JPS61247110A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To decrease the quantity of transmission distortion of the 2nd harmonic by connecting a capacitor between an input terminal inputting an oscillated wave from a VCO to a phase comparator and an an earth. CONSTITUTION:The VCO12 has the oscillating capability with frequency variable and the oscillated wave extracted from a collector of a transistor Q is inputted to a phase comparator 9 via a coupling capacitor C2 as a reference signal. The phase comparator 9 compares the reference signal with the 2nd IF signal, the result is converted into an error signal and then outputted. A capacitor C3 applies impedance matching between an output terminal of the VCO12 and an input terminal of the phase comparator 9.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to a PLL used in a satellite broadcasting receiving system, etc.
The present invention relates to a (phase-locked loop) type FM demodulation circuit.

(Prior Art) FIG. 2 is a system diagram showing the configuration of a general satellite broadcast receiving system. The code ANT in the figure is a parabolic antenna, 1
is L N 3 (Low No.1se Blockco
nverter), the output terminal of LNBl is connected to a receiver 2, which is an indoor unit, via a coaxial cable, and a TV receiver F14 3 is connected to the rear of the receiver 2. The reception IN 2 includes an RF converter 4, a bandpass filter 5 for removing random noise, a PLL type FMI adjustment circuit 6,
and a baseband processing circuit 7.

Then, the 4GH2 band broadcast signal received by the parabolic antenna ANT is amplified by LNBl to 950~1450HH.
After being converted into a first intermediate frequency signal of 2, the signal is introduced into the receiver 2 via a coaxial cable. The first intermediate frequency signal (hereinafter referred to as the 11th F signal, etc.) includes 24 channels, and when this 11th F signal is introduced into the receiver 112, the RF converter 4 selects one channel from among them. A channel is selected and converted to a second IF signal at 600 MHz.

Since the second IF signal contains a lot of noise (random noise) near its center frequency, the noise is removed by a bandpass filter 5 having a passband of 6008H7±15 MHz before inputting it to the FM demodulation circuit 6. be done.

The FM demodulation circuit 6 demodulates the noise-removed 21st F signal into a baseband signal, which is sent to the TV receiver 5 via the baseband processing circuit 7, and the TV receiver 11!
3, the desired broadcast can be viewed.

FIGS. 3 and 4 are diagrams showing in more detail the internal circuit configuration of the conventional FM demodulation circuit 6 using such a PLL system. In FIG. 3, 8 is a buffer amplifier, 9 is a phase comparator, 10 is a loop filter, 11 is a loop amplifier,
12 is a voltage controlled oscillator (hereinafter referred to as vCo). FIG. 4 shows the circuit configuration of VC012 in more detail, in which VC is a variable capacitance diode, Q is an oscillation transistor, 13 is a reactive element such as a strip line, and this reactive element 13 Collector voltage Vcc is supplied to the collector of transistor Q via. C1 is a bypass capacitor. Further, a coupling capacitor C2 is interposed in the transmission line 14 connecting the collector of the transistor Q and the input end 9a of the phase comparator 9.

The operation of the FM demodulation circuit 6 will be described first.

12 has a frequency variable oscillation ability of ±208H2 or more with a center frequency of 300 MHz (this frequency value will be described later). and transistor Q
The oscillation wave taken out from the collector of is inputted as a reference signal to the phase comparator 9 via the coupling capacitor C2.

The phase comparator 9 uses this reference signal and the buffer amplifier 8
The phase of the signal is compared with the 21st F signal of 600 Hllz which is input through the input signal, and the signal is converted into an error signal and output. Here, the demodulation method of this FM demodulation circuit 6 is such that a second harmonic of 600 MHz is generated in the phase comparator 9 by a fundamental wave of 300 Hllz oscillated by the VCO 12, and this second harmonic is used for FM modulation. The phase of the second IF signal is compared with that of the second IF signal. The error signal output as a result of the phase comparison is band-limited by the loop filter 1o, amplified by the loop amplifier 11, and then outputted as a demodulated signal from the FM demodulation circuit 6, and a part of it is negatively fed back to the V C012. V C0
12 is controlled by this negative feedback error signal so that its oscillation frequency follows the instantaneous frequency of the second MF signal. The error signal outputted from the phase comparator 9 corresponds to a baseband signal obtained by demodulating the frequency change of the 21st F signal, and this demodulated signal is sent to the baseband processing circuit 7.
The signal is then output to the TV receiver 3.

