JP2007243252A - Cable modem - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cable MODEM wherein a tuner can receive a signal received by an input terminal, apply frequency conversion to the signal, a QAM demodulator can demodulate the resulting signal and normally output, and even when a signal with an excess level is given to the input terminal, the tuner can receive the signal with a proper level and can be normally in operation. <P>SOLUTION: An attenuation means provided with attenuator elements whose attenuation is adjustable is interposed between the input terminal and the tuner and configured to transmit the normally attenuated signal to a next stage, the tuner includes: an electric field strength measurement means for measuring an input signal level and outputting it; and a control means for executing a step wherein measured signal level information items are received, the total sum is calculated, and the total sum of the signal level information items is compared with a first threshold value, a step wherein a level of an optional wave in the signal level information items is compared with a second threshold value, and for outputting a switching control signal for decreasingly switching the attenuation amount of the attenuator elements to an attenuation means when the total sum is equal to or less than the first threshold value and the signal level of the one wave is equal to or less than the second threshold value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a cable modem for connecting to a CATV network and supplying data communication signals.

A conventional cable modem includes a tuner and a QAM (Quadrature Amplitude Modulation) demodulator inside, and after converting the frequency of a CATV signal (downstream signal: downstream) input from the input end by the tuner, the QAM demodulator Demodulate with
Also, assuming that the level of the signal input from the input terminal becomes an abnormal value that prevents normal operation of the internal tuner and the QAM demodulator, the tuner is adjusted in order to adjust it to a normal level. An AGC function is provided. (For example, see Patent Document 1)

JP 2002-16524 A

However, in the conventional cable modem, the input signal level (hereinafter also referred to as “practical input level”) in a range where the tuner can operate normally is input due to various causes such as interference of the input signal. In some cases, a tuner having an AGC function with a narrow AGC range may be distorted and cannot be properly handled.
In addition, when an excessively high level signal is input in this manner, there is a reflection in the tuner, and the signal that the tuner tries to detect is disturbed by the reflection, and the tuner cannot operate normally.
Furthermore, when a plurality of tuners (for example, a QAM tuner in a set-top box with a built-in cable modem) are adjacent to each other in addition to the cable modem tuner, other tuners are affected by reflection from the tuner. There was also a problem that it interfered with the operation of other tuners.

An object of the present application is to provide a cable modem that can receive a signal input to an input terminal into a tuner, frequency-convert it, demodulate it with a QAM demodulator, and output it normally. Another object of the present invention is to provide a cable modem in which a tuner accepts an appropriate level signal and can operate normally even when an excessive level signal is input to the input terminal.
Other objects and advantages will be readily apparent from the drawings and the following description associated therewith.

  The cable modem according to the present invention includes a tuner 5 that frequency-converts and outputs a signal input from the input terminal 2, and a QAM demodulator 8 that receives the output signal and performs QAM demodulation and outputs the signal. 1, an attenuating means 10 having an attenuator element capable of adjusting the amount of attenuation is interposed between the input terminal 2 and the tuner 5, and the attenuating means 10 always receives an input signal via the attenuator element. Is configured to be attenuated and sent to the next stage, and the tuner 5 is provided with electric field strength measuring means 6 for measuring the level of the input signal and outputting the signal level information 17. The step of accepting the measured signal level information 17 and calculating the sum of the signal level information 17 and comparing the sum of the signal level information 17 with the first threshold value T1. And a step of comparing the level value of one arbitrary wave in the signal level information 17 with the second threshold value T2, and when both are equal to or less than the first threshold value T1 and the second threshold value T2, the above attenuation Control means 16 is provided which outputs a switching control signal 18 for switching so as to reduce the attenuation amount of the attenuator element toward the means 10.

  Preferably, the control means 16 further compares the level value previously stored for the arbitrary one of the signal level information 17 with the level value currently received. If the difference between the two exceeds a predetermined value, the signal level information 17 is again output without outputting the switching control signal 18 for switching the attenuation means 10 so as to decrease the attenuation amount of the attenuator element. And the above three comparison steps are performed, and the sum of the signal level information 17 is less than or equal to the first threshold value T1, and the level value of one wave in the signal level information 17 is less than or equal to the second threshold value T2, When the difference between the measured level value and the previously stored level value is equal to or smaller than a predetermined value, a switching control signal 18 for switching the attenuation means 10 to decrease the attenuation amount of the attenuator element. Output As long as it was to so that.

