JP3234479B2 - FM detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an F-type filter suitable for IC.
The present invention relates to an M detection circuit, and more particularly, to an FM detection circuit having improved S / N.

[0002]

2. Description of the Related Art Conventionally, a phase shift circuit for shifting the phase of an IF signal serving as an input signal, and a multiplier for multiplying the IF signal and an IF signal having a phase shifted by the phase shift circuit are provided. There is known a quadrature type FM detection circuit for detecting an audio signal. In such an FM detection circuit, the phase shift characteristic of the phase shift circuit for shifting the phase of the IF signal is shifted by 90 degrees at the center frequency (455 KHz) of the IF signal, and the phase difference between the two input signals of the multiplier is determined. The detection is carried out by using.

Meanwhile, there is a phase shift circuit using an active filter for facilitating IC integration. An active filter includes an amplifier such as an operational amplifier and a capacitor, and a filter characteristic is obtained from a time constant set by the conductance of the amplifier and the capacitance of the capacitor. When an active filter is used, the number of external elements can be reduced, so that the FM detection circuit can be integrated into an IC.

[0004]

However, since an amplifier is used for the active filter, noise is generated from the differentially connected transistors constituting the first-stage amplifier of the operational amplifier, and noise is generated in the output signal of the phase shift circuit. included. The noise is directly generated at the output terminal of the multiplier, and there is a problem that the S / N of an audio signal obtained by detection is deteriorated.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and comprises a phase shift circuit comprising an active element for shifting the phase of an input signal, and an input signal and an output of the phase shift circuit. A FM detection circuit comprising a multiplication circuit for multiplying the input signal and an input terminal to which the input signal is applied and an amplifier for amplifying the input signal. I do.

[0006] A filter is provided between the phase shift circuit and the multiplier, and is provided with a filter for passing only a signal in a predetermined pass band. Further, the filter is a passive filter composed of passive elements.

[0007]

FIG. 1 is a diagram showing an embodiment of the present invention. (1) is an input terminal, (2) is an amplifier for amplifying an input signal, and (3) is a phase of an output signal of the amplifier. An active filter circuit serving as a phase shift circuit for shifting
(4) is a passive filter circuit that passes a signal of a predetermined band, (5) is a multiplier that multiplies an output signal of the filter circuit by an input signal, and (6) is an output terminal.

In FIG. 1, I is applied from a previous stage circuit.
After being amplified by the amplifier (2), the F signal is phase-shifted by the active filter circuit (3) using, for example, an amplifier and a capacitor. The active filter circuit (3)
Has a characteristic that the phase of an IF signal is shifted by 90 degrees at a center frequency (for example, 455 KHz). The output signal of the active filter circuit (3) is applied to the filter circuit (4), and the cutoff frequency of the filter circuit (4) is equal to the IF.
Since the signal is set near the center frequency of the signal, a signal of only a predetermined band of the output signal is filtered by the filter circuit (4).
Pass through. The output signal of the filter circuit (4) is applied to the multiplier (5), and is multiplied by the IF signal applied to the other input terminal of the multiplier (5). Since the phase difference between the two input signals of the multiplier (5) is a phase difference corresponding to the frequency of the IF signal, an output signal having a level corresponding to the phase difference is generated as a result of the multiplication. The output signal of the multiplier (5) is applied to the subsequent circuit as an output signal of the FM detection circuit.

In FIG. 1, an IF signal is amplified by an amplifier (2) and then applied to an active filter circuit (3). Here, NF (noise figure) of the amplifier (2)
Is set low. The amplifier (2) and the amplifier constituting the active filter are connected in multiple stages. The NF of the multi-stage connected amplifier is F
Therefore, the total NF of the amplifier (2) and the amplifier of the active filter circuit (3) is determined by the NF of the amplifier (2). Therefore, the noise in the output signal of the active filter circuit (3) is small.

The output signal of the active filter circuit (3) is applied to the filter circuit (4). The filter circuit (4) has a band for blocking signals other than the frequency band of the IF signal. Therefore, noise existing outside the IF signal band can be removed, and the S / N of the audio signal generated from the multiplier (5) can be improved.

