JPS63309006A - Fm demodurator - Google Patents

Abstract

PURPOSE:To correct the amplitude characteristic of a 90 deg. phase shifter without increasing the circuit scale by providing a digital filter of linear phase which has an amplitude characteristic which is approximated in a frequency band required for the amplitude characteristic of the 90 deg. phase shifter. CONSTITUTION:An FM signal which is digitized by an A/D converter 1 is shifted in phase by 90 deg. through the 90 deg. phase shifter 3. Further, the signal which is digitized by the A/D converter 1 is inputted to a digital filter 7 as well. This digital filter 7 is the digital filter of linear phase designed so as to the amplitude characteristic which is approximated in the frequency band required for the amplitude characteristic of the 90 deg. phase shifter 3. Consequently, the digital filter corrects the amplitude characteristic of the 90 deg. phase shifter into the amplitude characteristic approximated in the frequency range required for the amplitude characteristic of the output of the 90 deg. phase shifter, so a more accurate demodulated signal is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Applicability g'''I] The present invention relates to an FM demodulator that demodulates an analog FM signal converted into a digital signal.

[Prior Art] When configuring an FM demodulator using digital signal processing technology, when the FM frequency is X and the signal obtained by shifting the phase of signal X by 90 degrees is Y, the signal X is demodulated. This has been known for some time.

4th [A] is a block circuit diagram of a conventional FM demodulator that realizes this using hardware;
1) is an A/D converter that converts the input analog FM signal into a digital signal, and (3) is a 90 degree phase shifter.
The FM signal digitized by the D converter (+) is phase-shifted by 90 degrees to obtain signal Y. (2) is a delay compensator,
The output of the L A/D converter (1) is input, and the above 9
This is to delay the delay time of the signal Y generated by the 0 degree phase shifter () and the digital filter (by the amount of rotation).
The output signal X of this 'N-interpolating iE device (2) and the output signal Y of the 90-degree phase shifter (3) can be regarded as the same signal quantized to 1. (4) is an @ calculator which performs the operation jan''('') on both the input signals X and Y. (5) is a differentiator which differentiates the output of the arithmetic unit (4). (6) is a D/A converter which converts the digital signal output from the differentiator (5) into an analog signal.

In this way, the output of the arithmetic unit (4) is used as the differentiator (5), and dt
an-'(') fN calculation wo A-1'' If we differentiate by seven steps, we get -
7t-YA, the output of the differentiator (5) is converted to D/A.
If converted to an analog signal by the converter (6), F
An output obtained by demodulating the M signal is obtained.

As an example of the configuration of the arithmetic unit (4), for example, signals It can be constructed by obtaining low 1 (-X-).

Further, the 90 degree phase shifter (3) is configured by, for example, Hilbert transform. In an ideal Hilbert transform, the amplitude characteristic is always 1 regardless of the frequency of the input signal, but
If this is implemented using hardware, it will cost $14.00 because it is configured with a finite number of taps. ), for example, as shown in Figure 5.

In order to obtain more ideal amplitude characteristics for the 90-degree phase shifter (3) having such characteristics, it is necessary to increase the number of taps, which increases the circuit scale.

Also, in Fig. 5, 1. For example, when a signal with frequency ω0 is input to the FM demodulator shown in Fig. 4, if the output of the ideal 90-degree phase shifter is Y, the output of the 90-degree phase shifter (3) is aY (in this example , 0<a<I), and the arithmetic unit (4) calculates L a n -'(k), outputting an incorrect result.

[Problem to be solved in Part 1] As mentioned above, in conventional FM demodulators, in order to make the amplitude characteristics of the 90-degree phase shifter (3) closer to the ideal 90-degree phase shifter, Another problem was that it was necessary to increase the number of taps, which meant that the circuit scale had to be increased.

Additionally, if the amplitude characteristics of the 90 degree phase shifter (3) are not close to ideal, an incorrect result may be output when an FM signal with a frequency for which ideal amplitude characteristics cannot be obtained is input. There was a point.

This invention was made in order to solve the problems mentioned above, and it is possible to correct the amplitude characteristics of the 90-degree phase shifter without increasing the circuit scale of the 90-degree phase shifter, thereby making it more accurate. An object of the present invention is to provide an FM demodulator that can obtain a demodulated signal.

[Means for Solving the Problems] The FM demodulator according to the present invention has an amplitude characteristic approximated within a frequency band required for the amplitude characteristic of a 90-degree phase shifter that shifts the phase of a digital signal by 90 degrees. A digital filter with a linear phase is provided, and this digital filter is configured so that the amplitude characteristics of the digital signal can be approximated within the frequency band a required for the amplitude characteristics of the output of the 90 degree phase shifter. Features.

[Function] According to the present invention, - the amplitude characteristic of the digital filter or the 90-degree phase shifter is compensated for by an amplitude characteristic approximated within a frequency band required for the amplitude characteristic of the output of the 90-degree phase shifter; I', so a more accurate demodulated signal can be obtained.

Example of 1 shot 1j1] An embodiment of the present invention will be described below.

FIG. 1 is a block circuit diagram of an FM demodulator according to an embodiment of the present invention. In the figure, the same components as the 4M configuration in FIG. Omitted.

