JPH02141127A - Frequency control circuit - Google Patents

Abstract

PURPOSE:To obtain frequency control suitable for various signals with different transmission speeds by constituting a low pass filter provided to a feedback loop with plural low pass filters connected in parallel and with different cut-off frequencies and switching the filters selectively with a controller in response to the transmission speed. CONSTITUTION:A phase frequency detector 4 is so constituted that 3 low pass filters 6A-6C are connected in parallel and a voltage controlled oscillator 9 is connected to one of the filters selectively by a switch 8. The cut-off frequency of the low pass filters 6A-6C differs from each other and the cut-off frequency is set larger in the order of the low pass filters 8. In the case of selecting the low pass filter 6A whose cut-off frequency is lowest, since the loop band of the feedback loop is made narrow to increase the frequency stability for a long period thereby enabling to pass through a signal of a low transmission speed. In the case of selecting the low pass filter 6C whose cut-off frequency is highest, since the loop band is widened, the frequency lock range is widened, the circuit is immune to the frequency drift and suitable for the high speed transmission.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to satellite communication network technology, and particularly to a frequency control circuit that is effective when the transmission speed of communication signals is diverse.

[Conventional technology]

Generally, in a satellite communication network composed of a plurality of earth stations, the receiving station 13 is connected to the transmitting station 1, as shown in FIG.
The communication satellite 14 not only receives the signals transmitted from the pilot signal transmission station 11 using the communication satellite 14, but also receives the pilot signal, which is an unmodulated signal transmitted from the pilot signal transmission station 11.
are being received via.

The conventional receiving station 13 has an antenna 1 as shown in FIG.
The frequency converter 2 converts the received pilot signal to an appropriate frequency. Further, this output signal is input to the mixer 3, frequency-converted to an appropriate IP frequency by the output signal of the voltage controlled oscillator 9, and output to a subsequent circuit (not shown).

Furthermore, this signal is phase-compared with the output signal of the reference signal generator 5 in the phase frequency detection 4, and an error component between the two is detected. Then, this output error signal is passed through a low-pass filter 6 in order to eliminate unnecessary waves and determine a frequency pull-in range, and is then negatively fed back to the voltage control terminal of the voltage controlled oscillator 9.

By forming this feedback loop, the output signal of the mixer 3 can always be kept constant regardless of fluctuations in the received pilot signal.

[Problem to be solved by the invention]

In the conventional frequency control circuit described above, the low-pass filter 6 that determines the frequency pull-in range of the negative feedback loop is uniquely determined as a predetermined frequency range. As a result, it is no longer possible to respond to changes in the transmission speed of communication signals, it is not possible to maintain long-term frequency stability when performing automatic frequency control for signals with low transmission speeds, and it is impossible to cope with frequency drift for high-speed transmission. There is a problem. This poses a problem in that it is not possible to set an optimal loop band when there are signals of a plurality of transmission speeds on one transmission path.

SUMMARY OF THE INVENTION An object of the present invention is to provide a frequency control circuit that can set optimal loop bands for signals of a plurality of transmission speeds.

[Means to solve the problem]

The frequency control circuit of the present invention includes a plurality of low-pass filters with different cut-off frequencies, which filter the error signal obtained by comparing the phase of a received signal with a reference signal and negatively feed it back to the voltage-controlled oscillator. The low-pass filters are connected in parallel, and these low-pass filters are configured to be selectively switched by a controller operated by a transmission rate signal.

[Effect]

In the above configuration, the controller selects one of the plurality of low-pass filters according to the transmission speed, connects it to the voltage controlled oscillator, and sets the pull-in frequency of the feedback loop to a value suitable for each transmission speed. Change to frequency.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing the main parts of an embodiment of the present invention. In the figure, ■ is an antenna, 2 is a frequency converter,
3 is a mixer, 4 is a phase frequency detector, 5 is a reference signal generator, and 9 is a voltage controlled oscillator, which are the same as before. Here, a plurality of low-pass filters 6A, 6B, and 6C are connected in parallel to the phase frequency detector 4, and in this case, three low-pass filters 6A, 6B, and 6C are connected in parallel.
It is configured so that it can be selectively connected to the voltage controlled oscillator 9 by a switch 8.

The three low-pass filters 6A, 6B, and 6C have different cutoff frequencies, and here, the cutoff frequencies are set to be larger in the order of the low-pass filters 6A to 6B. Further, the switch 8 can be switched by the controller 7. This controller 7 is configured to detect the transmission speed of a signal in, for example, a subsequent circuit (not shown), and to selectively change over a switch using this transmission speed signal as input.

In the receiving station with this configuration, a pilot signal received by an antenna 1 is converted to an appropriate frequency by a frequency converter 2, and converted to a desired IF frequency by a mixer 3 using an output signal of a voltage controlled oscillator 9. This IF multiplied signal is output to a subsequent circuit.

Further, a portion of this IF multiplied signal is input to a phase frequency detector 4, where the phase is compared with a reference signal from a reference signal generator 5, and a difference component between the pilot signal and the reference signal is detected. This output error signal is passed through the low-pass filters 6A, 6B, and 6C.
The output signal of the low-pass filter selected by the controller 7 and the switch 8 is negatively fed back to the voltage control terminal of the voltage controlled oscillator 9, from which unnecessary waves are removed.

Thereby, the output of the mixer 3 is always kept constant.

The selection of the low-pass filters 6A, 6B, and 6C is performed as follows. For example, a low-pass filter 6A with a small cutoff frequency
When switched to , the loop band of the feedback loop becomes narrower, which increases long-period frequency stability and allows low transmission rate signals to pass. Moreover, when switching to the low-pass filter 6C with a high cutoff frequency, the loop band is widened, so the frequency pull-in range is widened, and the filter is resistant to frequency drift, making it suitable for high-speed transmission.

In the above embodiment, three low-pass filters are switched, but the configuration may be such that two, four or more low-pass filters can be switched depending on the signal transmission speed. .

〔Effect of the invention〕

As explained above, the present invention comprises a low-pass filter provided in the feedback loop of a voltage-controlled oscillator that converts the frequency, consisting of a plurality of parallel-connected low-pass filters with different cut-off frequencies, and Since it can be selectively switched by the controller according to the transmission speed, the low-pass filter can be set to a frequency corresponding to the transmission speed, and it can be set to a frequency suitable for various signals with different transmission speeds. This has the effect of realizing frequency control.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the main parts of a receiving station to which the automatic frequency control circuit of the present invention is applied, Fig. 2 is a conceptual system diagram of a communication satellite network using the automatic frequency control circuit, and Fig. 3 is a diagram of the conventional communication satellite network. FIG. 2 is a block diagram of a portion of a receiving station using an automatic frequency control circuit. 1... Antenna, 2... Frequency converter, 3... Mixer, 4... Phase frequency detector, 5... Reference signal generator, 6.6A, 6B, 6G... Low Area filter, 7...
- Controller, 8... Switch, 9... Voltage controlled oscillator, 11... Pilot transmitting station, 12... Transmitting station, 1
3... Receiving station, 4... Communication satellite.

Claims (1)

[Claims] 1. Mix the received signal with the output of the voltage controlled oscillator, convert the frequency, compare the phase of this output signal with a reference signal to obtain an error signal, and pass this error signal through a low-pass filter to the output of the voltage controlled oscillator. In a frequency control circuit configured to provide negative feedback to a control terminal, the low-pass filter has a plurality of low-pass filters connected in parallel with different cut-off frequencies, and these low-pass filters are controlled by a transmission speed signal. A frequency control circuit characterized in that it is configured to be selectively switched by a controller operated.