Patent Publication Number: US-2009227220-A1

Title: Channel detecting apparatus and tuner testing apparatus comprising same

Description:
TECHNICAL FIELD 
     The present application claims priority based on Japanese Patent Application No. 2004-333941, filed Nov. 18, 2004, entitled “Channel Detecting Apparatus and Tuner Testing Apparatus Comprising the Same,” the disclosure including the specification, drawings, and claims of which is incorporated herein by reference in its entirety. 
     Disclosed embodiments generally relate to an apparatus for detecting a channel, a channel detecting apparatus for a circuit having channels such as a tuner, and a tuner testing apparatus including such a detecting apparatus. 
     BACKGROUND ART 
     As disclosed, for example, in Laid-Open Japanese Patent Application No. 55-100781, a conventional tuner testing apparatus for testing the characteristics of a tuner has a function of detecting each channel of the tuner. This function permits the tuner testing apparatus to determine whether or not a channel exists in the tuner at a frequency, depending on an output which the tuner generates at a frequency on the output side of a specified channel (or an intermediate frequency (IF)) thereof in response to an input at a frequency on the input side of the specified channel of the tuner (or a radio frequency (RF)). The tuner testing apparatus also determines an RF frequency associated with the channel based on the frequency of the IF circuit output when it determines that the channel exists. Thus, the tuner testing apparatus supplies the tuner with a frequency sweep signal generated thereby for sweeping a certain range of RF frequencies in order to search for the RF frequency of the specified channel of the tuner. 
     The detection of a channel using the frequency sweep signal experiences a skew phenomenon which causes the frequency characteristic of a channel under detection to shift depending on a frequency sweep speed of the sweep signal. The channel detection is variably affected by the skew phenomenon depending on the bandwidth of the frequency characteristic of the channel, the steepness of cut-off slope, and the like. 
     In recent years, with the inauguration of digital broadcasting, tuners which are compatible with digital broadcasting, for example, a ground digital broadcasting tuner, have emerged for practical use. The digital broadcasting compatible tuner presents an extremely steep slope in cut-off regions of the channel frequency characteristic, as compared with conventional tuners which are compatible with the analog broadcasting. For this reason, the aforementioned skew more gravely affects the detection of channels in a digital broadcasting compatible tuner, which often results in no detection of channels when a sweep signal at a sweep speed as high as before is used. 
     DISCLOSURE OF THE INVENTION 
     Means for Solving the Problem 
     While the following various aspects and embodiments will be described and explained in connection with apparatuses, circuits, and methods, they are meant to be exemplary and illustrative, and not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     In one embodiment, an apparatus for detecting a channel comprises a channel search circuit that determines whether or not a frequency detected through a search sweep using a frequency sweep signal for the channel falls within a frequency range associated with the channel. 
     In another embodiment, the channel search circuit may include a search sweep controller, and the search sweep controller may comprise a sweep stop timing circuit that stops the search sweep when an output of the channel is generated in response to the search sweep using the frequency sweep signal, and a sweep resume timing circuit that resumes the search sweep by determining that the detected frequency does not fall within the frequency range associated with the channel when a level of the output of the channel generated in response to the frequency sweep signal fixed at the detected frequency is below a threshold. 
     Also, in another embodiment, the channel search circuit may further comprise a search sweep circuit that generates the frequency sweep signal which is supplied to the channel for searching the channel, and the search sweep circuit comprises a sweep signal generator that is capable of generating the frequency sweep signal at a first sweep speed and a second sweep speed lower than the first sweep speed, wherein the channel can be first searched at the first sweep speed, and subsequently searched at the second sweep speed. 
     Further, in another embodiment, the channel detecting apparatus may comprise a channel frequency measuring instrument that measures the frequency of the channel when a level of the output of the channel generated in response to the frequency sweep signal fixed at the detected frequency is not below a threshold. 
     Also, in another embodiment, the present invention provides a tuner testing apparatus for testing a tuner for a channel, wherein the tuner testing apparatus comprises the channel detecting apparatus. 
