JPS6172417A - Fm noise level detection circuit - Google Patents

Abstract

PURPOSE:To detect the noise level of an FM reception signal with less number of elements by extracting a signal having mainly white noise from an FM detection output and converting the signal into an AMIF signal. CONSTITUTION:The signal consisting mainly of white noise is extracted from the FM detection output of an FM detector by using an HPF102. Then a frequency converting circuit 101 adds a signal from an oscillator 103 to the noise component extracted by the HPF102 to obtain an AMIF signal. The converted AMIF signal is amplified by an AMIF amplifier 104, then rectified by an AM detection circuit 105 and a rectified output is extracted from an output terminal E. Thus, the noise level of the FM reception signal is detected by the rectified output obtained at an output terminal E with less number of elements.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides an AM/
The present invention relates to an FM receiver, and particularly to an F'M noise level detection circuit.

[Conventional technology]

Conventionally, as a detection method for FM electric field strength, for example, as shown in FIG.
．． After rectification at step 9, these are combined at a synthesis circuit 10, and detected by the DC level at terminal C.

[Problem that the invention seeks to solve]

However, this method has the disadvantage that the number of elements required for FM electric field strength detection accounts for about half of the number of elements required for the intermediate frequency processing circuit, resulting in an unavoidable increase in cost.

Therefore, an object of the present invention is to provide an FM noise level detection circuit that can significantly reduce the number of elements and has a wide range of applications (i.e., can detect FM electric field strength).

[Means for solving problems]

The present invention provides an FM detection output terminal (FM detector 5 in Fig. 8).
Among the noise components included in the FM detection output of output terminal B), the level of noise components (mainly white noise) of approximately 100 (IQ(z) or higher) corresponds to the electric field strength of the received M signal, In other words, when there is a strong electric field, the amplitude and noise component of the noise component are suppressed by the limiter action of the FM receiver and become smaller, and when the electric field is weaker, the limiter action becomes smaller and the noise component becomes thicker (about 1
This makes it possible to detect the electric field strength of the FM received signal by detecting the level of noise components of 0.00 KHz or higher.

In order to extract this noise component, the present invention is equipped with a bypass filter and a frequency conversion circuit. The bypass filter extracts a signal mainly consisting of white noise from the FM detection output, and the extracted signal is converted into an AM signal by the frequency conversion circuit.
After converting into an intermediate frequency signal, the signal is amplified and detected using an existing AM intermediate frequency amplifier and an AM detector, respectively.

〔Example〕

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

FIG. 1 is a block diagram of the main parts of an AM/FM receiver equipped with an embodiment of the noise level detection circuit of the present invention.

The same reference numerals as in FIG. 8 indicate the same components. The bypass filter 102 is composed of a resistor and a capacitor, and removes an audio signal component from the FM detection output of the FM detection output terminal B, and extracts a noise component of about 100 (■2) or more. Oscillator 108 oscillates at a predetermined oscillation frequency. A frequency conversion circuit 101 applies a signal from an oscillator 108 to the noise component extracted by a bypass filter 102 to obtain an AM intermediate frequency signal. Here, the AM intermediate frequency is set to, for example, 450 (
kHz), and the oscillation frequency of the oscillator 10B is, for example, 3 KHz.
00 (KI(z)), in a 4M signal processing circuit, Hinoki is 450 (KHz) - 300 (KHz) kq
A noise component around the frequency of 150 (KHz) is selected. The noise component frequency-converted in this way is applied to the input terminal of the AM intermediate frequency processing circuit. Here, the AM intermediate frequency processing circuit usually contains approximately ±10
A frequency conversion circuit such as an intermediate frequency transformer and a ceramic filter is included to provide a gain band of (KHz). The signal applied to the input terminal is amplified by an AM intermediate frequency amplifier 104, rectified by an AM detection circuit 105,
A rectified output is taken out from output terminal E. Here, the oscillation circuit 108 may include an oscillator and an arbitrary stage of frequency dividing circuits.

From the rectified output obtained at output terminal E after 5K, FM
The noise level of the received signal, and therefore the electric field strength, can be detected.

