JP2575057B2 - FM modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an FM modulator used for recording a video signal, performing wireless communication, and the like, and particularly applicable to a large frequency shift. It is intended to obtain a wide and stable FM modulation output.

[Related Art] When a video signal is recorded on an optical disk, a VTR, or the like, the video signal is usually once FM-modulated and then recorded.

As the FM modulator used in such a case,
The multivibrator type shown in the figure and the frequency conversion type shown in FIG. 5 are well known.

The FM modulator 10 shown in FIG.
The base terminal 3 is supplied with a video signal as a modulation signal in common, and an FM modulation output is obtained from the terminal 4.

The FM modulator 10 shown in FIG. 5 includes an FM modulator 6 having a center frequency of f1 and an FM modulator 7 having a center frequency of f2.
These FM modulation outputs are frequency-mixed by the frequency converter 8. The low-pass filter 9 extracts only the difference frequency (f1-f2) from the FM modulation output after frequency mixing.

The direction of the frequency shift is opposite between the FM modulators 6 and 7, and when one of the FM modulation frequencies increases for a positive input,
The other acts to decrease. Therefore, the final
The difference frequency (f1-f2), which is the FM modulation output, is actually the sum frequency of each.

[Problems to be Solved by the Invention] In the above-described FM modulators 10 conventionally used, since all are processed in a pure analog manner, particularly: (1) Non-linearity of input voltage versus output frequency ( 2) Higher-order distortion, especially second-order distortion, included in the FM modulated output wave. (3) Oscillation frequency stability, especially stability due to temperature characteristics. Absent.

In order to improve these, various adjustments and compensations are required, but sufficient accuracy has not yet been obtained.

Therefore, the present invention proposes an FM modulator that solves such a problem.

Means for Solving the Problems The present invention is an FM modulator that performs FM modulation by integrating a modulation signal and then performing phase modulation, and includes an adding unit and a register unit. Integrating means for converting the input analog modulated signal into a digital integrated output by cyclically adding the analog modulated signal with the adding means; carrier signal generating means for generating a carrier signal; Phase modulating means for phase modulating the output of the integrating means. The phase modulating means, based on the output of the integrating means,
Digital modulation signal output means for outputting first and second digital modulation signals having a quadrature phase relationship with each other;
And a carrier signal converting means for converting the signal into a second carrier signal, a first digital modulation signal and a first carrier signal, and multiplying the first digital modulation signal by the first carrier signal. A first D / A converter with a multiplying function for outputting an analog signal corresponding to the result, a second digital modulation signal and a second carrier signal are input, and the second modulation signal and the second carrier signal are input. Second D / A conversion means with a multiplication function for outputting an analog signal corresponding to a result of multiplication with a signal, and first and second D / A with a multiplication function
A calculating means for adding or subtracting the output of the converting means.

[Function] The FM modulator according to the present invention converts the input analog modulated signal into a digital integrated output by cyclic addition, and based on the integrated output, the first and second signals having a quadrature phase relationship with each other. An analog signal corresponding to the result of multiplication of the first digital modulated signal and the first carrier signal by converting the carrier signal into first and second carrier signals having a quadrature phase relationship with each other And outputs an analog signal corresponding to the result of multiplication of the second digital modulation signal and the second carrier signal. By adding or subtracting these outputs, the phase of the carrier signal is changed every cycle. ,
It can be changed according to the amplitude of the input means modulation, and an FM modulation output equivalent to the frequency modulation of the carrier signal by the modulation signal can be obtained.

Embodiment An example of an FM modulator according to the present invention will be described below in detail with reference to FIG.

The FM modulator 10 includes an integrator 20 for integrating a modulation signal supplied to a terminal 21 and a phase modulator 30 for phase-modulating the integrated output. The modulation signal may be a video signal or the like.

This FM modulator 10 is a digital process and an integrator 20
Are also digitally processed. Therefore, the integrator 20 has an A / D converter 22, and converts the video signal supplied to the terminal 21 into a digital signal having a predetermined number of bits, in this example, 8 bits.

The digitized video signal is added to the video signal one clock before output from the register 23 in the adder 24.

The adder 24 is a 2n-bit (n is an integer) configuration adder. In this example, n = 5. Therefore, the 8-bit video signal is input to the lower 8 bits, and the remaining 2 bits are input as 0. Then, this addition output (10-bit configuration) is input to the register 23 again.

By sequentially adding the video signals one clock before in this way, an integrated digital video signal is obtained from the register 23.

