JP2005167860A - Digital television broadcast receiving module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terrestrial wave digital television broadcast receiving module which is hardly interfered by unwanted waves of adjacent channels or three adjacent channels. <P>SOLUTION: The digital television broadcast receiving module includes a signal level detection circuit 12 which detects the level of a signal outputted from an IF tuner circuit 6 and outputs an AGC signal for applying feedback to an RF amplifier circuit 3 so that the detection level becomes constant. Furthermore, AGC is applied from a demodulation circuit 17 to first and second IF amplifier circuits 14, 16 so that the demodulated signal level of a desired channel becomes constant. Since the AGC is applied to the RF amplifier circuit on the basis of the level of a signal after IF tuning, an interference caused by the unwanted waves of the adjacent channels can be suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a receiving module for digital television broadcasting using terrestrial waves, and more particularly to an RF and IF circuit portion.

  Conventionally, in terrestrial analog television broadcasting, in order to prevent interference from adjacent channel signals, two adjacent channels are not used for broadcasting in the same region as much as possible, but for example, channels are used by skipping one channel at a time. Policies are in place.

  Currently, measures are being taken to switch terrestrial television broadcasting from analog to digital. Unlike terrestrial analog television broadcasting, terrestrial digital television broadcasting has a placement policy that broadcasts are almost continuously from 6 to 10 channels in each region from 13 to 62 channels in the UHF band. Yes. However, in some areas, it is difficult to assign continuous channels, and there are cases where discrete channels are assigned. In such areas, the broadcast radio waves (unwanted waves) between the allocated channels are transmitted from another transmission facility close to the transmission facility of the allocated channel and are stronger than the broadcast waves of the allocated channel. It can come. In this case, since the frequency of the unnecessary wave is close to the frequency of the broadcast radio wave that is originally required, there is a high possibility that the broadcast radio wave that is originally required becomes an interference wave. In particular, during analog-to-digital switching, analog television broadcasts in the UHF band may be temporarily mixed in or near the channel range of digital television broadcasts, and digital broadcasts are transmitted more policy than analog broadcasts. The signal level at that time is set low, and it can be expected to be as low as 30 dB in regional conditions, so the problem is likely to become serious.

  Here, the disturbance mechanism will be briefly described.

  In general, as shown in a block diagram in FIG. 7, the television broadcast receiving module 1 includes an input tuning circuit 2 and an interstage tuning circuit 4 which are frequency-variable RF tuning circuits for selecting a signal of a desired channel. A variable gain RF amplifier circuit 3 positioned, a frequency converter circuit 5 for converting an RF signal into an IF signal having a fixed frequency, an IF tuning circuit 6 for further attenuating a signal of a channel other than a desired one, and after IF tuning SAW filter 7 for selecting only a desired channel from among the above signals, a variable gain IF amplifier circuit 8 for amplifying the signal to a desired level, and a demodulation circuit (digital demodulation circuit) for extracting a television signal from the IF signal ) 9 and the like.

  The input tuning circuit 2 is a one-stage tuning circuit, and has a pass band corresponding to a desired channel and three channels on each side (total of 7 channels). In general, the pass band refers to a band whose insertion loss is within 3 dB with respect to the center frequency. Since the interstage tuning circuit 4 is a two-stage tuning circuit, it has a correspondingly narrow band, and has a passband for the desired channel and two channels on each side (total of 5 channels).

  The frequency conversion circuit 5 includes a mixer circuit, a voltage controlled oscillator, and a PLL circuit, and converts a signal of a desired channel in the RF signal into an IF signal having a fixed frequency. At this time, the signal of the channel near the desired channel is also converted into an IF signal while maintaining the frequency difference from the desired channel.

  The IF tuning circuit 6 is further narrowed by a single stage tuning circuit, and has a pass band corresponding to a desired channel and 1.5 channels on both sides (4 channels in total).

  Then, the demodulation circuit 9 outputs two AGC signals (voltages) AGC1 and AGC2 according to the level of the demodulated desired channel signal (demodulation signal), which are output to the RF amplification circuit 3 and the IF amplification circuit 8, respectively. Have been entered. Thus, AGC control is performed so that the level of the signal of the desired channel input to the demodulation circuit 9 becomes a desired level when the signal is input to the demodulation circuit 9. The RF amplifier circuit 3 and the IF amplifier circuit 8 are operated so as to reduce the gain based on the AGC voltage with a state having a predetermined gain as a reference point. Therefore, although the amplification operation is basically performed, the gain change due to the AGC voltage is expressed as an attenuation amount because it is the attenuation direction from the reference point.

