JP2794707B2 - Frequency shift keying modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a frequency shift keying type modulator suitable for high-speed data communication.

[Conventional technology]

Conventionally, many frequency shift keying (FSK) modulators of this type use a phase locked loop (PLL) circuit. Historically, this PLL circuit appeared in 1932 as what could be called an electronic servo system for stabilizing the local oscillation frequency in superheterodyne receivers.

 FIG. 3 shows a basic configuration of the PLL circuit.

In the figure, a voltage controlled oscillator (VCO) 11 is a free-running type, and its oscillation frequency is usually determined by an external element. Frequency of the output signal 12 of the VCO is fed back to the phase detector 13 where it is compared with the input frequency f _i of the input signal 14. The output 15 of the phase detector 13 is called an error voltage, and includes a frequency difference (f _i −f _O ), an input signal 14 and an output signal 12 of the VCO 11.
Is provided as an average DC voltage in proportion to the phase difference Δθ. The low-pass filter 16 removes the high-frequency noise of the error voltage 15 and converts the error voltage 15 into a VCO 11 to form a loop.
It works to return to. Thus, when the phase lock state is established, the output frequency f _O of the VCO 11 becomes the input frequency f _i
Is equal to

FIG. 4 shows a conventional FSK modulator using such a PLL circuit.

The input signal 21 is input to the variable frequency dividing circuit 22. The variable frequency dividing circuit 22 changes the frequency dividing ratio 1 / N according to the mark and space included in the input signal 21 and sends out the frequency divided output 23 to the phase comparator 24. The phase comparator 24 further receives a divided output 29 of a frequency dividing circuit 28 for dividing the reference output 27 of the reference frequency oscillation circuit 25. Then, the phases of the divided outputs 23 and 29 are compared, and an error output corresponding to the phase difference is output.
31 is transmitted to the voltage controlled oscillator 33 via the low-pass filter 32. The voltage controlled oscillator 33 provides an oscillation output 35 having a frequency according to the output voltage 34 of the low-pass filter 32, and a part of the output 35 is fed back to the variable frequency dividing circuit 22. Thereafter, as in the case of FIG. 3, when the phase lock state is entered, two divided outputs 23,
29 frequencies will now match.

[Problems to be solved by the invention]

As described above, in the conventional FSK modulator,
A low-pass filter is provided between the phase comparator and the voltage-controlled oscillator to remove noise causing unnecessary radiation and to narrow the occupied bandwidth. For this reason, the conventional FSK modulator has a problem that the transient characteristics are deteriorated and it is not suitable for high-speed digital communication.

Therefore, an object of the present invention is to provide an FSK modulator in which waveform distortion characteristics and adjacent frequency interference characteristics do not deteriorate even when used in high-speed digital communication.

[Means for solving the problem]

According to the present invention, (a) a voltage-controlled oscillator that generates a mark frequency and a space frequency in accordance with a mark and a space of digital modulation data, (b) a storage circuit that stores a control voltage value that controls the voltage-controlled oscillator,
(C) a switch for switching the control voltage of the voltage-controlled oscillator to one of the mark frequency and the space frequency; and (d) a switch disposed on the output side of the switch to appropriately remove noise. A digital filter that blocks high-frequency components and shapes the waveform, a D / A converter that converts the output of this digital filter to an analog signal, and an analog filter that is placed on the output side of the D / A converter A waveform shaping unit that adjusts these circuit parameters so that the signal band is shaped into an ideal waveform without interference between codes in a predetermined range, and (e) a voltage shaping unit based on a predetermined reference frequency. Counter means for detecting the output frequency of the control oscillator and outputting detection data; and (f) detection data output from the counter means and stored in the storage circuit. Comparing the pressure value, thereby and a comparator circuit for correcting the voltage value stored in the storage circuit in accordance with the deviation in the frequency shift keying modulating device.

Therefore, using the FSK modulator according to the present invention, before input data is applied to the voltage-controlled oscillator, rather than a simple low-pass filter as in the prior art, appropriately cut off high-frequency components that cause unnecessary radiation, In addition, a waveform shaping unit that outputs a shaped waveform suitable for a voltage-controlled oscillator is provided, so that even when the FSK modulator is used for high-speed digital data communication, the waveform distortion characteristics and the adjacent frequency interference characteristics are not deteriorated. can do.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to examples.

 FIG. 1 is a block diagram showing the configuration of this embodiment.

