JPH09181628A - Double super tuner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a frequency of a tuner output signal stable by correcting frequency fluctuation and offset of a 1st local oscillator in the inside of the double super tuner. SOLUTION: An output signal of a voltage controlled oscillator 11 of a 1st local oscillation circuit varying a frequency over a wide range and an output signal of a voltage controlled oscillator 24 of a 2nd local oscillation circuit supplying a fixed frequency are mixed by a 3rd mixer 21 and a difference between frequencies of the 1st local oscillation signal and the 2nd local oscillation signal is detected. The oscillating frequency of the voltage controlled oscillator 24 of the 2nd local oscillation circuit is controlled by a PLL circuit 23 to correct frequency fluctuation and offset of the 1st local oscillation signal varied over a broad band.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tuner for receiving a digitally modulated signal without a carrier wave, and more particularly to a CAT.
The present invention relates to a double super tuner (International Patent Classification H04B 1/26) suitable for receiving a signal in which a large number of channels are frequency-multiplexed in a wide band such as a V signal.

[0002]

2. Description of the Related Art A conventional double super tuner for receiving an NTSC video signal will be described with reference to the block diagram of FIG. In FIG. 3, 1 is an RF signal input terminal, 2
Is an RF filter, 3 is an AGC circuit, 4 is a first mixer, 5
Is a first band filter, 6 is a first intermediate frequency amplifier, 7 is a second mixer, 8 is a second band filter, 9 is a second intermediate frequency amplifier, 10 is an intermediate frequency signal output terminal of a tuner, and 11 is a first Voltage-controlled oscillator of local oscillation circuit, 12 is PLL circuit of first local oscillation circuit, 13 is reference oscillator of first local oscillation circuit, 14 is voltage-controlled oscillator of second local oscillation circuit, 15
Is a voltage control terminal of the second local oscillation circuit, 16 is a video intermediate amplifier circuit, and 17 is an AFC which is a part of the video intermediate amplifier circuit.
(Automatic frequency control) circuit, 18 is a voltage control terminal of the AGC circuit, and 19 is a control terminal of a PLL circuit of the first local oscillation circuit.

In the above configuration, the RF signal input from the RF input terminal 1 suppresses the high frequency noise mixed on the transmission line by the RF filter 2 and is amplified by the AGC circuit 3 according to the control voltage of the voltage control terminal 18, The signal from the voltage controlled oscillator 11 of the one local oscillation circuit causes the first mixer 4 to
Is converted to a first intermediate frequency signal by the first band-pass filter 5, band-limited by the first band-pass filter 5, amplified by the first intermediate-frequency amplifier 6, and second-mixed by the second mixer by the signal of the voltage controlled oscillator 14 of the second local oscillator circuit. After frequency conversion into an intermediate frequency signal,
It is output to the intermediate frequency signal output terminal 10 through the second bandpass filter 8 and the second intermediate frequency amplifier 9.

In the first local oscillator circuit, in order to set the frequency corresponding to the desired channel, the P of the first local oscillator circuit is set.
A predetermined value corresponding to CH is set in the PLL circuit 12 of the first local oscillation circuit from the LL circuit control terminal 19. Normally, the first intermediate frequency signal band is
It is set on the higher frequency side than the RF signal band, and therefore the first
The output frequency of the local oscillation circuit needs to be set higher than the first intermediate frequency band by the frequency of the desired channel. Then, by setting the frequency of the first local oscillation circuit to a predetermined value according to CH, the first bandpass filter 5 can be a fixed bandpass filter (BPF), and the desired channel can be selected. Can be stationed.

Since the second local oscillator circuit frequency-converts the first intermediate frequency signal in the fixed frequency band into a predetermined second intermediate frequency band, it has a fixed oscillation frequency. However, if the frequency of the first local oscillation circuit fluctuates or is offset from the predetermined frequency, the frequency of the second intermediate frequency signal fluctuates, and a phase shift occurs when demodulating to a video signal. To solve this problem, the video intermediate amplifier circuit 16
The frequency carrier of the first local oscillation circuit is detected by separating the video carrier from the AFC circuit 17 and performing FM detection to detect the frequency fluctuation and inputting the detection signal to the voltage control terminal 15 of the second local oscillation circuit. Fluctuations and offset are corrected.