  Here, it is assumed that a signal (unnecessary wave) having a higher signal level than the desired channel has arrived in a channel adjacent to the desired channel. In this case, the unnecessary wave passes through the input tuning circuit 2, passes through the interstage tuning circuit 4, and further passes through the IF tuning circuit 6. Since the AGC voltage applied to the RF amplifier circuit 3 in the RF band is applied only based on the level of the demodulated signal, that is, the signal level of the desired channel, even if there is an unnecessary wave having a high level in the adjacent channel, Accordingly, the amount of attenuation does not increase. Therefore, unnecessary waves are input to the RF amplifier 3 and the frequency conversion circuit 5 without being attenuated. Since these circuits use active elements, the circuit saturates due to unwanted waves and the desired channel signal is distorted, or the signal of the desired channel is mixed into the desired channel signal due to cross modulation. . This is a basic mechanism of interference (adjacent interference), and an unnecessary wave in such a case is called an interference wave.

Since such a problem is not basically limited to the digital television broadcast receiving module, many countermeasures have been proposed. For example, in Patent Document 1, the level of the entire input RF signal is detected, and from both the value and the level of the demodulated signal, an RF band variable gain amplifying circuit or a variable attenuation provided in the preceding stage is used depending on the situation. Control is performed so that the optimum AGC is applied to the circuit. As a result, the variable gain amplifier and the frequency conversion circuit are prevented from being saturated by unnecessary waves, and cross modulation is prevented.
JP 2001-326549 A

  The receiving module of Patent Document 1 is effective as a countermeasure when an unnecessary wave having a higher level than the desired channel arrives in the channel adjacent to the desired channel. However, in the receiving module of Patent Document 1, the level of the entire RF signal is detected, and an AGC signal is created based on both the level of the demodulated signal. Therefore, a signal of a channel that is far from the desired channel and does not necessarily become an interference wave may also affect the AGC. In this case, the signal of the desired channel is attenuated more than necessary, and there is a possibility that a sufficient signal level necessary for demodulation may not be obtained when it is input to the demodulation circuit.

  Further, since the AGC signal is generated based on both the RF signal level and the demodulated signal level, the determination circuit for determining how the AGC signal is to be used when the two levels are combined becomes complicated. There is.

  Furthermore, when detecting the level of the RF signal in order to apply AGC, some kind of detection circuit is required, but such a detection circuit generally has frequency characteristics. Therefore, even if the signal level of the desired channel is the same, the magnitude of the AGC signal may differ depending on the channel of the signal, which may cause insufficient prevention of interference depending on the channel. Also included.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problems, and provides a receiving module that is not easily disturbed by unnecessary waves of adjacent channels or further three adjacent channels.

  In order to achieve the above object, a digital television broadcast receiving module of the present invention includes an RF tuning circuit that tunes to a frequency near a desired channel from an input RF signal, and a variable gain RF that amplifies the RF signal. An amplification circuit, a frequency conversion circuit that converts an RF signal into an IF signal having a fixed frequency, an IF tuning circuit that selects a signal in the vicinity of a desired channel from the IF signal, an IF filter circuit that selects a desired channel from the IF signal, and IF A variable gain IF amplifier circuit that amplifies the signal, a demodulator circuit that demodulates the signal of the desired channel from the IF signal, and a level of the signal output from the IF tuning circuit are detected and the detection level becomes constant A signal level detection circuit that outputs an AGC signal for applying feedback to the RF amplifier circuit. The features.

  In addition, the digital television broadcast receiving module of the present invention, when output from the IF tuning circuit, has an RF tuning circuit and a channel so that the attenuation of the adjacent channel is 2 to 5 dB larger than the attenuation of the signal of the desired channel. The frequency characteristic of the IF tuning circuit is set.

  In addition, the digital television broadcast receiving module of the present invention includes an RF tuning circuit and an RF tuning circuit so that the attenuation amount of three channels away from the attenuation amount of a signal of a desired channel is within 12 dB when output from the IF tuning circuit. The frequency characteristic of the IF tuning circuit is set.

