JPS61274495A - Measuring system for cn ratio - Google Patents

Abstract

PURPOSE:To allow the CN ratio of FM signal regarding a satellite broadcasting obtained by receiving to be measured from receiving signal which is modulated through a video and a sound signal as it is by extracting noise components of frequency domains which are occupied by signals to be measured. CONSTITUTION:The CN ratio is taken by the following two electric powers; main carriers' powers during the pedestal time for a period of an equalizing pulse within the vertical flyback time of TV video signal as well as the powers obtained after extracting the noise components of the frequency domains which are exclusive of the frequency domains of side band signal caused by main carriers' frequency domains during the above pedestal time and other signals. For example, the following times are utilized as times for extracting noises; pedestal level times a1-a6 during the equalizing pulse on the front side of vertical synchronous signals; peak value level times of vertical synchronous signals b1-b6, the pedestal level times during the equalizing pulse on the rear side of the vertical synchronous signals c1-c6 as well as several horizontal scanning times taken from the pedestal level time d1 at the horizontal scanning times during which no picture signals of the vertical interval as well as mere picture signals are transmitted.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a ratio between the power of a main carrier signal of a received satellite television broadcast signal and the noise power contained within the frequency bandwidth occupied by the main carrier signal. The present invention relates to a CM ratio measurement method for measuring the CM ratio (hereinafter referred to as 0M ratio).

[Summary of vA Indication] The present invention is based on the power C of a main carrier wave in a main carrier signal obtained by frequency-modulating the main carrier wave by frequency multiplexing a television video signal and an audio subcarrier signal and the frequency band occupied by the main carrier wave. C) Calculating the ratio C/N with the noise power N included in the width C) In the ratio measurement method, the power of the main carrier wave at the pedestal time of the equalization pulse period within the vertical retrace period of the television video signal; By determining the C/N from the frequency band of the main carrier wave at the time and the noise power obtained by extracting noise components in the frequency band excluding the frequency band of the sideband signal generated by the audio subcarrier signal, the C/N of the satellite broadcast received signal is determined. The present invention discloses a technique that allows the 0M ratio to be easily measured in the form of a broadcast signal as it is without preparing an unmodulated signal for measurement when measuring the CN ratio.

Note that this summary is provided solely for the purpose of quickly accessing the technical content of the present invention, and does not have any influence on the technical scope of the present invention or the interpretation of rights.

[Prior Art] A main carrier signal emitted from a broadcasting satellite as a television broadcast is frequency-positioned It (hereinafter referred to as satellite broadcast Fll!) by a modulation signal that is frequency-multiplexed with a television video signal and an audio sub-carrier signal. The signal is in the 12 GHz band, and its occupied frequency bandwidth is 27 MHz.

The most common receiving device to receive this signal is.

An antenna device in which a receiving antenna (hereinafter referred to as BS antenna) and a frequency conversion converter (hereinafter referred to as BS converter) are integrated, and a signal converted to an intermediate frequency in the IGHz band by this antenna device (hereinafter referred to as B5-4F signal) The receiver is equipped with a demodulator (which also has a channel selection function and will hereinafter be referred to as a BS tuner) for demodulating television video signals and audio signals by inputting the same.

Considering that the power flux density of satellite broadcasting radio waves on the ground is weak, the most popular BS antenna for home use is relatively small, so the antenna gain is small and the received wave strength is not very high. Therefore, the BS-IF signal contains a relatively large amount of random noise generated inside the OS converter in addition to external thermal noise.

It is known that in order to receive satellite broadcasting as a preferable image, the CN ratio of the received signal needs to be 14 dB or more. Therefore, in popular receiving devices, the 0M ratio is 1.
The current situation is that we recommend something that provides about 4 dB or a slight margin.

However, satellite broadcasting FM signals have the property that even if the CN ratio is less than 14 dB, the received image quality gradually deteriorates up to about 9 dB, but when it decreases further than 9 dB, the received image quality deteriorates rapidly. There is. Therefore, the range in which practical image quality can be obtained is only a few dBL.

Furthermore, the radio waves in the 12 GHz band used for satellite broadcasting have the property of being easily attenuated by rain, so great care must be taken to provide as much margin as possible in the 0M ratio, including attenuation caused by rain. The reality is that the construction of receiving equipment is done at a cost.