     Further, in another embodiment, a method of detecting a channel comprises determining whether or not a frequency detected through a search sweep using a frequency sweep signal for the channel falls within a frequency range associated with the channel. 
     Further, in another embodiment, the present invention provides a tuner testing method for testing a tuner for a channel, wherein the tuner testing method comprises the channel detecting method. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a channel detecting apparatus according to one embodiment. 
         FIG. 2  is a block diagram illustrating a tuner testing apparatus according to one embodiment, which more specifically implements the channel detecting apparatus of  FIG. 1 . 
         FIG. 3  is a circuit diagram illustrating in detail a tuner shown in  FIG. 2 . 
         FIG. 4  includes diagrams for describing a method of detecting a channel, wherein (a) illustrates a tuner, (b) a range of sweep frequency for a sweep signal supplied to the tuner, and (c) the IF frequency characteristic of the tuner. 
         FIG. 5  includes diagrams showing the frequency characteristics of IF outputs of tuners, where (a) shows the frequency characteristic of an analog tuner, and (b) the frequency characteristic of a digital tuner. 
         FIG. 6  is a diagram showing skew which occurs in relation to the frequency characteristic of a circuit depending on a frequency sweep speed. 
         FIG. 7  is a circuit diagram illustrating in detail an IF receiver circuit in  FIG. 2 . 
         FIG. 8  is a flow chart illustrating a channel detection flow executed by a CPU in  FIG. 2 . 
         FIG. 9  is a timing chart showing a search sweep according to one embodiment, showing a variety of signals in a variety of periods in a tuner test. 
         FIG. 10  is a diagram showing the IF frequency characteristic of a digital tuner, showing the relationship between the frequency characteristic and search sweep. 
         FIG. 11  is a timing diagram similar to  FIG. 9 , showing a search sweep according to another embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, a variety of embodiments will be described in detail with reference to the drawings. 
       FIG. 1  illustrates a channel detecting apparatus according to one embodiment in a block diagram. The illustrated channel detecting apparatus is an apparatus for detecting the presence or absence of each of one or more channels of a channel circuit  1 . Also, the channel detecting apparatus can detect the frequency on the input side of each of the channels. Here, the channel encompasses an arbitrary path which can be defined by frequency characteristics such as a tuning characteristic, a frequency conversion characteristic, a filtering characteristic and the like. Each channel of the channel circuit  1  comprises an input frequency band and an output frequency band, and a channel of a tuner has an input frequency band in an RF (radio frequency) frequency band, and an output frequency band in an IF (intermediate frequency) band. In this connection, these input and output frequency bands may be different as is the case with the tuner, or may be in the same band or in partially overlapping bands. 
     As illustrated, the channel detecting apparatus of  FIG. 1  comprises a channel search circuit  3  for searching channels, and a channel frequency measuring instrument  7  for measuring the frequency of a channel detected through the search. More specifically, the channel search circuit  3  comprises a search sweep circuit  30  and a search sweep controller  32  for search sweep for channels. The search sweep circuit  30  comprises a sweep signal generator  300  which generates a frequency sweep signal at its output to be supplied to the channel circuit  1  for use thereby in searching channels, and is configured to have the ability to generate the frequency sweep signal at two sweep speeds, i.e., a higher sweep speed and a lower sweep speed. In this connection, a search sweep for a given channel is conducted initially at the high sweep speed and subsequently at the lower sweep speed. The channel circuit  1  receives a frequency sweep signal from the search sweep circuit  30  at its input and generates a response thereto at its output. 
