JPH03207143A - Filter circuit for data reception - Google Patents

Abstract

PURPOSE:To facilitate a base band process including a noise state by detecting a preamble signal, and then varying the in-band ripple of a base-band filter and varying the in-band gain. CONSTITUTION:When the output of a comparing circuit 110 is a preamble signal with a predetermined bit rate, a preamble signal detecting circuit 111 detects it and a switch 112 is controlled with its output. The switch 112 is in the OFF state until the preamble signal is detected and turned on when the preamble signal is detected. When the switch 112 is in the ON state, this circuit operates as a normal tertiary LPF, but when the switch 112 is in the OFF state, the ripple in the band increases, so that the gain nearby a cutoff frequency increases. Consequently, the passing band gain is high in a period from the noise state at the stop of transmission to the preamble signal detection and noise detection in the base band behind the comparing circuit 110 is easy; and in-passing-band group delay characteristics become stable after the preamble signal detection and the filter which is free from inter-code interference is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] 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 is suitable for receiving asynchronous data.

[Prior Art] Conventionally, a baseband filter for data reception has been designed so that the entire transmission path satisfies the Nyquist condition and the influence of intersymbol interference is reduced. In addition, especially when the data receiving terminal is small and portable, such as a radio selective calling receiver, this type of baseband filter can be used without using a large and complicated circuit, instead using one operational amplifier and a resistor. A simple active filter consisting of a capacitor and a capacitor is often used.

[Problems to be Solved by the Invention] The electrical characteristics (frequency characteristics) of the conventional baseband filter for data reception as described above are fixed in terms of hardware and cannot be easily changed. There were drawbacks such as:

(1) When receiving data transmitted asynchronously (burst mode) as shown in Figure 2 using the FM modulation method, the characteristics of the receiver are FM characteristics, so the S/M modulation method as shown in Figure 3 is used. It becomes N characteristic. From Figure 3, when transmission is stopped (
The demodulated output level at the time of carrier off) and the demodulated output level at the time of signal transmission are different. Therefore, the level diagram in the baseband after demodulation will be different, and signal processing must be devised.

(2) In particular, in (1), if a baseband level diagram optimized for the demodulation output level during signal transmission is used, no noise will be detected when transmission is stopped, and the data output will be all 1s (or all O's). In this case, it becomes difficult to distinguish between transmission stop and all I (or all O) data.

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

[Means for Solving the Problems] In order to achieve the above object, the data reception filter circuit according to the present invention includes a preamble signal detection circuit for bit synchronizing asynchronous data and a preamble signal detection circuit for bit synchronizing asynchronous data in the data reception baseband filter circuit. , and a filter that is activated by the output of the preamble signal detection circuit and changes the passband characteristics. Further, in the data reception filter circuit according to the present invention, the filter changes characteristics by changing in-band ripple, and the filter includes at least one summation amplifier and at least three The filter is a third-order Sallen-Key type filter composed of three resistors and three capacitors, and has a switch for switching the AC-grounded capacitor at the input stage.

[Example] Next, the present invention will be explained with reference to the drawings.

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

In the figure, an FM modulated signal is applied to an input terminal 101 and demodulated by a demodulation circuit +02.

The demodulated signal is transmitted through an operational amplifier 109, resistors 103, 1
04, 105, and capacitors 106, 107, and 108, high frequency components are removed, and the comparator circuit 110 converts the data into data 170. 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 is used as the preamble signal for the switch 112.
is controlled. The switch 112 is in the OFF state until the preamble signal is detected, and becomes the ON state when the preamble signal is detected. When the switch 112 is ON, it becomes a normal third-order LPF, but when the switch 112 is OFF, the in-band ripple increases and the gain near the cutoff frequency increases. FIG. 4 shows a third-order Bessel filter with a cutoff frequency of 300 Hz, and shows the difference in characteristics when the switch 112 is turned on and off.

Due to the above structure, the pass band gain is high from the noise state when transmission is stopped until the preamble signal is detected, and the filter becomes easy to detect noise after the baseband comparator circuit 110, and after the preamble signal is detected, it is within the pass band. The filter has stable group delay characteristics and less intersymbol interference. Since the preamble signal is a regular signal of 1/0 and has a single frequency component, there is no problem of intersymbol interference even if the preamble signal has characteristics similar to those of a bandpass filter as shown in FIG.

By switching the switch 112, the group delay characteristic of the filter changes rapidly, but since the amount of this change is known in advance, deterioration of the error rate can be prevented by correction in the signal processing section 113.

[Effects of the Invention] As explained above, the present invention changes the demodulated output level and the demodulated output level in a noise state when transmission is stopped by changing the in-band ripple of the baseband filter by detecting a preamble signal and changing the in-band gain. This has the effect that the difference in signal demodulation output level in the transmission state can be reduced, and baseband processing including the noise state is facilitated.

[Brief explanation of 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, and FIG. 3 is a block diagram showing an example of an asynchronous signal.
/N characteristic diagram, FIG. 4 is a diagram showing the frequency characteristics of the filter according to the embodiment of the present invention. +01...Input terminal 102...Demodulation circuit 1
03, 104, 105...Resistance 106, 1
07, 108... Capacity +09... Operational amplifier
110... Comparison circuit II1... Preamble signal detection circuit 112... Switch 113... Signal processing unit

Claims (1)

[Scope of Claims] (1) A baseband filter circuit for data reception includes a preamble signal detection circuit that synchronizes bits of asynchronous data, and a filter that is activated by the output of the preamble signal detection circuit and changes passband characteristics. 1. A data reception filter circuit comprising: (2) The data reception filter circuit according to claim (1), wherein the filter changes characteristics by changing in-band ripple. (3) The filter is a third-order Salleen-Key type filter composed of at least one operational amplifier, at least three resistors, and three capacitors, and its input stage is grounded in the AC direction. 2. The data reception filter circuit according to claim 1, further comprising a switch for switching capacitance.