In this way, it is natural that the level of the received BS-IF signal at each point of the receiving equipment must be set to the required value, but the 0M ratio of the signal has the greatest effect on the quality of the received image. This is very easy to do, so it is extremely important to check this.

Conventionally, there are the following methods for measuring the CN ratio.

One method is to use test signals. To measure the 0M ratio, if the signal under test is modulated by a television video signal, it is difficult to know only the noise power within that band. Then, measure the noise power in a frequency band with no signal component within the occupied band.

However, this method is not suitable for modulated broadcast waves.

The other method is to receive a modulated broadcast wave, measure the noise power near the occupied bandwidth (27 MHz) of the signal under test, and assume that the same noise power exists within the signal band under test. Estimate the CPI ratio. However, with this method, measurement becomes impossible if the signal to be measured is inserted with a filter that allows only the occupied bandwidth to pass due to transmission reasons in the receiving equipment. At present, this method is most commonly used as there is currently no OM ratio measuring device that can measure the noise power within the occupied band.

The other method is applied when estimating the CN ratio of the BS antenna device output signal, and uses a receiving device with known electrical performance values such as the gain of the BS antenna and the noise figure and gain of the BS converter. This is a method for estimating the 0M ratio. This method is just a guideline and is unreliable.

[Problems to be Solved by the Invention] Therefore, an object of the present invention is to
An object of the present invention is to provide a CM ratio measurement method that can measure the 0M ratio of an M signal from a received signal modulated with video and audio signals.

Another object of the present invention is to provide a method capable of measuring the CN ratio by extracting noise components within the frequency band occupied by the ratio measurement signal.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a main carrier signal in which a television video signal and an audio subcarrier signal are frequency-multiplexed and the main carrier is frequency-modulated. In the C/N ratio measurement method that calculates the ratio C/N between the power C and the noise power N contained within the frequency bandwidth occupied by its main carrier, the pedestal of the equalization pulse period within the vertical retrace period of the television video signal is used. The power of the main carrier wave at the time of
It is characterized in that the C/N is determined from the frequency band of the main carrier wave in time and the noise power obtained by extracting noise components in the frequency band excluding the frequency band of the sideband signal generated by the audio subcarrier signal.

[Function] According to the present invention, when measuring the Cal ratio of a satellite broadcast FM signal, the 0M ratio can be easily measured using the broadcast signal as it is without preparing an unmodulated signal for measurement.

Here, regarding the noise measurement method, the advantages and disadvantages of measuring the SN ratio and measuring the 0M ratio as in the present invention will be explained.

Originally, there is a need to measure the Cal ratio, which involves measuring the signal level of the signal under test that remains at a high frequency and the noise level within the bandwidth occupied by that signal to find the ratio.

This is one means for determining the extent to which noise is included in the demodulated signal after demodulating the signal.

Therefore, there is a relationship between the CN ratio at the high frequency stage and the SNN ratio of, for example, a television video signal obtained by demodulating the CN ratio.

The former 0M ratio is based on the high-frequency signal level (power) of the signal being handled, and the noise level (power) existing within the bandwidth occupied by that signal (however, even though it is called noise, it is band-limited, so , which is actually a high frequency signal with instantaneously random amplitude).

In the latter case, the high frequency signal is passed through, for example, an FM detector to demodulate the modulated signal, and then passed through a circuit with de-emphasis characteristics, as in the case of satellite broadcasting signals, to the pre-modulated TV video signal on the transmitting side. The voltage from the pedestal of the video signal to the 100% self level, that is, the voltage corresponding to the amplitude of the image component, is treated as S. Further, the noise N is the effective value voltage of the noise contained in the video signal thus restored.

Therefore, even though there is a relationship between the 0M ratio value and the SN ratio value, there is a demodulator between them and they are affected by their electrical contract characteristics, so it is difficult to convert them. requires sufficient caution. That is, the problem is what should be defined as the standard characteristic of the demodulator.

However, the original purpose of measuring the 0M ratio is to secure the 0M ratio necessary to ensure a certain amount of S/N ratio after demodulation when designing a satellite broadcasting system line. This can be said from the fact that it is used as a guideline.

However, in reality, it is not easy to obtain the theoretical characteristics of a demodulator, and considering that it is manufactured within a range that does not cause any practical problems, it is treated as the average performance. This can be said in some cases. Furthermore, it must be taken into consideration that the SN ratio has a property that it does not have a linear relationship with the 0M ratio.The FN@ signal has a nearly linear relationship with the SN ratio up to a certain 0M ratio when noise is mixed. However, the 0M ratio has a characteristic that it no longer maintains a linear relationship beyond a so-called threshold value.

