JPH02104107A - Fm demodulator - Google Patents

Abstract

PURPOSE:To stabilize temperature characteristics by connecting the output terminal of a DC voltage amplifier directly to the base of one of two transistors(TRs) having almost the same temperature characteristics to constitute a differential amplifier and connecting the output terminal to the base of the other TR through an inductor having an inductance value almost suppressing the passage of a video signal. CONSTITUTION:The output terminal of a video amplifier 26 is directly connected to the base of a TR 29 in the differential amplifier 27 constituted of two TRs 29, 30 having almost the same temperature characteristics, resistors 31, 32 connected to the collectors of the TRs 29, 30 and a constant current source 33 connected to the emitters of the TRs 29, 30 and the output of the amplifier 26 is also connected to the base of the TR 30 through the inductor 34 having an inductance value almost suppressing the passage of a video signal. Thereby, a change in an output offset voltage due to the temperature change of the video amplifier is canceled and only the video signal is amplified by the differential amplifier. Thus, the temperature change of a DC offset voltage from the video amplifier can be compensated and the temperature characteristics can be stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an FM demodulator for demodulating FM-modulated signals for the purpose of information transmission, such as satellite broadcasting.

2. Description of the Related Art Satellite broadcasting receiving systems are desired to be compact and high-performance, and in addition to conventional BS tuners, indoor receivers are now available with built-in televisions or videos.

Figure 3 is a schematic diagram showing a satellite broadcasting reception system.
Ku-Band (12GHz) transmitted from broadcasting satellite 1
) band or C-Band (4GHz) band, and a low-noise antenna that is attached to this parabolic antenna 2 and converts the output signal from the parabolic antenna 2 into a signal in the IGHz band called 1st IF. An outdoor unit 3 such as a converk, a tuner unit 6 built into the indoor receiver 5 for converting the 1st IF signal drawn into the room via the coaxial cable 4 to 2st IF, and the tuner unit 6
an FM demodulator 7 that demodulates the output signal from the FM demodulator 7, and a monitor television 8 that inputs the demodulated signal after signal processing with a de-emphasis circuit, a video amplifier circuit, etc. (not shown).
It is made up of.

In order to reduce the cost of the satellite broadcasting receiving system mentioned above, the most effective way is to make the parabolic antenna 2 smaller, but on the other hand, this lowers the received C/N and causes a deterioration in image quality. .

Therefore, in order to prevent deterioration in image quality and greatly contribute to reducing the total cost of a satellite broadcast receiving system, it is required to lower the threshold eyelid level of the demodulator. For this reason, FM demodulators based on the PLL system, which can lower the threshold level, have come to be used in the demodulation section.

FIG. 4 is a block diagram showing the configuration of a conventional PLL-based FM demodulator, showing the frequency and phase of an input signal input from an input terminal 9 through an amplifier 10 and a bandpass filter (B, P, F). and voltage controlled oscillator (hereinafter referred to as v
a phase comparator 13 that compares the oscillation frequency and phase of the phase comparator 12 (abbreviated as CO), and a loop filter (hereinafter abbreviated as LPF) 14 that removes unnecessary noise and interference signals from the output signal of this phase comparator 13. and a DC voltage amplifier (hereinafter abbreviated as video amplifier) 15 that amplifies the output signal of this LPF 14. 16 amplifies the FM demodulated signal from the video amplifier 15 and outputs it to an output terminal 17
This is a buffer amplifier that supplies

Among the components of the FM demodulator using the PLL method, the video amplifier 15 is a temperature-compensated video amplifier for the purpose of reducing the board mounting area and making it possible to arbitrarily set the PLL loop gain. In the conventional FM demodulator described above, even if the video amplifier is temperature compensated for the gain in the video signal band, the DC offset voltage may change with temperature, which changes the free run frequency of the PLL circuit and causes the carrier color signal to change. This causes deterioration of temperature characteristics of FM demodulation characteristics such as differential gain, differential phase, and video output frequency characteristics.

Since the amount of change due to temperature in the DC offset voltage of an IC used as a video amplifier varies individually, it is necessary to compensate for this even if the VCO 12, phase comparator 13, etc. that make up the PLL circuit are intentionally given temperature characteristics. The problem was that it was also difficult.

The present invention solves these conventional problems,
It is an object of the present invention to provide an FM demodulator that compensates for temperature changes in the DC offset voltage of a video amplifier and stabilizes its temperature characteristics.

