JPH01179550A - Signal generating circuit - Google Patents

Abstract

PURPOSE:To miniaturize the title circuit by switching the frequency of a clock signal to a low-pass filter LPF to be high when an FM signal is inputted and to be low when the high group frequency signal of a DTMF(Dual Tone Multiple Frequency) signal is inputted. CONSTITUTION:For LPFs 13 and 14, a switched capacitor filter SCF, which can cause a cut-off frequency to be variable by the frequency of the clock signal, is used. To one LPF 13, the FM signal or the high group of the DTMF signal is switched and inputted. On the other hand, since the frequency of the clock signal to the LPF 13 is switched to be high when the FM signal is inputted and to be low when the high group of the DTMF signal is inputted, a rectangular wave signal to be inputted can be always converted to a sine wave. Thus, for the LPF to conventionally need three circuits, the two circuits are enough. Then, by using the SCF, the whole signal generating circuit is miniaturized and the cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a signal generation circuit particularly suitable for facsimile machines.

[Prior Art] A facsimile machine conforming to the G1 standard recommended by the CCITT is equipped with a circuit that performs FM modulation on image information and outputs an FM signal. In addition, facsimile machines that are connected to a telephone line subscribed to a telephone line and have a dialing function are equipped with a DTMF (dualTone Mu), which is a dialing signal to the telephone line.
It is equipped with a circuit that outputs a ltiple frequency) signal.

FIG. 3 shows a conventional example of a signal generating circuit that outputs the above-mentioned FM signal and DTMF signal. In the figure, a DTMF high group signal generation circuit 1 is a DTMF high group signal generating circuit 1 that transmits a DTMF
Generate four types of high-frequency rectangular wave signals of MF signals, and oTiF
The low group signal generation circuit 2 generates four types of rectangular wave signals of the low group of DTMF signals. The gate circuits 3 and 4 are opened and closed by the gate signal GSI, and when transmitting the DTMF signal,
A rectangular wave signal output from the DTMF high group signal generation circuit 1.2 is input to an LPF (low pass filter) 5.6. The LPF 5.6 converts the input rectangular wave signal into a sine wave signal, and each sine wave signal outputted from the LPF 5.6 is synthesized by an adder circuit 7 and then converted into a predetermined DTMF signal.
taken out as a signal.

On the other hand, the FM modulation circuit 8 FM modulates the image information to be transmitted and outputs a rectangular FM signal. The gate circuit 9 is opened and closed by a gate signal GS2, and inputs the rectangular FM signal to the LPFIO when transmitting image information. LPFIO
1 converts a rectangular FM signal into a sine wave signal, and the output signal is taken out as a transmission signal via an adder circuit 7.

By the way, conventionally, in general, the above-mentioned LPF5,6
．． 10 was composed of discrete components such as resistors, capacitors, and operational amplifiers, and had a relatively large external size.

Further, as described above, three LPFs were provided, one for the FM signal and two for the DTMF signal.

For this reason, conventional signal generation circuits that generate FM signals and DTMF signals have a problem of being large in size and high in cost.

[Objective] It is an object of the present invention to solve the above problems and provide a signal generating circuit for a facsimile machine that is small and inexpensive.

[Configuration] For this purpose, the present invention uses a 5CF (switched capacitor filter) whose cutoff frequency can be varied depending on the frequency of the input clock signal, while also Select a rectangular wave signal and input it to one LPF, and
The frequency of the clock signal is switched to be high when an FM signal is being input, and to be low when a rectangular wave signal corresponding to the high group frequency of the DTMF signal is being input. It is characterized by this.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a block diagram of a signal generating circuit of a facsimile machine according to an embodiment of the present invention. In the figure, the same symbols as in FIG. 3 indicate the same circuits, and the DTM
The F high group signal generation circuit 1 generates high group frequencies of DTMF signals of 1209 Hz, 1336 Hz, and 1477 Hz.