  As described above, according to the present invention, even when an excessive level signal is input to the input terminal, the input signal is always attenuated by the attenuation means and input to the tuner. The tuner can be accepted, and the tuner can be operated normally without causing the conventional problems.

  Further, in the present invention, even when a low level signal is input to the input terminal and the signal input to the tuner is equal to or less than the threshold value T2, the attenuator element in the attenuation means is switched by the instruction from the control means. As a result, the amount of attenuation is reduced, so that an appropriate level signal is input to the tuner and the tuner can be operated normally.

Embodiments of the present invention will be described below with reference to the drawings. 1 to 3, reference numeral 1 denotes the presence of a known cable modem that is connected to a CATV network and supplies data communication signals.
In the figure, the attenuating means 10, the tuner 5, the QAM demodulator 8 and the like are those used for the downstream signal in the cable modem 1. On the other hand, as is well known in this type of cable modem 1, the related member for the upstream signal is substantially present, but the illustration and description thereof are omitted because of known matters.

In the cable modem 1, 2 is an input terminal for inputting a well-known CATV signal, and 3 is an output terminal for outputting a data signal to a terminal device such as a PC.
Next, 5 is a known tuner, and as is widely known, tunes to a predetermined frequency (also referred to as one wave) within the frequency band of the CATV signal (downstream signal) input from the input terminal 2, The frequency (one wave) signal is frequency-converted and output.
6 indicates the presence of a received signal strength indicator (RSSI) built in the tuner 5, and measures the level of an arbitrary predetermined frequency (one wave) in the signal input to the tuner 5. is there. Further, the signal level information measured by the tuner 5 is output to the control means 16 described later.

  Reference numeral 8 denotes a known QAM demodulator for receiving the signal 5b outputted from the tuner 5, QAM demodulating it, and outputting it.

Denoted at 10 is an attenuating means interposed between the input end 2 and the tuner 5, and includes a first attenuator element 11, a second attenuator element 12, and a third attenuator element 13 having different attenuation amounts. The first to third attenuator elements 11, 12, and 13 are configured to be selectively switchable by a switching control signal 18 from a control means 16 described later. The attenuation amounts of the first to third attenuator elements 11, 12, and 13 are selectively switched to each other to adjust the level of the signal input to the input terminal 2 to the practical input level of the tuner 5, A signal may be transmitted to the input point 5a of the tuner 5, for example, 0 dB, 10 dB, and 20 dB, respectively.
Note that the number of attenuator elements may be increased or decreased as necessary. For example, the first attenuator element 11 and the second attenuator element 12 may be configured to be switched and used.

Next, reference numeral 15 denotes a CPU which executes various applications for operating the cable modem as is generally known.
16 indicates the presence of the control means, reads the signal level information 17 output from the electric field strength measuring means 6, and based on the signal level information 17, the first to third attenuator elements 11, A switching control signal 18 for selectively switching between 12 and 13 is output.
As a means for switching the first to third attenuator elements 11, 12 and 13, a plurality of attenuator elements can be selectively switched using a normally known switching circuit using a PIN diode. Just keep it.
Next, 20 indicates a flash memory for storing a program and data for operating the cable modem, and 21 indicates a RAM used for temporary storage of data when the CPU 15 and the control means 16 are operated.

In the above configuration, the operation of the control means 16 will be described in detail with reference to FIGS. In addition, in the attenuation means 10, the case where two of the 1st attenuator element 11 and the 2nd attenuator element 12 are used is demonstrated.
A case where the signal input to the input terminal 2 is at an appropriate level (practical input level of the tuner 5, for example, 24 dBmV) will be described.
First, the second attenuator element 12 (for example, attenuation: 10 dB) of the attenuation means 10 is set to be interposed (S101). The signal input to the input terminal 2 is attenuated by the attenuation means 10 and sent out toward the input point 5a. “Week signal” (a parameter indicating whether or not the signal is weak) is set to “1” (weak signal) (S102).
Next, all frequencies (for example, 117 waves) within the frequency band of the downstream signal input to the tuner 5 are scanned (S103, S104). The electric field strength measuring means 6 measures the level for each of all frequencies and outputs the signal level information 17 thereof. The control means 16 reads the signal level information 17 (S105) and stores it in the RAM 21 (S106).
Next, it is determined whether or not all the stored signal level information 17 is greater than a predetermined value P1 (for example, −7.8 dBmV) (S107). It is determined that the value is equal to or less than the predetermined value P1 (N in S107), and the parameter “week signal” remains “1” and the process proceeds to the next process.
Next, after determining whether the downlink signal is in Japan or the like (S109), input level unit conversion (S110), and input level integration (S111), the process proceeds to the next process.
Next, it is determined whether or not the reading of the signal level information 17 relating to all frequencies has been completed (S112). If it has not been completed (N in S112), the process returns to S104, and the processes from S104 onward are repeated.

