KR0159440B1 - A circuit and method for restorating frequency offset of receiver - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

A device for receiving a modulated signal in a residual sideband manner.

2. The technical problem to be solved by the invention

In an apparatus for demodulating a signal modulated in the residual sideband method, a band filter for detecting a pilot signal is independently implemented to remove frequency offset.

3. Summary of the Solution of the Invention

A device for receiving a signal modulated by the residual sideband method, comprising: tuner means for frequency-converting the received signal and converting the received signal into a signal of an intermediate frequency band, and inputting the intermediate frequency signal, and converting a band of the pilot signal from the input signal. Filter means for filtering and carrier recovery means for removing a frequency offset by controlling the oscillator of said tuner means using a pilot signal output from said filter means.

4. Important uses of the invention

Always reliably remove the carrier's frequency offset in devices receiving signals modulated in the residual sideband scheme.

Description

Frequency offset recovery circuit and method of receiver

1 is a diagram showing a configuration of a general receiver.

2a-c show the relationship between the filter and the pilot signal of the received carrier in FIG.

3 is a diagram showing a configuration of a circuit for recovering a carrier of a signal received at a receiver according to the present invention.

FIG. 4 is a diagram showing characteristics of recovering a carrier by bandpassing a pilot signal of a carrier received in FIG.

The present invention relates to a circuit and a method for recovering a frequency offset of a device for receiving a radio signal, and more particularly, to a pilot signal of a carrier wave received in a device using a residual side band (VSB) modulation scheme. The present invention relates to a circuit and a method for detecting and recovering an offset of a carrier wave.

Recently, the GA (Grand Alliance) committee adopted and proposed VSB modulation as the modulation method of HDTV system. The VSB modulation method is used as an analog video signal modulation method in conventional TV broadcasting, and is used for digital signal modulation in HDTV of GA. In the early days of Digital Spectrum Compatible (DSC) -HDTV, 2-VSB and 4-VSB using two and four levels were selected as modulation methods, but in GA HDTV, 8-VSB and high-speed cable mode using eight levels (high 16-VSB using 16 levels in the speed cable mode was selected as the modulation method.

In order to demodulate the VSB signal, the GA committee has proposed a schematic structure of a television receiver, and the proposed television receiver has the following characteristics. First, unlike the demodulator of other digitally modulated signals, the VSB receiver detects data using only I-channel signals and performs sampling by symbol rate. Therefore, VSB receiver is simpler to implement than QAM receiver which uses Q (Quadrature) channel simultaneously. Since it processes data with symbol rate, it can detect even relatively low processing speed compared to fractional rate receiver. This has the advantage of being possible. And GA HDTV receiver uses synchronous detection (coherent detection). The synchronous detection method has the advantage that it can be detected at a lower error rate at the same signal-to-noise ratio than the asynchronous detection method, but the structure of the receiver is complicated by the carrier recovery circuit. Therefore, accurate structure identification and optimization of the carrier recovery circuit is an essential problem. Therefore, the phase detection of the transmission signal for synchronous detection in the HDTV receiver of the GA committee consists of two stages using frequency and phase locked loop (FPLL) and phase tracking loop (PTL) circuits. The FPLL recovers the carrier component by using the pilot signal included in the VSB signal of the GA committee. This FPLL can be easily implemented as a conventional PLL and frequency error detection circuit.

FIG. 1 shows a typical structure of a part for receiving a radio signal in a receiver for recovering a signal of the residual sideband modulation scheme as described above. Referring to FIG. 1, a tuner 11 receives a signal transmitted in a selected channel band and down-converts the intermediate frequency signal to an intermediate frequency signal. The signal is modulated in the residual sideband method input to the tuner 11. The filter 12 is a band pass filter of the received signal, and filters and outputs a video signal and an audio signal band signal from the tuner 11. The filter 12 may use a SAW filter. Amplifier 13 amplifies and outputs a weak signal level output from filter 12. Multiplier 14 receives the outputs of amplifier 14 and oscillator 15. The multiplier 14 performs a demodulation function by mixing the received IF signal and the oscillation signal. A carrier recovery circuit 16 inputs a demodulation signal outputting the multiplier 14, recovers a pilot signal from the received demodulation signal, and controls a frequency of the oscillator of the tuner 11 to obtain a frequency offset. Remove