  In the digital television broadcast receiving module of the present invention, the IF amplifier circuit is a variable gain amplifier circuit, and an AGC signal is supplied to the IF amplifier circuit so that the level of a signal of a desired channel input to the demodulator circuit is constant. Is applied.

  The digital television broadcast receiving module of the present invention is characterized in that the IF filter circuit includes first and second SAW filters and the IF amplifier circuit includes first and second IF amplifier circuits.

  In the digital television broadcast receiving module of the present invention, the first and second IF amplifier circuits are configured such that delay points with respect to an applied AGC signal are different.

  In the digital television broadcast receiving module of the present invention, the pass band of the first and second SAW filters is shifted or narrowed to such an extent that the signal of the adjacent channel is not passed even if the pass band is moved due to temperature change. It is characterized by being.

  According to the digital television broadcast receiving module of the present invention, since the AGC is applied to the RF amplifier circuit based on the signal level after IF tuning, the adjacent channel as in the case where the AGC is applied based only on the signal level of the desired channel. Interference due to unnecessary waves can be suppressed.

  Further, since the attenuation amount of the adjacent channel is set to be 2 to 5 dB larger than the attenuation amount of the signal of the desired channel after IF tuning, the signal level of the desired channel is small and the signal of the unnecessary wave of the adjacent channel is set. Even if the level is high, the desired channel can be received.

  Further, since the attenuation amount of the three adjacent channels with respect to the attenuation amount of the signal of the desired channel after IF tuning is set to be within 12 dB, it is possible to prevent the interference due to unnecessary waves having a large signal level of the three adjacent channels. it can.

  Further, since the gain of the IF amplifier circuit is AGC-controlled, the level of the signal of the desired channel input to the demodulation circuit is made constant without applying the AGC of the RF amplifier circuit based on the signal level of the desired channel. Can do.

  Further, since two SAW filters are used as the IF filter circuit, it is possible to sufficiently suppress signals of channels other than the desired channel from signals input to the demodulation circuit.

  In addition, since two IF amplifier circuits are provided and the delay points with respect to the applied AGC signal are different from each other, CN deterioration of the signal at the IF stage can be prevented.

  Furthermore, since the passbands of the two SAW filters are shifted or narrowed to such an extent that the signals of the adjacent channels do not pass even if the passbands move due to temperature changes, the passbands of the SAW filters shift with temperature. However, it is possible to prevent interference due to unnecessary waves in adjacent channels.

  FIG. 1 shows a block diagram of an embodiment of the terrestrial digital television broadcast receiving module of the present invention. In FIG. 1, the same or equivalent parts as in FIG.

  In FIG. 1, the terrestrial digital TV broadcast receiving module 10 detects the level of the signal after broadband tuning 11 and IF tuning in addition to the TV broadcast receiving module 1 of FIG. 7, and outputs an AGC signal to the RF amplifier circuit 3. An RSSI (Radio Signal Strength Indicator) circuit 12 which is a signal level detection circuit to be created is included. The output of the RSSI circuit 12 is also connected to the RF side AGC monitor terminal RFm. Further, instead of the SAW filter 7 and the IF amplifier circuit 8, the first SAW filter 13 and the second SAW filter 15 which are IF filter circuits for selecting only a desired channel from the signal after IF tuning, A first IF amplifying circuit 14 which is a variable gain type IF amplifying circuit provided between and after the two SAW filters in order to amplify the signals of the channels to a desired level and input them to the demodulating circuit of the next stage; The second IF amplifying circuit 16 and the attenuating means 18 provided between the first SAW filter 13 and the first IF amplifying circuit 14 are provided. The attenuating means 18 is not essential and may be omitted depending on the specifications. Further, in place of the demodulating circuit 9, a demodulating circuit (digital demodulating circuit) 17 for extracting a television signal from the IF signal of the desired channel is provided. The demodulating circuit 17 outputs AGC signals (AGC voltages) from the two AGC terminals (AGC1, AGC2) according to the level of the demodulated desired channel signal (demodulated signal), respectively. 2 is input to the IF amplifier circuit 16. The output of the AGC 2 terminal is also connected to the AGC monitor terminal IFm on the IF side.

