JPS6149846B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase-shift synergistic FM demodulator.

FIG. 1 is a circuit diagram showing a conventional phase-shift synergistic FM demodulator. In FIG. 1, 1 is an FM intermediate frequency signal input terminal, and 2 is an amplifier for amplifying the FM intermediate frequency signal supplied to the input terminal 1. The amplifier has, for example, a limiter function that limits the amplitude of the intermediate frequency signal. A differential direct-coupled amplifier (hereinafter referred to as
IF amplifier) is used.

3 shows a phase-shift synergistic demodulator that demodulates the signal from the IF amplifier 2, and this demodulation circuit consists of a differential pair of transistors 311 and 312 that are switched by the FM intermediate frequency signal _S1 from the IF amplifier 2. A phase shift device consisting of a first switching circuit 31 and a resonant circuit 322 that converts the phase of the FM intermediate frequency signal S' ₁ (frequency) shifted by the phase shift element 321 that shifts the phase of the FM intermediate frequency signal. 32, and a second switching circuit consisting of a differential pair of transistors 331 and 332 connected in series to the differential transistor 311 of the first switching signal 31 and switching with the signal S' ₁ phase-converted by the phase shifter 32. It has a magnitude of 33. 4 is a low-pass filter, and 5 is a demodulation output terminal.

In the demodulator shown in FIG. 1, the first switching circuit 31 is driven by the FM intermediate frequency signal S ₁ supplied to its input terminal, and the first switching circuit 31 is driven by the signal S' ₁ phase-shifted by the phase shifter 32. By driving the second switching circuit 33 and conducting the constant current I ₀ of the constant current source circuit 351 only during the period when the input signal is present in the resistors 341 and 342 provided at the output stage of the second switching circuit 33, the resistors 341 and 342 are connected. 342 has the above two signals
A current signal with a pulse width corresponding to the phase difference between S ₁ and S' ₁ flows, and the resistances 341 and 34 generated by this signal flow.
By passing the voltage change signal No. 2 through the low-pass filter 4 and taking out its average value, a change in DC voltage according to the phase difference can be obtained, so that the input signal is
If it is FM modulated, it will be FM demodulated.

By the way, when the demodulator shown in Fig. 1 is applied to a receiver, in addition to the FM intermediate frequency, an amplitude modulated (hereinafter referred to as AM) broadcast signal with a different carrier frequency may be input to the input terminal 1 of the IF amplifier 2. be. This is especially true when receiving an FM signal near a medium wave band broadcasting station that is transmitting an AM signal.

Here, if the amplitude of the AM signal is large, the differential amplifier of the IF amplifier 2 has an amplitude suppression effect, so the AM component is sufficiently suppressed, and only the carrier wave, which has become a rectangular wave, is the difference between the phase shift synergistic demodulator 3. Since the signal is input to the first switching circuit 31 composed of a dynamic pair of transistors 311 and 312, the phase-shift synergistic demodulator 3 does not output the AM component. However, when the input level of the AM signal is very small, the differential amplifier 2 of the IF amplifier 2 has no amplitude suppression effect and operates as a linear amplifier, and the AM signal is input to the first switching circuit 31. Since the first switching circuit 31 operates as a differential amplifier for weak signals, the AM signal is amplified by the first switching circuit 31. This amplified AM signal is transferred to the resistor 34
3 and through the differential pair transistors 331 and 332 of the second switching circuit 33 to the resistor 34.
It flows to 1,342. The AM signal flowing to the resistor 343 is grounded by the resonance circuit 322 because the impedance of the resonance circuit 322 becomes extremely low with respect to the AM signal.
3 is not applied. Therefore, the transistor 31
The minute AM signal applied to the base of transistor 11 appears at the collector of transistor 332, which serves as a load for transistor 11. However, by extracting this AM signal through the low-pass filter 4, no AM component is demodulated at the output terminal 5.