[0006]

However, in the case of a digitally modulated signal without a carrier, if the conventional method is used, after demodulating into a digital signal, the carrier is reproduced from the demodulated baseband signal by digital processing,
The frequency variation of a predetermined carrier frequency must be calculated, and the data must be digital-to-analog converted and then input to the control voltage terminal of the second local oscillation circuit of the double super tuner.

Normally, the voltage-controlled oscillator used for the local oscillator always fluctuates in frequency from the set frequency of the PLL circuit even if it is controlled by the PLL circuit to have a specific frequency. Further, in the case of wideband frequency multiplex transmission such as a CATV signal, the number of channels is large, and it is difficult to keep the characteristics of the voltage controlled oscillator of the first local oscillation circuit uniform in the entire reception band. The frequency fluctuation range also changes. To control these frequency fluctuations by digital signal processing requires a very complicated circuit configuration and is difficult to realize.

[0008]

In order to solve the above-mentioned problems, the first local oscillation circuit is configured to be variable to a predetermined oscillation frequency over a wide range in accordance with channel selection, and the first local oscillation circuit thereof is configured. A signal of the frequency difference between the two outputs is detected through a third mixer that mixes the output of the oscillator circuit and the output of the second local oscillator circuit, and the second signal is detected by the detected signal.
By controlling the local oscillation circuit so that the frequency difference becomes a predetermined difference frequency according to channel selection using a PLL circuit, the frequency fluctuation and the offset of the first local oscillation circuit can be controlled by the second local oscillation. Corrected by the circuit and output the predetermined second local oscillation frequency to the second mixer to stabilize the second
We can provide a double super tuner that can obtain an intermediate frequency.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention according to claim 1 is a first local oscillation circuit for frequency-converting an RF input signal frequency-multiplexed over a wide band into a first intermediate frequency,
A first mixer for converting a frequency to a first intermediate frequency by a signal from the first local oscillator; and a second local oscillator for converting an output from the first mixer to a second intermediate frequency. In a double super tuner having a second mixer that frequency-converts to a second intermediate frequency by the signal of the second local oscillation circuit, the first local oscillation circuit has a predetermined oscillation frequency over a wide range according to channel selection. Is configured to be variable, and a signal of the frequency difference between the two outputs is detected through a third mixer that mixes the output of the first local oscillation circuit and the output of the second local oscillation circuit,
The second local oscillator circuit is controlled by the detection signal so that the frequency difference becomes a difference frequency predetermined according to channel selection by using a PLL circuit. The P of the second local oscillation circuit is adjusted so that the frequency difference between the oscillation signal and the second local oscillation signal becomes a predetermined value.
Since the LL circuit is controlled, even if the first local oscillation frequency or the second local oscillation frequency fluctuates, the frequency difference is converged to a predetermined value and the frequency fluctuation and offset are compensated to stabilize the second intermediate frequency signal. It can be transformed into.

Next, a double super tuner according to a second aspect of the present invention extracts a frequency difference signal from a mixed signal of the first local oscillation signal and the second local oscillation signal output from the third mixer. Therefore, the output from the third mixer is selected by outputting to the PLL circuit of the second local oscillation circuit through a variable band filter having a bandwidth corresponding to the selected channel. Since the frequency corresponding to the station CH is set to the minimum necessary feasible bandwidth, noise can be reduced and the PLL circuit operation of the second local oscillation circuit can be stabilized.

Embodiments of the invention described in claims 1 and 2 of the present invention will be described below with reference to FIGS. 1 and 2. The same or similar constituent blocks as those of the conventional example shown in FIG. 3 are designated by the same reference numerals.

The difference between the present embodiment and the conventional configuration in FIG. 1 is that a third mixer 21 is provided and a filter is provided in order to extract a difference signal between the output frequencies of the first local oscillation circuit and the second local oscillation circuit. The PLL circuit 23 of the second local oscillation circuit via 22
Is to control. Reference numeral 20 is a PLL circuit control terminal of the second local oscillation circuit, and 25 is a reference oscillator of the second local oscillation circuit.