  The most significant difference between the terrestrial digital television broadcast receiving module 10 and the conventional television broadcast receiving module 1 is that the level of the IF signal after IF tuning is detected by the RSSI circuit 12, and the AGC signal is sent to the RF amplifier circuit 3 based on the detected level. It is the point comprised so that may be applied. Therefore, the AGC based on the level of the demodulated signal of the desired channel is not applied to the RF amplifier circuit 3 as in the prior art. Note that the broadband amplifier 11 is provided in front of the input tuning circuit 2 because the reception sensitivity is increased to suppress NF and the connection to the CATV is assumed to reduce the reflection of the input tuning circuit out of band. This is to serve as a buffer amplifier so as not to affect the input side terminal. In addition, the IF stage has two stages of SAW filters, and the IF amplifier circuit during and after that is a variable gain type, and a different AGC is applied from the demodulator circuit 17 based on the demodulated signal. The reason why the SAW filter has two stages is that, assuming that there is an analog television broadcast signal having a level 30 dB higher in the adjacent channel, the suppression of the adjacent channel is not sufficient with only one stage. Since the insertion loss is increased by providing the SAW filter in two stages, the IF amplifier circuit is also provided in two stages. Further, since the AGC in the RF band is not applied only based on the level of the demodulated signal, that is, the signal level of the desired channel, the signal level of the desired channel after IF tuning is not necessarily constant. Therefore, AGC control is also performed in the IF band so that the input signal to the demodulation circuit 17 becomes a predetermined level.

  First, the AGC operation of the RF amplifier circuit 3 will be described. As described in the background art, the wideband RF signal is approximately equal to the desired channel and 1.5 channels on each side (4 channels in total) by the input tuning circuit 2, the interstage tuning circuit 4, and the IF tuning circuit 6. Bandwidth is limited. Then, the RSSI circuit 12 detects the level of the band-limited IF signal, outputs an AGC signal based on the detected level, and controls the RF amplifier circuit 3.

  As a result, when an unnecessary wave having a high signal level is present in a channel adjacent to the desired channel, AGC is applied to the RF amplifier circuit 3 in accordance with the unnecessary wave level, so that the RF amplifier 3 and the frequency conversion circuit 5 It is possible to prevent the signal of the desired channel from being distorted and the signal of the unwanted wave from being mixed into the signal of the desired channel by performing cross modulation.

  On the other hand, since AGC is applied based on the band-limited IF signal, a signal (unnecessary wave) of a channel that is far from the desired channel and does not necessarily become an interference wave does not affect the AGC.

  Furthermore, since the IF signal has a fixed frequency and a relatively narrow band, the RSSI circuit 12 can be substantially free of frequency characteristics. That is, the AGC level differs depending on the desired channel, and the prevention of interference is not insufficient.

  In this way, in the terrestrial digital broadcast receiving module 10 of the present invention, since AGC is applied in the RF band based on the band-limited signal after IF tuning, the RF signal as in Patent Document 1 is applied. Problems included in the configuration in which AGC in the RF band is applied based on the level can be solved.

  Although not shown as a patent document in the background art, a configuration in which the level of a signal after IF tuning is detected and used for gain control in the RF band is disclosed in Japanese Patent Laid-Open No. 2003-46353. Yes. However, in the case of what is disclosed in Japanese Patent Application Laid-Open No. 2003-46353, it is only detected whether the received signal is saturated, and the LNA gain in the RF band is switched in two steps accordingly. This is completely different from AGC for keeping a general signal level constant.

  By the way, as already described, in terrestrial digital television broadcasting, it is conceivable that the signal level at the time of reception is nearly 30 dB lower than terrestrial analog television broadcasting. For this reason, it is necessary to assume that the adjacent channel signal in the terrestrial digital television broadcast receiving module is 30 dB larger than the desired channel. As an example, when the signal level at the input end of the module is set to −50 dBm or more and AGC is applied (attenuation operation is performed), for example, −65 dBm to the desired channel at the input end of the module, adjacent channel When a signal of −35 dBm is input to the signal, the RSSI circuit 12 generates an AGC signal based on the level of the adjacent channel having a high signal level. In this case, the attenuation amount due to AGC is −35 [dBm] − (− 50 [dBm]) = 15 dB when input to the RSSI circuit 12 with the level difference as it is. Since this attenuation is also applied to the signal of the desired channel, equivalently (assuming that the attenuation is zero), the input signal of the desired channel is −65 [dBm] −15 [dB] = − at the module input end. It means 80 dBm. In Japan, as will be described later, the minimum reception level of a desired channel when receiving 13 segments in terrestrial digital television broadcasting is defined as -75 dBm at the input end of the module, and is actually about -78 dBm. In this state, it is impossible to receive the signal by lowering the reception level than the adjacent interference level.

