JPH0330531A - Fm receiver - Google Patents

Abstract

PURPOSE:To improve a characteristic for the S/N of a demodulated signal-to-the C/N of a reception signal by detecting the signal intensity of the received FM signal, controlling the band width of a variable band limit filter according to the output of this detection and demodulating the reception signal with an FM demodulator. CONSTITUTION:A C/N detecting circuit 7 detects the C/N of the reception signal and supplies the output to a controller 9. The C/N detecting circuit 7 is composed of an FM detector, for example, and the output is supplied to a local oscillating circuit 5 and the controller 9. The oscillating frequency of the local oscillating circuit 5 is controlled by the output of a frequency detecting circuit 8. A control signal corresponding to the C/N of the reception signal and deviation from a central frequency is outputted from the controller 9 and the output is supplied to a variable band limiting filter 6. The central frequency of a passing band is controlled so as to be coincident with the central frequency of an output signal from a mixing circuit 4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an FM receiving device used, for example, in a BS tuner for receiving satellite broadcasting.

[Summary of the invention]

The present invention detects the signal strength of a received FM signal in an FM receiving device used in a BS tuner for receiving satellite broadcasting, controls the bandwidth of a variable band-limiting filter based on this detection output, and controls the band width of a variable band-limiting filter. The received signal passed through the limiting filter is supplied to an FM demodulator for demodulation, thereby improving the characteristics of the S/N ratio of the demodulated signal relative to the C/N ratio of the received signal.

[Conventional technology]

In satellite broadcasting, a carrier wave in the 12 GHz band is FM-modulated with a signal in which a video signal and an audio PCM signal subcarrier are frequency division multiplexed, and then transmitted. FM modulation has a wider occupied radio wave bandwidth than AM modulation, but it produces less distortion during transmission (and is less susceptible to interference waves.
FM modulated signals have the advantage that less transmission power is required to obtain a demodulated output with the same S/N ratio on the receiving side. For this reason, FM modulation is adopted in satellite broadcasting.

Incidentally, the FM modulated signal has a characteristic that when the C/N ratio of the received signal becomes lower than a certain value, the S/N ratio of the demodulated signal rapidly deteriorates. The level of the C/N ratio at which the S/N ratio of the demodulated signal begins to deteriorate rapidly is called the threshold level. When receiving an FM modulated signal, it is desirable to lower this threshold level.

It is known that it is effective to use a tracking filter type FM demodulator such as a PLL type to improve the threshold level.

In other words, Figure 6 shows the C/N ratio and S/N ratio in the FM demodulated signal.
This shows the relationship with the N ratio. In Figure 6, the horizontal axis shows the C/N ratio of the received signal, and the vertical axis shows the S/N ratio of the demodulated signal.
It shows the ratio.

In Figure 6, the AIO is a normal FM without feedback loop.
This is the characteristic when using a demodulator. As shown in FIG. 6, when an FM demodulator is used, when the C/N ratio of the received signal is equal to or higher than the threshold level THIO, the C/N ratio of the received signal is
The N ratio and the S/N ratio of the demodulated signal are approximately proportional. When the C/N ratio of the received signal becomes below the threshold level THIO,
The S/N ratio of the demodulated signal deteriorates rapidly.

In FIG. 6, All indicates the characteristics when using a tracking filter type FM demodulator such as a PLL type. When a tracking filter type FM demodulator such as a PLL type is used, the threshold level is lowered to THll, and the threshold level can be improved.

[Problem to be solved by the invention]

By using a tracking filter type FM demodulator such as a PLL type, the threshold level can be lowered as described above. Therefore, it is possible to improve the S/N ratio of the demodulated signal when the C/N ratio is below the threshold level.

However, as shown in FIG. 6, even if a tracking filter type FM demodulator such as a PLL type is used, the S/N ratio of the demodulated signal when the C/N ratio is above the threshold level is
The characteristics are the same as when using a normal FM demodulator without a feedback loop. In this way, when using a tracking filter type FM demodulator such as a PLL type, the threshold level can be improved, but if the C/N ratio is below the threshold level, the S/N ratio of the demodulated signal cannot be improved. I can't do it.

Therefore, an object of the present invention is to provide an FM receiving device that can improve the threshold level.

Another object of the present invention is to provide an FM receiver capable of improving the S/N ratio of a demodulated signal even when the signal is above a threshold level.

(Means for Solving the Problems) The present invention comprises: a detection means 7 for detecting the strength of a received FM signal from a received FM signal; a variable band uniform filter 6 for receiving the FM signal and extracting a predetermined band component; This is an FM receiving device comprising: control means 9 for controlling the bandwidth of the variable band-limiting filter 6 based on the output of the means 7; and an FM demodulator lO to which a signal is input after passing through the variable band-limiting filter 6. .

