JPS6027446B2 - Quadruple FM detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a quadrature type FM that uses a two-terminal resonator to reduce the distortion rate in the demodulation band and of the demodulated signal.
Regarding detection circuits.

Normally, a quadrature type FM detection circuit is used as the final stage of an FM intermediate frequency amplifier, as shown in Figure 1.
The output of the J Mitsutta multiplier 1 is used as one input signal (reference signal) of the phase comparator 2, and the output signal of the yielding Mitsuta multiplier 1 is converted to FM by the tank circuit 3 and phase shifter 4.
A signal whose phase is shifted approximately 90 degrees at the center frequency of the intermediate frequency is used as the other input signal of the phase comparator 2, and the phase comparator 2 outputs a signal corresponding to the amount of change in the phase difference between the two input signals. After converting the pulse train signal into a pulse train signal in which pulses vary, the pulse train signal is passed through an integrating circuit (not shown) and its average value is obtained as an FM demodulated signal.

Nowadays, limiter multipliers and phase comparators are integrated into ICs.
A tank circuit and a phase shifter are attached externally to the IC, and some use a three-terminal ceramic resonator as the tank circuit.

However, recently, ICs with built-in phase shifters have appeared. Therefore, if a ceramic resonator is to be used as the tank circuit of this IC, a conventional three-terminal type ceramic resonator cannot be used, and a two-terminal type ceramic resonator must be used. In addition, since the phase shifter built into the above-mentioned integrated circuit uses a capacitor, it can be used as a tank circuit to adjust the temperature from zero to -90 degrees depending on the frequency deviation of the applied signal.
In this case, a two-terminal resonator, which can be made into an unadjusted tank circuit, is used as the tank circuit, and the phase characteristics of the resonator with respect to the frequency range from zero to zero. When using the phase change characteristic of -90 degrees, as is clear from the impedance characteristics and phase characteristics of the two-terminal resonator shown in Figure 2, with two-terminal resonance alone, the resonant frequency 8 at both ends of the resonator In the frequency band on the low side or the high side of the anti-resonant frequency wave number, that is, the frequency band where the phase change is from 0 degrees to minus 90 degrees, the frequency band where the phase change has good linearity with respect to frequency is narrow, and therefore two terminals are used. The quadrature type FM detection circuit which conveniently uses a shaped resonator has the disadvantage that the demodulation X-band and the 1% distortion band are narrow. The present invention is a quadrature type FM using a capacitor as a phase shifter and a two-terminal resonator as a tank circuit.
This was made in order to eliminate the above-mentioned drawbacks in the detection circuit, and by connecting an inductance element in parallel to the two-terminal resonator that constitutes the tank circuit, a frequency lower than the resonant frequency of the two-terminal resonator is detected. By creating an anti-resonance point in the band or high frequency band and correcting the impedance characteristics of the two-terminal resonator, the slope of the phase change from zero to minus 90 degrees in the phase change characteristic with respect to frequency is made gentler, and demodulation is performed. It is an object of the present invention to provide a quadrature type FM detection circuit which has a wide band range and a low distortion rate of a demodulated signal.

The present invention will be described in detail below with reference to the drawings showing embodiments of the invention.

In the tank circuit according to the present invention, as shown in FIG.
The equivalent circuit of the tank circuit is:
As shown in FIG. 4, when the mechanical vibration of the two-terminal resonator 10 is replaced with an electric circuit, the two-terminal resonator 10 is connected to a circuit in which an equivalent mass L, an equivalent compliance C, and an equivalent resistance R are connected in series. It has an equivalent circuit 10' of a two-terminal resonator 10 in which ten capacitances Co are connected in parallel, and an equivalent circuit in which an inductance of a coil 11 is connected in parallel.

As is clear from the equivalent circuit shown in FIG. As shown in FIG. 5, the resonance point Pr occurs at the frequency fr, and as the frequency increases, the influence of the capacitance Co appears, and the parallel resonance of the L, C, and Co occurs at the frequency. When this occurs, the impedance between the electrodes 10a and 10b becomes high, and an anti-resonance point Pa occurs.In addition, the addition of the coil 11 increases the parallel resonance between the inductance Lo of the coil 11 and the capacitance Co. Therefore, the above inductance Lo
If the value of is set to an appropriate value, an anti-resonance point P'a will newly occur at a frequency f'r in a frequency band lower than the resonance frequency h.

If a new anti-resonance point P'a is formed in the two-terminal resonator 10 as described above and its impedance characteristics are corrected,
As shown in FIG. 5, in the range of zero degrees to minus 90 degrees in the phase characteristics of the two-terminal resonator 10, the phase characteristics become less harsh and the linearity of the nuclear phase characteristics is improved. I understand.