     The search sweep controller  32  comprises a sweep stop timing circuit  320  and a sweep resume timing circuit  322  for controlling the search sweep circuit  30  in a search sweep. The sweep stop timing circuit  320  has an input connected to the output of the channel circuit  1 , and generates a sweep stop timing signal at its output to be supplied to the search sweep circuit  30  for stopping a search sweep when the channel circuit  1  generates an output in response to the search sweep based on a frequency sweep signal. Specifically, when the channel circuit  1  detects that the frequency of a sweep signal, which is being swept, falls within a frequency range associated with a channel under search, the sweep stop timing circuit  320  determines that the channel is found and stops the search sweep. Also, when a threshold is not exceeded by the level of the output of the channel generated in response to the frequency sweep signal which is fixed at a frequency when the sweep is stopped, the sweep resume timing circuit  332  determines that the fixed frequency does not fall within the channel associated frequency range, and generates a sweep resume timing signal at its output for resuming the search sweep. This is because the aforementioned skew of the frequency characteristic of the channel, or other factors (for example, a response delay of the channel circuit and the like) can cause a phenomenon which disables the channel circuit to generate an output at a level high enough for detection after the sweep is stopped, even though it generates the output at a level high enough to be detected during the sweep. Accordingly, a channel which has been once determined to be found is actually determined not to be found, causing the sweep resume timing signal to be supplied to the search sweep circuit  30  which then resumes the search sweep. In the resumed search sweep, the lower search speed than the first search sweep is used in order to reduce the influence of skew due to the sweep speed. In this connection, the subsequent search sweep can be performed in a direction opposite to that of the first search sweep (for example, when the first sweep was performed from a higher frequency to a lower frequency, the subsequent search sweep can be performed from a lower frequency to a higher frequency), or in the same direction as the first sweep. 
     Next, the channel frequency measuring instrument  7  included in the detecting apparatus of  FIG. 1  comprises a frequency measurement timing circuit  70  for determining a timing at which a frequency measurement is conducted, and also comprises a channel output frequency counter  72  and a channel frequency correction circuit  74  for the channel frequency measurement. More specifically, the measurement timing circuit  70  has an input connected to the output of the channel circuit  1 , and generates a frequency measurement start timing signal at its output for measuring the frequency of a channel when a threshold is exceeded by a detected level of the output of the channel circuit, particularly, the level of the output of the channel circuit  1  generated at the time when the frequency sweep signal is maintained at the above-mentioned fixed frequency. In this way, when the output of the channel circuit is not at a level high enough to measure the frequency, the measurement timing circuit  70  delays the measurement of the frequency until the frequency is detected by further search sweep, and starts measuring the frequency when the level reaches a sufficiently high level. The frequency counter  72 , which receives the output from the measurement timing circuit  70 , also has an input for receiving the output of the channel circuit  1 . Upon receipt of a frequency measurement start signal, the frequency counter  72  counts and outputs the frequency of the output from the channel circuit. The correction circuit  74 , which receives the frequency count output at one input, also receives, at another input, the value of the fixed frequency of the sweep signal, when the frequency measurement is conducted, from the search sweep circuit  30 . Then, the correction circuit  74  corrects the fixed frequency in accordance with the difference between the counted channel output frequency and a certain frequency to determine the channel frequency. The determined channel frequency output by the correction circuit  74  can be used in a channel testing apparatus such as a tuner testing apparatus. According to this embodiment, a channel can be detected at high speeds despite the influence of skew. 
     Here, the certain frequency can be a known center frequency of a channel under search. For example, the known center frequency of a channel of a tuner is the center frequency in an IF band of the tuner, i.e., a known IF frequency. However, since the certain frequency is a frequency for use in correcting the fixed frequency of the sweep signal while the sweep is stopped, another arbitrary value may be used instead as long as it can be used for the correction. Referring next to  FIG. 2 , a tuner testing apparatus will be described in one embodiment which more specifically implements the channel detecting apparatus of  FIG. 1 . It should be noted that components corresponding to those in  FIG. 1  are designated with the same reference numerals suffixed with a character “A.” As illustrated, the tuner testing apparatus adjusts and tests a tuner module  1 A such as a TV receiver set, an FM receiver or the like, and comprises components for this purpose, including a tuner power supply/controller  2 , an IF receiver circuit  4 , a CPU  6 , a sweep signal generator  300 A and a display  8 . 
