JPH04264816A - Fm signal detecting device using filter - Google Patents

Abstract

PURPOSE:To improve S/N and to realize high sensitivity without being affected by the response delay inseparable from FM demodulation. CONSTITUTION:The frequency-converted input signal is inputted to plural narrow bands BPF 21 to 23 by a mixer 12, and these output levels are compared by signal exchanges 101 and 102, and the signal with the maximum level is taken out by switching. When the FM reception signal is between the adjacent band characteristics, the maximum level synthetic signal is outputted by synthesizing devices 31 and 32 and a signal changeover device 103. According to the detection signal with the frequency of the above-mentioned FM reception signal, the synthetic signal or the maximum level signal is selected by a switch 81. The band is narrower than the original FM demodulation band width, the C/N is improved, and the S/N can be improved by demodulating this narrow band signal.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM signal detection device using a plurality of filter switching methods, and more particularly to an FM signal detection device that obtains a narrowband FM signal by switching to the maximum level filter output during FM demodulation.

0002

2. Description of the Related Art A PLL demodulation method is known as one of the effective systems for increasing the sensitivity of an FM receiver. Figure 6 shows
In the block diagram showing the system of this PLL demodulation method, 1
is an input terminal, 2 is a phase comparator, and 3 is a loop filter (L
PF), 4 is an amplifier, 5 is a VCO, and 6 is an output terminal. In FIG. 6, the phase of the FM signal input from the input terminal 1 is compared with the signal from the VCO 5 by the phase comparator 2, and an error voltage corresponding to the phase difference is output. 4, the output of the amplifier 4 controls the oscillation frequency of the VCO 5, while the output is taken out from the output terminal 6 as a demodulated signal.

0003

[Problems to be Solved by the Invention] As described above, in the PLL demodulation system, it is possible to track the target signal buried in noise components, but the response characteristics of the PLL are influenced by the band characteristics of the loop, and this The loop bandwidth is determined by the time constant of the LPF. If you want to obtain more faithful tracking characteristics, it is necessary to widen the loop bandwidth, but in this case, the loop becomes more susceptible to noise as the bandwidth becomes wider. On the other hand, in order to reduce the influence of noise,
It is necessary to narrow the loop bandwidth, but in this case, the influence of response delay in the loop increases. An object of the present invention is to provide an FM signal detection device that improves S/N (signal-to-noise) and achieves high sensitivity without being affected by response delays that are inevitable in FM demodulation.

0004

[Means for Solving the Problems] In order to achieve such an object, the present invention provides frequency conversion means for converting an FM reception signal into a signal of intermediate frequency components, and a frequency converting means for converting an FM reception signal into a signal of intermediate frequency components, filter means for extracting a plurality of signals in a frequency band, band signal selection means for comparing the levels of the signals in the plurality of frequency bands and selecting and outputting only the signal in the frequency band having the maximum signal level; FM reception frequency detection means for detecting whether or not the FM reception signal exists between adjacent frequency band characteristics among a plurality of frequency bands based on a comparison result by the signal selection means, and outputting a detection signal; a frequency band synthesizing means for synthesizing the signals of the two frequency bands, making the FM reception signal exist approximately in the center of the synthesized frequency band characteristics, and outputting a synthesized signal having a maximum signal level; and based on the detected signal,
output signal selection means for selecting a composite signal from the frequency band synthesis means when the FM received signal exists between adjacent frequency band characteristics, and selecting a signal from the band signal selection means for other times; It is characterized by having

[0005]

[Operation] In the present invention, when demodulating an FM signal, the FM modulation band is divided by a plurality of narrow band filter means, and the output level of each filter means is compared and the output having the maximum level is selected. Since the FM signal is obtained, the carrier-to-noise ratio (C/N
), and by demodulating this signal, it is possible to improve the signal-to-noise ratio (S/N).