In Fig. 1, (7) is a digital filter, and this digital filter (7) is a linear phase filter whose amplitude characteristic is approximated within the frequency band required for the amplitude characteristic of the 90 degree phase shifter (3). It is a digital filter. Further, in the embodiment shown in FIG. 1, the delay time (111) of the digital filter (7) is longer than that of the 90-degree phase shifter (3).

Next, the operation of the above configuration will be explained.

The FM signal digitized by the A/D converter (1) is input to a 90-degree phase shifter (3) and phase-shifted by 90 degrees.

Let Y be the output signal when the amplitude characteristic of this 90 degree phase shifter (3) is ideal. For example, if the actual amplitude characteristic is as shown in Fig. 5, then the input FM signal When the frequency is ω0, the output of the 90-degree phase shifter (3) becomes aY, which is a signal with a smaller amplitude than the original signal Y (where 0<a<1 in this case).

However, the signal digitized by the A/D converter (1) is also input to the digital filter (7). This digital filter (7) is a linear phase digital filter designed to have an amplitude characteristic approximated within the frequency band required by the amplitude characteristic of the 90 degree phase shifter (3). If the output of the delay corrector (2) that performs only correction and outputs is X, then the filter (7
) is within the required frequency band, in this example, the swing l! of the 90 degree phase shifter (3) at ω0! l! The characteristics are designed to be as similar as possible. Therefore, the filter (7
) is designed to approximate aX.

Also, the digital filter (7) is better than the 90 degree phase shifter (3).
), a delay compensator (2) is provided after the 90 degree phase shifter (3) in order to synchronize the delay time with the digital filter (7).

The faith obtained in this way"s'y aX, aY
is input to the arithmetic unit (4), and La n-' (expanded)
= L a n -' (+)1Y output, which is an ideal 90 degree phase shifter (3)
In the case where the output is obtained by using the digital filter (7), the amplitude characteristics are corrected by the digital filter (7).

After that, as explained in Fig. 4, use the differentiator (5).
, and is converted into an analog signal by a D/A converter (6) to obtain a more accurate demodulated signal.

In addition, in the above embodiment, the case where the digital filter (7) has a longer extension time than the 90 degree phase shifter (3) is shown.
On the other hand, the 90 degree phase shifter (3) or the digital filter (7)
If the delay time is longer than , as shown in Figure 2,
By providing the delay compensator (2) after the digital filter (7), the same effects as in the above embodiment can be achieved.

Also, if the delay times of the 90 degree phase shifter (3) and the digital filter (7) are the same, as shown in Figure 3,
There is no need to provide a delay compensator.

Furthermore, the 90-degree phase shifter (3) is not limited to a Hilbert transformer, and may be any 90-degree phase shifter whose amplitude characteristics can be flat within the frequency band required for implementation.

[Effects of the Invention] As described above, according to the present invention, a linear phase digital filter having an amplitude characteristic approximated within a frequency band required for the amplitude characteristic of a 90-degree phase shifter constituting an FM demodulator is provided. By providing this, the amplitude characteristics of the 90-degree phase shifter can be corrected without increasing the number of taps and increasing the circuit scale, and the internal circuit configuration allows for more accurate FM than the conventional example. The effect of demodulation (obtaining No. 3) is increased.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram of an FM demodulator according to an embodiment of the present invention, FIGS. 2 and 3 are block circuit diagrams of an FM demodulator according to other embodiments of the invention, and FIG. 4 is a block circuit diagram of a conventional FM demodulator. FM demodulator block circuit IA, FIG. 5 is a diagram showing the amplitude characteristics of a 90 degree phase shifter. (+)... A/D converter, (3)... 90 degree phase shifter, (4)... Arithmetic unit, (5)... Differentiator, (6
)...D/A converter, (7)...Digital filter. In the drawings, the same reference numerals indicate circuits or corresponding parts. Agent Masuo Oiwa Procedural amendment (voluntary) Kuchishowa 6 Cloth November 1st, 11th, at the Office of the Chief Judicial Licensor's Palace.
Indication of the case: Japanese Patent Application No. 145854/1982 2, Name of the invention FM demodulator 3, Representative Moriya Shiki 4, Agent 5, Detailed explanation of the invention I'll refer to the description of the subject matter of the amendment Column 6 of J, content of correction A, details: (1) Page 1, line 20 to page 2, line 1; "Digital signal...FM signal" was replaced with "analog FM signal" Convert it to a digital signal and correct it. (2) Page 9, line 5; ``Cannot'' be changed to ``Not''. (3) Page 9, line 6; “Good” should be corrected to “effective.” that's all

Claims (1)

[Claims] (1) An A/D converter that converts an input analog FM signal into a digital signal, a 90 degree phase shifter that shifts the phase of the converted digital signal by 90 degrees, and a 90 degree phase shifter that inputs the digital signal. When a linear phase digital filter has an amplitude characteristic approximated within the frequency band required for the amplitude characteristic of the phase shifter, the output of the digital filter is X, and the output of the 90 degree phase shifter is Y, When inputting these X and Y, tan^-^1(X/Y)
FM demodulation characterized by comprising: a computing unit that performs calculations; a differentiator that differentiates the output of this computing unit; and a D/A converter that converts a digital signal output from the differentiator into an analog signal. vessel.