[Effect]

The signal bandwidth of the FM received signal changes depending on the C/N ratio. That is, when the C/N ratio is high, the signal bandwidth is wide, and when the C/N ratio is low, the signal bandwidth is accordingly narrow.

On the other hand, the noise spectrum in the received signal is uniformly spread. The amount of noise input to the FM demodulator 10 is proportional to the passband width of the variable band-limiting filter 6.

From this, if the passband width of the variable band limiting filter 6 is controlled according to the C/N ratio of the received signal and signal components in bands other than the signal band are removed, the C/N ratio of the received signal can be controlled.
The characteristics of the S/N ratio of the demodulated signal with respect to the N ratio can be improved.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

In Fig. 1, the BS antenna l receives a 12Gl (Z band signal) sent from a satellite. This is an FM modulated signal.

This received signal is supplied to the BS converter 2. The BS converter 2 converts the received signal into a BS-intermediate frequency signal in the IGHz band.

The output of the BS converter 2 is sent to the mixing circuit 4 via the amplifier 3.
is supplied to A local oscillation signal is supplied to the mixing circuit 4 from a local oscillation circuit 5 . Mixing circuit 4, 1 GHz band B
The S-intermediate frequency signal is converted into a second intermediate frequency signal of 400M seven bands.

The output of the mixing circuit 4 is supplied to a variable band limiting circuit 6, as well as a C/N ratio detection circuit 7 and a frequency detection circuit 8.

The C/N ratio detection circuit 7 detects the C/N ratio of the received signal. The C/N ratio detection circuit 7 may have a configuration that detects the carrier wave level of the received signal, a configuration that detects the noise level in the received signal, or a configuration that detects the carrier wave level and the noise level. good.

The carrier wave level can be detected by detecting the signal level within the signal band. The noise level can be detected by detecting the signal level outside the signal band. The output of the C/N ratio detection circuit 7 is supplied to the controller 9.

The frequency detection circuit 8 detects deviation from the intermediate frequency. This frequency detection circuit 8 can be composed of, for example, an FM detector. The output of the frequency detection circuit is supplied to the local oscillation circuit 5 and also to the controller 9. The oscillation frequency of the local oscillation circuit 5 is determined by the frequency detection circuit 8.
is controlled by the output of This performs AFC control.

The controller 9 outputs a control signal according to the C/N ratio of the received signal and the deviation from the intermediate frequency. The output of the controller 9 is supplied to the variable band limit filter 6.

The passband width of the variable band-limiting filter 6 is controlled according to the C/N ratio of the received signal. Further, the center frequency of the passband of the variable band limit filter 6 is determined by the frequency detection circuit 8.
The frequency is controlled according to the deviation from the intermediate frequency detected by the frequency.

That is, the passband width of the variable band-limiting filter 6 is C
When the /N ratio is high, it is widened, and when the C/N ratio is low, it is narrowed. For example, if the C/N ratio is 30 dB, the passband width of the variable band limit filter 6 is about 33M7, and the C/N ratio is 2.
If it is 0 dB, the passband width of the variable band limit filter 6 is 30.
If the frequency is about MHz and the C/N ratio is 10 dB, the passband width of the variable band-limiting filter 6 is set to about 27 MHz. Then, the center frequency of the passband is controlled to match the center frequency of the output signal of the mixing circuit 4.

The output of variable band-limiting filter 6 is supplied to FM demodulator IO. The FM demodulator 10 demodulates the video signal and audio PCM signal subcarriers. As the FM demodulator 10, one having a configuration without a feedback loop may be used, or one having a tracking filter method such as a PLL method may be used. This demodulated signal is output from the output terminal 11.

Incidentally, in an FM signal, there is a level at which the S/N ratio of a demodulated signal rapidly deteriorates compared to the C/N ratio of a received signal. This level is called the threshold level.

In one embodiment of the present invention, a variable band-limiting filter 6 whose passband width is controlled according to the C/N ratio is provided. Therefore, the threshold level can be improved. Further, in one embodiment of the present invention, it is possible to improve the S/N ratio of the demodulated signal when the C/N ratio is equal to or higher than the threshold level.

In other words, Figures 2A to 2C each have a CZN ratio of 3.
0dB, a C/N ratio of 20dB, and a C/N ratio of 10dB.

