JPH01101010A - Voice modulation circuit - Google Patents

Abstract

PURPOSE:To always apply a feedback and to correct the dislocation of a central frequency by adding a PLL circuit and a loop filter circuit composed of ICs for a PLL to an LC generating type FM modulation circuit composed of a mixer circuit and a VCO circuit, and forming an APC circuit. CONSTITUTION:At the time of a phase more advancing than the value of a normal oscillation frequency, an error signal is obtained from a phase comparator 21. The error signal passes a loop filter 34, and the voltages of both edges of a variable capacity diode 36 are lowered. Thus, the central frequency is lowered, and it approaches the value of the normal oscillation frequency. To the contrary, at the time of a lagging phase, the voltages of both edges of the variable capacity diode 36 is raised in the same work. Thus, the central frequency is also increased, and it approaches the normal value. In such a way, by feeding back the dislocation of the central frequency of the oscillation, the correct oscillation frequency can be always obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to equipment such as VTRs and CATV converters that FM modulates and outputs baseband audio signals.

2. Description of the Related Art Conventionally, an audio modulation circuit 51 used in an RF modulator
As shown in Figure 3, the oscillation frequency is set to LC or SA.
A method was adopted in which the LC constant of the W oscillation circuit 52 or the SAW oscillator was used.

Problems to be Solved by the Invention In the conventional configuration as described above, the oscillation frequency changes due to the addition of external factors such as temperature changes.
The problem was that it was impossible to achieve stability within the range of 5 MHz±I KH2.

The audio modulation circuit of the present invention solves the above-mentioned conventional problems, and aims to provide an audio modulation circuit whose oscillation frequency does not change even with external changes such as temperature changes.

Means for Solving the Problems In order to solve the above problems, the audio modulation circuit of the present invention has the following features:
An APC circuit is formed by adding a PLL circuit and a loop filter to an LC oscillation type FM modulation circuit, which is composed of a mixer circuit and a VCO circuit, and performs FM modulation of an audio signal.

Effect With the above configuration, when the center frequency of oscillation of the VCO circuit is higher than the normal frequency, the APC circuit lowers the tuning voltage of the VCO circuit, thereby lowering the oscillation frequency and moving the center frequency to a lower side. , Conversely, if the frequency is lower than the normal frequency, the APC circuit increases the tuning voltage of the VCO circuit, thereby increasing the oscillation frequency and moving the center frequency to a higher side. In this way, the shift in the center frequency of the oscillation of the VCO circuit is constantly fed back, so the stability against external factors such as temperature changes is good, and the modulation circuit always operates at the normal center frequency. Furthermore, since the comparison reference signal of the PLL circuit uses a crystal resonator, accurate frequency stability can be obtained.

EXAMPLE An example of the present invention will be described below based on the drawings.

FIG. 1 is a circuit diagram of an audio modulation circuit using the APC method, showing an embodiment of the present invention. In FIG. 1, an audio input terminal 1 to which an audio signal is input is connected to one end of a resistor 2 and a capacitor 3, and the other end of the resistor 2 is connected to one end of a capacitor 4 and one end of a resistor 5. The other end is connected to the base of transistor 6 and one end of resistor 7, the emitter of transistor 6 is connected to one end of resistor 8, the other end of resistor 5, 7.8 and capacitor 3 is grounded, and the other end of transistor 6 is connected to one end of resistor 7. The collector is connected to one end of the primary coil 9 of the transformer, and the power terminal 10 is connected to the base of the transistor 6 via a resistor 11, forming an audio signal mixer circuit 14.

Further, the power supply terminal 10 is grounded through resistors 12 and 13, and the connection point between the resistors 12 and 13 is connected to the other end of the primary coil 9 of the transformer.

A secondary coil 15-@ of the transformer is grounded, and the other end is connected to a PLL circuit 17 via a capacitor 16. P.L.
The L circuit 17 includes a reference counter 19 that divides the output of the crystal oscillator 18, a programmable counter 20 that sets a division ratio according to the center frequency of the modulation frequency, and the reference counter 19 and the programmable counter 20.
The output of the secondary coil 15 of the transformer is input to a programmable counter 20.

