JPH05344020A - Frequency control system - Google Patents

Abstract

PURPOSE:To receive a signal of a satellite line in which a frequency fluctuation is very large by using a simple and low-cost oscillation circuit. CONSTITUTION:A control section 6 sets a frequency of an oscillated output signal L2 of a voltage controlled oscillator section (VCXO) 7 to a center frequency at the reception initial stage and controls a frequency synthesizer section 5 to allow the section 5 to sweep the frequency of a local oscillation signal L1 at a prescribed step over a frequency fluctuation range of a reception signal S1. The section 6 observes an AGC control signal Vc from a demodulation section 4 during the frequency sweeping to detect a point where a reception signal level is maximized thereby setting the frequency of the local oscillation signal L1. Then the control section controls the voltage controlled oscillator section 7 to allow the section 7 to sweep the frequency of the oscillation power supply L2 in a fine range and it is continued till the demodulation section 4 takes synchronization with a desired signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency control method for selectively receiving a desired signal from a group of frequency division multiplexed modulated waves, and particularly to satellite communication etc. in which the frequency fluctuation of a transmission system and equipment is extremely large. The present invention relates to an applied frequency control method.

[0002]

2. Description of the Related Art Generally, when a desired signal is selectively received from a group of frequency-division-multiplexed modulated waves, the receiver side is required to cope with large frequency fluctuations in the transmission system and equipment.
An FC oscillator is provided. A method is used in which the frequency of the AFC oscillator is changed by means such as sweeping, and when the desired signal is detected, the sweeping is stopped and received.

By the way, in a satellite circuit, one transponder may be shared by a plurality of signals. When the frequency fluctuation of each signal is large, adjacent signals are adjacent to each other if they use the same modulation method. It becomes impossible to distinguish from the signal, resulting in incorrect synchronization.

In order to prevent such a situation, a predetermined frequency interval is set between adjacent signals. Now, assuming that the signal frequency interval (channel separation) is Fs and the frequency fluctuation is ΔF, the maximum frequency sweep width Fw of the AFC oscillator for preventing an adjacent signal from being captured.
Is Fw <Fs-ΔF (1).

On the other hand, the frequency stability of the AFC oscillator is α
(Α is a positive real number), Fw = (1 + α) ΔF ...
… (2)

Therefore, from equations (1) and (2), Fs> F
w + ΔF = 2ΔF + αΔF (3)

That is, the channel separation Fs
Indicates that at least twice the frequency variation ΔF is required, and a larger interval is required depending on the frequency stability α of the AFC oscillator.

The frequency stability of the oscillator for AFC is determined by the temperature characteristics and the temporal variation, and if the frequency stability is good, the channel separation can be reduced, and therefore the frequency utilization efficiency is improved.

[0009]

As described above, in order to improve the utilization efficiency by sharing one transponder with a plurality of signals in a satellite line whose frequency fluctuation is extremely large,
As the performance of the AFC oscillator provided on the receiving side, a large frequency sweep width and high frequency stability are required. However, in general, the greater the frequency sweep width of the AFC oscillator, the worse the frequency stability. Therefore, there is a problem in that the circuit configuration becomes complicated and the cost becomes high in order to secure high frequency stability.

It is an object of the present invention to use a simple low-cost oscillator without increasing the frequency stability of the AFC oscillator provided on the receiving side, and to control the frequency of a satellite line signal with extremely large frequency fluctuations. To provide a method.

[0011]

The frequency control system of the present invention comprises an AGC section which operates according to an AGC control voltage and controls the level of a received signal to be constant, and an output signal of this AGC section and a local oscillation signal. A mixer for mixing and converting to an intermediate frequency signal, a demodulator for demodulating the intermediate frequency signal to output a desired signal and generating the AGC control voltage, and a reference signal to generate the local oscillation signal. A frequency synthesizer unit, a voltage control oscillator unit that sends an oscillation output signal as the reference signal, and a control unit that controls the frequency synthesizer unit and the voltage control oscillator unit while observing the AGC control voltage. It Further, the control unit controls the frequency synthesizer unit by setting the oscillation output signal of the voltage controlled oscillation unit to a center frequency in an initial stage of reception, and controls the frequency synthesizer unit to a predetermined frequency step over a frequency variation range of the received signal. To sweep the frequency of the local oscillation signal, observe the AGC control voltage, set the frequency of the local oscillation signal to the point where the received signal level becomes maximum, and then control the oscillation frequency of the voltage controlled oscillation unit. Then, the sweep is performed in a minute range slightly larger than the predetermined frequency step, and the sweep is continued until the demodulator synchronizes with the desired signal.

[0012]

The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. The reception signal S1 is converted into a signal of a constant level by the AGC unit 1, and converted into an IF signal (intermediate frequency signal) S2 by the mixer unit 2. I passed through the bandpass filter unit 3
The F signal S3 is demodulated by the demodulation unit 4.