  Therefore, in the terrestrial digital broadcast receiving module 10 of the present invention, as shown in FIG. 2, in the output of the IF tuning circuit 6, the attenuation from the module input end is 2 to 5 dB in the adjacent channel rather than the desired channel. It is set to decrease. This is set as the frequency characteristics of the entire three tuning circuits of the input tuning circuit 2, the interstage tuning circuit 4, and the IF tuning circuit 6. FIG. 2 shows a relative value (relative attenuation amount) of the attenuation amount of the adjacent channel with respect to the attenuation amount of the desired channel from the module input end.

  In this case, even if the signal level of the adjacent channel is 30 dB higher than the desired channel at the module input end, the signal level of the adjacent channel at the output of the IF tuning circuit 6, that is, the input end of the RSSI circuit 12, is relative to the desired channel. Thus, the state becomes higher by 25 to 28 dB. Therefore, in the above example, the attenuation amount due to AGC is 10 to 13 dB, and the input signal level of the desired channel is equivalently −75 to −78 dBm at the input terminal of the desired channel. Reception is possible.

  In this way, the problem of interference due to unwanted waves in adjacent channels can be solved. By the way, in an actual terrestrial digital television receiving module, interference due to unnecessary waves becomes a problem not only in adjacent channels but also in a channel (three adjacent channels) that is three channels away from a desired channel. Specifically, as shown in FIG. 3, for example, there is no unnecessary wave having a large signal level in the channel adjacent to the desired channel and the channel adjacent to the desired channel, and levels such as signals of terrestrial analog television broadcasting are present in the three adjacent channels. This is interference when there are high unnecessary waves.

  That is, the IF signal input to the RSSI circuit 12 is band-limited to approximately the desired channel and 1.5 channels on each side (total of 4 channels). As described above, when the relative attenuation amount of the adjacent channel with respect to the desired channel in the output of the IF tuning circuit 6 is set to 2 to 5 dB, in the normally designed tuning circuit, the relative attenuation amount of the three adjacent channels is about 15 to 20 dB. . Therefore, even if unnecessary waves with high levels exist in the three adjacent channels, they are greatly attenuated when they are input to the RSSI circuit 12, and the level of the unnecessary waves does not contribute much to the AGC signal. Therefore, when the level of the unwanted wave in the adjacent channel or the adjacent channel is low and only the level of the unwanted wave in the three adjacent channels is considerably higher (for example, 30 dB) than the desired wave, it is attenuated too much. Without being input to the RF amplification circuit 3 or the frequency conversion circuit 5. As a result, there is a possibility that each circuit is saturated and the signal of the desired channel is distorted, or the signal of the interference wave is mixed into the signal of the desired channel by performing cross modulation.

  Therefore, in the terrestrial digital broadcast receiving module 10 of the present invention, as shown in FIG. 4, the attenuation of the three adjacent channels with respect to the desired channel from the module input end is within 12 dB at the output of the IF tuning circuit 6. Is set to That is, while the adjacent channels are set so that the attenuation is reduced by 2 to 5 dB as described above, the attenuation is set to be within 12 dB for the three adjacent channels. This is set as the frequency characteristics of the entire three tuning circuits of the input tuning circuit 2, the interstage tuning circuit 4, and the IF tuning circuit 6. FIG. 4 shows relative values (relative attenuation amounts) of the attenuation amounts of the three adjacent channels with respect to the attenuation amount of the desired channel from the module input end.

  In this case, if the signal level of the three adjacent channels is 30 dB higher than the desired channel at the module input end, the signal level of the three adjacent channels is set to the desired channel at the output of the IF tuning circuit 6, that is, the input end of the RSSI circuit 12. On the other hand, it is 18 dB higher. That is, the signal level of the three adjacent channels is 3 to 8 dB higher than that in the case where the attenuation is set to decrease by 2 to 5 dB for the adjacent channels. Therefore, the amount of attenuation of the RF amplification circuit 3 by AGC also increases correspondingly, each circuit is saturated and the signal of the desired channel is distorted, or the signal of the interference wave is mixed into the signal of the desired channel by performing cross modulation. The possibility of doing is reduced.

  Next, the AGC of the two variable gain IF amplifier circuits 14 and 16 in the IF stage will be described.