The clock CK used in the A / D converter 22 and the register 23 is a reference oscillator composed of a crystal oscillator or the like (in this example, a video signal whose time axis is expanded by a factor of 15 is used. 2.5MHz is used as the frequency.) The output from 50 is used. Clock CK is terminal 25
Supplied by

A clear signal is supplied to a terminal 26 provided in connection with the register 23, whereby the contents of the register 23 are initialized.

This also takes into account that a video signal from which a DC component is lost is supplied to the terminal 21. Even when there is no DC component, once the contents of the register 23 are reset once every horizontal cycle in the sync chip portion of the horizontal synchronization signal, the initial value of the register 23 is fixed, so the integrated value at the sync chip level Can be fixed.

The digitally integrated video signal is supplied to the phase modulator 30.

The phase modulator 30 is provided with a pair of waveform conversion ROMs 32 and 33, and converts an input digital video signal into two digital video signals having a quadrature phase relationship with each other.

That is, the amplitude values (digital signals) corresponding to the cosine wave and the sine wave as shown in FIG. 2 are stored in the respective waveform conversion ROMs 32 and 33, and the amplitude values corresponding to the level of the input digital video signal are simultaneously referred to. Then, two digital video signals (cosine digital video signal cos (c) and sine digital video signal sin (c)) which are orthogonal to each other are output. Here, the phase c corresponds to the level of the input digital video signal.

Each of the cosine digital video signal cos (c) and the sine digital video signal sin (c) functions as a modulation signal as described later.

The cosine digital video signal cos (c) and the sine digital video signal sin (c) are composed of first and second 2n-bit signals.
Are supplied to the D / A converters 35 and 36 of FIG. An analog carrier signal is supplied to the first and second D / A converters 35 and 36 in addition to the digital video signal.

In this example, the reference clock signal CK from the reference oscillator 50 is once supplied to the band-pass filter 51 and converted into a sine wave signal sin (2πfct) having the same frequency as the reference clock signal CK.

The reference oscillator 50 and the band pass filter 51 constitute a carrier signal generating means.

 Here, π is pi, and t is time (the same applies hereinafter).

The sine wave signal sin (2πfct) is supplied to the quarter-period delay device 31, and the sine wave signal sin (2πfct) input thereto is
Cosine wave signal cos (2π
fct) is output.

With the presence of the quarter-period delay unit 31, the sine wave signal sin (2π
fct) has been converted into first and second carrier signals (sine carrier signal sin (2πfct) and cosine carrier signal cos (2πfct)) having a quadrature phase relationship.

The sine carrier signal sin (2πfct) and the cosine digital video signal cos (c) are supplied to the first D / A converter 35, and the cosine carrier signal cos (2πfct) and the sine digital video signal sin (c) are supplied to the first D / A converter 35. Are supplied to the second D / A converter 36.

The D / A converters 35 and 36 are designed to control the output amplitude by an input carrier signal when converting an input digital video signal into an analog signal. The D / A converter has a four-quadrant multiplying function. It is a converter.

 Accordingly, sin (2πfct) · cos (c) (1) is output from the first D / A converter 35.

 The second D / A converter 36 outputs cos (2πfct) · sin (c) (2).

Each analog conversion output is added by an analog adder 37. The output of the analog adder 37 is as follows.

sin (2πfct) · cos (c) + cos (2πfct) · sin (c) = sin (2πfct + c) (3) Thus, the sine carrier signal whose phase is advanced by c with respect to the sine carrier signal sin (2πfct) sin (2πfct +
c) is output. This sine carrier signal sin (2πfct
+ C) is band-limited by the band-pass filter 38.

In the sine carrier signal sin (2πfct + c) obtained at the output terminal 39 in this manner, the phase is rapidly increased (1) according to the amplitude of the input video signal with respect to this carrier signal every cycle of the carrier signal. / fc time), and as a result, FM modulation can be performed.

The sine carrier signal sin (2πfct) and the cosine carrier signal cos (2πf) input to the D / A converters 35 and 36 described above.
The phase resolution of ct) depends on the bit configuration of the D / A converters 35 and 36, respectively. For example, if the D / A converters 35 and 36 each have a 10-bit configuration, the phase resolution is 0.35 ゜ (= 360 ゜ ÷ 1023).

The relationship between the minimum phase change dc per unit time and the frequency change df is expressed by the following equation.

df = (1 / 2π) (dc / dt) (4) Accordingly, the relationship between the minimum phase change dc per unit time and the maximum frequency shift Δf is as follows.

Δf = df (2 ⁸ −1) (5) Therefore, dc = 6.14 × 10 ⁻³ radian (6) dt = 400 nsec (= 1 / fc = 2.5 MHz) (7), Δf = 0.623 MHz (8) df = 2443 Hz (9), and the input voltage and the output frequency have a completely linear relationship. That is, it has a linear characteristic.