Next, when it is determined as “complete” in S112 (in the case of Y in S112), the sum of the signal level information 17 regarding all frequencies is calculated (S113).
Next, the sum of the signal level information 17 and the first threshold value T1 (for example, 31.0 dBmV) are compared (S114). Since the signal input to the input terminal 2 is attenuated by the attenuation means 10 and sent to the input point 5a, it is determined that the sum of the signal level information 17 is equal to or less than the first threshold value T1 (N in S114). The process proceeds to the next process with the “Multi-channel” (parameter indicating whether the sum of the signal levels is high or low) being the initial value “0” (low level) (S115).

Next, an arbitrary frequency (arbitrary wave) in the frequency band of the downstream signal is read (S201), a switching control signal 18 is output to the attenuation means 10 so that the second attenuator element 12 is interposed, and the second The attenuator element 12 is set (S202). Next, the level L2 measured by the electric field strength measuring means 6 for the read arbitrary wave is read (S203). The level value L2 (also referred to as the current incoming level value) is compared with the level value L1 of the same frequency stored in S106, and the difference is calculated (S204). It is determined whether the difference is larger than a predetermined value G (for example, 4 dB) (S205). It is determined that the difference is equal to or smaller than the predetermined value G (N in S205), and the process proceeds to the next process (S206).
Next, it is determined whether or not the level value L2 read in S203 is larger than a predetermined value P2 (for example, −2.8 dBmV) and “week signal” is “1” (weak signal) (S206). The signal input to the input terminal 2 is determined to have no level fluctuation (N in S206), and the process proceeds to the next process (S207).

  Next, the level value of an arbitrary wave that has currently entered (the level value read in S203) is compared with a second threshold value T2 (for example, −1.8 dBmV). Then, it is determined whether or not the second threshold value T2 or less and the sum of the signal level information 17 is equal to or less than the first threshold value T1 (“Multi-channel” is “0”) (S207). In both comparisons, it is determined that both the first threshold value T1 and the second threshold value T2 or less (Y in S207), and the control means 16 causes the first attenuator element to decrease the attenuation amount of the attenuator element toward the attenuation means 10. A switching control signal 18 for switching to 11 (for example, attenuation: 0 dB (also referred to as through)) is output and switched (S208). The signal input to the input terminal 2 is sent to the input point 5a at the same level without being attenuated by the attenuation means 10.

Next, the control means 16 determines whether or not it is a signal that can be detected as a downlink signal (whether it is a QAM signal) as is normally done in a cable modem (S209), and if it can be detected (if S209 is Y) ) Performs upstream signal processing (S210). On the other hand, if it cannot be detected as a downlink signal (N in S209), it is determined whether or not the search has been performed once or more for all frequencies (S211). If not (N in S211), the process proceeds to S201 so as to search for the next frequency based on a predetermined channel plan (S212), and the processes after S201 are continued.
On the other hand, if it is performed (Y in S211), the process proceeds to S101, and the processes after S101 are performed again.

  As described above, after the cable modem 1 is activated, the level of the input signal is always reduced by the attenuation means 10 so as not to input an excessive level signal to the tuner 5 (S101). Can determine that the level of the signal input to the input terminal 2 is small (no excessive level is input) based on the steps S114, S207, etc., and based on that, the attenuation means 10 is determined in the step S208. Since the attenuator element in (1) instructs switching so that the attenuation amount is reduced, a signal of an appropriate level is input to the tuner 5 so that it can operate normally.

