JPS61127210A - Am detector - Google Patents

Abstract

PURPOSE:To obtain stable AM demodulation with less noise even if a small input signal is given or the modulation of an AM modulation signal is increased by using an ultra-narrow band filter amplifier to select a carrier only and multiplying it with an AM intermediate frequency signal to apply synchronous detection. CONSTITUTION:The AM intermediate frequency signal is amplified by an IF amplifier 1' and divided into two. One signal is inputted to a base of a transistor 13 constituting the ultra-narrow band filter amplifier and the carrier component of the extracted AM intermediate frequency signal is fed as one input of a multiplier 2. In this case, the other output of the IF amplifier 1' is fed to one input of the multiplier 2, where multiplication is applied and synchronous detection is attained. A DC component proportional to the intensity of the carrier is extracted from the output, the result is amplified and fed back to the IF amplifier 1' as an AGC control signal so as to obtain an AMIF signal at a prescribed output level and an AF demodulation output is obtained through a low pass filter comprising capacitors 3, 5 and a resistor 4.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an AM demodulation method for improving the characteristics of an AM receiver, and particularly to an AM detector thereof.

(Constitution of Conventional Example and Problems thereof) FIG. 1 is a schematic diagram of an AM detector using a synchronous detection method using a conventional PLL circuit to detect a carrier wave and a multiplier based on the signal. In FIG. 1, an AM intermediate frequency signal (AMIF signal) is amplified by an intermediate frequency amplifier (IF amplifier) 1, and then the signal is separated into two systems, one of which is connected to a phase comparator 6 and a low-pass filter 7 (LPF). The signal is supplied to a PLL circuit composed of a voltage controlled oscillator (VCO) 8 to obtain the output of the VCO 8 synchronized with the carrier wave of the AM signal wave, and this signal is combined with the other signal of the AM intermediate frequency amplifier to the multiplier 2. The output is multiplied by capacitor 5. A by the low-pass filter composed of resistor 4,
The AP output signal of the M demodulated signal is extracted. In this case, when the input signal is small or the degree of modulation of the AM modulation signal becomes large, the PLL circuit is likely to lose lock, making AM demodulation impossible. Further, there is a drawback that it causes unnecessary noise to be generated.

(Object of the Invention) The present invention is to solve the above-mentioned conventional problems in a synthesizer receiver, that is, to achieve stable AM demodulation with less noise even when the input signal is small or when the modulation degree of the AM modulation signal becomes large. The purpose is to obtain.

(Structure of the Invention) In order to achieve the above object, the present invention performs synchronous detection using a multiplier for demodulating an AM intermediate frequency signal. In order to create a synchronization signal at this time, a powerful high-speed AGC circuit suppresses the AMIF signal to a constant level regardless of changes in the input signal level, and then an ultra-narrowband filter amplifier that selects only the carrier wave is used to keep the AMIF signal at a constant level. The carrier wave signal is extracted.

That is, in the present invention, the AM signal demodulation circuit divides the output of the AM intermediate frequency amplifier into two systems, and one selects only the carrier wave using an ultra-narrow band filter amplifier and multiplies it with the AM intermediate frequency signal to perform synchronous detection. In order to make the conditions of this synchronous detector independent of input changes, the configuration is such that high-speed AGC using the detected output is applied to the AM intermediate frequency amplifier.

An AF demodulated output is obtained by passing the synchronously detected output under certain conditions through a low-pass filter.

In this way, an AM detector is provided which ensures stable reception with a low threshold level and deep modulation depth.

(Description of Embodiment) FIG. 2 shows an embodiment of the present invention. Components that are the same as those in FIG. 1 are given the same numbers.

1' is an IF amplifier whose amplification degree changes with high-speed AGO voltage;
2 is a multiplier, 3 and 5 are capacitors, and 4 is a resistor. 3.4.
The low-pass filter constituted by No. 5 operates in the same manner as the conventional example.

In FIG. 2, the AM intermediate frequency signal is transmitted to the IF amplifier 1'
After being amplified by , one of the signals is input to the base of transistor 13 via capacitor 9 . A bias is applied between the collector and the base of the transistor 13 with a high resistance 11, and a crystal resonator 10 is connected in parallel, and the load is a resistor 12.
shall be. At this time, if the anti-resonance point of the crystal oscillator 10 is selected to be the intermediate frequency, the transistor 13 operates as a selective amplifier that exhibits the maximum amplification since the anti-resonance point of the crystal oscillator 10 becomes the maximum feedback resistance. In this way, an ultra-narrow band filter amplifier having a high Q characteristic of the crystal resonator 10 is constructed. The carrier wave component of the AM intermediate frequency signal thus extracted is transmitted to the multiplier 2.
is supplied as one of the inputs.

At this time, the other output of the IF amplifier 1' is supplied as one input of the multiplier 2, multiplication is performed in the multiplier 2, and synchronous detection is performed.

One of these outputs is connected to the base of transistor 15. High resistance 1 between the base and collector of transistor 15
A capacitor 18 which is biased at 6 and simultaneously passes the AF multiplied signal which is modulated into an AM intermediate frequency signal.
are connected in parallel. The load at this time is the resistor 17. In this case, the AF multiplier signal capacitor 18 short-circuits the transistor 15, and the transistor 15 has no amplification effect. At this time, the DC component is amplified, that is, only the DC component proportional to the carrier wave intensity is taken out, amplified, and fed back to the IF amplifier 1' as an AGC control signal.
An IF signal can be obtained at a constant output level.

In this way, the synchronous detection performed by multiplier 2 is performed at a constant level, so this output is passed through a low-pass filter consisting of a capacitor 3.5 and a resistor 4, so that the AF
Obtaining demodulated output. This configuration makes it possible to realize stable synchronous detection.

(Effects of the Invention) As described above, the present invention extracts only the carrier wave by using an ultra-narrow band filter amplifier.

At the same time as obtaining stable synchronous detection output, the AGC circuit makes it possible to always perform synchronous detection under stable conditions. In particular, unlike the PLL system, there is no noise generation due to loss of synchronization, and it has become possible to obtain a stable demodulated output without distorting the output even during high AM modulation.

Particularly, in a frequency synthesizer system with a stable tuning point receiver, its ability as a stable synchronous detector system can be demonstrated to a great extent.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an AM detector using a synchronous detector method using carrier wave detection using a conventional PLL method and a signal obtained therefrom and a multiplier. FIG. 2 is a configuration diagram of an AM detector in an embodiment of the present invention. 1.1'... IF amplifier, 2... Multiplier,
3゜5... Capacitor, 4... Resistor, 6
...Phase comparator, 7-LPF, 8-VCO19,
14, 18... Capacitor, 10... Crystal resonator,
4, 11, 12, 16, 17...resistance, 13.1
5...Transistor. Patent applicant: Matsushita Electric Industrial Co., Ltd.
--J

Claims (1)

[Claims] A high-speed AGC circuit that utilizes a detection output, an AM intermediate frequency amplifier controlled by the high-speed AGC circuit, and an ultra-narrow amplifier that selects only a carrier wave from an AM intermediate frequency signal output from the AM intermediate frequency amplifier. A bandpass filter amplifier, a multiplier that performs synchronous detection by multiplying the carrier signal obtained through the ultra-narrowband filter amplifier and the AM intermediate frequency signal output from the AM intermediate frequency amplifier, and a demodulated signal from the synchronous detection output of the multiplier. An AM detector characterized in that it is equipped with a low-pass filter that obtains.