     Referring first to  FIG. 3 , the tuner  1 A under measurement will be described in brief. As illustrated, the tuner  1 A comprises an antenna (ANT) input terminal  10 , a local oscillator (LO)  14 , a mixer  15 , an IF amplifier  16 , an IF filter  17  and an IF output terminal  18 , as is well known in the art. More specifically, an RF tuning circuit  11  receives an input from the antenna input terminal  10  in an RF frequency range, and generates an RF output which is tuned to a particular RF frequency band within this input. The local oscillator  14 , which is a PLL type oscillator, receives at an input  13  data for specifying a channel to which the tuner  1 A should tune, i.e., PLL data (data for defining a division ratio for a PLL within the local oscillator) for defining an oscillation frequency of the local oscillator corresponding to the specified channel, and generates at its output a local oscillator output having the oscillation frequency. The mixer  15  receives the local oscillator output at one input and the RF output at another input, and converts the RF output to an IF frequency band. The output of the mixer  15  is amplified by the IF amplifier  16 , and bandpass filtered by the IF filter  17  to generate a final IF output at the IF output terminal  18 . Such a tuner module is adjusted for the frequency characteristic and the like of its RF circuit and IF circuit components, and the once adjusted tuner module is subjected to a final test. 
     Turning back to  FIG. 2  for describing the tuner testing apparatus in detail, the tuner power supply/controller  2  included in the testing apparatus is connected to the tuner  1 A and CPU  6  for powering the tuner  1 A and supplying the same with the aforementioned PLL data for specifying a channel of the tuner. The operation of the tuner power supply/controller  2  is controlled by the CPU  6 . The sweep signal generator  300 A is connected to the tuner  1 A and CPU  6 , and supplies the tuner  1 A with a frequency sweep signal in an RF frequency range (for example, in a range of 1 GHz to 25 MHz) in which the tuner can receive. The sweep signal is generated at either of two sweep speeds, a higher sweep speed V S1  (for example, 487.5 GHz/s) or a lower sweep speed V S2  (for example, 243.75 GHz which is one-half of the higher sweep signal) in the aforementioned search sweep. It should be noted that the higher sweep speed is a speed close to an upper limit speed at which a conventional analog tuner can detect an IF output. On the other hand, for measuring the characteristic of a tuner, the tuner testing apparatus uses a sweep speed V S3  (for example, 9.375 GHz/s) significantly lower than the sweep speed used in the search sweep in order to avoid the occurrence of the skew in the frequency characteristic. The sweep speed, sweep frequency range, and operation of the sweep signal generator are controlled by the CPU  6 . 
     Next, the IF receiver circuit  4  will be described, also with reference to  FIG. 4 . The IF receiver circuit  4 , which is connected to the tuner  1 A and CPU  6 , receives at its input the IF output generated by the tuner  1 A in response to the frequency sweep signal, and relies on the IF output to conduct search sweep for channel frequencies and counts the channel frequencies. More specifically, as illustrated in  FIG. 4(   b ), the sweep signal is swept in a range from an upper limit RF frequency f RU  to a lower limit RF frequency f RL  in order to detect a specified channel of a tuner and the frequency thereof. As illustrated in  FIG. 4(   a ), the sweep signal is supplied to the antenna input  10  of the tuner  1 A, and an IF output corresponding thereto appears at the IF output terminal  18 . The IF circuit of the tuner, particularly, the IF filter  17  exhibits an IF output frequency characteristic, for example, as shown in  FIG. 4(   c ). Here, the detection of a channel and its frequency by the tuner testing apparatus involves detecting an RF frequency f RC  at the input of the tuner which will generate at its output the center frequency f IC  of the IF characteristic at a certain specified channel of the tuner. When this f RC  is detected, an RF frequency range centered at f RC  can be determined such that a sweep signal is generated which varies in frequency within this range, thereby making it possible to measure the IF frequency characteristic within a certain frequency range centered at f IC    
     Referring now to  FIG. 5 , the frequency characteristic of the IF output of a tuner will be described.  FIG. 5(   a ) shows a typical IF frequency characteristic of a conventional analog tuner, and  FIG. 5(   b ) shows a typical characteristic of a digital tuner. As can be also understood from the figures, the IF characteristic of the analog tuner has a Gaussian distribution which has a slow slope (with an attenuation rate of 45 dB/oct by way of example) in a cut-off region. Therefore, the channel has a bandwidth of 6 MHz at −3 dB, and a frequency range associated with this channel, for example, a frequency width at −10 dB (for example, approximately 20 MHz) is considerably wider than 6 MHz. On the other hand, the IF characteristic of the digital tuner shown in  FIG. 5(   b ) has a flat region, and cut-off regions steeply falling from both ends of the flat region (with an attenuation rate of 1680 dB/oct, by way of example). Thus, even if a bandwidth at −3 dB extends over 6 MHz, a frequency range associated with a channel, for example, a frequency width at −10 dB (for example, approximately 6.5 MHz) is fairly narrow as compared with that of the analog tuner. Therefore, with the use of a level detector capable of detecting levels up to −10 dB, a very narrow frequency range can only be practically used for detecting the level for the digital tuner (it should be noted that a frequency range substantially available for the level detection varies depending on the detection capabilities of a particular level detector). This narrow range makes the IF output level detection more susceptible to the skew in the frequency characteristic caused by the frequency sweep. 