Generates four types of rectangular wave signals of 1633Hz, DTM
The F low group signal generation circuit 2 has a low group frequency of 697.
1 (z, 770Hz, 852Hz, 941Hz)
It generates a type of square wave signal. The modulation circuit 8 is
It modulates image information to generate a rectangular wave-like FM signal that changes between 1500 Hz and 2430 Hz.

The operation unit 11 is used to perform various operations on a numeric keypad for inputting a destination telephone number and a facsimile machine. The control circuit 12 performs control for outputting a DTMF signal corresponding to a destination telephone number entered manually using the numeric keypad, and control for modulating image information and outputting a predetermined FM signal.

L P F 13.14 is constructed by SCF. The cutoff frequency of the LPF 13 can be switched between frequencies fc and 2fc as shown in FIG. 2 by switching the input clock signal. The frequency fc is set, for example, to 1633 Hz, the highest frequency of the DTMF signal. - way, LPF
14 is a fixed frequency clock signal CLk whose cutoff frequency is set to, for example, the maximum frequency of the low group of DTMF signals, 941 Hz.

The 172 frequency divider circuit 15 divides the signal frequency of the clock signal CLk2 into 1/2, and the switching circuit 16 selects the clock signal CLk2 or the signal frequency-divided to 1/2. , is input to the LPF 13.

The OR circuit 17 inputs the signals output from the gate circuit 9 and the gate circuit 3 to the LPFL 3.

With the above configuration, the signal generation circuit of this embodiment can perform DTM
When extracting the F signal, the control circuit 12 extracts the gate signal G
Turn on S'l and turn off gate signal GS2.
On the other hand, one of the four frequencies is selected for each of the DTMF high group signal generation circuit 1 and the DTMF low group signal generation circuit 2.

When the gate signal GSI is turned on, the gate circuits 3 and 4 open their gates, while when the gate signal GS2 is turned off.

Gate circuit 9 closes the gate. The DTMF high group signal generation circuit 1 outputs a rectangular wave signal of the selected frequency, and the signal is passed through the gate circuit 3 and the OR circuit 17 to the LP
It is input to F13.

The switching circuit 16 selects the frequency dividing circuit 15 side when the gate signal GSI is on. As a result, the signal frequency of the clock signal CLk2 is divided into 172 and input to the LPF 13. At this time, the cutoff frequency of the LPF 13 becomes the frequency fc. This allows the LP
In F13, the frequency f of the input rectangular wave signal is
Components higher than c are blocked.

This frequency fc is 1633 Hz as described above.

On the other hand, from the DTMF high group signal generation circuit l, 1209~
A rectangular wave signal in the range of 1633 Hz is output. Since a rectangular wave signal becomes a fundamental wave when its third harmonic or higher is blocked, the rectangular wave signal output from the DTMF high group signal generation circuit 1 is converted into a sine wave by the LPF 13. . The sine wave is input to the adder circuit 7.

On the other hand, the DTMF low group signal generation circuit 2 outputs a rectangular wave signal of the selected frequency, and the rectangular wave signal is input to the LPF 14 via the gate circuit 4. The DTMF low group signal generation circuit 2 outputs the rectangular wave signal in the range of 697 to 941 Hz, and the cutoff frequency of the LPF 14 is set to 941 Hz. The signal is converted into a sine wave and input to the adder circuit 7. Then, from the adder circuit 7, 2
A predetermined DTMF signal with mixed frequencies is output.

Next, when extracting an FM signal using this signal generation circuit, the control circuit 12 turns on the gate signal GS2 and turns off the gate signal GSI. At this time, image information PS is input to the modulation circuit 8 from an image information processing circuit (not shown). The modulation circuit 8 modulates the image information PS to 1500Hz and 2:
It outputs a rectangular wave FM signal that changes at 30Hz.

Now, since the gate signal GS2 is on, the gate of the gate circuit 9 is open, and the above FM signal is sent to the OR circuit 1.
7 to the LPF 13. Furthermore, at this time,
Since GSl is off, gate circuit 3.4 closes its gate and has no output.