Here, as mentioned above, set the oscillation frequency of V C012 to 300.
The reason why it is set to 8H2 is that this FM demodulation circuit 6
The PLL circuit used in this is originally for 450 HN3, and its oscillation frequency stop range is approximately 700 M.
It's about llz. But from VC'012 600H
If the I3 signal is directly oscillated, the oscillation output amplitude slope and the followability by the modulation signal (demodulation signal) will change, which is not preferable. Therefore, the oscillation frequency of V C012 is 3008H2
, the second harmonic of 6008H2 is detected by the phase comparator 9.
This generates an oscillation wave, which is used as a reference signal.

(Problem to be solved by the invention) In the conventional FM demodulation circuit, 300
The oscillation wave of HI3 is oscillated, and this oscillation wave is sent to the phase comparator 9.
This 300 MH
The second harmonic of Goo HI3 is generated using the oscillation wave of z as a fundamental wave, and this is compared in phase with the 21F signal of 600)4H2 as a reference signal. By the way, 3008H
The second harmonic of 600 HI3 for the fundamental of V
It also occurs inside CO12. Furthermore, phase distortion occurs when such high frequency waves are transmitted through the transmission line 14, and the occurrence of this phase distortion becomes larger as the frequency becomes higher. Therefore, the 600 Hz second signal generated inside V C012
A relatively large amount of phase distortion occurs in the harmonics before they reach the phase comparator 9. On the other hand, the second harmonic of 600 MHz generated inside the phase comparator 9 has a small amount of phase distortion.

Then, the second harmonic of 600 HI3, which is synthesized by mixing the second harmonic on the V CO12 side, which has a relatively large phase distortion, and the second harmonic on the phase comparator 9 side, which has a small phase distortion, is the original one. The reference signal is dominated by the amount of phase distortion of the second harmonic with larger phase distortion among the two second harmonics, resulting in a reference signal having relatively large phase distortion. If there is phase distortion in the reference signal as described above, there is a problem in that the differential phase of the demodulated signal output that is generated thereby deteriorates.

Here, regarding the differential phase, the color television signal (N
This will be explained using a waveform diagram of the TSC method.

In the figure, reference numeral 15a is a color bust signal, and 15b is a 3.5
8 Htlz color subcarrier, 15c is a luminance signal. Color burst signal @15a on TV receiver 3 side
An oscillator is provided that is phase-synchronized with the oscillator, and it is determined how much the color subcarrier 15b increases, decreases, or advances or lags in amplitude or phase based on the output of this oscillator.
, B are detected.

When this subcarrier 15b is not modulated (that is, when there is no color), it is preferable that this increase/decrease or advance/delay is small. Differential gain and differential phase are used to express this preference. Of these, the differential phase is the degree (de(1)
). When the phase change from point a to point b remains unchanged (constant phase difference), it is expressed as a differential phase of 0 degrees.

The purpose of this invention is to solve the problems that occur in the conventional FM demodulation circuit as described above.
An object of the present invention is to provide an FM demodulation circuit capable of minimizing the amount of phase distortion of an oscillation signal inputted to a phase comparator 9 from the second side and outputting a demodulated signal with less phase distortion.

(Means for Solving the Problems) In order to achieve the above object, an FM demodulation circuit according to the present invention uses a voltage controlled oscillator whose frequency is controlled by a phase-locked loop and the phase of an FM-modulated frequency signal. In an FM demodulation circuit that outputs a demodulated signal by comparing the phase of an oscillation wave oscillated by the voltage controlled oscillator with a phase comparator, the oscillation wave whose phase is compared by the phase comparator is the same as the fundamental wave oscillated by the voltage controlled oscillator. A capacitor for impedance matching is connected between the input terminal of the phase comparator which inputs the oscillation wave from the voltage-controlled oscillator to the phase comparator and the ground, which is the second harmonic.