     Referring to  FIG. 6 , the relationship between the frequency sweep speed and skew in the frequency characteristic will be described. Assume first that a certain circuit has a frequency characteristic curve C 0  as shown which represents a frequency characteristic such as a tuning characteristic or a filtering characteristic. When such a circuit is supplied with a frequency sweep signal to measure the frequency characteristic, the frequency characteristic continues to assume the curve Co until the sweep speed exceeds a certain limit when the sweep is performed in a direction from a lower frequency to a higher frequency. However, it is known that as the sweep speed is increased beyond the limit value, the frequency characteristic apparently changes, for example, to assume curves C 1 , C 2 , C 3  one after another. This phenomenon is called the skew. The skew causes a peak frequency to shift in the sweeping direction with an additional reduction in level and increase in bandwidth as the sweep speed is higher, i.e., as the frequency characteristic changes to the curves C 1 , C 2 , C 3  one after another. Supposing herein that the output level of the circuit is detected at the shown detection level or higher, the position of the first frequency at which the level can be detected shifts to positions P 1 , P 2 , P 3  as the sweep speed is increased. In this scenario, the characteristic curve C 0  has gains higher than the detection level at the frequency positions P 1  and P 2 , whereas the characteristic curve C 0  has a gain fairly lower than the detection level at the frequency position P 3 . For this reason, a response of the circuit to the sweep signal when the sweep is stopped cannot be detected at the position P 3 . Such skew is more prominent in the digital tuner illustrated in  FIG. 5 . This embodiment enables the level of an IF output to be detected despite such skew. 
     Referring next to  FIG. 7 , the IF receiver circuit in  FIG. 2  will be described in detail. As illustrated, the IF receiver circuit  4  comprises an input terminal  40  for receiving an IF output of a tuner, an automatic gain control (AGC) amplifier  41 , an IF output detector  42 , a buffer amplifier  43 , an IF output level detector  44 , and a frequency measuring instrument  45 . The IF output detector  42  has an input for receiving the IF output through the AGC amplifier  41 , and generates at its output an IF trigger pulse for starting the processing on the IF output (involving detecting the level of the IF output, and measuring the frequency of the IF output) when it detects an IF signal in the IF output. For implementing the operation, the IF output detector  42  comprises a waveform shaper  420  and a pulse generator circuit  422 . The waveform shaper  420 , which can be comprised, for example, of a Schmitt trigger circuit, receives the IF output as a response of a tuner to a sweep signal, and outputs a certain voltage when it detects that the level of the IF output exceeds a certain threshold. Upon generation of this voltage, the subsequent pulse generator circuit  422 , which can be comprised, for example, of a mono-multivibrator, responds to the voltage by generating a pulse of a certain width which serves as the IF trigger pulse. Upon generation of the trigger pulse, the subsequent CPU  6  determines that a channel is found, forces the sweep signal generator  300 A to stop the sweep to fix the sweep signal to the frequency at that time, and also controls the IF output processing. 