[0006]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 is a block diagram of an embodiment of an FM signal receiving apparatus according to the present invention. In the figure, 11 is an input terminal, 12 is a mixer, 13 is an oscillator, and frequency conversion means (
IF means). 21 to 23 are band pass filters (BPF). Signal switchers 101 and 102 constitute band signal selection means. 31 and 32 are synthesizers, and 103 is a signal switcher, which constitutes frequency band synthesis means. 41 and 42 are absolute value circuits, 51 and 52 are comparators,
61 and 62 are reference voltage sources, 71 is an AND gate, and FM
This constitutes reception frequency detection means. A switch 81 constitutes output signal selection means. 14 is an output terminal.

In FIG. 1, a mixer 12 converts the frequency of an FM signal (fc±75 kHz, where fc represents a carrier frequency) input to an input terminal 11. The mixer 12 outputs fc and the oscillation frequency fA of the local oscillator 13.
The frequency components of the sum and difference, i.e., (fc+fA) ±75kHz
…(1) (fc
-fA) ±75kHz
...(2) is output.

Among the outputs of the mixer 12 described above, (2)
The band components of the equation are passed through a plurality of band pass filters (BPFs) (FIG. 1 shows a case of three BPFs 21 to 23, but more than three BPFs may be provided). As mentioned above, the reason why the frequency is lowered by frequency conversion is that it is easier to narrow the band of the BPF. As shown in FIG. 2, the FM signal represents changes in the baseband signal as a frequency shift from the carrier wave. Therefore, if you compare the output levels of each filter and extract the filter output that has the maximum level, that is the filter output that includes the FM signal component, and the band is narrowed with respect to the original FM modulation bandwidth. /N can be improved. By demodulating this narrowband signal, the S/N ratio can be improved.

The signal whose frequency has been converted by the mixer 12 is input to the BPFs 21 to 23, and their output levels are compared. First, the signal switch 101 selects the BPF
The output levels of 21 and BPF 22 are compared, and the signal with the higher level is switched and taken out as described later. Further, this switched signal and the output of the BPF 23 are compared by the signal switch 102, and switching is performed. When increasing the number of BPFs, comparisons are made in the same way and switching is performed.

FIG. 3 shows the signal switchers 101 and 102 of FIG.
, 103, the AM detector 1
11, 112, a comparator 113, and a switch 114. 1 to 2 of the signal switch 101 in FIG. 1 correspond to input/output terminals 1 to 3 in FIG. 3, respectively. In FIG. 3, signals S1 and S2 are input to input terminals ■ and ■. Each is branched into two systems, one is inputted as it is to the switch 114, and the other is input to the AM detectors 111 and 112 to determine the amplitude (
DC) level and then input to the comparator 113. The comparator 113 calculates the difference between the two signals, and controls the switch 114 depending on whether the difference is + (comparator output: +V voltage) or - (comparator output: -V voltage). , take out the filter output with the higher level (if the comparator output is +V, turn on the S1 side switch; if it is -V, switch to the S2 side). In this way, the maximum level filter output is taken out from the output terminal (2). The output of the output terminal ■ will be described later.

FIG. 4(a) shows the amplitude characteristics of each BPF 21 to 23, and 201 to 203 are BPF 2, respectively.
1 to 23 amplitude characteristics. In this figure, if F
When the M signal is a frequency component called fs shown by the broken line, that is, when the signal is at the cross point between adjacent filters, when comparing the output levels of the filters with amplitude characteristics 201 and 202, they are almost the same level. This may cause problems with switching operations. but,
As shown in FIG. 4(b), if the filter has the characteristic 211, which is a combination of the outputs of the two filters having the characteristics 201 and 202, the filter can be easily detected because it is located in the center of the filters for the signal of the same fs.