Referring to the operation of the receiver having the configuration as shown in FIG. 1, the tuner 11 frequency-converts the received signal to generate an IF signal, and the filter 12 filters the output from the tuner 11 into the band of the signal. Here, the filter 12 may use a 6MHz bandpass filter. The signal output from the filter 12 is demodulated by the multiplier 14, output to the signal processor, and input to the carrier recovery circuit 16. At this time, the carrier recovery circuit 16 detects a pilot signal from the demodulated signal, and then recovers the carrier using the pilot signal.

In this case, when the received signal is normally received without a frequency offset, the frequency spectrum of the IF signal output from the tuner 11 is as shown in FIG. 2a. Here, reference numeral 21 denotes a band of the signal output from the tuner 11, reference numeral 22 denotes a band filtered by the filter 12, and 23 denotes a pilot signal. Here, the spectrum of the pilot signal 23 is located about 300 KHz away from the start position at which the signal exists. The carrier recovery circuit 16 controls the oscillator of the tuner 11 using the pilot signal 23 to remove the carrier offset. In this case, the carrier recovery circuit 16 detects a pilot signal by using the signal filtered by the band 12, and has the following problems depending on the type of the signal filtered by the filter 12.

First, as shown in FIG. 2B, when the IF signal 21 output from the tuner 11 is converted to a higher frequency than the set band and out of the pass band 22 of the filter 12, but the pilot signal 23 is located within the pass band 22 of the filter 12, The pilot signal 23 can be detected by the carrier recovery circuit 16. Accordingly, the carrier recovery circuit 16 may remove the frequency offset by controlling the oscillator of the tuner 11 based on the detected pilot signal 23, and thus, when the frequency offset shown in FIG. 2b is generated, the carrier recovery circuit 16 may tune the tuner 11. The output of is able to remove the frequency offset as shown in Figure 2a. That is, if the pilot signal 23 exists in the pass band 22 of the filter 12, the frequency offset can be removed.

However, when the IF signal 21 output from the tuner 11 is converted to a lower frequency than the set band as shown in FIG. 2C, it is out of the pass band 22 of the filter 12 and the pilot signal 23 has a frequency lower than the pass band 22 of the filter 12. In the carrier recovery circuit 16, the pilot signal 23 cannot be detected. Then, the carrier recovery circuit 16 cannot use the pilot signal 23, and when the frequency offset shown in FIG. 2c is generated, the carrier recovery circuit 16 cannot recover the frequency offset of the carrier. As in the above description, the frequency offset that carrier recovery circuit 16 can remove is essentially limited. That is, if the offset of the frequency exists more than the difference between the cut off frequency of the filter 12 and the pilot frequency, such frequency offset cannot be removed.

Therefore, in order to solve the above problems, the tuner 11 must perform the tuning function correctly, and in this case, the price of the tuner 11 is increased. There is also a method of extending the band of the filter 12 to detect the pilot signal 23. In this case, the pilot signal 23 can be detected, but the signal of the noise component is also filtered in accordance with the band determination, which causes a problem of complicated signal processing. In addition, since the carrier recovery signal must be performed with the 6MHz band signal output from the filter 12, there is a problem that the video signal may operate as noise during carrier reproduction.

Accordingly, an object of the present invention is to provide a circuit capable of stably removing a frequency offset by extending the amount of frequency offset in a device receiving a VSB signal.

Another object of the present invention is to provide a circuit that includes a filter for detecting a pilot signal in an apparatus for receiving a VSB signal and which can remove an offset of a frequency by using a pilot signal output through the filter.