Although this is generally true, in reality, depending on the method of the demodulation circuit, there are also factors such as the difference between the SN ratio and the CN ratio.

Therefore, when evaluating the so-called signal quality of a received high-frequency signal, it is desirable to define it by the 0M ratio, and it is often necessary to measure the CN ratio.

When calculating the CN ratio, it is theoretically possible to convert it from the SN ratio, but the measurement results are still only estimates and are somewhat unreliable. If we were to realize this as a somewhat practical 0M ratio measuring device, we would need a demodulator whose electrical characteristics were well controlled, and what's more, we would need a demodulator with sufficiently controlled electrical characteristics. Since the circuit configuration required for the 0M ratio measurement of the present invention is equivalent to or slightly more complicated, it is better to perform the 0M ratio measurement method at a high frequency as in the present invention.

[Example] The present invention will be described in detail below with reference to the drawings.

In the present invention, when extracting a noise component within a frequency band of a signal under test, it is utilized that there is a frequency band in which there is no spectrum constituting the signal under test for a certain period of time.

Examples of time and frequency bands from which noise components in satellite broadcast reception signals can be extracted will be explained with reference to FIGS. 1 and 2.

FIG. 1 shows the vertical synchronization signal during the vertical blanking period of a television video signal and the waveforms before and after it, in order to explain the possible time for noise extraction in the present invention. The pedestal level times 1 to a6 during the equalization pulse period on the front side of the signal are the vertical synchronizing signal peak value level times.

~b also shows some images from the pedestal level time C1#C(,,,, and the pedestal level time dl in the horizontal scanning time that does not transmit the image signal in the vertical blanking period) during the equalization pulse period after the vertical synchronization signal. The horizontal scanning time during which no signal is transmitted can be used as the noise extraction time.

FIG. 2 is a diagram showing, as an example, the spectral distribution of the satellite broadcasting FM signal during the above-mentioned time 01 to c, and at this time, the main carrier signal component spectrum 1
There are upper and lower sideband spectra 2, 3, and 4.5 with the audio subcarrier signal (5,7373 MHz) and the audio subcarrier signal (5,7373 MHz). There is no spectrum component forming the signal under test at a frequency odd B, 7°8.9 between these spectra.

Here, the measurement of carrier wave power will be explained in detail.

The ratio measurement signal power is the sum of the powers of all signal spectrums within the 27 MHz band, and is therefore the power phase of spectra 1, 2, 3, and 4.5 in the example of FIG.

However, it is possible to know the power phase of the entire spectrum described above just by knowing the power of main carrier component 1 in FIG. The reason is as follows.

The satellite broadcast FM signal is an audio subcarrier signal (center frequency 5.
Since the frequency shift width of the main carrier signal by 727272NH2) is ±3.25MH2, the modulation index at this time is 0.5875.Therefore, the spectrum in Fig. 2 for the case where the main carrier signal is not modulated! The relative value of the amplitude is approximately 0.921. The amplitude of the upper and lower first sideband spectra due to the audio subcarriers of spectra 2 and 3 is approximately 0.271, and similarly the amplitude of spectra 4 and 5 is 0.05. The relationship is such that the power phase of these five spectra is 1.0. Here, focusing on spectrum 1, the relative amplitude of 0.921 means that if the signal level of the measured wave is 1.0, it is -0,715 dB lower than the original signal level. becomes.

When trying to measure the 0M ratio of the signal under test, it would be a good idea to separate the signal power C and the noise power, but
At this time, C1, that is, the signal power, is determined by the spectrum l in Fig. 2, and as described above, o is 715 dB.
You can find it by converting the minutes.

Indeed, in the case of measuring instruments implementing the invention.

There is no need to perform any particular conversion operation at the time of measurement, and when attaching the display scale of the measuring instrument, it is sufficient to attach the corresponding scale to the input signal of the known quantity.

'In order to obtain C, it is also possible to extract spectra 2, 3, and 4.5 in FIG.
It is also possible to ask for these.