Therefore, when there is a signal at the cross point between the filter characteristics, the outputs of the BPFs 21 to 23 in FIG. 101 (same configuration),
Extract signal components by switching. In this case, the band is broadened compared to the original filter characteristics 201, 202, and 203, but since the signal is centered between the filter characteristics, a sufficient amplitude level can be ensured, and there is no problem in C/N. . To determine whether a signal has a frequency at a cross point between the filter characteristics, the output of the comparator 113 in the signal switching device 101 described above (ie, the signal at the output terminal 2 in FIG. 3) is used.

As the levels of the two signals approach each other, the output of comparator 113 approaches zero. When the output level of the comparator becomes below a certain value (VT) as shown in FIG. 5, control of the switch 114 becomes impossible. In such a state, the signal is near the cross point between the filter characteristics, and at that time, the filter characteristics 201 and 202
(21 in Figure 4)
Switch to output 1) and extract the signal.

Furthermore, in FIG. 1, the signal switch 101,1
Since the signal at the output terminal ■ of the output terminal 02 has a ± component, the absolute value is extracted by the absolute value circuits 41 and 42. The comparators 51 and 52 compare this absolute value and the voltage VT of the reference voltage sources 61 and 62.
The comparator output is set so that if the difference between the two voltages is equal to or greater than VT, the comparator output becomes high level, and if it is equal to or less than VT, the comparator output becomes low level. The outputs of these comparators are connected to AND gate 7
Enter 1. The output of the AND gate 71 will be high level only when all inputs are high level, and will be low level if even one input is low level (the figure shows the case of 2 inputs, but The same applies to the above inputs). The switch 81 is controlled by the output of the AND gate 71. That is, the switch 81 compares the output of the original narrowband filter and outputs the switched output (signal switch 1
02 output) and the output of the synthesized filter, input the switched output (output of signal switch 103), and when the output of AND gate 71 is at a high level, switch to the output side of signal switch 102. However, when the output of the AND gate is at a low level, it is switched to the output side of the signal switch 103.

[0015] In this way, it is possible to obtain a narrowband FM signal by eliminating unstable switching operation when a signal exists at a cross point between filter characteristics. In addition, although three BPFs are provided in Figure 1, BPF, which has a narrower band,
By setting more PFs, the C/N can be further improved. However, the number of filters cannot be determined simply because it depends on the complexity of the characteristic circuit configuration of the filter itself.

[0016]

As described above, according to the present invention, in an FM signal detection device using a plurality of filters,
Since the modulation bandwidth is divided by a narrowband BPF and the filter output containing the signal is taken out, the C/N increases due to narrowing the band, and by demodulating this narrowband signal, the S/N can be improved. It is possible to increase the sensitivity of the receiving device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of an FM signal detection device according to the present invention.

FIG. 2 is a characteristic diagram showing the relationship between FM instantaneous frequency and level.

FIG. 3 is a block diagram showing a specific configuration example of the signal switch in FIG. 1;

FIG. 4 is an amplitude characteristic diagram of the filter in FIG. 1;

FIG. 5 is a characteristic diagram showing the output level of a comparator.

FIG. 6 is a block diagram of a conventional FM signal detection device.

[Explanation of symbols]

12 Mixer 21-23 BPF 31, 32 Combiner 51, 52 Comparator 71 And Gate 81 Switch 101-103 Signal switch

Claims (1)

[Claims] 1. Frequency converting means for converting an FM reception signal into a signal of intermediate frequency components, filter means for extracting a plurality of signals of different predetermined frequency bands from the signal of the intermediate frequency components, and the plurality of frequency bands. band signal selection means for comparing the levels of signals in the frequency band and selecting and outputting only the signal in the frequency band having the maximum signal level; FM reception frequency detection means detects whether or not the FM reception signal exists between the frequency band characteristics and outputs a detection signal; a frequency band synthesizing means for making an FM reception signal exist in the section and outputting a synthesized signal having a maximum signal level;
When the M received signals exist between adjacent frequency band characteristics, selecting a composite signal from the frequency band combining means;
An FM signal detecting device using a filter, characterized in that the device includes output signal selection means for selecting a signal from the band signal selection means at times other than the above.