However, the base input voltage 2 of the differential amplifier in Fig. 2
As shown in the two-pair collector current 21 relationship diagram, the differential pair transistor 311 of the first switching circuit 31,
312 and bias resistors 313, 314, etc., when the initial bias 23 is balanced, the initial bias 2
If it deviates from 1, the AM signal 24
is the transistor 3 of the first switching circuit 31
When input to the base of transistor 311,
312 base voltage versus collector current characteristic, that is, the second
From the curves 27 and 28 in the figure, the transistors 311 and 3
The 12 collector current waveforms become curves 25 and 26 in FIG. 2, respectively. Since the collector load of the transistor 311 is a differential amplifier composed of differential pair transistors 331 and 332, the transistor 33
The output waveform of 2 is proportional to that waveform. If this output is taken out through the low-pass filter 4, the output terminal 5
produces an AM demodulated output.

As described above, in the conventional circuit, if the initial biases of the base bias voltages of the differential pair transistors 311 and 312 deviate, an adverse phenomenon occurs in which the AM signal is demodulated during FM reception.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the disadvantage of AM demodulation during FM reception operation in the prior art described above, and to provide a stable phase-shift synergistic FM demodulator.

The present invention improves the AM demodulation effect when an AM signal is input to a phase-shift synergistic FM demodulator composed of a first switching circuit 31 and a second switching circuit 33, each of which is a differential pair of transistors. , a signal having a phase opposite to the interference AM signal inputted to the first switching circuit 31 is extracted, and a modulated signal obtained by demodulating the above signal is sent to the second switching circuit 33.
is fed back to the base of the switching circuit 31 to prevent the AM component caused by imbalance between the elements constituting the first switching circuit 31 from being demodulated.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 3 is a circuit diagram showing one embodiment of the present invention.

In FIG. 3, numeral 30 indicates a gate circuit whose gate opens and closes in response to the output signal _S1 from the IF amplifier 2.
The first switching circuit 31 includes a first switching circuit 31, and the second switching circuit 33 includes a differential pair of transistors 331 and 332. The common emitter of the differential pair transistors 311 and 312 is the constant current source circuit 3.
It is grounded through 51. transistor 3
The base of 11 is connected to input terminal 1 through amplifier 2, and is grounded through resistor 314 and bias power supply 315. The collector is connected to the common emitter of differential pair transistors 331 and 332.

The base of the transistor 312 is grounded through a resistor 313 and a bias power supply 315, and the collector is connected to the power supply terminal 7 through a resistor 343.

The common emitters of the transistors 311 and 312 are grounded through a constant current source circuit 351.
The base of the transistor 331 is grounded through a resistor 334 and a bias power supply 335. The base of the transistor 332 is grounded through a resistor 333 and a bias power supply 335. resistance 31
3, 314, 333, and 343 are bias resistances of the switching circuits 31 and 33. The resistor 334 also serves as a resonant resistor of the resonant circuit 322, and is set to a resistance value as small as possible within a range that allows the switching circuit 33 to perform stable switching operations. 341 and 342 are resistors, 32 is a phase shift coil 321 that shifts the phase of the FM intermediate frequency signal derived to the collector of the transistor 312, and a phase shift coil 321 that converts the phase of the phase shift signal (frequency) shifted by the phase shift coil. This is a phase shift device consisting of a resonant circuit 322 leading to a second switching circuit 33.

The resonant circuit 322 consists of a parallel circuit of a coil and a capacitor, one end of which is connected to the second switching circuit 3.
3, and the other end is grounded through a capacitor 324. The capacitor 324 prevents the AM signal flowing through the resistor 343 from flowing to the ground side, and the impedance is FM signal (carrier wave).
A capacitance value that has an impedance that is sufficiently small relative to the resonant resistance value of the resonant circuit 322 for the AM signal, and has an appropriate impedance that acts to prevent the AM signal from flowing into the ground side. selected. 4 indicates a low-pass filter connected to the collector of the transistor 332 of the second switching circuit 33;
The harmonic components generated at the collector of the switch, that is, the FM intermediate frequency signal S1 applied to the first and second switching circuits, its phase-shifted signal _S'1 , and _the high frequency components generated by switching are removed, and the high frequency components generated by switching are removed. 5, a demodulated signal (audio signal) is extracted.

Next, the operation of the demodulator will be briefly explained.