In terrestrial digital television broadcasting, broadcasting is performed in such a manner that one channel is divided into 13 segments, and signals are transmitted for each segment using one, some or all of them. Accordingly, there may be a signal in all 13 segments of the received desired channel, or there may be a signal in only one of the segments. In addition, even if signals exist in a plurality of segments, only one segment among them may be received. In this case, the reception sensitivity is 13 times (about 11 dB) higher when only one segment is selectively received than when the entire 13 segments are received regardless of the transmission rate. In the case of 13-segment broadcasting, 12 segments may be signals with a high transmission rate such as 64QAM, and the remaining one segment may be a signal with another transmission rate such as QPSK. Also in this case, the C / N required for the input signal in the demodulation circuit 17 is different, and as a result, the minimum input level as one channel to the receiving module is different. The minimum input level is
Minimum input level (dBm) = CN + 10 log (kTB) + NF + DCN
CN: C / N required for the input signal, B: noise bandwidth,
k: Boltzmann coefficient, T: measurement temperature (300K)
NF: Receiver NF, DCN: Receiver equipment degradation (total 9.3 dB)
It is represented by Comparing the case of 13 segment broadcasting (CN = 22 dB, B = 5.7 MHz) and the case of 1 segment broadcasting (CN = 4.9 dB, B = 0.43 MHz)
13-segment broadcasting Minimum input level = -75 dBm
1-segment broadcasting Minimum input level = -92 dBm
It becomes. That is, a level difference of 24 dB occurs in the minimum input level as one whole channel depending on the broadcasting form.

  On the other hand, the input level that is the upper limit as one channel to the receiving module is defined as -20 dBm. In this case, it is necessary to assume that the level of an analog television broadcast signal (interference wave signal) that may exist in the adjacent channel is +10 dBm, which is 30 dB higher than that.

  Furthermore, the digital demodulator in the demodulator circuit 17 needs to be controlled so as to have an input voltage of 0.3 Vpp (−13 dBm in terms of 250Ω termination) as a general ability value. On the other hand, when the second IF amplifier circuit 16 effectively inputs a signal having a level of −25 dBm or more, IM (intermodulation distortion) starts to deteriorate. Therefore, since the input level needs to be −25 dBm or less, the gain needs to be −13 dBm − (− 25 dBm) = 12 dB or more. Since the SAW filter generally has an insertion loss of about −18 dB, the output level of the first IF amplifier circuit 14 needs to be −25 dBm + 18 dB = −7 dBm in consideration of the loss of the second SAW filter 15. There is.

  Since the output of the IF tuning circuit 6 is often set to be 0 dBm, 18 dB which is the loss of the SAW filter 13 is taken into consideration, and a −7 dB attenuation means 18 is further provided to the first IF amplification circuit 14. The input level is set to −25 dBm. At this time, the gain of the first IF amplifier circuit 14 is −7 dBm − (− 25 dBm) = 18 dB. This is the gain distribution when the input to the receiving module is -50 dBm or more.

  On the other hand, when the input to the receiving module falls below −50 dBm, the output level of the IF tuning circuit 6 decreases accordingly. Actually, in the case of 13 segment broadcasting, the input to the receiving module has a width of -50 dBm to -75 dBm, and the output level of the IF tuning circuit 6 has a width of 0 to -25 dBm. In the case of 1-segment broadcasting, the input to the receiving module has a width of −50 dBm to −92 dBm, and the output level of the IF tuning circuit 6 has a width of 0 to −42 dBm.

  If the gain of the second IF amplifier circuit 16 is fixed at 12 dB, the attenuation amount other than the first IF amplifier circuit 14 is -18-7-18 + 12 = -31 dB. In order to amplify the output signal from the IF tuning circuit 6 having a value of −25 dBm to −13 dBm, the first IF amplifier circuit 14 needs a gain of −13 − (− 25−31) = 43 dB. This is a realizable range using an IC amplifier or the like. However, in order to amplify the output signal from the IF tuning circuit 6 of −42 dBm, which is the minimum value in one-segment broadcasting, to −13 dBm, the first IF amplifier circuit 14 has −13 − (− 42−31) = 60 dB. A gain is required, which is not practical with a single-stage IC amplifier.

  Therefore, in the present invention, the second IF amplifier circuit 16 is also a variable gain type, and both are configured to apply AGC from the demodulation circuit 17. As a result, the second IF amplifier circuit 16 can have a gain that cannot be covered by the first IF amplifier circuit 14, so that each IF amplifier circuit can be configured within a feasible range.