In order to achieve the object of the present invention, the frequency offset recovery circuit of the apparatus for receiving a signal modulated in the residual sideband method, the tuner means for frequency-converting the received signal to convert the intermediate frequency band into a signal, and the intermediate frequency A filter means for inputting a signal and filtering a band of a pilot signal from the input signal, and a carrier recovery means for removing a frequency offset by controlling an oscillator of the tuner means by using a pilot signal output from the filter means. It is done.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The term IF used below denotes an intermediate frequency of the signal band. PBS denotes the first band signal filtered by the first filter 32 to detect the pilot signal at the intermediate frequency IF. SBS represents a second band signal filtered by the second filter 37 to detect a video signal and an audio signal at the intermediate frequency IF. The OCS represents a control signal for controlling the frequency offset using the pilot signal detected from the first band signal. DMS represents a demodulated signal output from the multiplier 39.

3 shows a configuration of a circuit for removing an offset of a frequency in an apparatus for receiving a VSB signal according to the present invention.

Referring to FIG. 3, the tuner 31 is connected between the antenna and the node N1 to receive a radio signal and to receive a recovery control signal OCS. The tuner 21 removes the frequency offset of the signal received by the recovery control signal OCS and generates an intermediate frequency IF signal by frequency converting the received signal. The first filter 32 is connected to the node N1, inputs an intermediate frequency IF output to the node N1, filters the band of the pilot signal, and outputs the first band signal PBS. The first filter 32 has a pilot frequency as a center frequency and a bandwidth as wide as a desired frequency offset. The first amplifier 33 amplifies and outputs the first band signal PBS. The multiplier 34 inputs the output of the first amplifier 33 and the output of the oscillator 35. The multiplier 34 mixes and outputs the output of the first amplifier 33 and the output of the oscillator 35. The carrier recovery circuit 36 receives the output of the multiplier 34. The carrier recovery circuit 36 detects a pilot signal at the output of the multiplier 34, and generates the recovery control signal OCS to remove the frequency offset using the detected pilot signal, and converts the recovery control signal OCS to the tuner 31. Output to oscillator as oscillation control signal.

The second filter 37 connected to the node N1 filters the intermediate frequency IF output from the node N1 into a signal band and outputs a second band signal SBS. The second band signal SBS is a band including an audio signal and a video signal. The second filter 37 may use a SAW filter. The second amplifier 38 amplifies and outputs the second band signal SBS. The multiplier 39 inputs an output of the first amplifier 38 and an output of a phase locked loop (PLL) 40, and an output terminal is connected to the node N2. The multiplier 39 generates a demodulation signal DMS by mixing the output of the second amplifier 38 and the output of the phase locked loop 40 and outputs it to the node N2. The phase locked loop 40 is connected between the node N2 and the multiplier 39. The phase locked loop 40 compensates for phase errors of signals delayed differently by the characteristics of the first filter 32 and the second filter 37.

4 is a frequency spectrum characteristic diagram for explaining an operation of compensating an offset of a frequency according to the present invention, and reference numeral 41 shows a band of the first filter 32 having a predetermined bandwidth centered on a pilot frequency. Reference numeral 42 denotes a pilot signal, and reference numeral 43 denotes a band of the signal filtered by the second filter 37.

Referring to the present invention based on the above configuration, the tuner 31 frequency-converts the received signal to generate an intermediate frequency IF and outputs it to the node N1. Then, the first filter 32 connected to the node N1 filters the intermediate frequency IF into a pilot frequency band and outputs a first band signal PBS. As shown in FIG. 4, the first filter 32 sets the pilot signal 42 as the center frequency and sets the bandwidth to the desired band according to the offset amount to be removed. The bandwidth of the first filter 32 may be set as a narrow band band pass filter having a 2 MHz centering on the pilot signal 42. The first band signal PBS output from the first filter 32 is amplified by the first amplifier 33 and then applied to the multiplier 34. The multiplier 34 mixes the output of the first amplifier 33 and the output of the oscillator 35 to recover the carrier. Output to 36 The carrier recovery circuit 36 then detects a pilot signal at the output of the multiplier 34 and analyzes the offset amount according to the detected pilot signal. A recovery control signal OCS is generated to remove the frequency offset and output as a signal for controlling the oscillator of the tuner 31. The tuner 31 removes the frequency offset by the recovery control signal OCS and outputs the received signal by frequency conversion. As a result, the pilot signal is alive as long as the frequency offset is smaller than the bandwidth of the first filter 32, thereby allowing the carrier recovery circuit 36 to operate normally.

