JPH0752811B2 - Quadrature detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a quadrature detector as an FM detector in an FM receiver.

The quadrature detector is an intermediate frequency (IF) signal that is multiplied by a signal that has passed through a phase shifter whose output phase changes with respect to the input phase when the frequency changes, and then the intermediate frequency. The audio signal, that is, the FM detection output is obtained by attenuating the frequency component of (10.7MHz) or more, but the multiplier can be generally configured by the double balance type differential amplifier circuit, and the external circuit is phase-shifted. It is suitable for IC as it requires only a container.

Conventionally, there has been a detector of this type as shown in FIG. In the figure, 1 is a phase shifter, which is composed of inductance coils L ₁ and L ₂ , a resistor R ₁ and a capacitor C ₁ , and R ₁ and
C ₁ and L _{2 form} a simple single tuning circuit. Reference numeral 2 is a differential amplifier connected to the input side of the phase shifter 1 to drive it, and includes transistors Q _{1 to} Q ₃ , resistors R ₂ and R ₃ and a capacitor.
Consist C _2, to its input terminal Tin FM intermediate-frequency signal is input. The phase shifter 1 including the differential amplifier 2 is represented by an equivalent circuit as shown in FIG. 2. As shown in FIG. 3, at the intermediate frequency f ₀ of the signal to the input, 90 It causes a phase shift of ° and works to change the amount of phase shift by deviating from f ₀ .

3 is a multiplier, which includes transistors Q _{4 to} Q ₁₀ and resistors R ₄ to
The output signal of the phase shifter 1 applied to the base of the transistor Q ₄ and the transistor Q ₈ are constituted by a double-balanced differential amplifier circuit composed of R ₇ and constant voltage sources V _B1 and V _B2.
The FM signal is converted into an audio signal by multiplying it with the IF signal applied to the base of. Reference numeral 4 denotes an amplifier that amplifies the output signal of the multiplier 3 and outputs the amplified signal to the output terminal T _OUT , which includes an operational amplifier A and resistors R ₈ and R ₉ .

As shown in FIG. 1, as a phase shifter ₁ , a conventional quadrature detector using an inductance coil L ₁ and a single tuning circuit (R ₁ , C ₁ , L ₂ ) is used as shown in FIG. It had the drawbacks of a narrow output linear range and poor distortion characteristics. In order to eliminate this drawback, the phase shifter may be double-tuned in order to make the frequency-amplitude characteristic wideband, but this causes another problem that the adjustment becomes complicated.

The present invention has been made to solve the above-mentioned conventional problems, and is obtained by detecting the AM component generated by the amplitude characteristic of the single tuning circuit included in the phase shifter, and by this AM detection.
It is an object of the present invention to provide a quadrature detector in which the detection output characteristic and the distortion characteristic are improved by controlling the gain of the multiplier to increase according to the decrease in the AM detection output.

The present invention will be described below with reference to FIG. 5 and subsequent figures, but the same parts as those of the conventional example are designated by the same reference numerals.

FIG. 5 is a block diagram showing the principle of the quadrature detector according to the present invention. In the figure, the FMIF signal is input to the phase shifter 1 and the multiplier 3. The output of the phase shifter 1 is connected to one input of the multiplier 3 and the AM detector 5.
The output of the AM detector 5 is connected to the control input of the variable current source 7 via the inverting circuit 6. When a double-balanced differential amplifier circuit is used as the multiplier 3, the block 7 can be incorporated in the multiplier 3 by making the constant current source of the differential amplifier circuit variable. .

Since the single tuning circuit used in the phase shifter 1 has an amplitude characteristic, when the output thereof is AM detected by the AM detector 5, the detection characteristic as shown in FIG. 6 is obtained. Looking at this characteristic, the AM detection output drops when detuned to the + and-sides. Therefore, if this AM detection output is used to control the gain of the multiplier 3 after being inverted in the inverting circuit 6, the lowered detection output is corrected, and as shown in FIG. The linear range is expanded and the distortion characteristic is greatly improved (in FIG. 7, the dotted line indicates the conventional characteristic and the solid line indicates the characteristic of the present invention).

