JPS59188251A - Fm tuner - Google Patents

Abstract

PURPOSE:To eliminate a hop noise by eliminating the variation in the DC potential of a signal output in tuning operation, etc. CONSTITUTION:An IF signal is detected and converted into a composite signal by an FM detecting circuit 1 having S curve characteristics, and the signal is inputted directly to a postamplifier 2. The output of the amplifier 2 is inputted directly to a multiplex demodulating circuit 3 to separate a left and a right audio signal, which are applied directly to a trailing stage. The left and right signals are summed up by resistances R3 and R4 and applied to the output line of the detecting circuit 1 through a DC negative feedback circuit 4. The circuit 4 has an operational amplifier OP2 which is applied with the left and right signal at the negative-phase terminal and with the earth potential at the in- phase input through a resistance R5, and capacitors C1 and C2, and the output of an amplifier OP2 is fed back to the postamplifier through a resistance R6. The amplifier OP2 and capacitors C1 and C2 constitute a Miller integration circuit and the integral time constant is controlled freely by turning on and off a switch SW.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM tuner, and particularly to the FM tuner circuit configuration of the low frequency signal portion after the FM detection circuit.

In an FM tuner, F is used for FM detection of intermediate frequency signals.
It includes an M detection circuit, a post amplifier that amplifies the composite detection output, and an MPX (multiplex) demodulation circuit that demodulates the composite signal into left and right channel signals, respectively. It is a processing circuit.

From this low frequency signal processing circuit, FM detection circuit, to MPX
Up to the demodulation circuit, there are DC fluctuations in the FM detection output due to various factors, so in order to generate pop noise due to the DC fluctuations, a coupling capacitor etc. is inserted in the middle to cut the DC current, which is called a non-directly coupled type. The circuit configuration must be adopted. As a result, the sound quality deteriorates, making it difficult to obtain a so-called T1l-F1 tuner.

Furthermore, in an FM tuner, when tuning or detuning, the S curve, which is the detection characteristic of the FM detection circuit, is swept, so DC is generated in the detection output, and DC fluctuations occur. This causes large pop noises and prevents direct connection of the FM tuner stage.In order to prevent DC generation and fluctuations during such tuning operations, a bypass filter consisting of a capacitor and a resistor is inserted in the signal line. However, this also contributes to the deterioration of sound quality and prevents the conversion to l-1i-Fi.

An object of the present invention is to provide an FM tuner that can be directly connected by suppressing the DC potential of the signal output to a constant value.

Another object of the present invention is to provide a direct-coupled FM tuner that eliminates pop noise by eliminating direct current potential fluctuations in signal output during tuning operations.

The FM tuner of the present invention includes a multiplex demodulation circuit that separates and demodulates the first and second channel signals from the detection output of the FM detection circuit, and an integrator that integrates the demodulated output of this demodulation circuit. It is characterized in that it includes a negative feedback circuit that provides negative feedback to the detection output line of the FM detection circuit, and enables direct connection by suppressing DC potential fluctuations of the demodulated output.

Furthermore, the FM tuner of the present invention includes a control means for controlling the integration time constant of the integrator to a small value in response to a channel selection operation, and the circuit system is equivalently configured as a bypass filter. The system is characterized in that DC fluctuations that occur during station tuning are not transmitted to the audio amplifier stage.

The present invention will be explained below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which an IF (intermediate frequency) signal is detected by an FM detection circuit 1 having S-curve characteristics and converted into a composite signal. This detection output is directly connected and input to the post amplifier 2, where it is amplified. As shown in the figure, this post-amplifier has a negative feedback amplifier OP1 having resistors R1 and R2 as feedback resistors, and the output of this amplifier is directly connected and input to the multiplex demodulation circuit 3. The demodulation circuit 3 separates left and right audio signals, which are first and second channel signals, respectively, and directly connects and applies them to an audio amplifier (not shown) at the next stage.

Furthermore, these left and right channel signals are added and combined by resistors R3 and R4, respectively, and applied to the output line of the detection circuit 1, ie, the anti-phase input of the post amplifier 2, via the DC negative feedback circuit 4. In this negative feedback circuit 4, the previous left and right channel signals are applied to the negative phase input, and the resistor R5 is applied to the positive phase input.
It has an operational amplifier OP2 to which a ground potential is applied via a resistor R6, and capacitors C1 and C2 connected in parallel between the opposite-phase input and output of the operational amplifier OP2. is fed back to the input of the Boston amplifier.

Here, a so-called Miller integration circuit is configured by the operational amplifier OP2 and the capacitors C1 and C2, and its integration time constant is determined by the parallel resistances of the resistors R3 and R4 and the capacitors C1 and C2.
This is the product of two parallel capacitances, and this integration time constant can be freely controlled by turning on and off the switch SW.

The control of this switch SW is configured to detect a channel selection operation and generate an off control signal for the switch SW.
Normally, this switch SW is on.