As shown in FIGS. 2A to 2C, the signal bandwidth of the FM modulated received signal changes depending on the C/N ratio. For example, at a C/N ratio of 30 dB, the signal bandwidth f is 33 MHz, and at a C/N ratio of 2 odB, the signal bandwidth f is 30 MHz.
At a C/N ratio of 10 dB, the signal bandwidth r is 27 M.
It is Hz.

On the other hand, the noise spectrum in the received signal is uniformly spread. The amount of noise input to the FM demodulator 10 is proportional to the passband width of the variable band-limiting filter 6.

From this, if the passband width of the variable band uniform filter 6 is controlled according to the C/N ratio of the received signal and signal components in bands other than the signal band are removed, the C/N ratio of the received signal can be controlled.
The characteristics of the S/N ratio of the demodulated signal with respect to the N ratio can be improved.

In one embodiment of the present invention, the C/N ratio in the received signal is detected by the C/N ratio detection circuit 7, and the band of the variable band limit filter 6 is controlled according to this C/N ratio.

That is, when the C/N ratio is 30 dB, the variable band limit filter 6 has characteristics as shown in FIG. 3A. As a result, as shown in FIG. 4A, the FM demodulator 10 has:
A signal from which signal components other than the signal band f1 of the received signal with a C/N ratio of 30 dB have been removed is input.

When the C/N ratio is 20 dB, the variable band limit filter 6
is assumed to have the characteristics as shown in Figure 3.

As a result, as shown in FIG. 4B, the FM demodulator 10 has a signal band f2 of the received signal when the C/N ratio is 20 dB.
The signal from which all other signal components have been removed is manually input.

For a C/N ratio of 10 dB, a variable band uniform filter 6
is assumed to have the characteristics as shown in FIG. 3C.

As a result, as shown in FIG. 4C, the FM demodulator IO has a signal band f3 of the received signal when the C/N ratio is 10 dB.
A signal from which all other signal components have been removed is input.

As a result, the S of the demodulated signal relative to the C/N ratio of the received signal is
/N ratio characteristics can be improved.

FIG. 5 shows the C/N ratio and S/N ratio in one embodiment of the present invention.
The relationship with the N ratio is shown in comparison with the relationship between the C/N ratio and the S/N ratio in a conventional FM demodulator. In FIG. 5, the horizontal axis shows the C/N ratio of the received signal, and the vertical axis shows the S/N ratio of the demodulated signal. In Figure 5,
A1 shows the relationship between the C/N ratio and the S/N ratio in a conventional FM demodulator, and A2 shows the relationship between the C/N ratio and the S/N ratio in an embodiment of the present invention.
It shows the relationship between the N ratio and the S/N ratio.

As shown in FIG. 5, the threshold level TH2 of the embodiment of the present invention is lower than the threshold level T)If of the conventional FMI adjuster, and the threshold level is improved. In addition, even if the C/N ratio of the conventional FM demodulator is higher than the threshold level of 781, the C/N ratio and S/N
You can improve your relationship with the ratio. As described above, in one embodiment of the present invention, not only the threshold level is improved, but also the C/N ratio and the S/N ratio are improved even when the C/N ratio is higher than the threshold level 781.

Note that if a tracking filter method such as a PLL method is used as the FM demodulator 10, the threshold level can be further improved.

〔Effect of the invention〕

According to this invention, by controlling the passband width of the variable band limiting filter 6 according to the C/N ratio of the received signal and removing signal components in bands other than the signal band, the C/N ratio of the received signal is
The characteristics of the S/N ratio of the demodulated signal with respect to the ratio are improved. As a result, the threshold level is improved, and even if the C/N ratio is higher than the threshold level, the characteristics of the S/N ratio of the demodulated signal relative to the C/N ratio of the received signal are improved.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a spectrum diagram used to explain an embodiment of the invention, and Fig. 3 is a block diagram of an embodiment of the invention.
Figure 2 is a frequency characteristic diagram used to explain a variable band limit filter in an embodiment of this invention. Figure 4 is a spectrum diagram used to explain an embodiment of this invention. Figure 5 is an explanation of an embodiment of this invention. The graph used in Figure 6 is the conventional FM
It is a graph used to explain a demodulator. Explanation of main symbols in the drawings 6: Variable band limit filter, '7: C/N ratio detection circuit,
9: Controller, 10: FM demodulator.

Claims (1)

[Scope of Claims] A detection means for detecting the strength of a received signal from a received FM signal; a variable band limit filter for receiving the FM signal and extracting a predetermined band component; An FM receiver comprising: a control means for controlling the bandwidth of a limiting filter; and an FM demodulator into which a signal is input after passing through the variable band limiting filter.