Here, the mixer section 2 receives the local oscillation signal L1 generated by the frequency synthesizer section 5, and receives the received signal S.
The channel including the desired signal is selected from 1 and transmitted as the IF signal S2. Further, the demodulation unit 4 uses the IF signal S3.
And the level of the IF signal S3 is detected, A
The GC control voltage Vc is generated to generate the AGC unit 1 and the control unit 6.
Send to. This AGC control voltage Vc changes according to the input level of the reception signal S1.

The frequency synthesizer section 5 receives the oscillation output signal L2 from the voltage controlled crystal oscillation section (VCXO) 7 and uses this as a reference signal to generate a local oscillation signal L1. The voltage controlled crystal oscillator 7 can change the frequency in a minute range while ensuring frequency stability. In addition, the control unit 6
Controls the frequency synthesizer unit 5 and the voltage controlled crystal oscillator unit 7.

Next, the operation will be described.

In the initial stage of reception, the control unit 6 sets the oscillation frequency of the voltage controlled crystal oscillation unit 7 as the center and controls the frequency synthesizer unit 5 so that the frequency range of the received signal S1 or more is exceeded. The frequency of the local oscillation signal L1 is swept in a predetermined step. During this frequency sweep, the control unit 6 observes the AGC control voltage Vc from the demodulation unit 4 and selects the channel including the desired signal by detecting the point at which the received signal level becomes the maximum. Set to the frequency of the signal L1.

After that, the voltage controlled crystal oscillating unit 7 is controlled to sweep the frequency of the oscillation output signal L2 in a minute range, and the demodulating unit 4 sweeps until the desired signal is synchronized. It is sufficient that the sweep frequency range at this time is slightly larger than the frequency step interval of the frequency synthesizer unit 5. Therefore, the frequency variable range of the voltage controlled crystal oscillating unit 7 can be small, and the stability can be secured even with a simple circuit configuration. Note that once the demodulator 4 is synchronized, the frequency of the variable oscillator 8 may be controlled by the frequency error information Se of the demodulator.

[0019]

As described above, according to the present invention, the received signal, which has a constant level by the AGC section operating according to the AGC control voltage from the demodulation section, is mixed with the local oscillation signal to obtain the intermediate frequency signal. When receiving the desired signal by demodulating this intermediate frequency signal by the demodulation unit, a local synthesizer signal is generated by the frequency synthesizer, and the oscillation signal of the voltage controlled crystal oscillator unit is set to the center frequency at the initial reception stage. A local oscillation signal that is set and supplied to a frequency synthesizer, generates a local oscillation signal that sweeps at a predetermined frequency step over the frequency fluctuation range of the received signal, and a channel including a desired signal can be selected by observing the AGC control voltage. After setting the oscillation frequency, sweep the frequency of the voltage-controlled crystal oscillator in a very small range (a little larger than the specified frequency step). By synchronizing to the desired signal by, even a simple variable oscillation circuit such as a voltage controlled crystal oscillator, it is possible frequency variable to ensure frequency stability, cost reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

[Explanation of symbols]

 1 AGC section 2 Mixer section 4 Demodulation section 5 Frequency synthesizer section 6 Control section 7 Voltage control crystal oscillation section (VCXO) L1 Local oscillation signal L2 Oscillation output signal S1 Received signal S2, S3 IF signal (intermediate frequency signal) Vc AGC control voltage

Claims (2)

[Claims] 1. An AGC section which operates according to an AGC control voltage to control a level of a received signal to be constant, and a mixer section which mixes an output signal of the AGC section and a local oscillation signal and converts the mixed signal into an intermediate frequency signal. A demodulation unit for demodulating the intermediate frequency signal to output a desired signal and generating the AGC control voltage; a frequency synthesizer unit for receiving a reference signal to generate the local oscillation signal; and an oscillation output signal for the reference signal. A frequency control system, comprising: a voltage control oscillator for transmitting as a signal and a controller for controlling the frequency synthesizer unit and the voltage control oscillator while observing the AGC control voltage. 2. The control unit, in an initial stage of reception,
The oscillation output signal of the voltage controlled oscillation unit is set to the center frequency to control the frequency synthesizer unit, and the frequency of the local oscillation signal is swept at a predetermined frequency step over the frequency fluctuation range of the received signal, By observing the AGC control voltage, the frequency of the local oscillation signal is set to a point where the received signal level becomes maximum, and then the oscillation frequency of the voltage controlled oscillation unit is controlled to be slightly larger than the predetermined frequency step. 2. The frequency control method according to claim 1, wherein the sweeping is performed in a minute range, and the sweeping is continued until the demodulation unit is synchronized with the desired signal.