[0011]

FIG. 2 is a diagram showing an embodiment of the present invention.
(7) and (8) are differentially connected transistors,
(9) is a load resistor connected to the collector of the transistor (8), (10) is an operational amplifier, (11) and (12)
Denotes first and second input resistors connected to the positive input terminal of the operational amplifier (10), and (13) denotes first and second input resistors (1).
A capacitor connected between the connection point of 1) and (12) and the output terminal of the operational amplifier (10); (14) a capacitor having one end connected to the positive input terminal of the operational amplifier (10); (16) denotes a resistor and a capacitor constituting a passive type high-pass filter.

In FIG. 2, an IF signal is applied to the base of a transistor (7) and differentially amplified. The collector current of the transistor (8) changes according to the amplitude of the IF signal, and an output signal is generated from one end of the load resistor (9).
The output signal includes first and second input resistors (11) and (1).
2) is applied to the positive input terminal of the operational amplifier (10). FIG. 3 shows the phase characteristics of the active filter circuit (3).
(A), the phase of the IF signal is shifted according to the frequency of the IF signal. Active filter circuit (3)
Is applied to the filter circuit (4). Since the filter circuit (4) has a high-pass filter characteristic,
The signal on the lower frequency side than the IF signal is cut off. The output signal of the filter circuit (4) is multiplied by the IF signal to form a multiplier (5).
An audio signal detected at the output terminal of the above is obtained.

Here, if a transistor having a low NF is used as the transistors (7) and (8), the N
F can be reduced. In a multi-stage connected amplifier, the overall NF can be determined by the NF of the first stage amplifier, so that the noise of the output signal of the active filter circuit (4) can be reduced. Further, since the active filter circuit (3) has a low-pass filter characteristic as shown by the solid line (a) in FIG. 4 (b), the IF signal is transmitted to the filter circuit (4) and the frequency of the IF signal is lower than the IF signal. Noise is also transmitted to the filter circuit (4). Therefore, high frequency noise is cut off by the active filter circuit (3). The filter circuit (4) has a high-pass filter characteristic as shown by a dotted line (b) in FIG. 4 (b), the IF signal passes through the filter circuit (4), and noise on the lower frequency side than the IF signal is filtered. ). Therefore, the multiplier (5)
The audio signal band generated from the audio signal does not include the noise generated from the active filter circuit (4), and the S / N of the audio signal can be improved.

Here, the filter circuit (4) is connected to the resistor (1).
By using the passive elements 5) and the capacitor (16), the circuit configuration can be simplified, and the generation of new noise can be suppressed. Further, even if the active filter circuit (3) is configured to have a high-pass filter characteristic and the filter circuit (4) is configured to have a low-pass filter characteristic, noise in the detection output signal can be reduced.

[0015]

As described above, according to the present invention, an amplifier having a low noise figure is connected before the phase shift circuit, so that the phase shift characteristics of the phase shift circuit are not degraded.
Noise in the output signal of the phase shift circuit can be reduced. Further, since a filter circuit for cutting off unnecessary signals such as noise is connected to the subsequent stage of the phase shift circuit, noise in the audio signal band can be removed, and the S / N of the audio signal obtained by detection can be improved. can do.

Further, since the filter circuit is constituted by passive elements, generation of new noise can be suppressed and the circuit configuration can be simplified.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing one embodiment of the present invention.

FIG. 3 is a characteristic diagram for explaining the present invention.

[Explanation of symbols]

 1 input terminal 2 amplifier 3 active filter circuit 4 filter circuit 5 multiplier 6 output terminal

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-58-73209 (JP, A) JP-A-50-56140 (JP, A) JP-A-6-216652 (JP, A) JP-A-58-73 201408 (JP, A) JP-A 1-129910 (JP, U) JP-A 56-43916 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03D 3/00-3 / 06

Claims (3)

(57) [Claims] 1. An active filter including a first amplifier.
An FM detection circuit comprising: a phase shift circuit configured to shift a phase of an input signal; and a multiplication circuit configured to multiply the input signal by an output signal of the phase shift circuit. And a second amplifier connected between the phase shift circuit and amplifying the input signal , wherein the second amplifier has a low noise figure,
FM connected in multiple stages with the first amplifier
Detection circuit. 2. An FM detection circuit according to claim 1, further comprising a filter connected between said phase shift circuit and said multiplier, for passing only a signal in a predetermined pass band. 3. The FM detection circuit according to claim 2, wherein said filter is a passive filter composed of passive elements.