Next, a case where the signal input to the input terminal 2 is at an excessive level (for example, 34 dBmV: one digital signal wave is +5 dBmV and an analog signal 76 waves are each +15 dBmV) will be described.
In addition, about the process of S101-S106, S109-S113, S201-S206, and S209-S212, since it is the same process as the above-mentioned case, some overlapping description was saved.
First, the second attenuator element 12 (for example, attenuation: 10 dB) of the attenuation means 10 is set to be interposed (S101). An excessive level signal input to the input terminal 2 is attenuated by the attenuation means 10 and an appropriate level signal is sent out toward the input point 5a. “Week signal” (a parameter indicating whether or not the signal is weak) is set to “1” (weak signal) (S102). Next, the process proceeds to step S103, and the processing of S103 to S106 is continued.
Next, it is determined whether or not all the stored signal level information 17 is greater than a predetermined value P1 (for example, −7.8 dBmV) (S107). It is determined that the value is larger than the predetermined value P1 (Y in S107), and the parameter “week signal” is set to “0” (not a weak signal) (S108). Subsequently, the processes of S109 to S113 are performed.
Next, the sum of the signal level information 17 and the first threshold T1 (for example, the first threshold T1: 31.0 dBmV) are compared (S114). It is determined that the sum of the signal level information 17 is larger than the first threshold T1 (Y in S114), the parameter “Multi-channel” is set to “1” (environment with high input level) (S115), and the process proceeds to S201. Subsequently, the processing of S201 to S206 is performed.

  Next, in S207, the level value L2 of an arbitrary wave that has currently entered (the level value read in S203) is compared with a second threshold value T2 (for example, −1.8 dBmV), and is equal to or less than the second threshold value T2, and It is determined whether “Multi-channel” is “0” (the sum of the signal level information 17 is equal to or less than the first threshold value T1) (S207). In both comparisons, it is determined that both are not less than the first threshold T1 and the second threshold T2 (N in S207). The control means 16 proceeds to S209 without outputting the switching control signal 18 for switching so as to decrease the attenuation amount of the attenuator element toward the attenuation means 10, and subsequently performs the processing of S209 to S212.

  As described above, after the cable modem 1 is started, the attenuator element of the attenuation means 10 is set in S101, and the signal level from the input terminal 2 is attenuated and sent to the tuner 5. Therefore, the tuner can be operated normally without a conventional level signal being input.

Next, the operation of the control means 16 when the level of the signal input to the input terminal 2 is a low level when the cable modem is activated but then becomes an excessive level (when the input level varies). Will be described. Note that the processing of S101 to S106, S109 to S113, and s201 to S204 is the same processing as that in the case where the signal input to the input terminal 2 is at an excessive level, and thus some redundant explanations have been saved.
First, the second attenuator element 12 (for example, attenuation: 10 dB) of the attenuation means 10 is set to be interposed (S101). The signal input to the input terminal 2 is attenuated by the attenuation means 10 and sent out toward the input point 5a. “Week signal” (a parameter indicating whether or not the signal is weak) is set to “1” (weak signal) (S102). Next, the process proceeds to step S103, and the processing of S103 to S106 is continued.

Next, it is determined whether or not all the stored signal level information 17 is greater than a predetermined value P1 (for example, −7.8 dBmV) (S107). It is determined that the value is equal to or less than the predetermined value P1 (N in S107), and the parameter “week signal” remains “1” and the process proceeds to the next process. Next, the process proceeds to step S109, and the processes of S109 to S113 are continued.
Next, the sum of the signal level information 17 and the first threshold T1 (for example, the first threshold T1: 31.0 dBmV) are compared (S114). It is determined that the sum of the signal level information 17 is equal to or less than the first threshold T1 (N in S114), and “Multi-channel” (a parameter indicating whether the sum of the signal levels is high or low) is an initial value “0” (the level is low). The process proceeds to the next process (S201) as it is (S114). Subsequently, the processing of S201 to S204 is performed.

  Next, in S205, it is determined whether or not the difference calculated in the process of S204 is larger than a predetermined value G (for example, 4 dB). For any one wave in the signal level information 17, the gap between the level value L1 stored in S106 and the current incoming level value L2 (the value of S203) is larger than a predetermined value G (Y in S205). If so, the process proceeds to S101, and the processes after S101 are repeated again.

