JP2001156666A - Signal processing circuit for fm radio receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a signal processing circuit for an FM radio receiver that reduces the sampling frequency in its analog/digital conversion. SOLUTION: An integrator 16 detects the level of the output of an amplifier 14. The output of the integrator 16 controls the gain of the amplifier 14 so tat the output of the amplifier 14 is a sine wave output. Thus, a DSP 20 can accurately conduct demodulation by decreasing the sampling gain of an analog/ digital converter 18.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal processing circuit of an FM radio receiver that receives an FM radio broadcast wave, converts a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the signal.

[0002]

2. Description of the Related Art Conventionally, analog circuits have been used in FM radio receivers. In this circuit, a broadcast wave received by an antenna is amplified by an amplifier, and the frequency of a desired station is compared with the amplified RF signal. Local oscillation signals different by an intermediate frequency are mixed. Then, the signal of the desired station is obtained by extracting only the intermediate frequency component from the mixed signal, and the signal of the desired station frequency-converted to the intermediate frequency is subjected to FM.
A demodulation process is performed, and a demodulated signal is supplied to a speaker to output a sound.

[0003] Here, like a CD (compact disk) player, an audio signal is converted to a DSP (digital disc).
Signal processing (digital signal processing) systems have become widespread, and FM radio receivers
The number of cases in which signal processing is performed using a DSP is increasing.

[0004] In the processing of an audio signal by such a DSP, it is easy to achieve a graphic equalizer function, bass treble, loudness, low boost function, surround function, and the like, and a wider range of processing can be performed than analog processing. Become.

Here, in an FM radio receiver using a DSP, an intermediate frequency signal is converted into a digital signal,
The obtained digital signal is processed by a DSP to obtain an audio signal, and the obtained audio signal is D / A converted and supplied to a speaker.

In particular, in the case of an FM signal, since information on the frequency displacement of a carrier is desired, an intermediate frequency signal is
It is amplified to a rectangular wave by a limiter amplifier. Then, the intermediate frequency signal of this rectangular wave is converted into digital data by an A / D converter. Therefore, FM demodulation can be performed from a change state of digital data.

[0007]

Here, when the sampling frequency in the A / D converter is sufficiently large (for example, 10 times or more the frequency of the carrier wave), accurate demodulation can be performed from the digital data obtained by the conversion. I can do it. However, it is difficult to configure a circuit that operates with an ultra-high frequency clock in digital processing in practice, and there is a demand for lowering the sampling frequency. However, if the sampling frequency is lowered, the digital signal becomes inaccurate and the demodulation cannot be performed accurately.

An object of the present invention is to provide a signal processing circuit of an FM radio receiver that can perform relatively accurate demodulation even when the sampling frequency is lowered.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a signal processing circuit of an FM radio receiver which receives an FM radio broadcast wave, converts a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the signal. An amplifier for amplifying the intermediate frequency signal; an AGC circuit for controlling an amplification factor of the amplifier according to an output level of the amplifier; and an A / D conversion circuit for converting the output of the amplifier from analog to digital. ,
It is characterized in that the output of the amplifier is adjusted to be substantially a sine wave by the AGC circuit.

Thus, the output of the amplifier can be made sinusoidal, and accurate demodulation can be performed in the DSP even if the sampling frequency in the A / D conversion circuit is relatively low.

The present invention is also a signal processing circuit of an FM radio receiver which receives an FM radio broadcast wave, converts a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the signal. An amplifier that amplifies the frequency signal and an A / A that converts the output of this amplifier from analog to digital
A D conversion circuit, a processing circuit for detecting a waveform of the digital signal output from the A / D conversion circuit and outputting a digital control signal based on the detection result, and a D circuit for converting the digital control signal into an analog control signal / A conversion circuit;
Wherein the amplification factor of the amplifier is controlled by the output of the D / A conversion circuit, and the output of the amplifier is adjusted to be substantially a sine wave.

The present invention is also a signal processing circuit of an FM radio receiver which receives an FM radio broadcast wave, converts the frequency of a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the signal. A limiter amplifier for amplifying the frequency signal into a rectangular wave, a filter circuit for blunting the output of the limiter amplifier, and an A / D conversion circuit for analog-to-digital conversion of the output of the filter circuit; It is characterized in that it is adjusted so as to be substantially a sine wave.

[0013]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 is a diagram showing a circuit according to a first embodiment. An RF signal received by an antenna 10 is supplied to a front end 12. In the front end 12, after amplifying the received signal, the local oscillation signal is mixed, filtered and subjected to a predetermined intermediate frequency (I
F) Obtain a signal. Here, the local oscillation signal is adjusted to have a frequency different from the desired station frequency by an intermediate frequency, and a signal of the desired station is obtained as an intermediate frequency signal. In the FM radio receiver, the intermediate frequency signal is usually 10.7 MHz, but in the present embodiment, the frequency of the carrier is reduced to about 460 kHz. As a result, the intermediate frequency can be changed to a practical DSP.
Can be made dependent on the operation speed.

The intermediate frequency signal from the front end 12 is supplied to an amplifier 14 where it is amplified. Here, the output of the amplifier 14 is supplied to an integrator 16, and the amplification factor (gain) of the amplifier 14 is controlled based on the output of the integrator 16. That is, the integrator 16 detects the signal level of the output of the amplifier 14, and thereby controls the gain of the amplifier 14 by an AGC (auto gain control).
A circuit is formed. In particular, the gain of the amplifier 14 is controlled by the AGC so that the output of the amplifier 14 becomes substantially a sine wave.

The sine wave from the amplifier 14 is supplied to an A / D converter 18 and converted into a digital signal. here,
The sampling frequency of the A / D converter 18 is
The frequency is as low as about 1 MHz.