In the circuit shown in FIG. 3, when there is no signal, each transistor of the first and second switching circuits 31, 33 is in the ON state, and the transistors 311, 33 are in the ON state.
The transistors 312 are set so that DC currents having the same amplitude (1/2I ₀ ) flow through them, and the transistors 331 and 332 are set so that DC currents having the same amplitude (1/4I ₀ ) flow through the transistors 331 and 332, respectively.

Next, when an unmodulated FM intermediate frequency signal is supplied to input terminal 1, the input signal is transmitted to IF amplifier 2.
The signal is amplified and limited in amplification, and a rectangular waveform output signal _S1 is derived from the output terminal of the amplifier as shown in FIG. 6A. This signal S ₁ is supplied to the bases of the differential pair transistors 311 of the first switching circuit 31 . Then, the transistor 311 turns on and off every 1/2 period of the signal S ₁ , and in response to this transistor, the transistor 312 also turns on the signal S ₁ .
It turns on and off every 1/2 cycle.

That is, the transistors 311 and 312 are _1/1 of the signal S1.
It performs ON-OFF switching operation every two cycles and alternately. FIGS. 6B and 6C are waveform diagrams showing these switching operations. The signal S ₁ is also supplied to the phase shifter 32 through the transistor 312 of the first switching circuit 31 . Here, the phase shifter 32 is configured such that the signal S ₁ is at the resonant frequency of the resonant circuit 322.
₀ , the phase of the signal is approximately π/2 (90
゜) It is designed to shift. In the following description, in order to simplify the explanation, the phase shift shift caused by the phase shift device 32 is assumed to be π/2 (90 degrees). When the signal S ₁ is detuned from the resonance frequency ₀ of the resonance circuit 322, a phase shift occurs based on the frequency versus phase characteristic curve 322a of the resonance circuit 322 shown in FIG.

Therefore, when the signal S ₁ resonates in the resonant circuit 322 and has a resonant frequency of ₀ , the signal S ₁ supplied to the phase shifter 32 is phase-shifted by π/2 (90°) by the phase shifter, for example, the 6th As shown by the solid line waveform in Figure E, π/2 (90
゜) It is delayed. This slow phase signal S' ₁ is supplied to the bases of the differential pair transistors 331 of the second switching circuit 33. Then, the transistor 331 is turned ON and OFF every 1/2 period of the signal S' ₁ , and in response to this transistor, the transistor 332 is also turned ON and OFF every 1/2 period of the signal S' ₁ . That is, the transistors 331 and 332 alternately switch ON and OFF every 1/2 cycle of the signal _S'1 .

FIGS. 6E and 6F are waveform diagrams showing these switching operations.

In such a switching operation, the transistor 311 of the first switching circuit 31 is turned on.
Moreover, when the transistor 331 of the second switching circuit 33 is turned on, a current I ₃₄₁ flows through the resistor 341 in the 1/4 period region of the signal S ₁ as shown in FIG.
flows. The current I ₃₄₁ at this time is It is.

Further, the transistor 311 of the first switching circuit 31 is ON and the second switching circuit 332 is ON.
When the transistor 332 is turned on, a current I ₃₄₂ flows through the resistor 342 as shown in FIG. 6H, that is, in the 1/4 period region of the signal S ₁ . The current I ₃₄₂ at this time is It is.

Next, a modulated FM intermediate frequency signal is supplied to the input terminal 1, and the frequency of the input signal is shifted by the frequency △ from the resonance frequency ₀ of the resonant circuit 322 due to the modulation, and its phase is shifted by the dotted line in FIG. 6E. When the phase is shifted by θ as shown in FIG. 6, the transistors 311 and 312 of the first switching circuit 31 perform a switching operation with the phase shifted by the phase as shown by the dotted lines in FIG. 6E and F. Therefore, the regions of current flowing through the resistors 341 and 342 are as shown in FIGS. 6I and 6J. The current I ₃₄₁ flowing through the resistors 341 and 342 at this time,
I ₃₄₂ is becomes.

As can be seen from the above, the current flowing through the resistor 341 increases by I ₀ θ/2π due to the phase shift θ, and the current flowing through the resistor 342 decreases by I ₀ θ/2π due to the phase shift θ. do.

