The data receiving filter circuit according to claim 1, wherein the filter changes its characteristics by changing a ripple within a band. 3. The filter is a third-order Sallen-hoy type filter composed of at least one operational amplifier, at least three resistors and three capacitors, and the input stage is installed in an alternating current manner. The data receiving filter circuit according to claim 1, further comprising a switch for switching the generated capacitance.