JPH04152709A - Afc equipment - Google Patents

Abstract

PURPOSE:To simplify the circuit by fine-tuning an oscillating frequency to make the frequency of an intermediate frequency signal close to a prescribed value and fine-tuning the oscillating frequency for each about 1sec being a multiple of 10 of a prescribed period after the output of a frequency error detector is changed first through channel selection. CONSTITUTION:For example, when an intermediate frequency at channel selection is high, suppose that an operating point of an FM demodulator 3 is at a point A at channel selection. In such a case, since the AFC is at a high level, the intermediate frequency is controlled to be low and the operating point is moved to a point B. Then a frequency error is detected after a prescribed time and the operating point is moved to a point C. Then the operating point is moved to points D, E, F. When the operating point is moved to the point F, the level of the AFC is changed after channel selection and goes to a low level. Then the intermediate frequency is controlled to be high and the operating point is moved to the point E again. Thus, the operating point is repetitively moved between the points E, F at an interval of nearly 1sec being 10 times the prescribed period of the intermediate frequency. Thus, the AFC function is attained almost without giving any effect on the FM demodulation characteristic.

Description

[Detailed description of the invention] Industrial applications The present invention provides an AFC for a satellite broadcast receiver. (number control circuit) device.

Conventional satellite broadcasting technology converges radio waves in the 12 GHz band emitted from a broadcasting satellite using a parabolic antenna, converts the frequency to a first intermediate frequency in the IGHz band using a low-noise converter, and transmits the radio waves indoors via a coaxial cable. , channel selection and FM demodulation are performed to obtain images and sounds, which can be received. Generally, FM demodulation signal processing is 400 MHz
This is done in the second intermediate frequency circuit of the band. There are various methods for FM demodulation signal processing, but the center frequency of the input signal must always be maintained within ±500. In recent years, one type of frequency synthesizer using a phase-locked loop has become common as a channel selection circuit for satellite broadcasting receivers.

As a result, the local oscillation frequency of the tuning circuit in the IGHz band is stabilized with high precision. However, the frequency (automatic frequency stability) of the local oscillator of a low-noise converter is about ±1.5 MHz, and AFC has been used to remove this frequency drift.

The conventional AFC device described above will be explained below with reference to the drawings. FIG. 4 shows a conventional AFC device. (Refer to Japanese Unexamined Patent Publication No. 63-307992.) In Fig. 4, ■ is an FM signal input terminal, 2 is a mixer for frequency conversion, 3 is an FM demodulator, 4 is a frequency error detector, and 5 is a PLL. The frequency control circuit 6 is a local oscillator.

Figure 4 shows a typical configuration example of a satellite broadcasting receiver.
Functions not related to FC are omitted for simplicity.

The operation of the conventional AFC device configured as described above will be described below. The FM signal applied to the input terminal 1 is frequency-converted by the frequency mixer to become a signal in an intermediate frequency band, and is input to the FM demodulator 3. F
The M demodulator 3 performs FM demodulation of the input signal and outputs it. F
The demodulation characteristics of the M demodulator 3 are generally as shown in FIG. According to FIG. 5, it can be seen that as the frequency of the FM signal becomes higher, the potential of the demodulated output becomes higher, and the frequency change is output as a voltage change.

The role of the frequency error detector 4 is to determine whether the error of the intermediate frequency from the center frequency (fo) is larger than a predetermined value;
Another purpose is to determine whether the direction of the frequency error is higher or lower. In FIG. 5, if the predetermined value of the frequency error tolerance limit is Δf, the output voltage of the FM demodulator 3 can be considered to be V, , V at the frequency error tolerance limit.

Therefore, a configuration as shown in FIG. 6 can be considered as the frequency error detector 4. 8 and 9 are comparator ICs,
12.13 is a reference voltage. As shown in FIG. 6, if the reference voltages are selected as □1□, respectively, the comparator turns on when the demodulated output voltages are VH and V. Therefore, the output AFCI of the frequency error detector 4, AFC2
has input/output characteristics as shown in FIG. According to FIG. 7, the intermediate frequency is lower than the center frequency (Fo) by a predetermined frequency error (Δf) (if AFCl, AFC2
It can be seen that both are at low levels, and as they rise, both become high levels.