In other words, the average current flowing through the resistors 341 and 342 is determined by the phase shift θ, that is, the frequency shift △ from the resonance frequency ₀ of the resonant circuit 322, as seen from equations (3) and (4) above.
changes in proportion to.

By passing this changing current I ₀ θ/2π through a low-pass filter 4, a demodulated signal is obtained at an output terminal 5.

That is, in the circuit configuration shown in FIG. 3, the first switching circuit 31 is switched by the signal _S1 , and the second
By switching the switching circuit 33 with the signal _S'1 , the resistors 341 and 342 receive the signal _S1.
A current proportional to the phase difference between the signal S'1 and the signal _S'1 flows through the low-pass filter 4, so that the FM signal can be demodulated.

Next, to explain the case when a weak AM signal, which is a carrier wave different from the FM intermediate frequency, is input to input terminal 1 of IF amplifier 2, in this case, IF amplifier 2
has no amplitude suppression effect, and the AM signal is input to the base of the transistor 311 constituting the first switching circuit 31. As explained in the conventional circuit of FIG. 1, if there is unbalance in the first switching circuit 31,
AM demodulated. However, the second switching circuit 3
The collector of the transistor 312 is connected to the base of the transistor 331 constituting 3 through the coil 321, and the resonant circuit 322 and the capacitor 324
Since the transistor 31 is grounded through
Due to the impedance of the capacitor 324, the collector output of No. 2 is input to the base of the transistor 331 without being AC grounded as in the conventional circuit. Since this input signal is a negative feedback signal to the transistor 332, when the collector current flowing through the differential pair transistors 331 and 332 increases in proportion to the collector current of the transistor 311, the transistor 332
The collector current of the transistor 332 cannot increase and remains constant, and the increased collector current of the transistor 332 flows as the collector current of the transistor 331. Therefore, the collector output of the transistor 332 can only produce a constant AC output, and the output passed through the low-pass filter 4 is not subjected to AM demodulation.

FIG. 4 shows an embodiment in which the output is taken out as a differential pair output. Originally, when outputting a differential pair as shown in FIG. 4, AM demodulation is not performed even if the operating point of the first switching circuit 31 shifts as shown in FIG.

If the elements constituting the second switching circuit 33 are balanced, even when an AM signal is input to the base of the transistor 311 of the first switching circuit 31, the emitters are commonly connected to the collector of the transistor 311. dynamic pair transistor 3
The outputs of 31 and 332 are in phase and have the same amplitude. This output is passed through the low pass filters 4, 4' and the subtraction circuit 6.
If the AM demodulated output is taken out through the output terminal 5, no AM demodulated output will be generated at the output terminal 5. However, in reality, the resistor 341,
342, an imbalance occurs. In this case, the outputs of the differential pair transistors 331 and 332 do not have the same amplitude, so these two signals are subtracted by the subtraction circuit 6.
An AM demodulated output corresponding to the difference is generated at the output terminal 5 even through the . In this embodiment, as explained in the embodiment of FIG.
This method applies negative feedback using the AM demodulated signal. Here, the output amplitude of the transistor 331 is always set to be smaller than the output amplitude of the transistor 332. This setting method can be performed, for example, by setting the resistors 341 and 342 connected to the collectors of the transistors 331 and 332 so that the value of the resistor 341 is smaller than the value of the resistor 342. When the AM signal is input to the base of the transistor 311 in this state, the output of the transistor 312 is input to the base of the transistor 331. The output of the transistor 312 acts to equalize the output amplitudes of the transistors 331 and 332, so the low pass filter 4,
4', the signals passed through the subtraction circuit 6 are canceled out, and no AM demodulated signal is generated at the output terminal 5.

FIG. 5 shows measurement data of the effect of the present invention in the embodiment of FIG. 4. The horizontal axis is the capacity 32 in Figure 4.
4, the vertical axis is AM at a capacity value of 324
This represents the demodulated output. Here, on the vertical axis
OdB indicates the FM demodulation output at 100% FM modulation,
The AM carrier wave is 1 MHz, the modulation frequency is 400 Hz, the modulation depth is 30%, and the AM carrier input level at the input end of the intermediate frequency amplifier is 20 μV. The data in FIG. 5 shows that there is an appropriate capacitance value. This shows that as the capacitance value increases, that is, the degree of attenuation decreases when equivalent to the conventional circuit, and if the capacitance value is selected to an appropriate value in the circuit of the present invention, it can be greatly improved.

