JPH0353824B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a modulation circuit using FSK (Frequency Shift Keying).

[Conventional technology]

Various types of FSK modulation circuits have been proposed in the past.

FIG. 3 is a block diagram showing an example of a conventionally used so-called digital modulation circuit.
31 is an input terminal for a modulated signal; 32 is a switching circuit;
35 is an oscillator that outputs a clock signal of a predetermined frequency; 36 and 37 are pre-frequency dividers; 38 is a main frequency divider;
39 is a low-pass filter, and 40 is an output terminal for a modulation signal (sine wave).

The prescaler 36 divides the clock signal from the oscillator 35 by a first frequency division ratio _N1 , and then divides the clock signal from the oscillator 35 by a first frequency division ratio N1.
is the clock signal from the oscillator 35 by the second frequency division ratio.
Divide by _N2 . Further, the switching circuit 32 sets the contact of the switch 33 to the A side when the modulated signal applied via the input terminal 31 is "H", and sets the contact of the switch 33 to the B side when the modulated signal is "L". Make it.
Therefore, when the modulated signal is "H", the output signal (rectangular wave) of the prescaler 36 is applied to the main frequency divider 38, and when the modulated signal is "L", the output signal (rectangular wave) of the prescaler The output signal (rectangular wave) of the frequency divider 37 will be applied to the main frequency divider 38. The main frequency divider 38 divides the signal from the pre-frequency divider 36 or 37 by a predetermined frequency division ratio N and applies it to the low-pass filter 39.
As a result, a sine wave having a frequency corresponding to "H" and "L" of the modulated signal is output from the low-pass filter 39.

However, in the conventional example described above, the modulation signal (sine wave) is obtained by applying the output signal (rectangular wave) of the main frequency divider 38 to the low-pass filter 39, and therefore there are the following problems. That is, the output signal of the main frequency divider 38 contains a third harmonic component of approximately -9.5 dBm and a fifth harmonic component of approximately -14.0 dBm. The level of the fundamental wave component (sine wave component) included in the output signal of 38 is set to 0 dBm], and in order to satisfy the criteria for unnecessary transmission level, it is necessary to use an expensive low-pass filter with good high-frequency rejection characteristics. Therefore, there was a problem that the equipment became expensive.

FIG. 4 is a block diagram of another conventional example called an analog system, in which 41 is an input terminal for a modulated signal, 42 and 48 are switching circuits, and 43 to 4
6 is a switch, 47 is a comparator, 49 is a low-pass filter, 50 is an output terminal, 51 is a capacitor, 5
2 to 55 are resistances. Note that the resistance values of the resistors 52 and 53 are different from each other, and the resistance values of the resistors 52 and 53 are different from each other.
The resistance values of 4 and 55 are also different.

The switching circuit 42 switches switches 43 and 4 when the modulated signal applied via the input terminal 41 is "H".
Both the contacts of switches 43 and 44 are set to the A side, and when the modulated signal is "L", the contacts of switches 43 and 44 are both set to the B side. The comparator 47 has the voltage V _c appearing across the capacitor 51 and the first and second thresholds V ₁ and V ₂ (V ₁ >V ₂ ).
The output signal is held at "L" from when the voltage V _c becomes above the first threshold V ₁ until it becomes below the second threshold V ₂ , and when the voltage V _c becomes the second The output signal is held at "H" from when it becomes equal to or less than the first threshold value _V2 until it becomes equal to or more than the first threshold value _V1 . Further, when the output signal of the comparator 47 is "H", the switching circuit 48 turns the switch 45 on and the switch 46 off, and when the output signal of the comparator 47 is "L", the switch 45 is turned off. , turns on the switch 46.

Therefore, when the modulated signal from the input terminal 41 is "H", the resistor 52, switches 43, 45
The capacitor 51 is charged by the charging current flowing through the capacitor 51, and when the voltage _Vc appearing across the capacitor becomes equal to or _higher than the first threshold value V1, the charge in the capacitor 51 is discharged via the switches 46, 44 and the resistor 54. Then, the voltage appearing across the capacitor 51 is
When V _c becomes less than the second threshold V ₂ , the resistor 52
Capacitor 51 is charged by the charging current flowing through switches 43 and 45, and the voltage V _c appearing across it gradually increases. By repeating the above operation, a triangular wave corresponding to the time constant of the resistors 52, 54 and the capacitor 51 appears at both ends of the capacitor 51.