In addition, the second filter 37 connected to the node N1 sets a bandwidth for filtering video and audio signals. Therefore, the intermediate frequency IF output to the node N1 is filtered by the second filter 37 into the signal band and output as the second band signal SBS, and the second amplifier 38 amplifies and outputs the second band signal SBS. In this case, the first band signal PBS and the second band signal SBS have different delay times according to the filter characteristics of the first filter 32 and the second filter 37. Therefore, there is a delay difference between the carrier recovery path and the main path of the signal. In order to compensate for the phase error caused by the delay difference between the two passes, in the present invention, a phase locked loop 40 is added. That is, the multiplier 39 connected between the second amplifier 38 and the node N2 feedbacks the output of the phase locked loop 40 connected to the node N2. Therefore, the multiplier 39 generates a demodulation signal DMS by mixing the output of the second amplifier 38 and the output of the phase locked loop 40. At this time, the demodulation signal DMS demodulates the intermediate frequency IF whose frequency offset is removed by the recovery control signal OCS. Therefore, the phase locked loop 40 connected to the node N2 generates an oscillating signal whose phase is synchronized to compensate for the phase error of the demodulation signal DMS.

As described above, when recovering a carrier signal in a device receiving a VSB signal, the pilot signal is set as the center frequency and the bandwidth is set so as to remove a desired frequency offset amount, thereby reducing the frequency of the filter that filters the signal band. Irrespective of this, the pilot signal can be stably detected at all times, and the bandwidth of the first filter 32 can be adjusted to extend the amount of removable frequency offset. In addition, a phase error that may be caused by a delay difference between the carrier recovery path and the main path of the signal may be compensated using the phase locked loop.

Claims (5)

An apparatus for receiving a signal modulated by a residual sideband method, comprising: tuner means for frequency-converting a received signal into a signal of an intermediate frequency band, and inputting the intermediate frequency signal, wherein the band of the pilot signal is input from the input signal And a carrier recovering means for removing a frequency offset by controlling the oscillator of the tuner means using a filter means for filtering the filter means and a pilot signal outputted from the filter means. 2. A frequency offset recovery circuit as set forth in claim 1, characterized in that it is a narrow band bandpass filter having a center frequency of said pilot signal of said filter means. An apparatus for receiving a signal modulated by a residual sideband method, comprising: a tuner for converting a received signal into a signal of an intermediate frequency band, and inputting the intermediate frequency signal, wherein the band of the pilot signal is input from the input signal. A carrier recovery circuit for removing the frequency offset by controlling the oscillator of the tuner means using a first filter for filtering, a pilot signal output from the filter means, and inputting the intermediate frequency signal, and converting the frequency signal into a signal band Inputs a second filter, a signal for demodulating the signal output from the second filter, and an output of the demodulation means, and compensates for a phase error due to a delay difference between the first filter and the second filter. And a phase fixing loop means for generating a phase-synchronized oscillation signal and outputting it to the demodulation means. Frequency offset recovery circuit. 4. The frequency offset recovery circuit of claim 3, wherein the filter means is a narrow band bandpass filter having a center frequency as the frequency of the pilot signal. A method of receiving a modulated signal in a residual sideband method, the method comprising: converting a received signal into a signal of an intermediate frequency band by converting the received signal into a carrier recovery signal; and narrowing the band from the intermediate frequency signal to a pilot signal Detecting a pilot signal by bandpass filtering, and generating the carrier recovery signal for canceling a frequency offset using the pilot signal.