JPS6057722B2 - FM modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency modulation device (hereinafter referred to as an FM modulation device) whose center frequency is stabilized by a reference crystal oscillator.

Generally, FM modulators apply FM modulation to a self-excited oscillator, so the stability of the center frequency is determined by the characteristics of the automatic oscillator itself, and the frequency stability of the FM modulator is relatively good.
Even with the M modulator, its stability is 1 when the operating temperature range is included.
It is widely known that the temperature is 0-1 degrees.

Therefore, depending on the purpose of use, automatic frequency control (hereinafter referred to as AF)
It is common practice to add a circuit (referred to as C) to improve the frequency stability of a self-excited oscillator.

As an AFC circuit, either a method using a frequency discriminator or a phase synchronization method in which a reference frequency and an automatic oscillator output are phase-detected are widely used.

Recent FM modulators are microwave band transmitters and receivers, and frequency stability is strictly required. Therefore, there is an increasing demand for devices with high frequency stability based on crystal oscillators. Hereinafter, a conventionally used FM modulation device will be explained in detail with reference to the drawings.

FIG. 1 is a block diagram showing an example of a conventional FM modulation device.

In the figure, 1 is a modulation signal input terminal, 2 is a signal level adjuster, 3 is a modulation signal amplifier, 4 is a low-pass filter, 5 is an FM modulator, 6 is an output terminal of the FM modulation carrier, and 7 is a reference crystal oscillator. , 8 and 9 are frequency dividers, 10 is a phase detector, 1 is an amplifier, and 12 is a loop filter, respectively. The outline of its operation is that a modulated signal supplied to a terminal 1 passes through a signal level adjuster 2 that sets the degree of FM modulation, and is amplified to a predetermined level by a modulated signal amplifier 3.

Further, harmonic components of the modulated signal are removed by a low-pass filter 4 having a predetermined frequency band, and frequency modulated by an FM modulator 5. The FM modulated carrier is taken out from the output terminal 6, but a part of the output is branched and supplied to the frequency divider 8. The crystal oscillator 7 is a reference frequency generator for controlling the center frequency stability of the pre-digital M modulator 5, and the output of the frequency divider 9 that follows it is such that the output frequency is the same as the output frequency of the frequency divider 8. selected as the frequency division ratio. The phase detector 10 detects the phase difference between the output of the frequency divider 8 and the output of the frequency divider 9, and its output passes through an amplifier 11 and a loop filter 12 that determines the loop characteristics of the AFC system. street, front
The signal is fed back to the M modulator 5 and constitutes an AFC system. It is widely known that the phase detector 10 operates normally as long as the phase difference between the two input signals does not exceed π/2. Therefore, it is necessary to prevent the phase difference between the outputs of both frequency dividers 8 and 9 from exceeding w/2 even instantaneously. Frequency divider 8 is provided for this purpose, and F
By frequency dividing the M modulated wave, the objective is to reduce the FM modulation index by the frequency division ratio. Therefore, this frequency division ratio is selected so that the instantaneous phase deviation position of the FM modulated wave at the input of the phase detector 10 is π/2 or less. On the other hand, since the input signal to the phase detector 10 is an unmodulated signal from the reference crystal oscillator and an FM modulated wave with a shallow modulation depth, the phase detector 10
The FM modulation signal component is demodulated to the output of the FM modulation signal component. For the purpose of AFC, a DC signal component corresponding to the fluctuation part of the center frequency of the carrier is required, and the demodulated signal component is unnecessary, so it is necessary to prevent it from returning to the FM modulator, which is blocked by the loop filter 12. be. For example, when the modulated signal is a multiplex telephone signal, the lower limit frequency is usually 4 KHz or higher, so the loop filter has a characteristic of blocking modulated signals of 4 KHz or higher.

On the other hand, in the case of an audio signal, the lower limit frequency is 300 Hz, and in the case of a program audio signal, the lower limit frequency is 50 Hz.

In this way, the characteristics of a loop filter exist at the lower limit frequency of the modulated signal, so in order to sufficiently block signal components of 50 Hz or higher, for example, the loop bandwidth should be set to 11% of the lower limit blocking frequency.
It is normal to design it to 10 to 1'2 degree, and 2.5 to 1'2 degree.
This results in a loop bandwidth of about 5 Hz. In the case of extremely narrow loop characteristics like this, sufficient consideration is required because the electrical or mechanical forces added to this circuit become weaker, and the structure becomes more electrically and mechanically complex. I become confused.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide an FM modulator having a stable N° C. system.

In order to achieve the above object, the FM modulation device according to the present invention connects a part of the output of the FM modulator to a phase detector via a frequency divider, performs phase comparison with a reference frequency, and detects the modulation input signal. A part of the signal is branched and added to the output of the phase detector in the opposite phase, canceling the demodulated signal component, and returning to the FM modulator via a loop filter to form an N° C. system.

FM of the present invention! According to the Ii modulation device, the demodulated signal can be canceled out and the characteristics of the loop filter can be sufficiently widened, so that the object of the present invention can be completely achieved. Next, the FM modulation device according to the present invention will be explained in detail with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of the FM modulation device according to the present invention.

Components with the same functions as those in FIG. 1 are given the same reference numerals.
In the same configuration as in FIG. 1, a level adjuster 21 made up of a variable resistor or the like, a phase inverter 22 and a synthesizer 23 made up of a transformer, a transistor, etc. are inserted. As described above, unnecessary modulation signal components are demodulated into the output of the phase detector 10. This demodulated signal is offset by supplying a signal that has the same amplitude as this demodulated signal component and is in an opposite phase relationship. Furthermore, this FM modulator has a stable λMC system by sufficiently widening the characteristics of the loop filter. A part of the input modulation signal is branched and sent to the level adjuster 21.
The amplitude is adjusted so that it has the same amplitude as the level of the demodulated signal demodulated by the phase detector 10. Further, a phase inverter 22 is provided to maintain an opposite phase relationship with the demodulated signal. Since this signal is combined with the demodulated signal in the combiner 23, ideally the demodulated signal from the phase detector will not appear in the combiner output. Therefore, as described above, the loop filter 12 does not depend on the lower limit frequency of the modulation signal, so it becomes possible to widen the band, and the operation of the M° C. system becomes more stable. As described above, according to the present invention, the FM modulator is provided to stabilize the center frequency of the FM modulator. In a phase-locked AFC system, by combining a part of the modulated input signal with the same amplitude and opposite phase to the signal demodulated by the phase detector and canceling out the demodulated signal, the band of the loop filter can be kept wide and stable. It is possible to provide an FM modulator having a M° C. system. The same effect can be obtained by using an operational amplifier instead of the phase inverter 22 and combiner 23 shown in FIG.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional FM modulation device. FIG. 2 shows an embodiment of an FM modulation device according to the present invention. 1...Modulation signal input terminal, 2...
Signal level adjuster, 3...Modulation signal amplifier, 4
...Low pass filter, 5...FM modulator,
6...FM modulation carrier output terminal, 7...
... Reference crystal oscillator, 8, 9 ... Frequency divider, 10
...Phase detector, 11...Amplifier, 12
...Loop filter, 21 ... Lebel adjuster, 22 ... Phase inverter, 23 ... Synthesizer.

Claims (1)

[Claims] 1 Connect a part of the output of the FM modulator to a phase detector via a frequency divider, perform phase comparison with a reference frequency, branch out a part of the modulation input signal, and perform the phase detection with the opposite phase. In addition to the output of the FM modulator, the demodulated signal component is canceled out and returned to the FM modulator via a loop filter to form an AFC system.