JPS60105304A - Carrier level detecting system of am signal - Google Patents

Abstract

PURPOSE:To detect quickly a carrier level of an AM signal by sampling a peak point of the carrier of the AM signal so as to demodulate and feeding back an inverted output of an AF signal so as to cancel the AF signal. CONSTITUTION:A PLL circuit consists of a phase comparator 11, an LPF 12, a voltage controlled oscillator 13 and frequency dividers 14, 15. An output of the frequency divider 16 samples the AM signal 27 via a differentiation circuit 17 and a sample-and-hold circuit 18 so as to obtain the AF signal 28. This signal is fed to a transistor 24 and the inverting signal of the AF signal produced at the collector is fed back so as to obtain an output 29 proportional to the carrier level.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] The present invention relates to a carrier level detection method for AM signals. FIG. 1 shows a conventional AM signal demodulation circuit and carrier level detection circuit using diode detection. In the same figure, 1 is a tuning transformer with AM
The signal is tuned and the detection circuit is matched. A diode 2 rectifies only the positive side component of the AM signal. A carrier wave component is removed by a low-pass filter composed of a capacitor 3.4 and a resistor 5, and the envelope of the AM signal is demodulated. Further, the modulated wave component is filtered by a resistor 6 and a capacitor 7, and the average level of the carrier is detected.

Note that 8 is an input terminal for an AM signal, 9 is an AP multiplied signal output terminal, and 10 is a carrier level detection terminal.

However, the conventional detection method described above has a drawback in that a long time constant is required to detect the carrier level.

The object of the present invention is to provide a carrier level detection scheme that does not have such drawbacks.

[Structure of the Invention] In the AM signal carrier level detection method and AM signal demodulation circuit of the present invention, P and L, which are different from a voltage controlled oscillator, a phase comparison circuit, and a low-pass filter, are used.
A reference signal to be locked to the carrier frequency by an L circuit is generated by dividing the output of the voltage controlled oscillator by 1/4, and a phase difference of 90° from the carrier wave of the AM signal is obtained from the frequency divided by 172 of the voltage controlled oscillator. This signal is differentiated to perform demodulation by sampling the peak point of the carrier wave of the AM signal using a simulator switch, and the AP multiplied signal of the amplifier which receives this demodulated signal as input is performed. It is characterized in that the inverted output is fed back to cancel the AP multiplied signal to detect the carrier level of the AM signal.

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

In the figure, 11 is a phase comparator, 12 is a low-pass filter (L-P-F), and 15 is a voltage-controlled oscillator (V-C-
O), 14.15.16 is 1/! When divided, V・0・01
5 oscillates at four times the frequency.

At this time, since the output of the 172 frequency divider 14 is guaranteed to have a 50% duty, the reference signal further divided by 1A by the 1A frequency divider 15 is 1/! The inverted output of the 1/2 frequency divider 14 is connected to the frequency divider 16, and the 1/2 frequency-divided output is 90% of the reference signal.
A phase difference of ° is ensured. This signal is passed through the differentiating circuit 17, and during the positive pulse period, the electronic switch of the sample and hold circuit 1B is turned on to sample the AM signal and held in the capacitor 19. Double signal D
C signal is changed.

This signal is connected to the pace of the transistor 24, the level is adjusted by the capacitor 22 and the resistor 23, and the inverted output of the AF multiplied signal generated at the collector is fed back so that there is no amplification effect on the AP multiplied signal. Only the 'fcD-C signal component can be routed to the collector of transistor 24.

The DC signal thus obtained is proportional to the carrier level, and its rate of change is much faster than in the conventional system shown in FIG. Note that the bias resistor and load resistor of the transistors 24 manufactured by No. 25 and No. 26 are provided, 27 is an input terminal for an AM signal, 28 is an output terminal for an AF' signal, and 29 is a carrier level detection terminal.

[Effects of the Invention] As described above, the carrier level detection method of the present invention can use a low-pass filter. Therefore, it has a number of excellent advantages, such as improved followability of the A-G-0 characteristic and application to muting detection signals and the like.

[Brief explanation of drawings]

Figure 1: Conventional carrier level detection circuit using diode detection and low-pass filter Figure 2: Block circuit diagram showing the configuration of the present invention [Symbol] 1
1... Phase comparator, 12... Low pass filter (L@P-F), 16...
・Voltage controlled oscillator (VC・0), 14.15.16・
...1/2 frequency divider, 17... Differential circuit, 18-... Sample hold circuit, 19.21.22... Capacitor, 20.25.25.26... Resistor, 24-- Transistor , 27...AM@ number input terminal, 28...AP
Double signal output terminal, 29...carrier level detection terminal

Claims (1)

[Claims] In an AM signal demodulation circuit, a reference signal that is locked to the carrier frequency is generated by dividing the output of the voltage controlled oscillator by 1/4 using an EL and L circuit similar to a voltage controlled oscillator, a phase comparison circuit, and a low-pass filter. , and 90° from the 1/2 frequency divided wave of the voltage controlled oscillator to the carrier wave of the AM signal.
A signal with a phase difference of An A'M signal carrier level detection method characterized by feeding back the double signal inversion output and canceling the AP double signal to detect the carrier level of the AM signal.