Also, the modulated signal from the input terminal 41 is “L”
In this case, the capacitor 51 is charged by the charging current flowing through the resistor 53 and the switches 43 and 45, and when the voltage V _c appearing across the capacitor becomes equal to or higher than the first threshold value V ₁ , the charge in the capacitor 51 is transferred to the switch 46, 44, and is discharged through the resistor 55. When the voltage V _c appearing across the capacitor 51 becomes less than the second threshold value V ₂ , the capacitor 51 is charged again by the charging current flowing through the resistor 53 and the switches 43 and 45, and the voltage appearing across the capacitor 51 is charged again.
V _c gradually increases. By repeating the above operations, a triangular wave corresponding to the time constant of the resistors 53, 55 and the capacitor 51 appears at both ends of the capacitor 51.

Then, by passing the triangular wave obtained as described above through the low-pass filter 49, a modulated signal (sine wave) having a frequency corresponding to "H" and "L" of the modulated signal is obtained at the output terminal 40.

In the above embodiment, if the level of the fundamental wave component of the triangular wave is 0 dBm, the third harmonic component and the fifth harmonic component contained in the triangular wave are approximately -
19.0 dBm and -28.0 dBm, which is cheaper than the low-pass filter 39 used in the digital embodiment shown in Fig. 3, and can still satisfy the unnecessary transmission level criteria. I had a problem like this. That is, in the above embodiment, since the frequency of the modulated signal corresponding to "H" and "L" of the modulated signal is determined by the time constants of the resistors 52 to 55 and the capacitor 51, the temperature There was a problem that the frequency of the modulated wave fluctuated due to change, aging, etc.

[Problem that the invention seeks to solve]

The present invention solves the above-mentioned problems, and its purpose is to provide a modulation circuit that is economical in structure and stable against temperature changes and the like.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention includes an oscillator that outputs a clock signal of a predetermined frequency, and a clock signal from the oscillator that is connected to a first clock and a second clock, respectively.
first and second frequency dividers that divide the frequency at a frequency division ratio, and switching that outputs one of the output signals of the first and second frequency dividers depending on the state of the modulated signal. first and second means for outputting first and second charging currents;
a charging circuit, first and second discharge circuits through which first and second discharge currents flow, and a state of the first and second discharge currents in the capacitor based on the output signal of the switching means and the state of the modulated signal. 2 charging circuit or the first charging circuit;
a control means for connecting one of the second discharge circuits to generate a trapezoidal voltage across the capacitor; and a low-pass filter for extracting a fundamental wave component from the trapezoidal voltage appearing across the capacitor. It was established.

[Effect]

Since the fundamental wave component is extracted from the trapezoidal wave appearing at both ends of the capacitor, the low-pass filter can have low high-frequency rejection characteristics. Furthermore, since the frequency of the trapezoidal wave appearing at both ends of the capacitor is determined by the frequency of the clock signal, the device can be made stable against temperature changes, normal changes, etc.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the present invention, in which 1 is an input terminal for a modulated signal, 2 and 7 are switching circuits, 3 is an oscillator such as a crystal oscillator that outputs a clock signal of a predetermined frequency, 4, 5 is a pre-frequency divider, 6 is a main frequency divider, 8 and 9 are charging circuits consisting of constant current circuits, etc., 10 and 11 are discharging circuits consisting of constant current circuits, etc., 12 is a capacitor, 13 is a low-pass filter, 14 is an output terminal for a modulation signal, and 15 to 19 are switches.

Further, FIG. 2 is an explanatory diagram of the operation of FIG. 1, and the operation of FIG. 1 will be explained below with reference to the same figure.

Prescaler 4 divides the clock signal from oscillator 3 by a first division ratio _N1 , and prescaler 5 divides the clock signal from oscillator 3 by a second division ratio N2 _. The switching circuit 2 sets the contacts of switches 15, 16, and 19 to the A side when the modulated signal applied via the input terminal 1 is "H", and when the modulated signal is "L" In this case, the contacts of switches 15, 16, and 19 are set to the B side. In addition, the main frequency divider 6
prescaler 4 added via switch 15
Alternatively, the output signal of the prescaler 5 is divided by a predetermined frequency division ratio N.
The frequency is divided by and applied to the switching circuit 7. Therefore, if the modulated signal shown in FIG. 2A is applied to the input terminal 1, the output signal of the main frequency divider 6 will be as shown in FIG. 2B.

Further, when the output signal of the main frequency divider 6 is "H", the switching circuit 7 turns on and off the switches 17 and 18, respectively, and when the output signal of the main frequency divider 6 is "L", the switch 17 and 18 are turned on and off, respectively. , 18 are turned off and on, respectively. Therefore, if the signal shown in FIG. 2B is applied from the main frequency divider 6, the states of switches 17 and 18 are shown in FIG. 2C and D, respectively. Become something.