When the gate signal GSI is off, the switching circuit 16 selects the clock signal CLk2. Therefore, LPF1
A clock signal CLk 2 is input to 3. As a result, the frequency of the clock signal to the LPF 13 is doubled compared to the above, and the cutoff frequency is set to 2fc,
In other words, it becomes 3266Hz.

Since the modulation circuit 8 outputs a rectangular wave signal of 1500 Hz or 2430 Hz, the LPF 13
The harmonics are rejected and converted to a sinusoidal signal.

This sine wave signal is output via the adder circuit 7 as an FM signal obtained by modulating one-picture information.

As described above, in this embodiment, an SCF whose cutoff frequency can be varied depending on the frequency of the clock signal is used for the L P F1a, 14. And one LPF
13, the high frequency group of FM signal or DTMF signal is switched and input, while the frequency of the clock signal to the LPF 13 is set high when the FM signal is input, and DTMF signal is switched high.
By switching the MF signal so that it becomes low when a high group of MF signals is being input, the input rectangular wave signal can always be converted into a sine wave.

As a result, the LPF, which conventionally required three circuits, now requires only two circuits. In recent years, SCF has also become a monolithic IC.
Since SCFs are commercially available at low cost, by using such SCFs, the entire signal generation circuit can be made small and inexpensive.

In this embodiment, the lower cutoff frequency of the LPF 13 was set to 1633Hz, and the cutoff frequency of the LPF 14 was set to 941Hz, but any frequency that can block the third harmonic of the input signal can be used. , it goes without saying that it can be set arbitrarily.

Furthermore, in the above embodiment, an example of a signal generation circuit used in a G1 standard facsimile machine has been described, but the present invention can also be applied to facsimile machines of other standards or other communication devices used on telephone lines. It is possible to apply it.

[Effect] As described above, according to the present invention, the LPF includes an SCF whose cutoff frequency can be varied depending on the input clock signal.
while selecting a high group frequency signal of the FM signal or DTMF signal and inputting it to one LPF,
The frequency of the clock signal to the LPF is switched so that it is high when an FM signal is input, and low when a high group frequency signal of a DTMF signal is input, so the LPF only needs two circuits. In addition, in recent years, SCFs have become inexpensive and small, so the entire signal generation circuit.

It is small and inexpensive.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram of a signal generation circuit of a facsimile machine according to an embodiment of the present invention, FIG. 2 is a graph diagram showing the filtering band of an LPF, and FIG. 3 is a block diagram of a signal generation circuit of a conventional facsimile machine. FIG. 1...-'DTMF high group signal generation circuit, 2...DTM
F low group signal generation circuit, 3, 4.9... gate circuit, 5
, 6, 10, 13.14...LPF, 7... Addition circuit, 8... Modulation circuit, 11... Operation unit, 12...
Control circuit, 15... Frequency dividing circuit, 16... Switching circuit,
17...OR circuit. Figure 2 Figure 3

Claims (1)

[Claims] An FM modulation circuit that outputs an FM signal that is a rectangular wave signal modulated by image information, a DTMF high group signal generation circuit that outputs a rectangular wave signal that corresponds to the high group frequency of the DTMF signal, and a low group frequency of the DTMF signal. a DTMF low group signal generation circuit that outputs a rectangular wave signal corresponding to the FM signal, and a low pass filter that converts the high group and low group rectangular wave signals of the FM signal and the DTMF signal into sine wave signals, respectively. In the circuit, a low-pass filter using a switched capacitor filter whose cutoff frequency is varied depending on the frequency of the input clock signal, and a rectangular wave signal corresponding to the high group frequency of the FM signal or the DTMF signal are selected for the low-pass filter. and selects a rectangular wave signal that is high when the selection circuit selects the FM signal and corresponds to the high group frequency of the DTMF signal, so that the frequency of the clock signal input to the low-pass filter is high when the selection circuit selects the FM signal. 1. A signal generation circuit comprising: a clock signal switching circuit which lowers the clock signal when the clock signal is being output.