JPS5840666Y2 - FM receiver noise cancellation circuit - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a noise cancellation circuit for an FM stereo receiver.

Conventionally, when switching between FM broadcasting and other inputs in an FM stereo receiver, there has been a drawback that pop noise appears due to the following reasons.

(a) When the power of the 1M front end is turned on and off as the input is switched, the FM
I due to the rise of the 1M front end when switching to broadcasting
F-fold signal frequency fluctuations and level fluctuations are detected and amplified to become pop noise.

(b) In devices with built-in PLL, MPX, and stereo demodulators, free running oscillation is often stopped at a position other than FM broadcasting.

In such a case, when switching to FM broadcasting from another input, pop noise will occur along with the rise of free running oscillation.

(c) A device with a built-in FM muting circuit that performs muting by applying a cutoff bias or saturation bias to the amplifying element for the IF multiplied signal or audio signal and losing the degree of amplification is not suitable for variations in the operating point. Pop noises occur accordingly.

The purpose of this invention is to provide a noise canceling circuit that solves the above-mentioned drawbacks by using transistor switching circuits to cut off the tuner's audio signal output line until each circuit becomes stable. .

Hereinafter, one embodiment of this invention will be described in detail based on the drawings.

In FIG. 1, Sl represents FM and other signals such as AM.
This is a switch that operates in conjunction with the input signal changeover switch (selector) that switches input signals such as Transistor Q1. Q2 and the transistors Q1. Bring Q2 into saturation instantly.

Transistors Q1 and Q2 constitute a switching circuit that cuts off the audio signal output line of the tuner,
Capacitor C1, resistors R1, R2 and transistor Q
1. The cut-off state is maintained for a time r determined by the input impedance Zi of Q2.

In order to eliminate the pop noises mentioned in a and b above, the cut-off time r of the switching circuit may be set to be longer than the time required for each circuit to stabilize.

In this case, the time r is expressed by the following equation.

However, C,,R,,R2 mean constants of capacitor and resistance, respectively, and vth is transistor Q□
, is the threshold voltage required to turn on Q2.

Next, the pop noise when the 1M muting switch described in C above is pressed is suppressed by closing the switch S2 linked to the 1M muting switch, and the muti give voltage is transferred to the terminal through the diode D1 and the resistor R3.
It can be erased in the same manner as in the above case by supplying the signal to the bases of transistors Q1 and Q2 through the .

Note that the resistor R4 and the diode D2 are connected to the switch S□
This is for discharging the charge in capacitor C1 when switching from the state shown in Fig.
□, this is to prevent malfunction of Q2.

The collector terminals c and d of the transistors Q□ and Q2 in the above circuit are connected to the terminal f between the input and output of the audio signal line shown in FIG. 2, and by grounding the audio signal line, pop noise is generated as described above. Delete.

As described above, according to this invention, it is possible to reliably eliminate pop noise when switching between FM broadcasting and other inputs with an extremely simple circuit, and it does not affect other circuits in any way.

Furthermore, the cutoff time can be easily set by simply changing the time constant of the time constant circuit.

In addition, it costs less and takes up less space than a structure that uses integrated relays, etc., it requires no special adjustments in manufacturing and is all chippable parts, and it is purely electronic, so there is little change over time. There are advantages such as:

[Brief explanation of drawings]

FIG. 1 is an electrical circuit diagram showing the configuration of a noise canceling circuit according to an embodiment of the invention, and FIG. 2 is a signal line to which the noise canceling circuit is connected. SL, S4...Switch, C1, C2...
... Capacitor, R1, R2 ... Resistor, Dl,
D2...Diode, Ql. Q2...Transistor.

Claims (1)

[Scope of utility model registration request] Consists of a transistor that becomes conductive when switched from a non-FM input supply state to an FM large input supply state or when a muting voltage is supplied, and when conductive, grounds the tuner's audio signal output line and cuts off the audio output signal. F from the switching circuit and the state of human power supply other than the above FM.
A time constant circuit that maintains the switching circuit in a conductive state until a predetermined time has elapsed from the time when the FM large input is supplied when the state is switched to the M large input supply state, and a reset circuit that resets this time constant circuit. FM receiver noise cancellation circuit.