It is a sideband component generated by the audio subcarrier signal, and therefore, it has the property of being directly affected by the modulation index, so it is not very desirable when considering the slight variation in modulation specifications on the transmitting side of satellite broadcasting. On the other hand, when spectrum 1 is used as described above, these effects are much smaller, so this is preferable in order to maintain measurement accuracy.

To further explain the noise power measurement in detail, 12GHz
The level of the received wave of the satellite broadcasting signal in the band is small, therefore,
This signal band contains relatively many noise components.

Sources of this noise include:

First, it is known that all substances in nature generate so-called thermal noise that is proportional to the temperature (absolute temperature) of the substance. This noise is frequency-independent and has a uniform distribution up to extremely high frequencies, and the noise power shows a value proportional to the frequency bandwidth handled.Noise power limited to a certain bandwidth fluctuates instantaneously. It is also known that the temporal probability distribution of the instantaneous power value has a Gaussian distribution. Therefore, such noise is called random noise, which is different from pulse noise in which noise energy is concentrated at a specific time.

The other type is noise generated from electronic circuits that generally use active elements, such as BS converters, and although this is often treated separately from the thermal noise, it is random noise in nature, similar to thermal noise. It is treated in the same way as thermal noise.

In general, the random noise contained in received satellite broadcast waves is higher in the latter than in the former. Therefore, in order to obtain received waves with less noise, it is necessary to use a BS converter with a performance that reduces the amount of noise generated within the BS converter (with a low NF value). small ones) are used.

In this way, measurement of thermal noise in which the noise power is distributed throughout the band can be obtained by knowing the amount of noise in a part of the bandwidth and converting it into a bandwidth. In other words, since the noise power is proportional to the bandwidth, noise within a part of the bandwidth where there are no signal components in frequency bands 8, 7, and 8.9 in Fig. 2 is extracted, and the bandpass used for the extraction is Filter passband width (in general, it is not possible to create an ideal filter, so
For example, when the bandwidth of the signal under test is 27 MHz, the bandwidth of the filter used for noise extraction is 1 MHz.
If so, the extracted noise power will be multiplied by 27. Generally, when the desired bandwidth is 85 and the bandwidth of the measuring device is B%, the decibel value can be found as the bandwidth conversion amount.

When the present invention is applied to an actual measuring instrument, when adding a display scale for noise power, the noise power should be calibrated using a known amount of input signal, similar to the conversion when calculating the signal power. For example, when using the measuring device, there is no need to perform an operation for converting the band.

The noise power within the measured signal band obtained by this measurement method is not the true noise power, but is a value assuming that the noise to be handled is uniformly distributed within the band. However, there are usually no problems even if it is assumed that the distribution is uniform, so this measurement method is sufficient in practice, and in that sense, it can be used as a simple measurement.

Regarding the total noise within the occupied band of the signal in F, as mentioned above, in reality, there is no conversion operation, and after the measuring instrument is manufactured, the signal power (level) and noise power (level) are independently calculated. Therefore, when calibrating the indicated value on each display, input signals (calibration signals) of known quantities for both signal and noise are used.
The indicated value is calibrated by .

As described above, by extracting the noise components in frequency bands 8, 7, and 8.9, it is possible to extract only the noise components.

In other words, if thermal noise or random noise with properties equivalent to thermal noise is distributed almost uniformly within the frequency band occupied by the signal under test, and the noise level exists without fluctuations other than the extraction time, There is no problem in finding the noise power in a specific frequency band at a specific time and calculating the noise power in the entire signal band under measurement.

As the noise extraction time, a1 to a&+J in WIJ1 figure
~b6, C1wC(,,dt~) can be arbitrarily determined to be used.

In addition, the frequency bands for noise extraction are frequency band 8,
It may be all or part of 7, 8, and 9.

From the noise power obtained by sampling in this way, the noise power included within the frequency bandwidth (2711 Hz) occupied by the signal under test can be calculated.

On the other hand, in order to know the signal power, only the main carrier signal component 1 shown in FIG. 2 is extracted using a band-pass filter during the same time as the time when the noise component is extracted.

According to this, signal power can be determined in the same way as noise power. Since the filter bandwidth is narrow, the signal power obtained in this way has the effect of reducing the noise power existing within that bandwidth, and therefore has the effect of suppressing errors in the signal power value due to noise power. ,desirable.