When setting a desired channel, the PLL circuit 12 of the first local oscillation circuit is set in the PLL circuit 12 of the first local oscillation circuit.
By inputting data of a predetermined first local oscillation frequency from the L circuit control terminal 19, as in the conventional case, the voltage controlled oscillator 11 of the first local oscillation circuit is oscillated to a predetermined frequency. At the same time, data of a predetermined frequency difference between the first local oscillation circuit and a predetermined frequency difference between the second local oscillation circuit is set in the PLL circuit 23 of the second local oscillation circuit from the PLL circuit control terminal 20. Even if the frequency of the first local oscillation circuit or the second local oscillation circuit fluctuates from the predetermined frequency, the PLL circuit 23 of the second local oscillation circuit can detect the frequency of the first local oscillation circuit and the frequency of the second local oscillation circuit. Since the frequency difference is controlled so as to become a predetermined value according to the channel CH,
The frequency difference between the first local oscillation frequency and the second local oscillation frequency is controlled to a predetermined value, and as a result, the tuner output terminal 1
The output signal of 0 is stabilized, and the desired intermediate frequency signal can be obtained.

For example, the first intermediate frequency shown in FIG. 2 is 15
US CA with 00 MHz and second intermediate frequency of 44 MHz
A case of selecting 2CH (center frequency 50 MHz) with a TV signal will be described. The first local oscillation frequency is 1550 MH
z, and the input RF signal is 15 in the first intermediate frequency band.
Converted to 00MHz ± 3MHz. Next, since the second local oscillation frequency is set to 1450 MHz, control data is input to the terminal 20 so that the frequency difference between the first local oscillation frequency and the second local oscillation frequency is 94 MHz, and the PLL circuit 23 is operated. , 1450 MHz to the second mixer 7. If the first local oscillation frequency is 1550MHz
If it is shifted to a higher frequency, the center frequency of the first intermediate frequency band becomes higher than 1500 MHz, so that the difference between the first local oscillation frequency and the second local oscillation frequency becomes large, and the control signal corresponding to the frequency difference becomes It is input to the PLL circuit 23, but since the PLL circuit 23 operates so as to converge to the first and second local oscillation frequency difference 94 MHz in 2CH,
The second local oscillation frequency is set higher than 1450 MHz, so that the center frequency of the second intermediate frequency is 44 MH.
will be converged to z. Even when the second local oscillation frequency is deviated, the PLL circuit 23 sets the difference frequency between the first local oscillation frequency and the second local oscillation frequency to 94 MHz.
The second intermediate frequency is converged to 44 MHz by controlling the.

FIG. 2 shows a signal band of an embodiment of the double super tuner. 50, 51, 52, 53 are 2, 3, 4
Channels and signal bands of 116 channels, 54 is a first local oscillation signal, 55 is a band characteristic of a first band filter, 56 is a second local oscillation signal, 57 is a band characteristic of a second band filter, and 58 is a second intermediate frequency. It is a signal.

In FIG. 2, (a) shows the spectral arrangement at the input end of the first mixer 4. In the case of a CATV signal in the United States, the RF input signal band has a center frequency of 50 MHz to 800 MHz and represents a signal band of 2 channels to 116 channels. FIG. 6B shows the signal spectrum arrangement at the output end of the first mixer 4 when 3CH is selected. The first local oscillation frequency 54 is 1556M.
The center frequency of 3CH of the RF input signal which is set to Hz and up-converted by the first mixer 4 is 1500 MHz.
Becomes FIG. 3C shows the spectrum at the input end of the second mixer 7, and the first intermediate frequency signal is selectively amplified by the first bandpass filter 5 and the first intermediate frequency amplifier 6. FIG. 3D shows the second intermediate frequency signal band, and the second intermediate frequency signal band is 4
It is 4 MHz ± 3 MHz, and the frequency of the second local oscillation signal 56 is 1456 MHz. Therefore 2CH to 116
When CH is selected, the frequency variable range of the first local oscillation signal 54 is 1550 MHz to 2300 MHz.

The difference between the first local oscillation frequency and the second local oscillation frequency is 94 MHz (155 MHz when the second channel is selected).
From 0MHz-1456MHz) to 844MHz (2300MHz-1456MHz) when the 116th channel is selected, and the filter 22 has an L of cutoff frequency 850MHz.
Set to PF. When amplifying the filter output to a voltage value that can control the voltage control circuit 24 for the second local oscillation and operating it, the second local oscillation frequency has a predetermined fixed frequency. Therefore, the frequency control of the PLL circuit 23 is performed. The characteristics can be made gentle, control can be performed with high precision, and high frequency stability can be achieved. That is, when the output of the frequency difference of the third mixer 21 is fed back to the first local oscillating unit which varies widely, and the PLL operation is performed, the voltage controlled oscillator 11
The control voltage vs. frequency characteristic of must also correspond to it,
The control voltage resolution becomes rough and the frequency cannot be controlled with high precision.