Further, the charging circuits 8 and 9 supply the first and second charging currents Ic1 and Ic2 to the capacitor 12, respectively, and the discharging circuits 10 and 11 supply the first and second charging currents Ic1 and Ic2, respectively, to the capacitor 12.
This is to flow second discharge currents Id1 and Id2. still,
In this embodiment, the capacitance and discharge of the capacitor 12, the charging current Ic1,
It is assumed that Ic2, Id1, and Id2 are defined. Therefore, if the modulated signal shown in FIG. 2A is applied to the input terminal 1, the "H" of the modulated signal as shown in FIG.
A trapezoidal voltage V _c having a frequency corresponding to "L" appears. Then, by passing this through the low-pass filter 13, a modulated signal (sine wave) having a frequency corresponding to "H" and "L" of the modulated signal as shown in FIG. F is obtained.

Here, the voltage appearing across the capacitor 12 is
When V _c is expanded by Fourier, V _c =V/ψπ{sin(2n+1)・/(2n+1) ²・sin(2n+1)ωt} (n=0, 1, 2,...). In addition, as mentioned above, in this example, ψ
Since the capacitance and discharging and charging currents Ic1, Ic2, Id2, and Id2 of the capacitor 12 are set so that = π/3, assuming that the fundamental wave component of V _c is 0 dBm, 3 times that, 5 When calculating the harmonic components, there is no third harmonic component at all, and the fifth harmonic component is approximately -
It becomes 28.0dBm. As can be seen, according to this embodiment, the 3rd and 5th harmonic components can be reduced compared to the conventional example shown in FIGS. 3 and 4, so that the high frequency Even if a simple low-pass filter with low blocking characteristics is used, it is possible to satisfy the criterion for the unnecessary emission level, and therefore, according to this embodiment, the apparatus can be constructed economically. Furthermore, since the frequency of the trapezoidal voltage appearing across the capacitor 12 is determined by the clock signal output from the oscillator 3, such as a crystal oscillator, it is different from the conventional example shown in FIG. The device can be made stable against temperature changes, secular changes, and the like.

〔Effect of the invention〕

As explained above, the present invention includes an oscillator such as a crystal oscillator that outputs a clock signal of a predetermined frequency, and a first clock that divides the clock signal from the oscillator by first and second frequency division ratios, respectively. a second frequency divider, and the first and second frequency dividers depending on the state of the modulated signal.
a switching means comprising a switching circuit 2, a switch 15, etc., which outputs one of the output signals of the frequency divider; and first and second charging currents which output the first and second charging currents.
a charging circuit, first and second discharge circuits through which first and second discharge currents flow, and a state of the first and second discharge currents in the capacitor based on the output signal of the switching means and the state of the modulated signal. 2 charging circuit or the first charging circuit;
A control means comprising a switching circuit 2, 7, a main frequency divider 6, etc. connected to one of the second discharge circuits to generate a trapezoidal voltage across the capacitor; It is equipped with a low-pass filter that extracts the fundamental wave component from the trapezoidal wave voltage that appears.Since the fundamental wave component is extracted from the trapezoidal wave that appears at both ends of the capacitor, an inexpensive low-pass filter with low high-frequency rejection characteristics is used. This has the advantage that the device can be constructed economically. Further, since the frequency of the trapezoidal wave appearing at both ends of the capacitor is determined by the frequency of the clock signal, there is an advantage that the device can be made stable against temperature changes, normal changes, etc.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG.
The figure is an explanatory diagram of the operation of FIG. 1, and FIGS. 3 and 4 are block diagrams of different conventional examples. 1 is an input terminal, 2 and 7 are switching circuits, 3 is an oscillator, 4 and 5 are pre-frequency dividers, 6 is a main frequency divider, 8 and 9
1 is a charging circuit, 10 and 11 are discharge circuits, 12 is a capacitor, 13 is a low-pass filter, 14 is an output terminal, and 15 to 19 are switches.

Claims (1)

[Claims] 1. An oscillator that outputs a clock signal of a predetermined frequency;
first and second frequency dividers that divide the frequency at a second frequency division ratio, and output one of the output signals of the first and second frequency dividers according to the state of the modulated signal. first and second charging circuits that output first and second charging currents; first and second discharge circuits through which first and second discharge currents flow; Based on the output signal and the state of the modulated signal, connect one of the first and second charging circuits or the first and second discharging circuits to the capacitor, and connect both ends of the capacitor. 1. A modulation circuit comprising: control means for generating a trapezoidal voltage; and a low-pass filter for extracting a fundamental wave component from the trapezoidal voltage appearing at both ends of the capacitor.