  In the present invention, the AGC signals applied to the first IF amplifier circuit 14 and the second IF amplifier circuit 15 are individually output from the demodulation circuit 17. The two AGC signals are made different. That is, as shown in FIG. 5, the AGC delay point 1 of the first IF amplifier circuit 14 is different from the AGC delay point 2 of the second IF amplifier circuit 16. In this case, when the signal level at the time of input to the first IF amplifier circuit 14 is relatively low, only the second IF amplifier circuit 16 performs an AGC attenuation operation, and the AGC switching point at which the signal level is a predetermined level. The first IF amplifier circuit 14 performs an attenuation operation by AGC. Note that the AGC delay point in FIG. 5 is the input signal level to the first IF amplifier circuit 14 at which the attenuation operation is started in each IF amplifier circuit. The AGC control level on the vertical axis in FIG. 5 is the AGC voltage value applied to the IF amplifier circuit.

  In this way, by making the AGC delay points of the two IF amplifier circuits different, it is possible to maintain a large gain of the first IF amplifier circuit 14 when the input level is low, and the CN of the signal at the IF stage Deterioration can be prevented.

  In the terrestrial digital television broadcast receiving module 10 of the present invention, both the RF stage and IF stage AGC are completed in the module. For example, some setting is made for each module on the set side on which the module is mounted. There is no need to make adjustments. Since the AGC monitor terminals RFm and IFm for monitoring the AGC signals of the RF stage and the IF stage are provided, the AGC operation state can be confirmed on the set side, and the reception data for auto-preset can be obtained. it can. In addition, the strength of the input signal is judged by monitoring the AGC signal of the RF stage and IF stage on the set side. Can be judged.

  FIG. 6 shows a block diagram of another embodiment of the terrestrial digital television broadcast receiving module of the present invention. 6, parts that are the same as or equivalent to those in FIG. 1 are given the same reference numerals, and descriptions thereof are omitted.

  6, the terrestrial digital television broadcast receiving module 20 includes a variable attenuator 21 using a PIN diode between the broadband amplifier 11 and the input tuning circuit 2 in the terrestrial digital television broadcast receiving module 10 of FIG. The attenuation is controlled by the AGC signal output from the RSSI circuit 12 together with the RF amplifier circuit 3. Note that the variable attenuator 21 may be disposed before the RF amplifier circuit 3.

  With this configuration, in the terrestrial digital television broadcast receiving module 20, it is possible to widen the AGC width in the RF stage and widen the selection range of gain distribution as the whole module. As a result, even when there is an analog input signal, there is a sufficient AGC variable width, and if there is interference due to a strong input signal, set the AGC start level to a lower level, or It is possible to cope with an increase in the AGC variable width.

  By the way, in the terrestrial digital television broadcast receiving modules 10 and 20 of the present invention, the first SAW filter 13 and the second SAW filter 15 are provided in the IF stage as described above. Finally, these two SAW filters will be described.

  As described above, in terrestrial digital television broadcasting, one channel is divided into 13 segments, and a signal is transmitted for each segment. When dividing a segment, 1/14 of the bandwidth of one channel is allocated as a frequency bandwidth for one segment, and a frequency bandwidth for ½ segment is opened on both sides, and then a frequency bandwidth for 13 segments in between. Is secured. Therefore, there is only a frequency band for one segment between the nearest segments of adjacent channels. Since the adjacent channels are very close in this way, in the first SAW filter 13 and the second SAW filter 15 in the IF stage, the pass band is almost equal to the frequency band of 13 segments of one channel. Must match.

  In general, the pass band of a SAW filter has temperature characteristics depending on the material used. For example, it is −28 ppm / degree C in the case of ZnO, −72 ppm / degree C in the case of LN, and −35 ppm / degree C in the case of LT. Therefore, when the pass band moves to the high frequency side or the low frequency side due to temperature change, there is a problem that the signal of the nearest segment of the adjacent channel is allowed to pass, thereby being easily disturbed during demodulation.

  Therefore, in the terrestrial digital television broadcast receiving modules 10 and 20 of the present invention, the passbands of the first SAW filter 13 and the second SAW filter 15 are set to adjacent channels even if the passband is moved due to a temperature change. For example, a shift of about +200 kHz or a narrowing of about 400 kHz is made to such an extent that no signal is passed. As a result, even if there is a temperature change, it is possible to prevent interference due to the signal of the adjacent channel.