[Brief explanation of the drawing]

Figure 1 is a conventional circuit diagram that connects a differential direct-coupled FM intermediate frequency amplifier and a phase-shift synergistic detector, Figure 2 is a characteristic diagram showing the input-to-output characteristics of the differential amplifier, and Figure 3
The figure is a circuit diagram showing an embodiment in which a differential direct-coupled FM intermediate frequency amplifier and a phase-shift synergistic detector according to the present invention are connected, FIG. 4 is a circuit diagram showing another embodiment of the present invention, and FIG. , FIG. 6, and FIG. 7 are characteristic diagrams for explaining the present invention. 311, 312, 331, 332...transistor, 321...coil, 322...resonant circuit, 31,
32...Switching circuit, 4,4'...Low pass filter, 324...Capacitor, 6...Subtraction circuit.

Claims (1)

[Claims] 1. An FM signal input terminal, first and second differential transistors that switch based on the FM signal supplied to the input terminal, and a phase shifter that shifts the phase of the FM signal.
a gate circuit consisting of third and fourth differential transistors that switches in response to an FM signal and switches an output signal flowing to the first differential transistor; and an FM signal supplied to the operating voltage source of the gate circuit and the input terminal. A circuit that guides a signal to the first and second differential transistors, a phase shift element that shifts the phase of the FM signal passed through the second differential transistor, and a circuit that resonates with the FM signal phase-shifted by the phase shift element. , the resonance
A phase shifter comprising a resonant circuit that converts the phase of the frequency of the FM signal and guides the phase-converted signal to the third and fourth differential transistors of the gate circuit, and extracts the gate output signal appearing at the output stage of the gate circuit. and a low pass filter for passing one end of the resonant circuit through the phase shift element to the output electrode (collector) of the second differential transistor of the gate circuit and the input electrode of the third differential transistor of the gate circuit. (base), and the other end is grounded through a capacitor.The above capacitor has an impedance smaller than the resonant resistance value of the above resonant circuit for the carrier wave of the FM signal, and for the AM signal, the ground side of the AM signal. A phase shift multiplier type FM demodulator characterized in that a capacitance value is selected to have an impedance large enough to prevent inflow into the phase synergistic FM demodulator. 2 FM signal input terminal, first and second differential transistors that switch according to the FM signal supplied to the input terminal, and a phase shifter that shifts the phase of the FM signal.
a gate circuit consisting of third and fourth differential transistors that switches in response to an FM signal and switches an output signal flowing to the first differential transistor; and an FM signal supplied to the operating voltage source of the gate circuit and the input terminal. A circuit that guides a signal to the first and second differential transistors, a phase shift element that shifts the phase of the FM signal that has passed through the second transistor, and a circuit that resonates with the FM signal phase-shifted by the phase shift element, and resonance FM
a phase shifting device comprising a resonant circuit that converts the phase of a signal frequency and guides the phase-converted signal to third and fourth differential transistors of the gate circuit; and output electrodes of the third and fourth differential transistors of the gate circuit. two low-pass filters that remove harmonic components from the gate output signal appearing at the gate output signal (collector) to obtain a demodulated (audio) signal; a subtraction circuit that subtracts the output signals of the two low-pass filters; , an impedance circuit coupled to the collector of the fourth differential transistor to make the output amplitude of the fourth transistor larger than the output amplitude of the third transistor, and one end of the resonant circuit is connected to the phase shift element. is connected to the output electrode (collector) of the second differential transistor of the gate circuit and the input electrode (base) of the third differential transistor of the gate circuit through the capacitor, and the other end is grounded through the capacitor. For the carrier wave, the impedance is smaller than the resonant resistance value of the above resonant circuit,
For AM signals, the phase shift synergistic type is characterized in that the capacitance value is selected to have an impedance large enough to prevent the AM signal from flowing into the ground side.
FM demodulator.