However, if the CN ratio of the signal under test is 7 to 818 or more, it is possible to pass almost the entire band of the signal under test without using the method described above to determine the signal power, and the signal can be used without any switching operation. Even if the power is treated as signal power, the error due to noise power is small, so it can be used as a simple method.

As described above, in measuring the 0M ratio of a received satellite broadcast signal, the present invention uses a signal obtained by frequency multiplexing a television video signal and an audio subcarrier signal and frequency modulating the main carrier wave. The main carrier signal spectrum and the audio subcarrier signal forming the modulated carrier signal at the time of the pedestal level excluding the equalization pulse during the equalization pulse period within the vertical retrace interval of , or the time of the peak level of the vertical synchronization signal. By extracting noise components in frequency bands excluding the frequencies of the upper and lower sideband spectra caused by this, the noise power contained in the signal under test can be known.

In order to implement this method, a switching circuit is configured to extract the noise component using a band-pass filter and pass the extracted signal only for a predetermined extraction time.

It goes without saying that the present invention is applicable not only to the above-mentioned satellite broadcasting, but also to the conventional terrestrial broadcasting AM@.

The first embodiment of the CN ratio measuring device implementing the present invention is described in the third embodiment.
As shown in the figure.

In Fig. 3, the input terminal lO of the signal to be measured is an IG signal that receives satellite broadcasting and whose frequency is converted by a BS converter.
I (Z band BJ-IF signal or [1 (F broadcast wave band (470 to 770 MHz) when the frequency is reconverted or for transmission to CATV, e.g. 222 to 470 MHz)
A satellite broadcast FM signal is input, although it may be a signal in any frequency band, such as in the MHz band.

The frequency converter 11 converts the input signal to a constant intermediate frequency that is easy to process within the measuring device, and when configured as a general-purpose measuring device, the frequency converter 11 may be provided with a frequency selection function. Become.

This output signal is input to the intermediate frequency amplifier 12, where:
Amplify to the required level and limit the bandwidth to the required signal and noise level measurements.

This necessary bandwidth is the minimum necessary bandwidth to demodulate the television multiplex synchronization signal to form a signal for switching only the noise component extraction time described above in the switching signal generation circuit 17 in FIG. It is sufficient that the noise extraction bandpass filter 15 has a bandwidth such that the band of the noise extraction bandpass filter 15 is included in the bandwidth.

That is, in the satellite broadcast FM signal, the main carrier wave is modulated into an occupied bandwidth of 27 MHz by a signal obtained by frequency multiplexing the television video signal and the audio subcarrier signal, but the video signal is de-emphasized before modulation. Therefore, the frequency shift width of only the synchronization signal is about 1.5 KHz.

However, in satellite broadcasting, in order to reduce interference with other communications, this frequency shift is 0. There are 0 MHz and about 2 MHz, which is the frequency at which the relative frequency of the synchronizing signal constantly changes due to changes in APL depending on the image content of the video signal.

Therefore, if the position is set to include the minimum signal spectrum l and band 6 in which only noise components exist in order to demodulate the synchronization signal, the TV synchronization signal is also included, so various necessary signals are included. It is.

In this way, the passband width of the intermediate frequency amplifier 12 is set to 27M.
Setting the frequency narrower than Hz has the advantage that it is easier to obtain a signal with a good S/N ratio when demodulating the synchronization signal with the frequency detector IB, and the signal level measurement consisting of the amplitude detector 13 and the signal level display section 14 This has the effect of reducing errors due to noise components in the results obtained from yarn I.

The output of this intermediate frequency amplifier 12 is subjected to amplitude detection by an amplitude detector 13 for displaying the signal level. The method for measuring the signal level C in this embodiment is a method in which signal components passing within the intermediate frequency signal band are handled without switching.

The FM signal is originally a signal with a constant amplitude, but since the frequency band of the input signal to the amplitude detector 13 is limited, this input signal has an amplitude change, and therefore,
It is necessary to have peak detection characteristics.

The output of the amplitude detector 13 is displayed on a signal level display section, so that the signal level applied to the signal input terminal 1O can be read as a corresponding level.

This signal level display section 14 can expand the dynamic range of the displayed value by logarithmically compressing the applied signal, but if compression of the dynamic range is not necessary, it may be equipped with a range switching function, or A step switching attenuator may be inserted between the signal input terminal 10 and the frequency converter 11.

The output of the intermediate frequency amplifier 12 is also supplied to the above-mentioned band pass filter 15 for noise extraction and the frequency detector 18 for demodulating the television synchronization signal.