In the figure, input data 41 as modulated data is input to a switch. The switch 42 sends an address signal 45 to a storage circuit 43 based on a voltage value corresponding to a mark and space frequency included in the input data 41, and extracts a control voltage signal 46 from the storage circuit 43. The control voltage signal 46 selected by the switch 42 is then sent to a digital filter 47. This digital filter 47 has the following
Together with the D / A converter 49 and the analog filter 50, as will be described later, it appropriately cuts off high-frequency components in order to remove noise, and outputs a shaped waveform suitable for the voltage-controlled oscillator 52 at the subsequent stage. Shaping output from these waveform shaping units 53
The voltage-controlled oscillator 52 receiving the signal 54 provides an output signal 55 as an FSK modulator, and a part thereof is sent to a counter 56. The counter 56 receiving this measures the frequency of the output signal 55 of the voltage controlled oscillator 52 based on the reference frequency signal 58 from the reference frequency oscillator 57. This measurement result is sent to the calculator 59. The arithmetic unit 59 compares this with the above-mentioned voltage value stored in the storage circuit 43. When there is a deviation between the two, the control voltage signal 46 including the mark frequency and the space frequency stored in the storage circuit 43 is corrected.

Next, the waveform shaping section 53 will be described in detail with reference to FIG. Input data 41 as modulated data (a in the figure)
Has a square wave, and when modulated as it is, the occupied frequency bandwidth is widened. In this state, noise that causes unnecessary radiation is easily picked up, so that high frequencies are cut off through the analog filter 50 that is a low-pass filter (b in the figure). As the analog filter 50, for example, a lag filter including a resistor and a capacitor is used.
However, the waveform in this case changes exponentially, as is apparent from the figure, and the high-frequency component (the rising edge of the waveform,
(Falling part)
Waveform distortion (square wave collapse) is large. From the viewpoint of effective use of radio waves, it is desirable that the signal band be within a predetermined range and that there be no interference between codes. In order to realize such an ideal waveform, it is conceivable to use the Blackman window used in the fast Fourier transform (c in the figure). For this reason, in the present embodiment, the circuit parameters of the digital filter 47, the D / A converter 49, and the analog filter 50 are adjusted to obtain a waveform close to the ideal waveform as described above. In that case, D /
The output waveform of the A converter 49 and the output waveform of the analog filter 50, that is, the input control voltage to the voltage controlled oscillator 52 are as shown in FIGS.

〔The invention's effect〕

As described above, according to the present invention, a mark frequency and a space frequency are generated according to the mark and the space of the digital modulation data, these are temporarily stored in the storage unit, and the stored control voltage value is taken out to obtain the voltage control oscillator. By setting the parameters of the specific digital filter such as the Blackman window and the D / A converter and analog filter so that it is suitable for I decided to shape it into an ideal waveform without interference. The shaped output is supplied to the voltage controlled oscillator, and a part of the output is fed back, the frequency of the feedback signal from the voltage controlled oscillator is detected by a counter using a predetermined reference frequency, and the detected signal and the storage circuit The stored control voltages are compared to sequentially correct the control voltage values stored in the storage circuit. Therefore, even in high-speed digital data communication,
There is an effect that an optimal modulation signal can be provided without deteriorating the waveform distortion characteristic and the adjacent frequency interference characteristic.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an FSK modulator according to the present invention, FIG. 2 is a diagram showing an operation of a waveform shaping unit in FIG. 1,
FIG. 3 is a block diagram showing a basic configuration of a PLL circuit, and FIG. 4 is a block diagram showing a conventional FSK modulator. 43 …… memory circuit, 53 …… waveform shaping section, 56 …… counter,
59 ... Calculation circuit.

Claims (1)

(57) [Claims] A voltage-controlled oscillator for generating a mark frequency and a space frequency in accordance with a mark and a space of digital modulation data; a storage circuit for storing a control voltage value for controlling the voltage-controlled oscillator; A switch for switching a control voltage to one of the mark frequency and the space frequency; and a switch disposed at an output side of the switch for appropriately cutting off high-frequency components and shaping a waveform to remove noise. A digital filter, a D / A converter for converting the output of the digital filter into an analog signal,
/ A converter consisting of an analog filter placed on the output side, and adjusting these circuit parameters to adjust the signal band within a predetermined range to form an ideal waveform without interference between codes. Counter means for detecting an output frequency of the voltage controlled oscillator based on a predetermined reference frequency and outputting detection data; detection data output from the counter means and a voltage value stored in the storage circuit And a comparator that corrects the voltage value stored in the storage circuit in accordance with the deviation.