JPH04243323A - Afc circuit - Google Patents

Abstract

PURPOSE:To realize the AFC circuit (automatic frequency control circuit) whose sweep speed is constant and implementing stable pull--in operation while the sweep frequency range is unchanged. CONSTITUTION:A frequency difference discrimination circuit 8 is provided in the AFC circuit provided with a mixer 1, a phase comparator 6, a sweep oscillator 2, a loop filter 3 and a voltage controlled oscillator 2, and the frequency difference discrimination circuit 8 discriminates a difference between the frequency of a reference signal outputted from the sweep oscillator 4 and an output frequency of the mixer, the sweep oscillator is connected to the loop filter 3 at the outside of the AFC loop lock range and the phase comparator 6 is connected to the loop filter in the AFC loop lock range.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AFC circuit (automatic frequency control circuit), and more particularly to an AFC circuit that stably controls a voltage controlled oscillator within the AFC circuit.

0003

2. Description of the Related Art Conventionally, this type of AFC circuit includes a mixer, a phase comparator, a sweep oscillator, a loop filter, and a voltage controlled oscillator, and controls the voltage controlled oscillator to control the output of the voltage controlled oscillator. The reception frequency is synchronized with the frequency of the reference signal output by the sweep oscillator in addition to the mixer.

In this conventional AFC circuit, a mixer is used in the PLL loop, and the output of the sweep oscillator is added to control the VCO, so the operating point of the sweep oscillator is A.
This is due to the influence of the FC loop, and a phenomenon has occurred in which when the direction in which the received signal is pulled in by the PLL loop and the sweep direction by the sweep oscillator are opposite, the speed becomes slower, and when they are in the same direction, the speed becomes faster.

[0005] For this reason, there are problems in that the speed of the pull-in operation after the sweep is performed varies, and the sweep frequency range also changes.

[0006]

[Problems to be Solved by the Invention] As mentioned above, the conventional A
According to the FC circuit, since the output of the sweep oscillator is added to the AFC loop by a mixer, there is a problem in that the sweep speed and frequency range vary in controlling the VCO.

[0007] The present invention was made in view of this problem, and provides an AFC capable of performing stable pull-in operation with a constant sweep speed and an unchanged sweep frequency range.
The purpose is to provide circuits.

[Configuration of the invention]

[0009]

[Means for Solving the Problems] A mixer, a phase comparator, a sweep oscillator, a loop filter, and a voltage controlled oscillator are provided, and the voltage controlled oscillator is controlled and the output of the voltage controlled oscillator is added to the mixer. In an AFC circuit that synchronizes a reception frequency with a frequency of a reference signal outputted by a sweep oscillator, the difference between the frequency of the reference signal outputted by the sweep oscillator and the output frequency of the mixer is determined from the output of the phase comparator. Based on the determination output of the frequency difference determination circuit, the sweep oscillator is connected to the loop filter, the phase comparator and the loop filter are disconnected, and the AFC loop is pulled in based on the determination output of the frequency difference determination circuit. Within this range, the present invention is characterized by comprising means for connecting the phase comparator and the loop filter without using the sweep oscillator.

0010

[Operation] In the AFC circuit of the present invention, the frequency difference determination circuit determines the difference between the frequency of the reference signal outputted by the sweep oscillator and the output frequency of the mixer, and based on this determination output, the AFC loop pull-in range is determined. Outside, the sweep oscillator is connected to the loop filter and the phase comparator and the loop filter are disconnected, and within the AFC loop pull-in range, the phase comparator and the loop filter are connected without going through the sweep oscillator. I try to do that.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

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

In FIG. 1, 1 is a mixer, 2 is a voltage controlled oscillator (VCO), 3 is a loop filter, 4 is a sweep oscillator, 5 is a sweep switching circuit, and 6 is a phase comparator. or,
12 is a reference oscillator.

FIG. 2 is a detailed block diagram of the sweep switching circuit 5. The frequency difference determining circuit 8 of the sweep switching circuit 5 includes a comparator circuit 9, a DC subdetection circuit 10, and a low-pass filter 11.

The configuration and operation of this embodiment will be explained below with reference to FIGS. 1 and 2.

In FIG. 1, the output of the mixer 1 and the output of the reference oscillator 12 are input to a phase comparator 6, and the phase comparison output of the phase comparator 6 is input to the frequency difference determination circuit 8 of the sweep switching circuit 5. .

In the frequency difference determination circuit 8, the output of the phase comparator 6 is applied to a low-pass filter 11 to extract its low-frequency component, and a detection circuit 10 further detects the DC component. The detected DC component is waveform-shaped by the comparator circuit 9 and becomes a control signal for the analog switch 7, which controls switching of the analog switch 7.

With this control, outside the AFC loop pull-in range, the contact of the analog switch 7 is set to the a side, the sweep oscillator 4 is connected to the loop filter 3, the loop filter 3 and the phase comparator 6 are disconnected, and the sweep The sweep by the oscillator 4 is started. After the start of this sweep, when the AFC loop pull-in range is reached, the contact point of the analog switch 7 is set to the b side by the control signal output from the frequency difference determination circuit 8, and the phase comparator 6 and the loop filter 3 are connected without going through the sweep oscillator 4. are connected to mixer 1, phase comparator 6, and reference oscillator 1.
2. An AFC loop consisting of a loop filter 3 and a VCO 2 is formed. This facilitates locking in the AFC loop, and the reception frequency input to the mixer 1 is synchronized with the frequency of the reference signal output from the reference oscillator 12. By doing as described above, switching during sweeping and maintaining phase synchronization is ensured, and control of the VCO 2 is stably performed.

[0019]

As explained above, in the AFC circuit of the present invention, the frequency difference determining circuit determines the difference between the frequency of the reference signal output from the reference signal oscillator and the output frequency of the mixer, and Based on the output, outside the AFC loop pull-in range, the sweep oscillator is connected to the loop filter and the loop filter and the phase comparator are disconnected, and within the AFC loop pull-in range, the phase comparison is performed without going through the sweep oscillator. The loop filter is connected to the loop filter.

[0020] Therefore, the control of the voltage controlled oscillator (VCO) becomes stable, and a stable pull-in operation can be performed in which the sweep speed is constant and the sweep frequency range remains unchanged.

[Brief explanation of the drawing]

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

FIG. 2 is a detailed block diagram of the sweep switching circuit according to FIG. 1;

[Explanation of symbols]

1...Mixer 2...Voltage control oscillator (VCO) 3...Loop filter 4...Sweeping oscillator 5...Sweep switching circuit 6...Phase comparator 7...Analog switch 8...Frequency difference judgment circuit 9...Comparator circuit 10...Detection circuit 11...Low pass filter 12...Oscillator

Claims (1)

[Claims] 1. A mixer, a phase comparator, a sweep oscillator, a loop filter, and a voltage controlled oscillator, the voltage controlled oscillator is controlled and the output of the voltage controlled oscillator is applied to the mixer, so that the sweep oscillator is controlled. In an AFC circuit that synchronizes a reception frequency with a frequency of a reference signal to be output, a frequency difference determination for determining the difference between the frequency of the reference signal output from the sweep oscillator and the output frequency of the mixer from the output of the phase comparator. and connecting the sweeping oscillator to the loop filter outside the AFC loop pull-in range, and connecting the phase comparator and the loop filter within the AFC loop pull-in range based on the determination output of the frequency difference determination circuit. It is characterized by comprising means.