     The level detector  44  included in the IF receiver circuit  4  receives at its input the IF output as a response of the tuner to the sweep signal maintained at the fixed frequency which was detected when the sweep was stopped, through the AGC amplifier  41  and buffer  43 , and generates a count start timing pulse for starting a measurement of the frequency of the IF output within a certain level detection period (approximately 10 microseconds) when the IF output is at a level equal to or higher than a threshold. On the other hand, when the level of the IF output is below the threshold, the level detector  44  does not generate the pulse within the level detection period. For implementing the foregoing operation, the level detector  44  comprises a level comparator  440 , a waveform shaper  442 , and a pulse generator circuit  444 . The comparator  440  compares the level of the IF output with a threshold L TH  (for example, a level attenuated by 10 dB from a peak of a filter waveform), and generates a certain voltage at its output when the level of the IF output is equal to or higher than the threshold L TH . The subsequent waveform shaper  442 , which can be comprised, for example, of a Schmitt trigger circuit, responds to the generated certain voltage by generating a certain voltage with hysteresis. The subsequent pulse generator circuit  444 , which can be comprised, for example, of a mono-multivibrator, generates a pulse of a certain width at its output upon receipt of the voltage from the waveform shaper  442 . This pulse serves as the count start timing pulse. The timing pulse is generated within the level detection period from the generation of the IF trigger pulse as described above, and thus indicates the start of a frequency measurement. On the other hand, when no timing pulse is generated within the level detection period, the absence of the timing pulse is interpreted by the CPU  6  as a sweep resume instruction. Once the CPU  6  recognizes the generated count start timing pulse, the CPU  6  supplies a counter gate signal to the frequency measuring instrument  45  which measures the frequency. 
     Next, the frequency measuring instrument  45  receives the IF output through the AGC amplifier  41  and buffer  43  as a response of the tuner to the sweep signal which is maintained at the fixed frequency, measures the frequency of the IF output, and generates a measured frequency value at its output. For implementing the operation, the frequency measuring instrument  45  comprises a waveform shaper  450  and a frequency counter  452 . The waveform shaper  450  shapes the sinusoidal IF output into a square wave. The subsequent frequency counter  452  receives the square wave at its input, and also receives the counter gate signal from the CPU  6  at a control input. The counter  452  comprises a gate (not shown) at the input, and counts the number of pulses in the square wave when the gate is opened in response to the counter gate signal, generates a count value at its output, and supplies the count value to the CPU  6 . 
     Referring next to  FIGS. 8 ,  9 , and  10 , the operation of the tuner testing apparatus in  FIG. 2  will be described when it tests a digital broadcasting tuner.  FIG. 8  illustrates a channel detection flow executed by the CPU  6 , and  FIG. 9  is a timing diagram showing a variety of signals in a variety of periods in the tuner testing.  FIG. 10  is a diagram showing the relationship between the IF frequency characteristic of the digital tuner and the sweep.  FIG. 10  also shows the characteristic of an analog broadcasting tuner represented by a dotted line for purposes of comparison. As illustrated in  FIG. 8 , the channel detection flow starts at step  800 , where the tuner testing apparatus receives an input for specifying an upper limit or a lower limit frequency value within an RF frequency range for starting a search sweep, and sends data indicative of the value to the sweep signal generator  300 A. By way of example, the upper limit is set at 1 GHz. At next step  802 , it waits until the RF output from the sweep signal generator  300 A is ready to start a search. Specifically, the flow loops until an RF oscillator (not shown) in the sweep signal generator reaches the set upper limit frequency. Upon determining that the upper limit frequency is reached (for example, after a known period of time (ex. about 2 milliseconds) has passed which is required for the sweep signal generator  300 A to sweep through a maximum width of variable frequency), the flow goes to next step  804 . At step  804 , the testing apparatus receives an input for specifying an RF step for a search sweep, and supplies the sweep signal generator with data for making this setting. Specifically, since the used sweep signal is in a digital form and changes the RF frequency in steps, the sweep signal generator sets a frequency increment per step and each step interval (by way of example, the frequency increment is 1.25 MHz, and the step interval is 2.6 microseconds). As described above, since the sweep signal generator  300 A uses the two sweep speeds V S1  and V S2  for the search sweep, the frequency increment and step interval are set for each of the speeds (the settings have been stored in a memory of the CPU, such that at step  804 , the settings for the search sweep are simply supplied to the sweep signal generator  300 A). In this way, the settings are completed for the sweep signal. Likewise, the sweep speed V S3  for measuring the characteristic can also be set at this step. 