FIG. 2 shows a preferred specific example of the MPX demodulation circuit 3 in FIG. 1, and is an example of a so-called well-known chopper type demodulation circuit. That is, the composite signal is branched to resistors R7 and R8 and turned on and off by gate switching transistors Q1 and C2. The gate transistors Q1 and C2 are controlled by the forward and reverse signals of the 38 KHz subcarrier signal obtained from the pilot signal in the composite 1~ signal, and are turned on and off alternately. The gate outputs are inputted via resistors R9 and R10 to low-pass 4° filters that control operational amplifiers OP4 and OF2, respectively. This filter further includes capacitors C3 and C4 and resistors R11 and R12, respectively, to separate the left and right channel signals from the outputs of the operational amplifiers OP4 and OF2.

Furthermore, an operational amplifier O with feedback resistors R13 and R14
P3 is provided, and the degree of separation can be adjusted by adjusting the resistor R14.

In the circuit configuration described above, the integration time constant control switch SW is closed during normal reception, and therefore this time constant is selected to be large. If the output line of the FM detection circuit 1 causes DC fluctuations for some reason in a darkening state,
Since all subsequent stages are directly connected, the DC level of the demodulated output varies accordingly. Here, the outputs of both channels are added together, applied to the negative phase input of the DC feedback amplifier 4, and integrated, so that a DC level corresponding to the DC fluctuation is output from the amplifier 4 and output to the post amplifier 2. It is fed back to the input. In this case, the post amplifier 2 is a negative phase amplifier, the demodulation circuit 3 is also a negative phase amplifier, and the DC feedback amplifier 4 is also a negative phase amplifier configuration, so the feedback loop becomes a so-called negative feedback configuration, and the feedback to the post amplifier 2 is By appropriately selecting the negative feedback level, it is possible to cancel DC fluctuations in the signal line. As a result, the detection circuit 1
The DC gain from the output of the demodulation circuit 3 to the output of the demodulation circuit 3 can be made almost zero, making direct connection possible.

Next, during a channel selection operation, the detection circuit 5 detects an operation such as turning the tuning knob, and generates a switch control signal.
Switch SW becomes open. Therefore, the only integral capacitance is substantially the capacitor C1, and the integral time constant becomes small.

Note that the resistor R7 provided in parallel with the switch SW prevents DC fluctuations due to charging and discharging of the capacitor C2 caused by turning the switch on and off, and is a high resistor. Therefore, the circuit system from the output of the detection circuit 1 to the output of the demodulation circuit 3 operates equivalently as a low-cut filter configuration as a whole, and has an LC frequency characteristic with the so-called ultra-low frequency band cut off. Since this frequency characteristic has a cutoff frequency of several cycles as shown by the dotted line in Figure 3, it is possible to sufficiently eliminate DC fluctuations when the IF frequency sweeps the so-called S curve during tuning operation, and automatically Pop noise caused by channel selection is removed.

Further, the solid line in FIG. 3 shows the frequency characteristics of the above circuit system during normal operation, and it can be seen that the gain extends to very low frequencies because no coupling capacitor or the like is used.

As described above, according to the present invention, everything after the detection stage of the FM tuner can be directly connected, and even the audio amplifier stage can be directly connected, thereby realizing an Hl-Fi tuner. Furthermore, even when tuning is detuned such as when selecting a channel, pop sounds can be removed without inserting a bypass filter into the signal line, making it possible to achieve even higher levels of Hi-Hi.

In the above, the input to the negative feedback amplifier is a composite signal of the left and right channel signals, but if the characteristics of the left and right channel amplifiers OP4 and OR3 in the stereo demodulation circuit 3 are uniform, either one of the left and right channel signals is input. You can select one as the feedback amplifier person horn. Furthermore, although a chopper-type circuit is shown as an example of the MPX demodulation circuit, it is not limited to this example as long as it is of a type that does not affect the DC level.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an example of an MPX demodulation circuit suitable for use in the present invention, and FIG. 3 is a diagram showing characteristics of the circuit of the present invention. Explanation of symbols of main parts 1... FM detection circuit 2... Post amplifier 3... MPX demodulation circuit 4... DC feedback amplifier 5... ...Tuning operation detection circuit Patent applicant Pioneer Co., Ltd. Figure 1 Figure 2 Figure 3

Claims (2)

[Claims] (1) A multiplex demodulation circuit that separates the first and second channel signals from the detection outputs of the F and M detection circuits, and whose integration time constant can be controlled to integrate the demodulated output of this multiplex demodulation circuit. a negative feedback circuit having an integrator and negative feedback of this integral ratio to the output line of the FM detection circuit; and control means for controlling the integration time constant to a small value in response to a channel selection operation; An FM tuner characterized in that DC potential fluctuations in demodulated output are suppressed. (2) The integrator includes an operational amplifier whose j terminal is grounded, and a capacitor connected between the input terminal and the output terminal of the operational amplifier, and the control means controls the capacitor. 2. The FM tuner according to claim 1, wherein the FM tuner controls a capacitance value of the FM tuner.