JP2901427B2 - FM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulator,
In particular, the present invention relates to an FM demodulator having a simplified configuration.

[0002]

2. Description of the Related Art As FM demodulators, various FM demodulators employing a digital signal processing format which is advantageous in terms of stability, reliability, and simplicity of adjustment as compared with an analog signal processing format have been proposed. I have.

FIG. 9 shows a typical configuration example, which is referred to as a TAN type.
Suitable as FM demodulator for M broadcast receiver. In FIG. 9, an input FM signal is converted into a digital signal by an A / D converter 101 and then input to a 90-degree phase shifter 102.
The arc tan calculator 103 obtains a phase component from two signal components having different 90-degree phases supplied from the 90-degree phase shifter 102. The differentiator 104 obtains a demodulated output by obtaining a difference between the phase components.

[0004]

As described above, the conventional digital demodulator includes a 90-degree phase shifter 102 and an arc tan.
Since the calculator 103 is included, there is a problem that the circuit scale becomes large. Since a sampling frequency of about 1 MHz is required, the arc tan calculation unit 103
Has not been put to practical use because of the problem that the calculation speed of must be high.

An object of the present invention is to provide an FM demodulator which solves the above problems and has a simplified configuration.

[0006]

In order to solve the above-mentioned problems, an FM demodulator according to the present invention comprises an input signal having a sampling frequency fs which is approximately 1/4 or 3/4 of a carrier frequency fc of an input FM signal. An A / D converter for sampling the FM signal, and a selector for alternately outputting a first output signal from the A / D converter and a second output signal that is -1 times the first output signal. A delay device for delaying an output signal from the selector for a predetermined time, a low-pass filter receiving the output signal from the selector as an input, and a phase component of the FM signal based on an output of the low-pass filter and an output of the delay device. , And a differentiator for substantially differentiating the output of the phase calculator and outputting a demodulated signal. The delay unit and the low-pass filter can be replaced with one low-pass filter.

[0007]

According to the present invention, a low-pass filter is provided based on a signal obtained by sampling an input FM signal at a sampling frequency fs substantially 1/4 or 3/4 of the carrier frequency fc of the input FM signal and its complement signal. , A phase calculator and a differential (difference) device.

[0008]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an FM demodulator according to the present invention. The input FM signal is converted into a digital signal by the A / D converter 1.
The complement selector 2 selects and outputs the output signal from the A / D converter 1 as it is or by multiplying the output signal by -1. The output of the complement selector 2 is sent to the delay unit 3 and the low-pass filter 4. The delay unit 3 includes a low-pass filter 4
The input signal is delayed by half the delay amount of the delay device represented by The arc tan calculator 5 obtains a phase component based on output signals from the delay unit 3 and the low-pass filter 4. The difference of the obtained phase component is calculated by the differentiator 6 to obtain a demodulated output. In this embodiment, the function of a 90-degree phase shifter is realized by the complement selector 2, the delay unit 3, and the low-pass filter 4.

The complement selector 2 can be configured as shown in FIG. 2, for example. The input digital signal is input to the minus input terminal of the subtractor 21 and the A input terminal of the selector. Subtractor 21
"0" is supplied to the + input terminal of the selector 22, and its output is supplied to the B input terminal of the selector 22. Selector 22
Outputs one of the signals supplied to the two input terminals. The above-described TAN-type FM demodulator performs an arc tan operation on an input FM modulated signal and its orthogonal signal to extract a phase component, and differentiates the phase component using a differentiator to output an instantaneous frequency. Demodulator. As a method for obtaining the quadrature signal, a configuration in which frequency conversion is performed by a two-phase quadrature signal as shown in FIG.

In FIG. 3, an input signal is multiplied by cos ω ₁ t and sin ω ₁ t by multipliers 31 and 32, respectively, and then predetermined frequency components are extracted by band-pass filters 33 and 34 and output. Is done. In such a configuration circuit, the gain difference and the phase difference error between the two signals degrade the demodulation characteristics, so that the required accuracy cannot be obtained by the analog processing. Further, in the digital processing, there is a problem that the circuit scale is further increased in the generation of the two-phase quadrature signal (cos ω ₁ t, sin ω ₁ t) and in the configuration of the band-pass filter than in the case of configuring the 90-degree phase shifter.

By the way, the frequency f ₁ = ω ₁ /
When (2π) is 1/4 of the sampling frequency fs,
ω ₁ t = 2πf ₁ k / fs = πk / 2 (k: integer 1), and cos ω ₁ t, si
nω ₁ t is simply 1,0, -1,0, ... and 0,1,0,-
It becomes a sequence of 1 ... Therefore, the input FM modulation signal x
When the carrier frequency fc of (k) is close to fs / 4, x
The sequence of (k), 0, -x (k + 2), 0, x (k + 4),... And 0, x (k + 1), 0, -x (k + 3), 0,. By extracting only the frequency component of / 4 or less, a quadrature signal can be obtained.

FIG. 4 shows the spectrum of the FM modulation signal, and FIG.
Shows the spectrum after multiplication by the sine wave of fs / 4. Since the signal sequence after multiplication is 0 every other signal, the multiplexed sequence x (k), x (k + 1), -x (k
+2), −x (k + 3), x (k + 4),...

