JP2985376B2 - Automatic frequency control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control circuit used for a local oscillator of a receiver.

[0002]

2. Description of the Related Art In general, when communication is performed using a signal having a small required bandwidth such as an SCPC, a secular change of a local oscillator frequency in a transmitter and a repeater or a frequency variation due to a Doppler effect in satellite communication is received. It is necessary that the frequency of the signal input to the demodulator be absorbed and absorbed by the demodulator. In satellite communication, a pilot signal other than the communication signal is transmitted from the master station, the receiving station detects the frequency variation of this pilot signal, and changes the output frequency of the local oscillator of the receiver with respect to the detected signal, Control is performed so that the signal frequency input to the demodulator is constant. This method is a so-called automatic frequency control method (AF).
Known as C).

Conventionally, in this type of AFC circuit, as shown in FIG. 2, a signal input to a receiver is composed of a signal from a voltage controlled oscillator (hereinafter referred to as VCO) 2 and a frequency mixer 1 (hereinafter referred to as MCO).
IX), and the difference frequency between the two signals becomes the output signal. A part of this output signal passes through the bandpass filter 3 for band-limiting the noise component, and passes through the frequency phase comparator 5
Supplied to On the other hand, the output signal of the reference signal generator 4 is also supplied to the frequency / phase comparator 5, where the frequency and the phase of the two are compared. The error component resulting from the comparison is VCO
2 to change the frequency of VCO2. This series of operations continues until the error component of the frequency phase comparator 5 becomes zero, and the output frequency of the AFC circuit always becomes a constant frequency by absorbing the change in the frequency of the input signal.

[0004]

In the conventional automatic frequency control circuit described above, when a signal having a narrow bandwidth such as an SCPC is to be received, another signal exists near the frequency of the signal. The AFC pull-in range cannot be made large. Therefore, when the frequency offset of the pilot signal exceeds the pull-in range of AFC, there is a disadvantage that AFC cannot be performed.

[0005]

SUMMARY OF THE INVENTION An automatic frequency control circuit according to the present invention comprises a voltage controlled oscillator, a frequency mixer for converting an input signal to an intermediate frequency by the voltage controlled oscillator, and an output signal of the frequency mixer. A frequency-phase comparator that compares an output signal of a reference signal generator with a band-limited signal that has passed through a band-pass filter by branching a unit to output an error signal; and feeding back the error signal to the voltage-controlled oscillator. In an automatic frequency control circuit having a loop for
A logic circuit for performing D-conversion and performing logic processing, and a step-like frequency conversion of the frequency of the input signal according to a control signal of the logic circuit, and as the voltage-controlled oscillator approaches the automatic frequency control range, Frequency change
A frequency converter provided with a synthesizer for controlling the dynamic width to be small is provided in front of the frequency mixer.

[0006]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of one embodiment of the present invention, and FIG. 3 is an explanatory diagram of an AFC range of the present embodiment. 1, the same reference numerals as those in the conventional example of FIG. 2 have the same functions and configurations.
That is, a signal obtained by subjecting the signal subjected to frequency conversion by the MIX 1 to band limiting by the band pass filter 3 and an output signal of the reference signal generator 4 are compared by the frequency phase comparator 5 until an error signal is extracted. Is the same as before. In the present invention, the extracted error signal is first subjected to digital conversion by the A / D converter 6. The converted digital signal is supplied to the logic circuit 7. The logic circuit 7 generates a control signal for the VCO 2 according to the digital signal. This signal is converted to an analog signal by the D / A
Added to O2. The logic circuit 7 sends a control signal to the frequency converter 9 so as to reduce the frequency fluctuation range of the VCO 2 when the frequency fluctuation range of the VCO 2 becomes close to the pull-in range of the AFC and the input frequency of the AFC circuit is reduced. Change.

Here, assuming that the frequency of the frequency synthesizer inside the frequency converter 9 controlled by the output signal of the logic circuit 7 is fs, the input frequency fr of the frequency converter 9
And the output frequency fi are expressed as in equation (1).

