JPS60259009A - Fm signal processing circuit - Google Patents

Abstract

PURPOSE:To simplify the integrated circuit by converting a meter voltage used for a meter drive circuit into a current and transmitting it through a current mirror circuit so as to give a mute amount and a mute sensitivity from one pin. CONSTITUTION:The meter voltage as the result of level detection of an output of an FM intermediate frequency amplifier and driving a meter drive circuit 27 by a DC detection voltage is converted into a current IS by a transistor Q1 and a resistor RS and this curren is transmitted as a KIS by the current mirror circuit in the constitution of a mute drive circuit 28, and a series circuit comprising a variable resistor 31 and a reference voltage source 32 is connected to a pin 30 for an externally mounted component. The gain of the gain variable control amplifier 29 is controlled by the variable resistor 31 so as to adjust the mute sensitivity and to set the mute amount by a voltage value of a reference voltage source 32. One pin is enough two pins for externally mounted components for conventional mute amount and mute sensitivity so as to simplify the integrated circuit.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an FM signal processing circuit, and is used, for example, in an FM intermediate frequency stage integration circuit such as a car radio.
Particularly when weak electric fields are input, the FM detection output is gradually reduced.
This relates to a circuit that sets the noise level to a certain value when there is no signal.

[Technical background of the invention]

2. Description of the Related Art Conventionally, there is an FM intermediate frequency amplification circuit that gradually changes the level of an FM detection output depending on the input electric field strength to create a mute state or a mute release state. This heel circuit has characteristics of electric field strength versus signal level detection output as shown in FIG.

That is, 1 is the detection output signal, and 12 is the noise level contained therein. The stronger the electric field strength, the lower the noise level and the higher the output signal level. but,
As the electric field strength becomes weaker, the noise level increases and the output signal level gradually decreases, eventually reaching the same level as the noise level. Here, setting the level L1 is called the mute amount, and setting the slope of the region M1 in which the output signal level gradually decreases is called the mute sensitivity.

The slope of mute sensitivity is obtained by gradually changing the automatic gain control voltage of a variable gain control amplifier for audio, and the lower end of this slope is the mute amount. This mute amount sets the mute limit of the variable gain control amplifier, and is set by an attenuator or the like that sets the limit of gain reduction.

[Problems with background technology]

According to the conventional FM signal processing circuit having the above-mentioned muting system, when it is integrated into an integrated circuit, at least two wires are required for connecting an external resistor for adjusting the mute sensitivity and for connecting an external resistor for adjusting the mute amount. An external bin was required. Meet sensitivity is achieved by converting the detected voltage 13 (used as the meter voltage of the level meter drive circuit) obtained by level detecting the FM intermediate frequency signal into a current using an external resistor, and transmitting the change to the gain control terminal of the variable gain control amplifier. giving. In addition, the amount of meat is determined by converting the meter voltage obtained by level detecting the FM intermediate frequency signal into a current using an external resistor, comparing the voltage of this resistor with the internal reference voltage, and determining the predetermined relationship. Then, the gain of the variable gain control amplifier is fixed by the output of the comparator. In this way, in order to set the mute sensitivity and mute amount separately, the fifth
When the mute amount is adjusted as indicated by the broken line 11A in the figure, the slope of the meat sensitivity (indicated by the broken line JIB) does not continue smoothly, and unnatural mute operations may occur.

[Purpose of the invention]

This invention has been made in view of the above circumstances, and aims to provide an FM signal processing circuit that allows adjustment of mute sensitivity and amount of mute from one external bin, and that allows the number of pins of an integrated circuit to be reduced. .

[Summary of the invention]

In this invention, for example, the meter voltage used in the meter drive circuit is converted into -phase current (transistor QJ, resistor R6
By transmitting this converted current to the bin 3o using a current mirror circuit and applying a reference voltage to the bin 30, the meet amount and meet sensitivity can be given from one bin. It was designed so that

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which the portion surrounded by broken lines is constructed as an integrated circuit. The FM intermediate frequency signal is amplified by an intermediate frequency amplifier 2 and then sent to a buffer circuit 22.
The signal is detected by the peak detection circuit 23 and converted into an audio signal.

This audio signal is amplified by variable gain control amplifier 24 and output to output terminal 25. The intermediate frequency amplifier 2
The signal No. 1 is also guided to the level detection circuit 26. This circuit detects the level of the intermediate signal and inputs the detected voltage to the meter drive circuit 27. The meter drive circuit 27 generates a drive signal for the display meter according to the level of the FM intermediate frequency signal. The meter voltage obtained by the meter drive circuit 27 is also given to the meter drive circuit 28. This mute sensitivity Eve circuit 28 is a circuit that controls the gain of the variable gain control amplifier 24 in response to changes in the meter voltage range.