If the outputs of these frequency error detectors 4 are input to the PLL frequency control circuit 5 as shown in FIG. 4 to control the oscillation frequency of the local oscillator 6, the intermediate frequency will always be the same as the predetermined frequency error detector. I can make it happen. That is, A
If both FCI and AFC2 are at a low level, the intermediate frequency is lower than the center frequency, so the local oscillation frequency is controlled to be higher. Conversely, AFCI and AFC2
If both are at a high level, the intermediate frequency is higher than the center frequency, so the local oscillation frequency is controlled to be lower. Further, if AFCI is at a high level and AFC2 is at a low level, the intermediate frequency is within a predetermined error range, so there is no need to change the local oscillation frequency. In this way, the AFC function can be achieved.

Problems to be Solved by the Invention However, in the above configuration, two sets of comparators are required as frequency error detectors, and it is necessary to send two signals to the PLL frequency control circuit, which is usually composed of a microcomputer. . In recent years, in order to further reduce the cost of satellite broadcast receivers, there has been an increasing demand for simpler circuits.

In view of the above problems, the present invention provides an AF system necessary for a satellite broadcast receiver.
The purpose is to provide a simpler C device.

! Means for Solving Problem 1 In order to solve the above problem, the AFC device of the present invention includes a frequency synthesizer tuning circuit consisting of a frequency control circuit using a phase-locked loop including a voltage control oscillator, and FM demodulates FM by inputting the intermediate frequency signal
a demodulator and a low-pass filter that smooths its output;
a frequency error detector comprising a comparator inputting the output of the low-pass filter and a reference voltage; and a frequency synthesizer tuning circuit that uses the output of the frequency error detector every predetermined period for each tuning. The transmission frequency of the intermediate frequency signal is finely adjusted to bring the frequency of the intermediate frequency signal close to a predetermined value, and after the output value of the frequency error detector changes for the first time after tuning, Approximately 1
It is equipped with a configuration that finely adjusts the transmission frequency every second.

Effect of the Invention With the above-described configuration, the present invention smoothes the modulation component of the FM demodulator output, and extracts a voltage corresponding to the average frequency of the intermediate frequency signal from the low-pass filter. A comparator compares this voltage with a reference voltage equal to the FM demodulated output voltage when the intermediate frequency is equal to the center frequency, and outputs a high level output if the intermediate frequency deviates higher than the center frequency. Furthermore, if the intermediate frequency deviates below the center frequency, a low level output is output.

The output of this frequency error detector is input to the frequency control circuit of the frequency synthesizer tuning circuit to control the oscillation frequency of the voltage controlled oscillator, which is a local oscillator of the tuning circuit. At this time, if the output of the frequency error detector is at a low level, the intermediate frequency is lower than the center frequency, so the local oscillation frequency is controlled to be high. Conversely, if the output of the frequency error detector is at a high level, the intermediate frequency is high, so the local oscillation frequency is controlled to be low.

In addition, the AFC function shortens the time from when a channel is selected until the intermediate frequency is pulled into the center frequency by shortening the interval at which the oscillation frequency of the local oscillator is changed by the frequency control circuit when selecting a channel. After the output value of the frequency error detector changes for the first time after tuning, the interval is set to about 1 second, which is about 10 times as large.

Note that when the AFC function is activated in this manner, the intermediate frequency changes approximately every second in the vicinity of the center frequency. The frequency width becomes the minimum frequency step of the frequency synthesizer. Since this is normally about 200 kHz in a BS tuner, it hardly affects the FM demodulation characteristics and poses no problem in practice.

EXAMPLE An example of the present invention will be described below with reference to the drawings. FIG. 1 shows an AFC device in one embodiment of the present invention. In FIG. 1, 1 is an FM signal input terminal, 2 is a mixer for frequency conversion, 3 is an FM demodulator, and 3 is an FM signal input terminal.
5 is a low pass filter, 40 is a frequency error detector, 50
is a PLL frequency control circuit and 6 local oscillators. FIG. 2 shows one embodiment of frequency error detector 40. FIG. In FIG. 2, 7 is a terminal for inputting the demodulated output signal, 20 is a comparator IC 121, which is the output of the frequency error detector (AFC
) Terminals, 22 units 1! Voltage (■.).

The operation of the AFC device configured as described above will be described below. The FM signal applied to the input terminal l is frequency-converted by the frequency mixer 2 to become a signal in an intermediate frequency band, and is input to the FM demodulator 3. The FM demodulator 3 performs FM demodulation of the human signal and outputs it. The demodulation characteristics of the FM demodulator 3 are as shown in FIG. 5, as shown in the conventional example.

According to FIG. 5, it can be seen that as the frequency of the FM signal becomes higher, the potential of the demodulated output becomes higher, and the frequency change is output as a voltage change.