  At this time, the first SAW filter 13 and the second SAW filter 15 may be made of materials having the same temperature characteristics, but they are made of materials having different temperature characteristics, and each has its own passband shift or By performing the narrowing, it is possible to control the passage characteristics more finely.

  In this case, depending on the temperature condition, there is a possibility of suppressing the signal that should be passed through in the outermost segment. However, since error correction is performed in terrestrial digital television broadcasting, a small amount of signal loss is allowed.

It is a block diagram which shows one Example of the terrestrial digital television broadcast receiving module of this invention. FIG. 3 is a characteristic diagram showing a relative value (relative attenuation amount) of an adjacent channel attenuation amount with respect to an attenuation amount of a desired channel from a module input end in the terrestrial digital television broadcast receiving module of FIG. 1. It is a relationship diagram which shows the relationship of the signal of a desired channel and the 3 adjacent channel which affects it. FIG. 3 is a characteristic diagram showing a relative value (relative attenuation amount) of attenuation amounts of three adjacent channels with respect to an attenuation amount of a desired channel from a module input end in the terrestrial digital television broadcast receiving module of FIG. 1. FIG. 6 is a characteristic diagram showing a difference between AGC delay points of the first and second IF amplifier circuits in the terrestrial digital television broadcast receiving module of FIG. 1. It is a block diagram which shows another Example of the terrestrial digital television broadcast receiving module of this invention. It is a block diagram which shows the conventional television broadcast receiving module.

Explanation of symbols

2 ... input tuning circuit 3 ... RF amplifier circuit 4 ... interstage tuning circuit 5 ... frequency conversion circuit 6 ... IF tuning circuit 10, 20 ... terrestrial digital TV broadcast receiving module 11 ... wideband amplifier 12 ... RSSI circuit 13 ... first SAW filter (IF filter circuit)
14: First IF amplifier circuit (IF amplifier circuit)
15: Second SAW filter (IF filter circuit)
16: Second IF amplifier circuit (IF amplifier circuit)
17 ... Demodulation circuit 18 ... Attenuating means 21 ... Variable attenuator

Claims (7)

  An RF tuning circuit that tunes to a frequency in the vicinity of a desired channel from the input RF signal, a variable gain RF amplifier circuit that amplifies the RF signal, and a frequency conversion circuit that converts the RF signal into an IF signal with a fixed frequency An IF tuning circuit that selects a signal in the vicinity of a desired channel from the IF signal, an IF filter circuit that selects a desired channel from the IF signal, a variable gain IF amplifier circuit that amplifies the IF signal, and a desired signal from the IF signal A demodulation circuit for demodulating a channel signal, and a signal for detecting the level of the signal output from the IF tuning circuit and outputting an AGC signal for applying feedback to the RF amplifier circuit so that the detection level is constant A digital television broadcast reception module comprising: a level detection circuit;   When output from the IF tuning circuit, the frequency characteristics of the RF tuning circuit and the IF tuning circuit are set so that the attenuation amount of the adjacent channel is 2 to 5 dB larger than the attenuation amount of the signal of the desired channel. The digital television broadcast receiving module according to claim 1, wherein:   When output from the IF tuning circuit, the frequency characteristics of the RF tuning circuit and the IF tuning circuit are set so that the attenuation of the three channels away from the attenuation of the signal of the desired channel is within 12 dB. The digital television broadcast receiving module according to claim 1, wherein   The IF amplifier circuit is a variable gain amplifier circuit, and an AGC signal is applied to the IF amplifier circuit so that a level of a signal of a desired channel input to the demodulator circuit is constant. The digital television broadcast receiving module according to any one of 1 to 3.   5. The digital television broadcast receiving module according to claim 4, further comprising first and second SAW filters as the IF filter circuit and first and second IF amplifier circuits as the IF amplifier circuit. 6.   6. The digital television broadcast receiving module according to claim 5, wherein the first and second IF amplifier circuits are configured so that delay points with respect to an applied AGC signal are different.   5. The pass band of the first and second SAW filters is shifted or narrowed to such an extent that a signal of an adjacent channel is not passed even if the pass band is moved due to a temperature change. Or a digital television broadcast receiving module according to 5;

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