On the other hand, if the difference is equal to or smaller than the predetermined value G in S204, the process proceeds to the next process (S206).
It is determined whether or not the level value read in S203 is greater than a predetermined value P2 (for example, −2.8 dBmV) and “week signal” is “1”. If both conditions are satisfied (Y in S206: if the signal level is weak at the time of S107 and if it is determined that the signal level of the incoming S203 is high), go to S101 Then, the process from S101 is repeated.

As described above, if the level of the signal input to the tuner 5 fluctuates during the calculation of the sum of the signal level information 17, for example, between 10 seconds and 20 seconds (for example, the user connects the cable modem) Even when the cable modem 1 is activated without connecting the RF connector and then connected, or when it changes due to poor contact of the RF connector), the change is detected in S205 and directed to the attenuation means 10 Since the switching control signal 18 that decreases the attenuation amount of the attenuator element is not output, the process proceeds to S101 and three comparison steps (S114, S205, S206, S207) are performed. Can be prevented from being input to the tuner 5, and the tuner can be operated normally.
Furthermore, since the presence / absence of fluctuation is also determined in S206, the tuner can be operated normally by detecting the presence / absence of fluctuation with higher accuracy.

Next, FIG. 4 showing an example different from FIG. 1 in terms of switching means for a plurality of attenuator elements will be described.
Reference numerals 25, 26 and 27 denote the same damping means, first attenuator element and second attenuator element as described above. Reference numeral 28 denotes an input point from the input terminal 2, 29 denotes an output point to the tuner 5, 30 denotes a contact point, and a first attenuator element 26 is connected via a known coaxial relay 31 by a control signal 18 from the control means 16. The second attenuator element 27 can be selectively used at two points.
In FIG. 4, the contacts 30 and 30 are provided on both sides of the attenuator element, but one of them may be short-circuited and only one of the contacts 30 may be switched.
Further, a switching circuit constituted by a known FET (field effect transistor) may be used to selectively switch at two points of the first attenuator element 26 and the second attenuator element 27.

1 is a schematic block diagram for explaining a cable modem. The flowchart for demonstrating operation | movement of a control means is shown. The flowchart for demonstrating operation | movement of a control means is shown. The schematic block diagram for demonstrating the example different from the attenuation | damping means of FIG. 1 is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Cable modem, 2 ... Input end, 3 ... Output end, 5 ... Tuner, 5a ... Input point, 6 ... Electric field strength measuring means (RSSI), 8 ... QAM demodulator, 10 ... attenuating means, 11 ... first attenuator element, 12 ... second attenuator element, 13 ... third attenuator element, 16 ... control means, 17 ... signal Level information, 18 ... switching control signal, 20 ... flash memory, 21 ... RAM

Claims (2)

A tuner that converts the frequency of the signal input from the input end and outputs the signal;
In a cable modem including a QAM demodulator that receives the output signal, demodulates and outputs the QAM,
An attenuating means having an attenuator element capable of adjusting the amount of attenuation is interposed between the input terminal and the tuner, and the attenuating means always attenuates the input signal via the attenuator element to the next stage. Configured to send out,
The tuner is provided with electric field strength measuring means for measuring the level of the input signal and outputting the signal level information,
Further, receiving the measured signal level information, calculating a sum of the signal level information, and comparing the sum of the signal level information with a first threshold;
Comparing the level value of any one of the signal level information with a second threshold value,
In both comparisons, there is provided control means for outputting a switching control signal for switching so as to decrease the attenuation amount of the attenuator element toward the attenuation means when both are equal to or less than the first threshold value and the second threshold value. A cable modem characterized by that. In the control means, for any one wave of the signal level information, a step of comparing the level value previously stored as a target with the currently received level value is performed, and the difference between the two is determined. When the value exceeds a predetermined value, the signal level information is received again without outputting a switching control signal for switching to reduce the attenuation amount of the attenuator element toward the attenuation means, and the three comparisons are made. Do the steps,
The sum of the signal level information is below the first threshold,
The level value of one wave in the signal level information is less than or equal to the second threshold value,
Switching control for switching to reduce the attenuation amount of the attenuator element toward the attenuating means when the difference between the current incoming level value and the previously stored target level value is below a predetermined value 2. The cable modem according to claim 1, wherein a signal is output.

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