The filter 22 is a tunable BPF.
Configuration, that is, 94MHz when selecting 2CH, 1
When 16CH is selected, the noise component is further improved by setting the bandpass center frequency corresponding to the selected CH and the feasible and necessary minimum band such as 844MHz, and outputting the control signal to the PLL circuit 20. It can be further reduced. Since the second intermediate frequency is set to a lower frequency than the input signal band, the frequencies of the second local oscillation signal and the first intermediate frequency signal are close to each other, and the phase noise component of the second local oscillation signal is 2 Converted to intermediate frequency signal S of signal
This is one of the causes for deterioration of the / N ratio (signal to noise ratio). Therefore, the spectrum purity of the second local oscillation signal becomes a problem, but since the second local oscillation frequency is not a variable frequency that can be varied over a wide range but a predetermined fixed frequency, the PLL loop of the second local oscillation circuit is Voltage controlled oscillator 2
4, PLL circuit 23, reference oscillator 25, third mixer 2
By optimizing the filter 22 and the filter 22, the spectrum purity of the second local oscillation signal can be improved.

[0019]

According to the present invention, in a double super tuner for frequency-converting a wideband frequency-multiplexed RF signal such as a CATV signal into a predetermined intermediate frequency signal and selecting a desired channel signal, the RF signal is In the case of a digital modulation signal without a carrier wave, it is possible to correct the frequency fluctuation and offset of the local oscillator generated inside the tuner and obtain a stable intermediate frequency signal. In addition, the detection signal of the frequency difference between the first local oscillation signal and the second local oscillation signal is fed back to the second local oscillation circuit having a fixed frequency instead of being fed back to the second local oscillation circuit having a fixed frequency, instead of being fed back to the PLL. By performing the operation, the amount of phase noise converted to the second intermediate frequency can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a double super tuner according to an embodiment of the present invention.

FIG. 2A is a diagram schematically showing a spectrum arrangement of a first mixer input end; FIG. 2B is a diagram schematically showing a spectrum arrangement of a first mixer output end; and FIG. 2C is a second mixer input end. Figure showing simulated spectrum (d) Figure showing simulated spectrum of double super tuner output end

FIG. 3 is a block diagram of a conventional double super tuner.

[Explanation of symbols]

 1 RF Input Terminal 4 1st Mixer 7 2nd Mixer 8 2nd Bandpass Filter 10 Intermediate Frequency Signal Output Terminal 11 Voltage Controlled Oscillator of 1st Local Oscillation Circuit 12 PLL Circuit of 1st Local Oscillation Circuit 13 1st Local Oscillation Circuit Reference oscillators 14, 24 Voltage-controlled oscillator for second local oscillator circuit 15 Control voltage terminal for second local oscillator circuit 16 Video intermediate amplifier circuit 17 AFC circuit 19 PLL circuit control terminal for first local oscillator circuit 20 Second local oscillator circuit PLL circuit control terminal 21 Third mixer 22 Filter 23 PLL circuit of second local oscillation circuit 25 Reference oscillator of second local oscillation circuit

Claims (2)

[Claims] 1. An RF frequency-multiplexed over a wide band
A first local oscillation circuit for frequency-converting an input signal to a first intermediate frequency; a first mixer for frequency-converting the input signal to a first intermediate frequency by a signal from the first local oscillation circuit; and a first mixer A double local tuner having a second local oscillation circuit for frequency converting the output of the second intermediate frequency to a second intermediate frequency, and a second mixer for frequency converting the signal of the second local oscillation circuit to a second intermediate frequency. The first local oscillation circuit is configured to be variable over a wide range to a predetermined oscillation frequency according to channel selection. The output of the first local oscillation circuit and the second local oscillation circuit
A signal of the frequency difference between the two outputs is detected through a third mixer that mixes the outputs of the local oscillation circuit, and the second local oscillation circuit is detected by the detected signal, and the frequency difference is channel-selected by using a PLL circuit. A double super tuner characterized in that it is controlled to have a predetermined difference frequency according to the above. 2. The frequency difference signal from the mixed signal of the first local oscillation signal and the second local oscillation signal output from the third mixer is extracted, so that it has a bandwidth corresponding to the channel selected. 2. The output to the PLL circuit of the second local oscillation circuit via a band variable filter.
The listed double super tuner.