FIG. 8 shows a concrete circuit example of the quadrature detector constructed based on the principle shown in FIG. The circuit shown is
The conventional circuit shown in FIG. 1 has a transistor Q ₁₁ and a resistor R ₁₀
And the voltage composed of transistor ₁₂ and resistor R ₁₁ −
Current conversion circuit, transistors Q ₁₃ , Q ₁₄ and transistor
Switching circuit, the transistors Q ₁₇ to Q ₁₉ made from each of Q _15, Q _16, a limiter circuit composed of a resistor R ₁₂ to R _15, and diode D _1, is obtained by adding a current mirror circuit consisting of transistors Q _20. The signal that has passed through the phase shifter 1 is converted into a current signal in the transistor Q ₁₁ , and then the transistors Q ₁₃ and Q ₁₄ and the transistor Q ₁₁ are connected.
It is switched by respectively switching circuits of Q _15, Q _16. The switched signal is applied to a limiter circuit composed of transistors Q ₁₇ , Q _18, etc., and the AM component is synchronously detected by the above switching. AM detection output is transistor Q of current mirror circuit
_It is applied to the collector of the current source transistor Q ₁₀ of the double-balanced differential amplifier circuit via ₂₀ to control the current source.
By controlling this current source, the gain of the multiplier is increased according to the decrease of the AM detection output. Therefore, the distortion characteristic can be improved by linearizing the detection output characteristic for the input signal deviating to the ± side from the center frequency f ₀ . An FM detection output signal is obtained at the output of the amplifier 4.

In the circuit shown in FIG. 8, a synchronous detection system is used as the AM detection circuit, but other detection systems such as diode detection may be used instead.

Also, the circuit that controls the current source by the AM detection output is the 8th
Any configuration other than that shown in the figure can be adopted, and the point is that it is possible to control the current source in the direction of correcting the distortion by the AM detection output.

As described above, according to the present invention, the AM component generated by the amplitude characteristic of the single tuning circuit included in the phase shifter is detected,
By controlling the gain of the multiplier to increase according to the decrease in the AM detection output obtained by AM detection, the gain of the multiplier increases at the part of the output of the phase shifter that decreases due to the amplitude characteristics of the phase shifter. Therefore, the linearity of the detection characteristic is improved by using a single tuning circuit that only needs to adjust one coil without using double tuning, which is troublesome to adjust.
In addition, it is possible to obtain an FM detector with improved distortion characteristics.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional quadrature detector, FIG. 2 is an equivalent circuit of the phase shifter in FIG. 1, and FIG. 3 is an input / output characteristic of the phase shifter in FIG. The graph shown in Fig. 4 is the first
5 is a graph showing the detection characteristic of the detector shown in FIG. 5, FIG. 5 is a block diagram showing the detector according to the present invention, FIG. 6 is a graph showing the detection characteristic of the AM detection circuit in FIG. 5, and FIG. 7 is the present invention. FIG. 8 is a graph showing the characteristics of the detector according to the present invention in comparison with those of the conventional example, and FIG. 8 is a circuit diagram showing a concrete circuit example of the detector according to the present invention. 1 ... Phase shifter, 3 ... Multiplier, 5 ... AM detection circuit, 8 ...
… Variable current source.

Claims (1)

[Claims] 1. A quadrature in which an FM intermediate frequency signal obtained at the output of a phase shifter having a single tuning circuit and an original intermediate frequency signal are added to a multiplier, and an FM detection output signal is obtained at the output thereof. A detector, which detects an AM component generated by the amplitude characteristic of the single tuning circuit, and increases the gain of the multiplier according to a decrease in the AM detection output obtained by the AM detection unit. And a gain control means for controlling the quadrature detector.