JPS6048819B2 - frequency modulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency modulation circuit that attenuates harmonics to obtain an FM modulation signal of only a fundamental wave.

In a VTR, a carrier wave of a predetermined frequency is generally FM-modulated with a video signal and a signal is recorded on tape.

As the FM modulator used in this case, a heterodyne modulator has conventionally been used. A heterodyne modulator changes the frequency of a VCO (voltage controlled oscillator) using a video signal.
By performing M modulation and converting the frequency of the modulated signal using the frequency of a local oscillator, an FM modulated wave of a desired frequency is obtained. This method requires a complicated circuit configuration and has problems with stability and linearity. Therefore, in a simple VTR, a multivibrator is used as a carrier wave oscillator. Therefore, the output is a rectangular wave and includes odd harmonics. In particular, the third harmonic has a high level and a wide sideband spread. On the other hand, signal transmission systems generally have frequency characteristics and phase characteristics, which cause some kind of distortion to the transmitted signal. In the transmission system of a VTR, when the rectangular FM signal is transmitted, even if the fundamental wave and its sidebands pass through, distortion occurs in the third harmonic and its sidebands. Such distortion cannot be removed even by demodulation on the receiving side. The present invention is intended to solve the above problems, and embodiments of the present invention will be described below with reference to the drawings.

A case will be described in which the third harmonic of frequency 3f included in this signal is removed from a rectangular carrier wave of fundamental frequency f. First, three waves are created, each having the same frequency as the third harmonic, different phases from each other, and each having a predetermined level. In this case, the phase relationship and level are selected so that when these three waves are combined, they become zero. An example of these three waves is three vectors a, , a as shown in FIG. , a. is possible. In FIG. 1, al and a2 are at level E and have a phase difference of π/4. Also, a3 has a level of KE and "a...
a. They have a phase difference of 3×π and −3×π, respectively. a, and a. When you combine them, the level! ' becomes vector a4 of 2E. Therefore, in order to cancel this a, it is sufficient to set it to -/2. The above relationship can be expressed by the following formula. Ee−j3l4π+KE+Eej3
I4π■E(+2cos3haπ 1 = E(Ni-ri/2) ■0 , . . ■VΣ Therefore, in order to remove the third harmonic, based on the carrier wave of the rectangular wave, A vector A,,a having the relationship as shown in .

, a3 and synthesize them. FIG. 2 shows an example of a circuit configuration for realizing the above principle.

In FIG. 2, an input terminal 1 receives a modulated signal S such as a video signal.

is added. A rectangular wave oscillator 2 composed of a multivibrator etc. oscillates at a frequency of 8f, and a modulated signal S
. The oscillation output is FMl modulated. FIG. 3A shows the output waveform of the oscillator 2. Note that this waveform is actually the signal S
. However, in order to simplify the drawing, it is shown as not being shifted. Therefore, each of the waveforms B to H in the figure actually undergoes a frequency shift corresponding to the shift of the output waveform. The modulated signal S shown in FIG. 3A obtained from the oscillator 2.

is applied to the 118 frequency divider 3 and triggers the D-type flip-flops 4, 5, 6. From frequency divider 3, the third
A rectangular FM signal S with a carrier frequency F, as shown in Figure B.
, is obtained. This signal S, contains the third harmonic of frequency 3f. This signal S, is the flip-flop 4
added to. This Q, output is applied to flip-flop 5, and its Q2 output is applied to flip-flop 6. Ru. These flip-flops 4, 5, 6 receive the signal S3
Therefore, each flip-flop 4, 5, 6 sequentially has a phase interval of π/4 as shown in FIG. 3C, D, and E. I get the output. These Q
, , Q2 and Q3 are shown in Fig. 3, respectively. The third harmonic of frequency 3f, as shown in F, G, H. Wave P,,P. ,P. Contains. In this case, these first and third harmonics P,,P2,
As shown in the figure, P3 advances by 314π with respect to P2, and P3
3 is delayed by 314π. That is, P,,
P. ,P. have the same level and the vector A in Fig.
,,a. ,a. They have a corresponding phase relationship.
Q, , Q2, Q3 outputs (in the diagram, these inverted outputs, Q
,,Q. , Q3 output) are combined in an adder 7. At this time, these are Q, :Q. :Q,=1:,/2:1
When synthesized at the ratio of , the third harmonics P,,P. , P3 is also 1
:, /2:1 ratio. As a result, in this adder 7, the vectors A, , a in FIG. , a3, that is, the calculation shown in the first equation above is performed, and P,,P. ，
P3 is canceled. As a result, the adder 7 outputs a signal S containing the fundamental wave from which the third harmonic has been removed, its sideband, and fourth and subsequent harmonic components. is taken out. This signal S4 is applied to a filter 8 to remove the fourth and subsequent harmonic components. As a result, a sinusoidal FM signal consisting only of the fundamental frequency F and its sidebands can be obtained from the output terminal 9. Note that the frequency of the oscillator 2 is set to 8 times the desired frequency F, but this is set to 4 times 4f, and the flip-flops 4, 5, and 6 are set to 4f,
The trigger may be triggered by the rising and falling edges of the rectangular wave. The present invention provides a frequency-modulated first square wave signal (for example, Q, output) and a second square wave signal (for example, Q, output) having a phase delay of π/4 with respect to the first square wave signal.
.

output) and a third rectangular wave signal (for example, Q. output) having a phase delay of π/4 with respect to the second rectangular wave signal, and Wave signal 1:V2:
This relates to a frequency modulation circuit characterized in that addition is performed at a rate of 1. Therefore, according to the present invention, it is possible to remove the third harmonic and obtain a sinusoidal FM signal consisting only of the fundamental wave and its sidebands, thereby removing distortion based on the harmonics. can.

[Brief explanation of the drawing]

FIG. 1 is a vector diagram showing the principle of the present invention, FIG. 2 is a circuit system diagram showing an embodiment of the invention, and FIG. 3 is a waveform diagram of each part of FIG. 2. In addition, in the symbols used in the drawings, 2...
・Square wave oscillator, 3... Frequency divider, 4, 5, 6.
. . . D-type flip-flop, 7 . . . Adder.

Claims (1)

[Claims] 1 A first frequency-modulated rectangular wave signal, a second rectangular wave signal having a phase delay of π/4 with respect to the first rectangular wave signal, and a π/4 phase delay with respect to the second rectangular wave signal. A third rectangular wave signal with a phase delay of /4 is generated, and the first and second
and a third rectangular wave signal at a ratio of 1:√2:1.