(Function) Since an impedance matching capacitor is connected between the input terminal 9a of the phase comparator 9 and the ground, V C012
output impedance and impedance of the transmission line 14,
Also, the impedance of the transmission line 14 and the input impedance of the phase comparator 9 can be matched, and the phase distortion of the oscillation wave of the VCO 12 transmitted through the transmission line can be minimized.

(Description of Embodiments) Embodiments of the present invention will be described below based on FIG. 1. In FIG. 1, the same or equivalent circuit elements as those in FIGS. 2 to 4 are designated by the same reference numerals and redundant explanation will be omitted.

First, to explain the configuration, in this invention, the VCO 12
A semi-fixed capacitor C for impedance matching is connected between the input terminal 9a, which inputs the oscillation wave from the oscillation wave to the phase comparator 9, and the ground.
The action of the 6th order to which 3 is connected will be explained. First, adjust the semi-fixed capacitor C3, and then adjust the input terminal 9a of the phase comparator 9.
The impedance of the transmission line 14 close to the input impedance of the phase comparator 9 is matched with the input impedance of the phase comparator 9. Since the impedance of the transmission line 14 near the output end of the VCO 12 follows the output impedance of the VCO 12, the impedance is matched between the output end of the VCO 12 and the input end of the phase comparator 9, and the impedance generated inside the VCO 12 is matched. The amount of transmission distortion of the second harmonic at 600 MHz is minimized.

Therefore, even if this second harmonic on the VCO 12 side is mixed with the second harmonic generated inside the phase comparator 9 and both are combined, the amount of phase distortion of the combined wave can be suppressed to a minimum.

As a result, deterioration in the differential phase of the demodulated signal output from the FM41 modulation circuit 6 can be minimized.

(Effects of the Invention) As detailed above, according to the present invention, since an impedance matching capacitor is connected between the input terminal for inputting the oscillation wave from vCO to the phase comparator and the ground, the vCO
Impedance matching is performed between the output end of the phase comparator and the input end of the phase comparator, and the transmission distortion R of the second harmonic generated on the vCO side and input to the phase comparator is minimized. Therefore, even if this wave is combined with the second harmonic generated inside the phase comparator, the phase distortion of the combined wave can be suppressed to a minimum, resulting in the effect that deterioration of the differential phase of the demodulated signal can be suppressed to a minimum.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a main part of an embodiment of an FM demodulation circuit according to the present invention, FIG. 2 is a block circuit diagram of a conventional satellite broadcast receiving system, and FIG. 3 is a block circuit diagram of a conventional FM demodulation circuit. , FIG. 4 shows v in the same FM demodulation circuit.
A circuit diagram showing the CO part, and FIG. 5 is a waveform diagram showing a color television signal. 6... FM demodulation circuit, 9... Phase comparator, 9a.
...Input end, 10...Loop filter, 11.
...Loop amplifier, 12...Voltage controlled oscillator, 14
...Transmission line, C3...Semi-fixed capacitor for impedance matching, VC...Variable capacitance diode. 1st gA fI2ga

Claims (1)

[Claims] In an FM demodulation circuit that outputs a demodulated signal by comparing the phase of an FM-modulated high-frequency signal and the phase of an oscillation wave oscillated by a voltage-controlled oscillator whose frequency is controlled by a phase-locked loop with a phase comparator, the phase The oscillation wave whose phase is compared by the comparator is the second harmonic of the fundamental wave oscillated by the voltage-controlled oscillator, and the input of the phase comparator inputs the oscillation wave from the voltage-controlled oscillator to the phase comparator. An FM demodulation circuit characterized in that an impedance matching capacitor is connected between the end and ground.