During the noise extraction time, the bandpass filter 15
The frequency is matched to approximately the center frequency of the frequency band 6 shown in FIG. 2, and its passband width is 0.2 to 1. , OMHz
is reasonable. The reason for this is that the main carrier signal spectrum 1 and the lower first sideband spectrum 2 of the audio subcarrier signal shown in FIG. Because of the fluctuation, the bandwidth in which only the noise component can be extracted stably at all times is about 2) l) Iz.

Hence the bandpass filter for noise extraction! Considering the out-of-band attenuation characteristics of 5, the bandwidth is 1-ONHz.
The following is true. Furthermore, if the bandwidth of this filter 15 is extremely narrow, each of the noise extraction times is less than half the horizontal scanning time as shown in FIG. 1, so a relatively high response speed is required. Ru. Therefore, it is desirable to have a bandwidth of at least 200KH2 or more, and the wider the bandwidth of the noise extraction filter 15, the greater the extracted noise level.

The output of this bandpass filter 15 is switched in a switcher 18 in response to a switching signal formed by a frequency detector IB and a switching signal generator 17 so that only the output during the noise component extraction time is extracted. Only the noise component of the filter 15 output is extracted.

The noise component extracted in this way is sent to the amplitude detector 19.
The signal is detected and rectified by This amplitude detector 19 is provided with an integration function and converts the detected rectified output into a DC voltage.

This DC voltage output is supplied to the noise level display unit 20, and the noise level converted to the noise level included in the 27 MHz band of the signal under test applied to the input signal terminal 10 is displayed. Although it is possible to expand the dynamic range by providing a logarithmic compression device to the noise level display unit 20, the display range may be changed using a range changeover switch.

Although the first embodiment has a relatively simple circuit configuration,
In order to determine the CN ratio, the signal level and noise level values displayed on the one-car display sections 14 and 20 are read individually, and the CN ratio is calculated.

Furthermore, the relationship between the center frequency of the noise extraction band-pass filter 15 and the frequency of the signal under test must be accurate.
It is necessary to stabilize the frequency of the local oscillation signal of the frequency converter 11.

A second embodiment of the invention in which these points are more easily handled is shown in FIG. 4 in this embodiment.

In order to perform stable noise extraction, AFC operation is adopted,
Furthermore, the configuration is such that the CN ratio can be directly read.

In FIG. 4, the frequency converter 21 has the same operating frequency as the frequency converter 11 used in the 1sl embodiment, but has a variable amplification function, is capable of AGO operation, and can change the oscillation frequency of one local oscillator. This enables AFC operation by being able to change it electronically. The intermediate frequency converted here may be the same as in the first embodiment.

Intermediate frequency amplification If! 22 may have a passband width equivalent to that of the first embodiment, but by providing an AFC function, it absorbs the frequency shift due to the dispersal signal and the frequency shift due to image signal APL fluctuation, and produces a stable intermediate frequency signal. Considering that the bandwidth can be obtained, it is possible to narrow the bandwidth to about half of the bandwidth of about 4)1z in the first embodiment. However, if the width is made too narrow, it becomes difficult for the AFC@ function to operate immediately with the input signal under measurement. Moreover, since the television synchronization signal needs to be demodulated to some extent, it is not advisable to make the bandwidth too narrow. Therefore, about 3 MHz is an appropriate bandwidth.

In this second embodiment, the AGO operation is performed so that the output level of the intermediate frequency amplifier 22 has a constant amplitude even if the input level of the signal to be measured is at an arbitrary level.
As for 3, it is not necessary that the amplitude detection characteristics of the amplitude detection be good, but the input signal exceeding a certain threshold value can be detected and rectified to obtain the ^GC signal 24 for operating the AGC. It is sufficient if it has a function.

Since the AG signal 24 obtained from the amplitude detector 23 changes in accordance with the signal level applied to the signal input terminal 10, the next stage signal level display section 26 displays this AG signal.
By reading the O signal 24, the signal level is known. In general, the AGC signal that changes the gain of the frequency converter 21 and the intermediate frequency amplifier 22 has a voltage that approximates logarithmic compression, so the scale of the signal level display section 26 only needs to be scaled with equally spaced dB values; At the same time, it also has the advantage of widening the dynamic range of signal levels. Therefore, the signal level display section 2B can be a simple circuit that displays a voltage with relatively little voltage change.

