JPH057132A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To attain AFC even when a receiver frequency offset is in excess of a pull-in range of the AFC by detecting the increase in the reception frequency offset so as to revise automatically the frequency of a pilot signal set to the receiver. CONSTITUTION:A frequency of a signal inputted to a receiver is mixed with a frequency of a signal from a VCO 2, the result is subjected to band limit and the result is compared with a reference signal and an error signal is outputted from a comparator 5. The error signal is digital-converted by an A/D converter 6 and fed to a logic circuit 7. The logic circuit 7 generates a control signal for the VCO 2 in response to the digital signal and a D/A converter 8 converts the signal into an analog signal, which is inputted to the VCO 2. When the frequency fluctuation of the VCO 2 is larger and is close to a pull-in range of the AFC, the logic circuit 7 gives a control signal to a frequency converter 9 so as to reduce the frequency fluctuation of the VCO 2. As a result, since the input frequency to the AFC circuit is changed, even when a reception frequency offset of the pilot signal is in excess of the pull-in range of AFC, the application of AFC is attained.

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 a signal having a narrow required bandwidth such as an SCPC is used for communication, a frequency variation due to a secular change of a local oscillator frequency in a transmitter or a repeater or a Doppler effect in satellite communication is received. It is necessary to keep the frequency of the signal that is absorbed by the machine and input to the demodulator constant. In satellite communication, the master station sends out a pilot signal other than the communication signal, the receiving station detects the frequency fluctuation of this pilot signal, and changes the output frequency of the local oscillator of the receiver for the detected signal, The frequency of the signal input to the demodulator is controlled to be 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 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 of both signals becomes the output signal. A part of this output signal passes through the bandpass filter 3 for band-limiting the noise component, and the frequency phase comparator 5
Is supplied to. On the other hand, the output signal of the reference signal generator 4 is also supplied to the frequency / phase comparator 5, and the frequencies and phases of both are compared. As a result of comparison, the error component generated is VCO
It is negatively fed back to 2 and changes 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 absorbs the frequency change of the input signal and is always a constant frequency.

[0004]

In the above-mentioned conventional automatic frequency control circuit, when an attempt is made to receive a signal having a narrow bandwidth such as in SCPC, another signal exists near the frequency of the signal. , AFC's pull-in range cannot be large. Therefore, when the frequency offset of the pilot signal exceeds the AFC pull-in range, there is a drawback that AFC cannot be applied.

[0005]

SUMMARY OF THE INVENTION An automatic frequency control circuit according to the present invention includes a voltage controlled oscillator, a frequency mixer for converting an input signal into an intermediate frequency by the voltage controlled oscillator, and an output signal of the frequency mixer. A frequency / phase comparator that outputs an error signal by comparing the band-limited signal that has passed through the band-pass filter and the output signal of the reference signal generator, and returns the error signal to the voltage-controlled oscillator. In the automatic frequency control circuit having a loop for
A logic circuit for D-converting and logically processing, and a frequency converter for controlling the frequency of the input signal stepwise by the control signal of the logic circuit so as to be within the automatic frequency control range of the voltage controlled oscillator. Is provided before the frequency mixer.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 3 is an explanatory diagram of an AFC range of this embodiment. 1, the same reference numerals as those in the conventional example of FIG. 2 have the same functions and configurations.
In other words, the frequency / phase comparator 5 compares the output signal of the reference signal generator 4 with the signal of which the frequency is converted by the MIX 1 and the band is limited by the bandpass filter 3, and the error signal is extracted. Is the same as before. In the present invention, the extracted error signal is first digitally converted 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 converter 8 and VC
Added to O2. Further, the logic circuit 7 sends a control signal to the frequency converter 9 so that the frequency fluctuation width of the VCO 2 becomes small when the frequency fluctuation width of the VCO 2 becomes large and becomes close to the pull-in range of the AFC, and the input frequency of the AFC circuit is changed. Change.

Here, if 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 is
And the output frequency fi is 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 as in equation (2).

[0009]   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 the oscillation center frequency is flo and the displacement amount from the center frequency is Δfl. . As shown in FIG. 3, when the AFC range is set to ± Δf, there is a relationship of formula (5).

[0011]   flo−Δf ≦ fl ≦ fl + Δf (5) By substituting the equation (4) into the equation (3), the equation (6) is derived.

Fr = fo + fs + flo + Δfl (6) In FIG. 3, when the frequency set value 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 calculated from the equation (6). (7)
It is expressed as an expression.

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

Fr = fo + fsi + fli = fo + fsi + flo + Δfli (8) However, fli = flo + Δfli. fli is V
It is the output frequency of CO2.

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

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

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

Further, referring to FIG. 3, the frequency switching 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 ½ of the frequency step of the synthesizer. I know it's good.

[0019]

As described above, the automatic frequency control circuit of the present invention detects an increase in the reception frequency offset of the pilot signal and automatically changes the frequency of the pilot signal set in the receiver accordingly. As a result, even if the reception frequency offset of the pilot signal exceeds the pull-in range of AFC, it is possible to apply AFC.

[Brief description of drawings]

FIG. 1 is a block diagram of an 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 according to this embodiment.

[Explanation of symbols]

1 Frequency mixer (MIX) 2 Voltage controlled oscillator (VCO) 3 band filters 4 Reference signal generator 5 Frequency phase comparator 6 A / D converter 7 logic circuits 8 D / A converter 9 Frequency converter

Claims (2)

[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. In an automatic frequency control circuit having a frequency phase comparator for comparing a band limited signal and an output signal of a reference signal generator to output an error signal, and a loop for returning the error signal to the voltage controlled oscillator, the error signal A logic circuit for A / D converting and logically processing, and controlling the frequency of the input signal to be within the automatic frequency control range of the voltage controlled oscillator by stepwise converting the frequency of the input signal by the control signal of the logic circuit. An automatic frequency control circuit comprising a frequency converter in a stage preceding the frequency mixer. 2. The automatic frequency control circuit according to claim 1, wherein the frequency converter has a synthesizer that changes the frequency stepwise by a control signal of the logic circuit.