The output terminal of the phase comparator 21 is connected to the base of a transistor 24 via resistors 22 and 23, the connection point between the resistors 22 and 23 is grounded via a capacitor 25, and the emitter of the transistor 24 is connected to the base of a transistor 26. Furthermore, the power supply terminal 28 is connected to the collector of the transistor 24 via a resistor 29 and to the collector of the transistor 26 via a resistor 30.31. The connection point is connected to the base of the transistor 24 via a series circuit of a resistor 32 and a capacitor 33.
The emitter of is grounded to form a loop filter circuit 34.

The connection point between the resistors 30 and 31 is connected to the cathode of the variable capacitance diode 36 and the capacitor 37 via the resistor 35.
The anode of the variable capacitance diode 36 is grounded, and the other end of the capacitor 37 is connected to the other end of the transformerless primary coil 9 via a capacitor 38 to connect the VCO circuit 39.
It consists of At this time, capacitors 40 and 41 are interposed between the collector and emitter of the transistor 6 and between the emitter and ground, respectively, and the VCO circuit 39 and the mixer circuit 14 are coupled by the primary coil 9 of the transformer to F'
This is aimed at stabilizing oscillation when performing M modulation.

In other words, the present invention provides an L
A PL consisting of a PLL IC is installed in the C oscillation type FM modulation circuit 42.
A by adding the L circuit 17 and the loop filter circuit 34
It is formed of a PC circuit, and is configured to constantly apply feedback to the deviation in the center frequency of the oscillation of the LC oscillation type FM modulation circuit 42, and to be able to correct the deviation in the center frequency.

Next, the effect of the above configuration will be explained. First, a frequency division ratio corresponding to the center frequency of the modulation frequency is set in the programmable counter 20 of the PLL circuit 17. ■CO circuit 3
A voltage is applied across the variable capacitance diode 36 of No. 9, and the capacitance varied by this voltage and the transformer perform FM modulation of the audio signal around the oscillation frequency. This modulation signal is again sent to the programmable counter 20 of the PLL circuit 17.
is input.

Let us now consider the case where the center frequency deviates from the normal one-oscillation frequency.

First, if the phase is ahead of the normal oscillation frequency value (if the center frequency is high), an error signal is output from the phase comparator 21, this error signal passes through the loop filter 34, and the voltage across the variable capacitance diode 36 is decreases, and as a result, the center frequency also decreases, approaching the value of the normal oscillation frequency. Conversely, when the phase is delayed (the center frequency is low), the voltage across the variable capacitance diode 36 increases due to the same effect, and the center frequency also becomes high and approaches the normal value.

By feeding back the shift in the center frequency of oscillation in this way, the correct oscillation frequency can always be obtained. Note that the output of the RF signal is from ■CO circuit 39 and mixer circuit 14.
LC oscillation type FM modulation circuit 42 consisting of
The signal is taken out from the link between the programmable counter 24 of No. 2 and the output terminal 44 via the capacitor 43.

Further, an LC oscillation type FM modulation circuit 42 consisting of a mixer circuit 14 and a VCO circuit 39, a loop filter 34, and a P
It is also possible to house the LL circuit 17 in one chip IC and provide a more streamlined model.

To give specific numerical values, FM modulation is possible under the following conditions.

1. Center frequency fa = 4.5M1-1z±1KH
z2, maximum frequency deviation ±75K (Go2 3, modulation signal range 301-1z ~ 100K 1-1
z.

Effects of the Invention According to the present invention, a PLL is added to the LL oscillation type FM modulation circuit.
By forming an APC circuit by adding a circuit and a loop filter circuit, it is possible to constantly provide feedback to deviations in the center frequency. Furthermore, since the center frequency of the crystal oscillator is compared, frequency stability comparable to that of a crystal oscillator can be obtained. In addition, since the frequency deviation is constantly fed back, it has excellent stability against external factors such as temperature changes, and can be used without any adjustment.

[Brief explanation of the drawing]

Figure m1 is a circuit diagram of an audio modulation circuit using the APC method showing an example of the present invention, Figure 2 is a block diagram of the same audio modulation circuit, and Figure 3 is a block diagram of a conventional FM modulation circuit. . 14...Mixer circuit, 17...PLL circuit, 34...
...Loop filter circuit, 39...VCO circuit, 42
...LC oscillation type FM modulation circuit. Agent Yahiro MorikiFigure 2Figure 3

Claims (1)

[Claims] 1. Consists of mixer circuit and VCO circuit, converts audio signal to FM
An audio modulation circuit in which an APC circuit is formed by adding a PLL circuit and a loop filter to an LC oscillation type FM modulation circuit.