In the above example, the carrier frequency fc is set to 2.5 MHz, this frequency is applied to the sync tip level of the video signal, and the maximum frequency shift Δf is set to 0.623 MHz, so that FM modulation is performed in a direction to increase the frequency. It is.

 FIG. 3 shows another example of the present invention.

In the figure, a sine carrier signal sin (2πfct) is supplied to an attenuator 41 and its input level is 1 / (2 ⁿ -1).
And then supplied to the third D / A converter 42.
n is the number of bits, and is 5 bits in this example.

As the third D / A converter 42, a D / A converter having a multiplying function as described above is used. This includes a cosine digital video signal cos composed of 2n bits, that is, 10 bits. The lower 5 bits of (c) are supplied.

Therefore, in this example, the upper 5 bits of the cosine digital video signal cos (c) are supplied to the first D / A converter 35, and the first and third D / A converters 35, 42 may have a 5-bit configuration.

In the third D / A converter 42, the cosine digital video signal cos
The amplitude of (c) is modulated by the sine carrier signal, and then supplied to the adder 37.

Similarly, the cosine carrier signal cos (2πfct) is
, The input level is reduced to 1 / (2 ⁿ
-1), and then supplied to the fourth D / A converter 44.

The fourth D / A converter 44 also uses a D / A converter having a multiplying function, and is supplied with the lower 5 bits of the sine digital video signal sin (c). Then, the upper 5 bits of the sine digital video signal sin (c) are supplied to the second D / A converter 36.

Now, the maximum amplitude of the carrier signal is n bits, that is, 5 bits.
Disassembling a bit, the size per bit becomes 1 / the maximum amplitude of the carrier signal (2 ⁵ -1). Therefore, the minimum resolution amplitude of the first D / A converter 35 is further decomposed by 5 bits by the attenuator 41 and the third D / A converter 42. As a result, the pair of D / A converters 35 and 42 and the attenuator 41 function as a 2n-bit D / A converter.

Therefore, according to this configuration, a D / A converter having a 5-bit configuration can be used, and the price is very low.

The present invention is not limited to the embodiment described above. For example, sine and cosine signals have a 1/4 phase
Since the signals are exactly the same just by shifting the period, even if the sine wave signal and the cosine wave signal are exchanged in the above-described embodiment, the same effect can be obtained.

Also, the D / A converters 35, 36, 42, and 44 may be configured to multiply sine waves and cosine waves.

The analog adder 37 may perform a subtraction process instead of the addition process.

[Effects of the Invention] As described above, according to the present invention, the first and second digital modulation signals, each having a quadrature phase relationship, and the first and second carrier signals, also having the quadrature phase relationship, Are converted into analog signals by multiplying each other, and an output obtained by adding or subtracting each analog signal can be used as an FM modulation output. Therefore, since FM modulation of digital processing, which is performed every one cycle of the carrier signal, is performed, a linear characteristic can be improved, a high-order distortion-free FM modulator with good temperature characteristics and high reliability can be realized. .

[Brief description of the drawings]

1 and 3 are block diagrams showing an example of an FM modulator according to the present invention, FIG. 2 is a diagram showing data contents of a ROM, and FIGS. 4 and 5 are systems of a conventional FM modulator. FIG. 10 FM modulator 20 Integrator 30 Phase modulator 31 Delay device 32,33 Sine and cosine ROM 35,36,42,44 D / A converter 50 Reference oscillator

Claims (1)

(57) [Claims] 1. An FM modulator for performing FM modulation by integrating a modulation signal and then modulating the phase by performing phase modulation, comprising an adding means and a register means, wherein an output of the register means and an input analog modulated signal are inputted. Integrating means for converting the input analog modulated signal into a digital integrated output by cyclically adding by the adding means; carrier signal generating means for generating a carrier signal;
And a phase modulating means for phase modulating the output of the integrating means based on the carrier signal, wherein the phase modulating means has first and second quadrature phase relations based on the output of the integrating means. Digital modulation signal output means for outputting a second digital modulation signal; carrier signal conversion means for converting the carrier signal into first and second carrier signals having a quadrature phase relationship with each other; A D / A converter with a first multiplication function for receiving a digital modulation signal and the first carrier signal and outputting an analog signal corresponding to a result of multiplication of the first digital modulation signal and the first carrier signal Means, and the second digital modulated signal and the second carrier signal are input, and the second modulated signal and the second carrier signal are A second D / A converter with a multiplying function that outputs an analog signal corresponding to the result of the calculation; and a calculating unit that adds or subtracts the outputs of the first and second D / A converters with a multiplying function. , FM modulator.