     Next, at step  806 , the CPU  6  supplies the sweep signal generator with a signal for starting a search sweep under the settings of the starting frequency, frequency increment, and step interval, to start a search sweep. In response, as shown in  FIG. 9(   a ), the sweep signal changes in frequency in steps at the higher sweep speed V S1  from 1 GHz to lower frequencies in a search sweep period. The vertical axis in  FIG. 9(   a ) represents the RF frequency, and the sweep signal is shown to smoothly vary for simplifying the illustration. Next, at step  808 , the CPU  6  determines whether or not the search sweep has ended by detecting whether or not an IF trigger pulse has been generated from the IF output detector  42 . When no trigger pulse is generated, the flow loops at step  808  to wait until a trigger pulse is generated. On the other hand, when the IF trigger pulse is generated, i.e., the search results in a hit as shown in  FIG. 9(   b ), the sweep stop timing circuit  320  generates a signal to the sweep signal generator  300 A to stop the search sweep. In this way, the search sweep period ends as shown in  FIG. 9(   a ), while the sweep signal generator  300 A continues to generate the sweep signal fixed at a search hit RF frequency f RH1  at that time, as shown in a level detection period in  FIG. 9(   a ). Next, at step  810 , the CPU  6  determines the IF detected level in the level detection period by detecting whether or not the level detector  44  generates a count start timing pulse within the level detection period. 
     At next step  812 , when a count start timing pulse is detected within the level detection period, the flow goes to step  816 . However, when no count start timing pulse can be detected as shown in  FIG. 9(   a ), the frequency cannot be counted. This state occurs when the search hit IF frequency f IH1  (corresponding to the search hit RF frequency f RH1 ) is at a position shown in  FIG. 10 , where the IF output level is below the threshold L TH . Such a phenomenon is not experienced in the conventional analog tuner because a level equal or higher than the threshold L TH  can be detected in the analog tuner even at the frequency f IH1 . In this example of the digital tuner, the determination at step  812  is NO, causing the flow to go to step  814 , where the CPU  6  again makes the settings for a second search sweep. Specifically, the CPU  6  supplies the sweep signal generator  300 A with data for specifying the RF frequency increment and starting RF frequency for the second search sweep to set them. For example, in the example shown in  FIG. 9 , the RF frequency hit in the first search sweep is specified to be the starting frequency, and the RF frequency increment, which is the resolution of the sweep, is set, for example, to one-half of that in the first search sweep (however, the step interval remains unchanged). In this way, the lower sweep speed V S2  in the second search sweep is one-half as high as the first higher sweep speed V S1 , resulting in a reduction in the influence of the skew as described previously with reference to  FIG. 6 . Also, the sweep speed direction is set to the inverse direction. 
     Subsequently, the flow returns to immediately before step  806 , from which the CPU  6  repeats steps  806 - 812 . In consequence, the sweep is performed in the opposite direction in the second search sweep period in  FIG. 9(   a ). When an IF trigger pulse ( FIG. 9(   b )) is again generated, indicating that the search results in a hit, the CPU  6  stops the search sweep at this search hit RF frequency f RH2 , and determines at step  812  whether or not the frequency can be counted. Since the skew exerts less influence this time, the count start timing pulse is detected within the level detection period (included in the frequency count period in  FIG. 9(   a )) as shown in  FIG. 9(   c ). This state occurs when the search hit frequency f IH2  (corresponding to the search hit RF frequency f RH2 ) is, for example, at a position shown in  FIG. 10 , where the IF output level is equal to or higher than the threshold L TH . As a result, the processing goes to step  816 . It should be noted that in  FIG. 10 , an IF frequency range related to a specified channel is in a range f IRL -f IRU  which is equal to or higher than the threshold level. This IF frequency range F IRL -f IRU  corresponds to an RF frequency range f RRL -f RRU  of a specified channel. However, these frequency ranges can vary from one tuner under testing to another, because they can differ in the IF frequency characteristic. These frequency ranges are also shown in  FIG. 4 . 