The role of the low-pass filter 35 is to smooth the modulation component of the output of the FM demodulator 3 and extract a voltage corresponding to the average frequency of the intermediate frequency signal.

The role of the frequency error detector 40 is to determine whether the intermediate frequency is higher or lower than the center frequency (fo). In FIG. 5, if the center frequency of the intermediate frequency is fo, then the output voltage of the FM demodulator 3 is 2 at the center frequency.

You can think about it.

Therefore, a configuration as shown in FIG. 2 may be considered as the frequency error detector 40. As shown in Figure 2, base 1! Voltage 2
The value of 2 is ■. If this is selected, the comparator IC20 is turned on when the demodulated output voltage is vo. Therefore, the output (AFC) of the frequency error detector 40 has human output characteristics as shown in FIG. According to FIG. 3, it can be seen that when the intermediate frequency becomes lower than the center frequency (ro), the AFC output is at a low level, and when it becomes higher, it is at a high level.

If the output of the frequency error detector 40 is input to the PLL frequency control circuit 50 as shown in FIG. 1 to control the oscillation frequency of the local oscillator 6, the intermediate frequency will always be within a predetermined frequency error. You can do it like this. That is, A
If the FC output is at a low level, the intermediate frequency is lower than the center frequency, so the local oscillation frequency is controlled to be higher.

Conversely, if the AFC output is at a high level, the intermediate frequency is higher than the center frequency, so the local oscillation frequency is controlled to be lower. In this way, the intermediate frequency gradually approaches its center frequency.

Here, the frequency control operation of the AFC device will be described, taking as an example the case where the intermediate frequency at the time of channel selection is high. Assume that the operating point of the FM demodulator 3 is at A at the time of channel selection, as shown in FIG. Since AFC is at a high level at this time, the intermediate frequency is controlled to be low and the operating point moves to B.

The frequency difference between AB is the frequency resolution of a frequency synthesizer, and is usually set to about 200 kHz in a BS tuner.

Next, the frequency error is detected after a predetermined time, and the operating point moves to C. Then the operating points become E and F. When the operating point reaches F, the value of AFC changes for the first time after channel selection and becomes a low level. Therefore, the intermediate frequency is controlled to become high and the operating point shifts to E again. The intermediate frequency will thus repeat between operating points E and F at intervals of about 1 second, which is ten times the predetermined period. Even if this interval is somewhat long, no particular problem will arise, but if it is too long, the AFC pull-in time will be too long.

Therefore, if the intermediate frequency deviates from the center frequency, at most E
The frequency difference between F and it is about 200 kHz, so FM? It has almost no effect on the JL tone characteristics,
There is no problem in practical use. In this way, AFC@ability can be achieved.

Effects of the Invention As described above, according to the present invention, by comparing frequencies at the center frequency of intermediate frequencies, AFC can be performed more easily.
The device can be realized.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an AFC device according to an embodiment of the present invention, Fig. 2 is a block diagram of a frequency error detector which is a basic component of the AFC device in the same embodiment, and Fig. 3 is a block diagram of the frequency error detector. 4 is a block diagram of the conventional AFC device, FIG. 5 is a characteristic diagram of the input frequency versus demodulated output voltage of the FM demodulator, and FIG. 6 is a diagram of the frequency error detector of the conventional example. The block diagram, FIG. 7, is an input/output characteristic diagram of the frequency error detector. 3...FM repeater mH120...Comparator IC822...Comparison base! 1! Voltage, 35
...low range i! 1 filter, 40...
Frequency error detector, 50...PLL frequency control circuit. Name of agent: Patent attorney Akira Okaji and two others Figure 02/ Figure FC Possible O Medium f No-v1ji*Sshi:

Claims (1)

[Claims] A frequency synthesizer tuning circuit consisting of a frequency control circuit using a phase-locked loop including a voltage controlled oscillator; and an FM system by inputting an intermediate frequency signal obtained from the tuning circuit.
A frequency error detector consisting of an FM demodulator for demodulating, a low-pass filter for smoothing the output thereof, and a comparator for inputting the output of the low-pass filter and a reference voltage, each time a channel is selected. Finely adjusting the oscillation frequency of the frequency synthesizer tuning circuit using the output of the frequency error detector every predetermined period to bring the frequency of the intermediate frequency signal close to a predetermined value, and adjusting the output value of the frequency error detector. The AFC device is characterized in that after the first change after tuning, the oscillation frequency is finely adjusted every 1 second, which is about 10 times as long as the predetermined period.