The frequency band used in the embodiment is between the frequency 8'r and the frequency fm at which the phase shift exhibits the maximum negative value. Therefore,
The tank circuit shown in FIG.
If connected to the M detection circuit, the distortion rate can be lowered over a wide frequency band and the demodulation band can also be expanded.

As can be seen from the above, the quadrature type FM detection circuit according to the present invention utilizes the capacitance (C property) of the two-terminal resonator 10.

The two-terminal resonator 10 alone cannot be used because the linearity of the phase characteristic of the C-type portion is poor.

Therefore, as described above, the coil 11 is connected in parallel to the two-terminal resonator 10 to create a new anti-resonance point Pa, thereby improving the linearity of the phase characteristic of the C-type portion.
Next, using a specific circuit example, a description will be given of how the distortion rate of the demodulated signal decreases when the demodulation band is expanded.

The tank circuit shown in FIG. 3 is assumed to have an inductance peak of the coil 11 = 5.6 days, and as shown in FIG. Combined with FM intermediate frequency 10.7MH
If the FM detection output characteristics and distortion rate characteristics at the time of z are measured, the maximum frequency deviation of the input signal 22 as shown in FIG.
．． FM detection output characteristics 40' and 40 and distortion rate characteristics 41' and 41 are obtained corresponding to the song level and 7 song Hz, respectively.

On the other hand, for comparison, the conventional quadrature method F
If the FM detection output characteristics and distortion rate characteristics of the M detection circuit are measured, FM detection output characteristics 42 and distortion rate characteristics 43 shown by solid lines and dotted lines in FIG. 8 are obtained.

As is clear from the above measurement results, in the case of the quadruple layer FM detection circuit according to the present invention, the frequency band range at which the output level is lower than the maximum value of the detection output is when the detection output characteristics are 40 and 40'. Tomo, 10. It has a demodulation phantom B band of from 10.92M to 10.92M, that is, 520K, whereas the conventional one without coil 11 has a range of 10.6 to 10. Up to the handling ratio, it only has demodulation of about 26Hz.

Such broadband characteristics could not be obtained with any conventional quadrature FM detection circuit. Also, even when compared in the distortion rate band of 1%, in the case of the present invention, the maximum frequency deviation of the input signal is, for example, 7 arcs, which is 10.63.
From M to 10.78 M, it has a distortion rate of 1% band of about 130KHz, whereas the conventional one has a distortion rate of 1%.
．． From 73MHz to 10.77M, it only has a distortion rate of 1% band of about 4 days.

Therefore, according to the present invention, compared to the conventional one, the demodulation It can be seen that it has been enlarged approximately 4 times.

As is clear from the detailed explanation above, is the present invention a quadrature system? An inductance element is connected in parallel to the two-terminal resonator used in the tank circuit of the M detection circuit, and an anti-resonance point is newly formed in a frequency band lower or higher than the resonance frequency of the two-terminal resonator. Since the impedance characteristics are corrected, the linearity of the phase change in the region of -90 degrees from zero in the phase characteristics of the two-terminal resonator is improved, and its slope is also made gentler, which reduces the demodulation illusion. The middle B band and the 1% distortion band are respectively expanded, and the FM signal can be stably demodulated.

In the above embodiment, an example using an energy-trapped two-terminal resonator utilizing thickness vibration has been described, but in the present invention, other two-terminal resonators having similar characteristics may be used. You can also do it.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a quadrature FM detection circuit, Fig. 2 is a diagram of phase characteristics and impedance characteristics of a two-terminal resonator, Fig. 3 is a tank circuit diagram according to the present invention, and Fig. 4 is a diagram of a 3-terminal resonator. 5 is a phase characteristic and impedance characteristic diagram of the tank circuit in FIG. 3, FIG. 6 is a circuit diagram of an embodiment of the present invention, and FIG. 7 is a distortion factor of the circuit in FIG. 6. and a detection output level characteristic diagram. FIG. 8 is a distortion factor and detection output level characteristic diagram of a conventional quadrature type FM detection circuit. lo. ．． ．． ．． ．． ．． 2-terminal resonator, 11... Coil. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8

Claims (1)

[Claims] 1. The output signal of the limiter amplifier is used as one input signal of the phase comparator, and the frequency deviation of the output signal is controlled by a tank circuit consisting of a two-terminal ceramic resonator and a phase shifter using a capacitor. The phase is set to 90
The signal shifted by ±Δφ by degrees is used as the other input signal of the phase comparator, and the FM detection signal is obtained by detecting a change in the phase difference between the signals applied to both input terminals of the phase comparator. An inductance element is connected in parallel to a two-terminal ceramic resonator, and an anti-resonance point is formed in a frequency range lower than the resonant frequency of the two-terminal ceramic resonator.
Quadruple layer method F characterized in that the slope of the phase change of the resonator from zero to minus 90 degrees is gentle.
M detection circuit.