Fi = fr−fs (1) Further, if the output frequency of MIX1 is fo and the output frequency of VCO2 is fl, it is expressed by equation (2).

Fo = fi−fl (2) Equation (3) is derived from equations (1) and (2).

Fr = fo + fs + fl (3) Here, the output frequency fl of the VCO 2 can be expressed as: fl = flo + Δfl (4), where Flo is the oscillation center frequency and Δfl is the displacement from the center frequency. . As shown in FIG. 3, if the AFC range is set to ± Δf, there is a relationship represented by Expression (5).

Flo−Δf ≦ fl ≦ Flo + Δf (5) By substituting equation (4) into equation (3), equation (6) is derived.

Fr = fo + fs + flo + Δfl (6) In FIG. 3, if the set value of the frequency of the pilot signal of the receiver is fci and the frequency of the frequency synthesizer in the frequency converter 9 is fsi, fci is given by equation (6). (7)
It is expressed like a formula.

Fci = fo + fsi + flo (7) Further, if the actual reception frequency of the pilot signal is defined as fr, fr is expressed by equation (8).

Fr = fo + fsi + fli = fo + fsi + flo + Δfli (8) where fli = flo + Δfli. fli is V
This is the output frequency of CO2.

Here, the frequency step of the frequency synthesizer in the frequency converter 9 is set to fs by the control signal from the logic circuit 7, and the output frequency is f (si + 1) = f
If the output frequency of the VCO 2 is f (li + 1) = flo-Δf (li + 1),
fr is expressed as follows from equation (3).

Fy = fo + f (si + 1) + f (li + 1) = fo + fsi + Δfs + flo−Δf (i + 1) (9) In the present invention, since equation (8) = (9), Δfs = Δfli + Δf (li + 1) (10) Here, Δfli ≦ Δf, and Δf (li + 1) ≦ Δ
Since there is a relationship of f, a relationship such as Expression (11) is derived from Expression (10).

Δfs ≦ 2 · Δf (11) Equation (11) indicates that the present invention can be realized if the frequency step of the frequency synthesizer in the frequency converter 9 is equal to or smaller than the AFC range.

FIG. 3 shows that the switching of the frequency of the frequency synthesizer in the frequency converter 9 from the logic circuit 7 is performed when the frequency offset of the pilot signal becomes one half of the frequency step of the synthesizer. It turns out to be good.

[0019]

As described above, the automatic frequency control circuit according to the present invention detects an increase in the reception frequency offset of the pilot signal and automatically changes the pilot signal frequency set in the receiver accordingly. Thus, there is an effect that AFC can be performed even when the reception frequency offset of the pilot signal exceeds the pull-in range of AFC.

[Brief description of the drawings]

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

FIG. 2 is a block diagram of a conventional automatic frequency control circuit.

FIG. 3 is an explanatory diagram of an AFC range of the present embodiment.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 frequency mixer (MIX) 2 voltage controlled oscillator (VCO) 3 bandpass filter 4 reference signal generator 5 frequency phase comparator 6 A / D converter 7 logic circuit 8 D / A converter 9 frequency converter

Claims (2)

(57) [Claims] 1. A voltage controlled oscillator, a frequency mixer for converting an input signal to an intermediate frequency by the voltage controlled oscillator, and a part of an output signal of the frequency mixer is branched and passed through a bandpass filter. A frequency-phase comparator for comparing the band-limited signal with an output signal of a reference signal generator to output an error signal, and an automatic frequency control circuit having a loop for feeding back the error signal to the voltage-controlled oscillator, a logic circuit for logic processing and a / D conversion, the logic circuit and the voltage according to the frequency of the input signal to frequency conversion in a stepwise manner by the control signal closer to an automatic frequency control range of the said voltage controlled oscillator Controlled oscillator
Synthesizer that controls to reduce the frequency fluctuation range
An automatic frequency control circuit, comprising a frequency converter provided with a frequency converter before the frequency mixer. 2. The automatic frequency control circuit according to claim 1, wherein said frequency converter has a synthesizer for changing a frequency stepwise according to a control signal of said logic circuit.