     At step  816 , the CPU  6  sets the counter gate signal to high, as shown in  FIG. 9(   c ), forcing the frequency counter  452  to start counting the frequency (frequency f IH2  in the example of  FIG. 10)  of the IF output. The frequency count period starts from this time, and the CPU  6  receives a resulting count which has been made by the counter  452  for a certain period. The CPU  6  calculates the frequency of the IF output from the count in the certain period. In the example shown in  FIG. 10 , the search hit frequency f IH2  is computed. Next, at step  818 , the CPU  6  acquires data on the RF frequency when the sweep was stopped, i.e., the search hit RF frequency f RH1  or f RH2  from the sweep signal generator  300 A, and calculates the RF center frequency f RC  of a specified channel currently under search from the measured IF output frequency (for example, f IH2 ) and known IF center frequency f IC . Specifically, since the difference between the measured IF output frequency and known IF center frequency, i.e., the difference between f IH2  and f IC  in the example of  FIGS. 9 and 10 , is equal to the difference between the RF search hit frequency f RH2  and RF center frequency f RC , the CPU  6  calculates the RF center frequency f RC  by adding the difference to the search hit frequency f RH2 . This RF center frequency f RC  is the frequency of the specified channel under search. Thus, the CPU  6  terminates the processing for detecting the presence or absence of a channel and detecting the RF frequency of the channel. 
     The tuner testing apparatus illustrated in  FIG. 2  uses the detected RF center frequency f RC  mentioned above to determine a sweep frequency range for measuring a variety of characteristics of a specified channel, and generates a sweep signal within the frequency range in a subsequent automatic sweep period for measurement to measure the characteristics. The CPU  6  displays the measured characteristics on the display  8 , such that the particular characteristics of a tuner under testing can be adjusted based on the displayed characteristics. As the tuner testing apparatus has completed the measurement for one specified channel, the tuner testing apparatus repeats the foregoing operations for each of sequentially specified channels to detect the channel and frequency thereof, and makes a variety of measurements on the channel. According to this embodiment, the tuner testing apparatus can test a tuner at high speeds. Particularly, the tuner testing apparatus can test even a digital broadcasting compatible tuner in a short time similar to that required for an analog broadcasting compatible tuner. 
     Referring next to  FIG. 11 , another embodiment of step  814  in  FIG. 8  will be described. Though similar to  FIG. 9 ,  FIG. 11  differs from  FIG. 9  in that the frequency is set back from the search hit frequency f RH1  by a certain amount Δf, and a second search sweep is resumed from this set-back frequency in the same direction as the first search sweep. Accordingly, step  814  may be modified to add processing for specifying this Δf and specifying that the sweep direction is the same as the first sweep. Here, the magnitude of Δf is only required to be large enough to detect the IF output level in the second search sweep. In the example of  FIG. 10 , the magnitude of Δf is large enough to bring the RF frequency f RH1  to an RF frequency point f RS  corresponding to a frequency point f IS  on the opposite side to f IH1  of the IF characteristic. However, the IF output frequency can also be counted when the RF frequency f RH1  is not even set back to a point on the opposite side such as f IS . 
     While one embodiment of the tuner testing apparatus in  FIG. 2 , particularly, components involved in the channel detection, has been described in detail, the channel detection technique in the foregoing embodiment can also be applied to the detection of a channel in circuits or apparatuses other than the tuner (for example, a tuned amplifier, a variable tuned filter and the like). Also, while the foregoing embodiment has used two sweep speeds, i.e., the higher and lower sweep speeds for the search sweep, three or more sweep speeds can be provided such that a sequentially lower speed is used. 
     While a number of exemplary aspects and embodiments have been discussed above, those skilled in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within the true spirit and scope.