Means for Solving the Problems In order to achieve the above object, the present invention connects the base of one transistor of a differential amplifier composed of two transistors having almost the same temperature characteristics directly to the output terminal of a video amplifier. , the base of the other transistor is connected to the output terminal of the video amplifier via an inductance large enough to prevent the video signal from passing through.

Therefore, according to the present invention, since the differential amplifier amplifies the difference component between the base voltages of the two transistors constituting the differential amplifier, a change in the output offset voltage due to a change in the temperature of the video amplifier is caused by this change being 2 It can be canceled by inputting it to the base of two transistors. Further, although the video signal is input to one transistor, the input to the other transistor is blocked by the inductance, so that only the video signal can be amplified by the differential amplifier.

Example FIG. 1 shows the configuration of an embodiment of the present invention.

In FIG. 1, 24 is a phase comparator, and this phase comparator 24 compares the frequency and phase of the input signal inputted from the input terminal 21 via the amplifier 22, the pass filter 23, and the oscillation frequency and phase of the VCO 28. compare.

25 is an LPF that removes unnecessary noise and interference signals from the output signal of the phase comparator 24; 26 is a video amplifier that amplifies the output signal of the LPF 25;

27 is a differential amplifier composed of two transistors 29 and 30 having almost the same temperature characteristics such as current amplification factor (haE), their collector resistors 31 and 32, and a constant current source 33 connected to their emitters. The output terminal of the video amplifier 26 is directly connected to the base of the transistor 29, and the output terminal of the video amplifier 26 is connected to the base of the transistor 3o via an inductance 34 large enough to prevent the video signal from passing through. ing.

Further, a buffer amplifier 35 is connected to the collector of the transistor 29 to amplify the demodulated signal and supply it to the output terminal 36.
is connected, and the collector of transistor 3o is connected to VCO28.
is connected to the oscillation frequency control terminal.

FIG. 2 shows an FM demodulator using the conventional PLL method.

3 is a diagram showing a free-run frequency temperature characteristic 18 and a free-run frequency temperature characteristic 38 of an FM demodulator using the PLL method of the present invention. FIG.

According to the FM demodulator having the configuration of the embodiment shown in FIG.
Cancelled by entering a base of 30,
As a result, as is clear from FIG. 2, the frequency is 0.2 MHz at -10°C.

At +60° C., the temperature characteristics are improved by 3 MHz.

In the above embodiment, an implementation method using an IC video amplifier has been described, but even if a DC voltage amplifier is used as a discrete DC voltage amplifier, the same effects as in the above embodiment can be obtained.

Effects of the Invention As is clear from the above embodiments, the present invention connects the output terminal of a DC voltage amplifier constituting a PLL circuit directly to the base of one of two transistors having approximately the same temperature characteristics constituting a differential amplifier. Since the other base is connected through an inductance large enough to prevent the video signal from passing through, changes in the output offset voltage due to temperature changes in the DC voltage amplifier are canceled by the differential amplifier, and only the video signal is connected to the base. is amplified and output, and the temperature characteristics of the FM demodulator can be stabilized.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an FM demodulator according to an embodiment of the present invention, and FIG. 2 shows the temperature characteristics of the free run frequency of the FM demodulator according to the present invention and the temperature characteristics of the free run frequency of a conventional FM demodulator. FIG. 3 is a schematic block diagram showing a satellite broadcast receiving system, and FIG. 4 is a block diagram of a conventional FM demodulator. 24... Phase comparator, 25... Loop filter, 2
6 DC voltage amplifier, 27... differential amplifier, 28...
・Voltage controlled oscillator, 29, 30...transistor, 3
4...Inductance. Name of agent: Patent attorney Shigetaka Awano, 1st person, 2nd person
Figure≧ ]:1 Figure 3

Claims (1)

[Claims] A phase comparator that compares the frequency and phase of an input signal with the oscillation frequency and phase of a voltage controlled oscillator, a loop filter that removes noise and interference signals from the output signal of the phase comparator, and amplifies the output signal of the loop filter. and two transistors with almost the same temperature characteristics, the output terminal of the DC voltage amplifier is directly connected to the base of one of the transistors, and the output is taken out from the collector of this transistor, and the other A differential amplifier in which the output terminal of the DC voltage amplifier is connected to the base of the transistor via an inductance large enough to prevent the video signal from passing through, and the collector of this transistor is connected to the oscillation frequency control terminal of the voltage controlled oscillator. FM demodulator equipped with.