JPH0153802B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a dropout detection circuit for an input FM signal.
Configured in combination with a quadrature demodulation circuit,
The present invention relates to this type of dropout detection circuit.

[Prior art]

For dropout detection in conventional FM demodulation circuits,
The PLL tracks the input FM frequency and
The input voltage of the VCO (voltage controlled oscillator) was used as the FM demodulation signal voltage, and dropout was detected by detecting that the phase lock was lost. However, since the PLL circuit requires a VCO, etc.
The disadvantages were that the cost was high.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a dropout detection circuit that eliminates the expensive PLL, which is the disadvantage of the conventional art, employs an inexpensive quadrature demodulation method, and can detect dropouts.

[Summary of the invention]

Dropout is a phenomenon in which the FM frequency drops instantaneously. Focusing on the fact that the speed and frequency of the drop far exceed the frequency and amplitude of the FM modulated wave, we have developed It was decided that dropout would be detected when the unnecessary high-frequency components of the signal that passed through the cutoff low-pass filter reached a certain level.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. In the figure, 1 is an FM signal input terminal, and the FM signal input here is amplified by a limiter 2 and becomes a rectangular wave. This output signal is shifted by 90° at the center frequency by a 90° phase shifter 3, and is multiplied by a multiplier 4 together with the signal that does not pass through the phase shifter 3. This output includes, in addition to the demodulated signal, a spectrum with a frequency twice that of the carrier. Most of this second harmonic wave is removed by a low-pass filter 5 and output. Furthermore, in order to improve the S/N ratio, this output is filtered by a low-pass filter consisting of a resistor 6 and a capacitor 7, so that a spectrum higher than the required frequency is removed. At this time, if the FM signal is lost due to dropout, the FM frequency will change for that period.
Since the signal drops extremely low, it passes through the low-pass filter 5, the resistor 6, and the capacitor 7, and is observed as dropout noise. However, since the base and emitter terminals of transistor 8 are connected to both ends of resistor 6,
When a dropout occurs and the frequency of the FM signal drops significantly, when the dropout noise passes through the low pass filter of the resistor 6 and capacitor 7, the transistor 8 becomes conductive because this noise contains high frequency components. This is because the high frequency component here refers to the above-mentioned second harmonic wave, and this second harmonic component also has a lower frequency and passes through the low-pass filter 5. However, although this second harmonic component has a frequency low enough to pass through the low-pass filter 5, it has a high frequency component compared to the cut-off frequency of the low-pass filter formed by the resistor 6 and capacitor 7 in the latter stage of the low-pass filter 5. Therefore, the voltage shared by the resistor 6 increases, and the voltage across the resistor 6 exceeds the VBE (base-emitter voltage) of the transistor 8, so the transistor 8 becomes conductive. Therefore, the transistor 11 becomes conductive via the resistor 9, momentarily discharging the charge in the capacitor 14, and the collector potential of the transistor 11 increases. Reference numeral 15 denotes an analog switch that shuts off when the control voltage rises. Therefore, when there is a dropout, the analog switch 15 opens and the voltage just before that is transferred to the hold capacitor 16.
Therefore, no dropout noise is generated at the output terminal 17. Even if the input dropout condition is canceled, its influence remains on the low-pass filter, but the analog switch 15 is determined by the time constant of the capacitor 14 and resistor 13, and the threshold voltage of the analog switch 15. Since the opening period continues, dropout noise is hardly detected. Note that the resistor 10 is for completely cutting off the transistor 11.

〔Effect of the invention〕

As described above, according to the present invention, expensive
Dropout can now be dealt with without using a PLL demodulation circuit, and an inexpensive FM demodulation circuit can be obtained.

[Brief explanation of drawings]

The figure is a circuit diagram showing one embodiment of the present invention. Explanation of symbols 1...Input terminal, 2...Limiter, 3...
Phase shifter, 4... Multiplier, 5... Low pass filter,
6,9,10,13...Resistor, 7,14...Capacitor, 8,11...Transistor, 15...Switch,
16...Holding capacitor, 17...Output terminal.

Claims (1)

[Claims] 1. A quadrature FM demodulation circuit that receives an FM signal, demodulates it, and outputs it; a first low-pass filter that receives the multiplication output from the demodulation circuit, suppresses the second harmonic of the carrier, and outputs the result; and an output from the filter. a second low-pass filter that cuts off and outputs frequencies higher than a predetermined frequency that are input, a switch circuit that leads the output of the second low-pass filter to an output side equipped with a hold capacitor, and a dropout that occurs in the FM input signal. and means for detecting a dropout noise signal that passes through the first low-pass filter and inputs into the second low-pass filter to open the switch circuit. A dropout detection circuit for a quadrature FM demodulation circuit, characterized in that the voltage is held by the hold capacitor and outputted.