FIG. 2 shows a specific circuit example of the bypass filter 102, frequency conversion circuit 101, and AM signal processing circuit in the block diagram of FIG.

Terminal B is the FM detection output terminal in FIG. A signal whose main component is white noise input from terminal B is passed through a bypass filter consisting of a capacitor C8 and a resistor R1, a constant current source IO1, and a transistor Q1゜Qz-Qs, Qa.
, a bias resistor island R8, a resistor R15e constituting a load circuit, a capacitor C8, and an amplifier circuit consisting of a coil L. Therefore, signal components having a predetermined frequency or higher are selectively output. Here, the present invention can be realized by simply using a resistive element as the load circuit consisting of the conductor C1, the coil L1, and the resistor RIS, but the characteristics of the system of the present invention can be improved in the next stage block.

The signal at terminal F is transmitted through transistor Q5. Qa + resistance R2
Level shifter and resistor R composed of ゜R,, R@, &
, *Transistor Qt * Qs * Qa * Ql
ce Qtt* Q+t+ Qt3e Ql is connected to the input terminal D of the AM intermediate frequency processing circuit (corresponding to the frequency conversion circuit 101K in FIG. 1) through a frequency conversion circuit (corresponding to the frequency conversion circuit 101K in FIG. supported). Here, terminals I and J are the other signal input terminals of the frequency conversion circuit (corresponding to the output terminals of oscillator 108 in FIG. 1). Transistor Qts* Qllll Ql? ? Q
aae QI*e Qn and resistance 1% + Rto
*The circuit block consisting of R11, Rtt, RIS, and R14 constitutes the frequency conversion circuit of the AM receiver.
K is an l input terminal, and L and M are terminals for applying oscillation voltage from the AM local oscillator. However, when receiving an FM broadcast, the AM frequency conversion circuit does not operate, and only the output of the frequency conversion circuit (frequency conversion circuit 101 '5? in FIG. 2) is output from the input terminal of the AM intermediate frequency processing circuit. Between the power supply terminal H and the AM intermediate processing circuit input terminal, an intermediate frequency selection circuit consisting of a capacitor F C3 and coils L, , L is connected, and from terminal G to the intermediate frequency selection circuit built in the AM intermediate frequency processing circuit. It leads to the frequency amplification circuit.

〔Effect of the invention〕

The present invention has the following advantages: 11) When the AM/FM f Yuna is configured with a semiconductor integrated circuit, the AM intermediate frequency processing circuit is also used during FM reception, so the elements required for the field strength detection circuit can be saved ( (2) F M
Using a pulse noise removal circuit after the detection stage, 4
When using an FM multiplex after the +31 FM detection stage, the output level of the FM noise level detection output terminal of the present invention facilitates DC control of the pulse noise removal circuit. The FM noise level detection output facilitates direct current control of a forced reduction circuit for stereo separation below medium electric fields, or a high frequency component removal circuit below small electric fields, which has recently become important as an accessory function of FM multiplex. It has the effect of

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the FM noise level detection circuit of the present invention, and FIG. 2 is a block diagram showing the bypass filter 102, frequency conversion circuit 101, and A in the block diagram of FIG.
FIG. 8 is a block diagram of a conventional example of an FM noise level detection circuit. 1.2, 8, 4: Amplifier, 5: F'M detector. 101: Frequency conversion circuit, 102: Bypass filter. 108: Oscillator. 104: AM intermediate frequency amplification circuit. 105: AM detection circuit. Percentage applicant NEC Corporation

Claims (1)

[Claims] In an AM/FM receiver including at least an FM intermediate frequency amplifier, an FM detector, an AM intermediate frequency amplifier, and an AM detector, FM transmitted through the FM intermediate frequency amplifier and detected by the FM detector Equipped with a bypass filter that extracts a signal mainly consisting of white noise from the detection output, and a frequency conversion circuit that applies a signal from an oscillator different from the AM reception system to the signal extracted by the bypass filter to obtain a frequency conversion signal of an AM intermediate frequency. . An FM noise level detection circuit, characterized in that the frequency-converted signal is amplified by an AM intermediate frequency amplifier and detected by an AM detector to detect the noise level of the FM received signal.