The obtained digital signal is supplied to a DSP 20, where an FM demodulation process is performed. This FM demodulation processing is performed by multiplying a signal whose phase is delayed by 180 °. As a result, an L + R signal is obtained, from which the LR signal on the 38 kHz subcarrier is separated,
These are added and subtracted to obtain an L signal and an R signal. Then, the L signal and the R signal are output to the speaker via the D / A converters 22a and 22b, respectively, so that sound is output. Here, since the audio signal is digitally processed in the DSP, it is easy to perform surround and various effect processings, and it is also easy to achieve volume adjustment for each frequency band, a graphic equalizer function, and the like. .

As described above, in this embodiment, AG
It has a C circuit so that the signal output from the amplifier 14 becomes a sine wave. Thereby, the A / D converter 1
At 8, the signal to be A / D converted is a sine wave. A
The / D converter 18 outputs an 8-bit digital signal, and can express a waveform even if the sampling frequency is low. That is, if the carrier wave is amplified by the limiter amplifier until clipping as in the conventional case, the sampling frequency of the digital signal representing the signal needs to be considerably high. Sampling frequency was required. That is, the clipped signal contains a high frequency of about 1 to 10 times, and a high sampling frequency is required to sample all of them. As in the present embodiment, by making the carrier of the signal to be A / D converted into a sine wave, sufficient demodulation can be performed even if the sampling frequency is about two to three times the carrier. At this time, the sine wave contains only high frequency of about 1 to 2 times,
Demodulation can be performed at a sampling frequency that is two to three times the carrier frequency. By lowering the sampling frequency, processing in the DSP 20 can be facilitated, and power consumption can be further reduced.

[Second Embodiment] FIG. 2 is a diagram showing a circuit according to a second embodiment.
4 are provided. The waveform detector 24 detects the level and distortion rate of a digital signal from a change in data. Then, as little distortion as possible according to the detected level and distortion rate, and the level is equal to or higher than a predetermined value,
The gain of the amplifier 14 is determined. Then, the determined gain is converted into an analog signal by the D / A converter 26 and supplied to the amplifier 14, and the gain of the amplifier 14 is controlled. In addition, the detection of the amplifier 1
Although the gain of 4 may be controlled, it is possible to perform highly accurate control for obtaining a sine wave by also considering the distortion factor.

By detecting the waveform from the digital data in the DSP 20, more accurate gain control of the amplifier 14 can be performed, and a sine wave can be obtained at the output of the amplifier 14. Therefore, a digital signal sufficiently representing a waveform is obtained at a low sampling frequency, and accurate demodulation can be performed based on the digital signal.

[Third Embodiment] FIG. 3 is a diagram showing a circuit according to a third embodiment. Similar to the conventional example, the output of the front end 12 is amplified by the limiter amplifier 30 until the signal is clipped. . As a result, a rectangular wave is obtained at the output of the limiter amplifier 30. Then, the output of the limiter amplifier 30 is supplied to the low-pass filter 32. The low-pass filter 32 serves to dull the waveform, so that a sine wave is obtained at the output of the low-pass filter 32. And this sine wave is A / D
Since the A / D conversion is performed in the converter 18, a digital signal sufficiently representing a waveform at a low sampling frequency is obtained, and accurate demodulation can be performed based on the digital signal.

[0022]

As described above, according to the present invention,
The output of the amplifier can be sinusoidal, and even if the sampling frequency in the A / D conversion circuit is relatively low, D
Accurate demodulation can be performed in the SP.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a first embodiment.

FIG. 2 is a block diagram illustrating a configuration of a second embodiment.

FIG. 3 is a block diagram illustrating a configuration of a third embodiment.

[Explanation of symbols]

10 antenna, 12 front end, 14 amplifier, 16 integrator, 18 A / D converter, 20 DSP,
22a, 22b D / A converter, 24 waveform detector, 2
6 D / A converter, 30 limiter amplifier, 32 low pass filter.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5D062 BB13 BB15 5D108 AB09 5K020 CC03 DD13 EE04 NN10 5K061 AA02 AA11 BB04 CC00 CC11 CC23 CC27 CC52 JJ00 JJ06 JJ24

Claims (3)

[Claims] 1. A signal processing circuit of an FM radio receiver for receiving an FM radio broadcast wave, converting a received signal into an intermediate frequency signal, converting the signal into a digital signal, and demodulating the digital signal, wherein the intermediate frequency signal is amplified. And an AGC circuit for controlling the amplification factor of the amplifier according to the output level of the amplifier; and an A / D conversion circuit for converting the output of the amplifier from analog to digital. A signal processing circuit for an FM radio receiver, wherein the signal processing circuit adjusts the signal to be substantially a sine wave. 2. A signal processing circuit of an FM radio receiver that receives an FM radio broadcast wave, converts a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the digital signal, wherein the intermediate frequency signal is amplified. And an A / D conversion circuit for converting the output of the amplifier from analog to digital. A process of detecting a waveform from a digital signal output from the A / D conversion circuit and outputting a digital control signal based on the detection result A circuit for converting the digital control signal into an analog control signal;
And an A / A conversion circuit, wherein the amplification factor of the amplifier is controlled by the output of the D / A conversion circuit, and the output of the amplifier is adjusted to be substantially a sine wave. Signal processing circuit. 3. A signal processing circuit of an FM radio receiver that receives an FM radio broadcast wave, converts a received signal into an intermediate frequency signal, converts the signal into a digital signal, and demodulates the signal. Limiter amplifier that amplifies the signal into a wave, a filter circuit that blunts the output of the limiter amplifier, and A that converts the output of this filter circuit from analog to digital
A signal processing circuit for an FM radio receiver, comprising: a / D conversion circuit; and adjusting the output of the filter circuit to be substantially a sine wave.