The intermediate frequency signal whose level has been made constant as described above is supplied to the two frequency detectors 1B and 27.The zero frequency detector 1B is used only for the purpose of demodulating the television synchronization signal as in the first embodiment. In order to realize such a frequency detector 1B, the frequency bandwidth of the demodulated signal only needs to be sufficient to reproduce the synchronization signal, and the linearity does not need to be particularly good. 1 For example,
A slope detection method using one resonant circuit is sufficient for practical use.

The frequency detector 27 is for obtaining a control signal for performing the above-mentioned AFC control. For example, when the frequency of the horizontal synchronizing signal peak value is to be constant, the frequency detector 27 is used to obtain a control signal for performing the AFC control described above. It is only necessary to have a device that is set so that the detection output of the value is the center of the detection characteristics.This output is supplied to the sample and hold circuit 29, and the switching signal generator 2
Sample only a short period of time in the center of the horizontal synchronization signal peak value generated at
An AFC signal 30 for performing FC operation is obtained. This A
FC signal 30 is supplied to frequency converter 21 .

The intermediate frequency signal whose level is constant and whose frequency is stabilized in this way is passed through the bandpass filter 15, the switcher 18, and the amplitude detector 19, and only the noise component is extracted as in the first embodiment. The level is displayed on the CM display section 31.

The level displayed on the display section 31 indicates the noise level included in the level of the input signal under test which has been made constant, so by directly attaching a dB value to the indicated value, It becomes possible to directly read the ratio in decibels regardless of the level of the signal under test applied to the signal, which improves the convenience of using the measuring instrument.

[Effects of the Invention] As is clear from the above, according to the present invention, when measuring the 0M ratio of the number of satellites transmitted and received @ signals, the broadcast signal can be used in its original form without preparing an unmodulated signal for measurement. The Cl1l ratio can be easily measured.

CA that receives satellite broadcasting signals and retransmits them as FM signals
In TV facilities, etc., bandpass filters that pass only the transmission signal band are used, but even for such signals, the conventional CM ratio measurement method cannot measure the noise level near the out-of-band. On the other hand, in the present invention, the ON ratio can be measured without any problem.

According to the present invention, it is possible to realize a portable measuring instrument capable of simultaneously measuring ratio C and signal level at t'1xyy, so the effect that it can be used on a daily basis in the field is extremely large.

Note that the present invention can be effectively applied not only to the 0M ratio measurement of satellite broadcast reception signals but also to conventional television terrestrial broadcast AM signals.

[Brief explanation of drawings]

FIG. 1 is a signal waveform diagram showing a vertical blanking period of a television video signal for explaining the present invention. FIG. 2 is a diagram showing an example of the spectrum distribution of a carrier signal during the vertical retrace period of a satellite broadcast FN signal for explaining the present invention in one pulse period pedestal bell value time, and FIG. FIG. 4 is a block diagram showing a first embodiment of the invention. FIG. 4 is a block diagram showing a second embodiment of the invention. ! 0...Signal input terminal. 11.21...Frequency converter, 12.22...Intermediate frequency amplifier, 13.19,23...Amplitude detector, 14.28-...Signal level indicator, 15...Band pass filter , 18.27...Frequency detector. 17.28...Switching signal generator, 18...
Switcher, 20... Noise level display section, 24... AGC signal, 29... Sample hold circuit. 30...AFC signal. 31...CM Hibiyo Gunbu

Claims (1)

[Claims] 1) Power C of the main carrier wave and noise power N included within the transmission frequency bandwidth in the main carrier signal obtained by frequency multiplexing the television video signal and other signals and modulating the main carrier wave.
In the C/N ratio measurement method for determining the ratio C/N, the power of the main carrier wave at the pedestal time of the equalization pulse period within the vertical retrace period of the television video signal, and the frequency of the main carrier wave at the time. A C/N ratio measurement method characterized in that the C/N is determined from a frequency band and a power obtained by extracting a noise component in a frequency band excluding a frequency band of a sideband signal generated by the other signal. 2) In the CN ratio measurement method according to claim 1, a band-pass filter for extracting noise components in frequency bands excluding the frequency band of the main carrier wave and the frequency band of sideband signals generated by the other signals; , a switching circuit for passing the output from the bandpass filter only during the period of time.