In this case, in the present invention, the mute drive circuit 28
Two external pins 30 are provided, and the mute sensitivity and the amount of mute can be adjusted via a variable resistor 31 and a reference voltage source 32 connected to the external pins 30. The reference voltage source 32 is configured such that the output voltage from the power supply circuit 33 can be taken out by a variable resistor 34. Note that 36 is a circuit for determining the level of the peak detection output (audio signal);
A predetermined relationship between detection output and frequency is monitored. In other words, the tuned or detuned state is determined, and the tuned frequency region in the so-called figure-8 detection output is set. When the tuning state is determined, a signal is sent to the stop signal generation circuit 37 to generate a stop signal for the frequency scanning operation.

FIG. 2 shows a basic feature of the present invention. The same parts as in FIG. 1 are given the same reference numerals and will be explained.
1 is converted into voltage and current by the emitter follower circuit configured by P1, and becomes a current flowing through the resistor Rs connected to the pin P1. The collector current of this transistor Q1 controls a current source 41. Current source 41 is connected between power supply Vcc and bin 30, and transistor Q1
Together, they form a current mirror circuit. Therefore, when current source 41 flows through transistor Ql,
From then on, current KI. is output. Here, since the variable resistor 31 is connected to the bottle 30, the K process is performed. ×RM
The voltage here increases by (RM; value of variable resistor 31). This increases the gain of the variable gain control amplifier 24, and as the input signal strength decreases, the detection output at the output terminal 25 increases. On the other hand, when the input signal level (electric field strength) is low, the voltage of bin P1 is Ov, and -
The voltage of 0 is fixed to the voltage of the reference voltage source 32. This means that the amount of meat has been set.

According to the above configuration, the mute sensitivity is always set to the voltage of the reference voltage source 32 (the mute amount setting value).
M is set, and its slope can be set by the value of resistor 3. FIG. 3 shows the relationship between the 5 voltage VM of the bottle 30 and the mute lending. In this circuit, the change in the mute amount is suppressed with respect to the change in the voltage VM.
We are considering the case of dB/1v.

FIGS. 4(a) and 4(b) show other examples of the above-mentioned circuits with the 1st line. In FIG. 4(,), a series circuit of resistors R1 and R2 is connected between a reference voltage and ground, and sets a voltage VM for the amount of muting. Further, the resistor R1'3 connected between the connection point of the resistors R1°R2 and the 2-pin 30 is the resistor for setting the mute sensitivity described above.

Furthermore, the circuit shown in FIG. 4(b) is an example in which the resistor R2 is used to also function as a mute sensitivity setting, and the resistor R3 mentioned above is omitted.

〔Effect of the invention〕

As described above, according to the present invention, the number of external bins for setting the mute amount and mute sensitivity can be reduced to one. Thereby, the mute sensitivity gradually changes the gain of the gain control amplifier over a range of about ]V or more based on the reference bias applied to this external bin. As a result, the characteristics of the mute amount and the characteristics of the mute sensitivity continue smoothly, and there is no steep change in the mute operation.

It is easy to adjust the sensitivity and mute amount by operating one resistor.Furthermore, current conversion is performed by using a resistor connected outside the integrated circuit. Therefore, it is possible to easily deal with variations in characteristics within the integrated circuit.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a circuit diagram showing the main part of the invention, and Fig. 3 shows the relationship between the control voltage and the muting amount of the variable gain control amplifier shown in Fig. 2. FIG. 4A and FIG. 4B are diagrams each showing an example of resistor connection of the main parts of the present invention, and FIG. . 2 No... FM intermediate frequency amplifier, 22... Peak detection circuit, 24... Variable gain control amplifier, 26... Level detection circuit, 27... Meter drive circuit, 28... Mute drive circuit, 3o...external pin, 3no...variable resistor, 32...reference voltage source. Applicant's agent Patent attorney Takehiko Suzue No. 3 Figure 4 (a) (b) Figure 5

Claims (3)

[Claims] (1) An F'M intermediate frequency amplifier that amplifies the FMM inter-frequency signal, a detection circuit to which the output of this amplifier is applied, and amplification of the detection output of this detection circuit according to the control voltage applied to the gain control terminal. a variable gain control amplifier; and the F M
A level detection circuit that detects the level of the output signal of the intermediate frequency amplifier to obtain a DC detection voltage, and a first resistor that converts the output DC detection voltage of the level detection circuit into a current and is connected to the first bin of the integrated circuit. a voltage-to-current conversion means for causing a current to flow through the integrated circuit; and a mute drive circuit in which a current source within the integrated circuit is controlled by the current flowing through the first resistor, and the current source from the current source is applied to a second pin of the integrated circuit; a second resistor connected to the second pin as an external component and biased by the reference voltage source; an FM signal processing circuit comprising means for applying a voltage generated by a current flowing through the resistor to the gain control terminal for use in mute sensitivity for gradually changing the gain of the variable gain control amplifier. (2) The FM signal processing circuit according to claim 1, wherein the voltage-current conversion means is a meter 1 oscillation circuit that uses the DC detection voltage from the level detection circuit as a meter voltage. (3) The FM signal processing circuit according to claim 1, wherein the reference voltage source is a resistor network to which an output of a stabilized power supply circuit within the integrated circuit is applied.