Digital FM obtained in consideration of the above points
FIG. 6 shows a configuration example of the demodulator. In the figure, components denoted by the same reference numerals as those in FIG. 1 are components having the same functions. FIG. 7 shows a case where the characteristics of the low-pass filter 4 are restricted.
In the case where the sampling frequency fs of the A / D converter 1 is decimated to １ ／, the input sequence of the 10th-order FIR filter shown in FIG. From this, since it is possible to divide into only an even-numbered sample and an odd-numbered sample, a configuration as shown in FIG. 8 is obtained.

In the half band full shown in FIG.
The even sample is multiplied by the coefficient a0 in the multiplier 52, and the multiplication result is output to the adder 54. Also, the odd samples are multiplier coefficients 46,47,48,49,50 and _{51 a 5, a 3, a} 1, a -1, are respectively multiplied with a _-3 and a _-5. The output of the multiplier 46 is sent to the adder 53 after being delayed by one clock in the delay unit 41. The delayed signal is
After being added to the output of the multiplier 47 by the adder 53, the delay is delayed by one clock by the delay unit 42. The adder 54 adds the output of the multiplier 52, the output of the delay unit 42, and the output of the multiplier 48. The output of the adder 54 is similarly delayed by the delay unit 43 and output to the adder 55. The addition signal from the adder 55 is delayed by the delay unit 44 and output to the adder 56. The result of the addition by the adder 56 is delayed by the delay unit 45, and added to the output signal of the multiplier 51 by the adder 57 to obtain an output signal.

In the low-pass filter configuration shown in FIG. 8, delay units 61, 62 and 63 for delaying even-numbered samples are connected in series. The odd-numbered samples are supplied to multipliers 69 to 74, delay units 64 to 68, and adders 75 to
79. Multipliers 69, 70, 71, 7
2, 73 and 74 have coefficients 2a ₅ , 2a ₃ , 2a ₁ ,
2a _-1 , 2a _-3 and 2a _-5 are supplied as multiplication coefficients. In the configuration shown in FIG. 8, the 90-degree phase shifter is connected to the FIR
Filter order is almost 1 compared to the case where filter is used.
/ 2. Therefore, in the configuration shown in FIG.
A demodulator can be realized.

The difference unit 6 in FIG. 1 calculates a difference value Δθ (k) = θ (k) −θ (k−1) with respect to the input θ (k), and approximates the differentiation. For example, when the intermediate frequency is 10.7 MHz, when Fs = 912 KHz, the carrier frequency Fc of the sampling signal is 668 KHz.
Hz, which is an appropriate value close to 3fs / 4 = 684 KHz. Even if fs = 640KHz, fc = 460KHz and 3fs / 4 = 4
This is an appropriate value close to 80k.

[0017]

As described above, the F according to the present invention is used.
According to the M demodulator, a function equivalent to a 90-degree phase shifter can be realized with a substantially half circuit scale, and since the sampling frequency is reduced to half of the normal, the processing speed is not restricted. 9 more
Since the gain characteristics of the low-pass filter can be set arbitrarily as compared with the 0-degree phase shifter, it is suitable for a configuration in which the pass band of the filter is adaptively changed according to conditions such as the input CN ratio.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an FM demodulator according to the present invention.

FIG. 2 is a block diagram showing a configuration example of a complement selector 2 in FIG. 1;

FIG. 3 is a diagram illustrating an example of a circuit for generating an orthogonal signal.

FIG. 4 is a diagram illustrating a spectrum of an input FM signal.

FIG. 5 is a diagram illustrating a spectrum of a signal obtained by multiplying the FM signal of FIG. 4 by a sine wave of fs / 4.

FIG. 6 is a block diagram showing another embodiment of the FM demodulator according to the present invention.

FIG. 7 is a configuration block diagram of a half-band filter according to the embodiment of the present invention.

FIG. 8 is a configuration diagram of a low-pass filter according to the embodiment of the present invention.

FIG. 9 is a configuration block diagram of a conventional FM demodulator.

[Explanation of symbols]

1,101 A / D converter 2 complement selector 3,41-45,64-68 Delay unit 4 Low-pass filter 5,103 Arc tan calculator 6,104 Difference unit 21 Subtractor 22 Selector 31,32,46-52, 69-74 Multiplier 33,34 Band-pass filter 53-57,75-79 Adder 102 90-degree phase shifter

Claims (3)

(57) [Claims] 1. An input FM signal having a carrier frequency fc of about 1.
An A / D converter that samples the input FM signal at a sampling frequency fs of or 3/4 times; a first output signal from the A / D converter and −1 of the first output signal; A selector for alternately outputting a doubled second output signal; a delay unit for delaying an output signal from the selector for a predetermined time; a low-pass filter receiving an output signal from the selector as an input; and an output of the low-pass filter And a phase calculator for obtaining a phase component of the FM signal based on an output of the delay unit; and a difference unit for substantially differentiating the output of the phase calculator and outputting a demodulated signal. An FM demodulator characterized by the following. 2. The carrier frequency fc of the input FM signal is substantially equal to one.
An A / D converter that samples the input FM signal at a sampling frequency fs of or 3/4 times; a first output signal from the A / D converter and −1 of the first output signal; A selector that alternately outputs a doubled second output signal; one or more low-pass filters that receive an output signal from the selector; and a phase component of the FM signal based on an output of the low-pass filter An FM demodulator comprising: a phase calculator; and a differentiator for substantially differentiating an output of the phase calculator to output a demodulated signal. 3. The method according to claim 1, wherein the sampling frequency is 912 [KH
The FM demodulator according to claim 1 or 2, wherein the frequency is z] or 640 [KHz].