Test and Analysis of Digital Signal Level

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1. The difference between digital signal level and analog signal level

  

The carrier modulation of analog TV is VSB, that is, residual sideband modulation. The image content is transmitted through amplitude modulation. The image content changes at any time, and the power of the channel changes continuously. Therefore, the channel power of analog TV depends on the image content. The analog TV line/field synchronization pulse level is relatively stable, so we use the peak level as the standard for judging the strength of analog TV signals.

  

Digital TV signals use QAM modulation, which has similar characteristics to double-sideband. It modulates both the amplitude and phase of the carrier. Because it is balanced amplitude modulation, the carrier is suppressed, so a digital TV channel does not have a so-called image carrier or sound carrier. The energy of the modulated signal of a digital channel is evenly distributed within the entire limited bandwidth. The channel power is relatively stable and does not change randomly with the content. Therefore, digital TV uses the average power level of the RF or IF signal within the effective bandwidth to represent the power of this channel. The average power level of the digital TV signal is also called the channel power, which is a completely different concept from the analog TV level.

  

2. Measurement method of digital signal level

  

According to the spectrum structure of digital TV signals, the level of QAM digital channels is expressed by the average power of the measured channel signal, that is, the average power of the digital channel. The measurement method is to scan and sample the entire channel, average the power value of each sample, and then integrate it within the bandwidth of the channel to obtain the average power of the channel. This requires a dedicated digital signal measuring instrument to measure.

  

At present, most small and medium-sized cable operators only have simple analog signal testing methods. When troubleshooting network problems, the most commonly used method is to measure the signal level. However, the power of the digital signal of a digital TV signal cannot be measured using peak power measurement because channel power is related to bandwidth. The wider the bandwidth, the higher the average power of the channel. The following introduces a method for estimating the digital signal level using an analog field strength meter.

  

When transmitting analog TV signals and digital TV signals simultaneously in the cable TV HFC network, they are generally transmitted under the same level value of each TV signal frequency band. If the level values ​​of analog signals and digital TV signals are measured separately with a general field strength meter, it will be found that the level value of digital TV signals is more than ten dB lower than that of analog TV signals. This is due to the different energy distribution characteristics of analog signals and digital signals in a specific bandwidth. The analog signal level value is the result of measuring the carrier of the analog TV signal with a field strength meter within the specified bandwidth (300 kHz). Because the carrier within this bandwidth concentrates the main energy of the TV signal in the channel (more than 98%), it can be considered that the measurement result of the carrier represents the level value of the signal in the channel. The peak of the analog signal appears at the carrier frequency point, while the carrier frequency point of the digital TV signal cannot be seen on the spectrum, so the measurement of the digital TV signal is similar to the measurement of noise. In analog transmission, the definition of carrier-to-noise ratio is: C /N = 20 Ig (effective value of image carrier level/root mean square value of noise level (within the specified bandwidth)), and the measurement of the root mean square value is related to the measurement bandwidth. From this, we can understand the difference between analog signals and digital TV signals when measured with an analog field strength meter. Because the energy of analog TV signals is concentrated in the 300 k frequency band (and concentrated in the center), the peak amplitude of its spectrum energy concentration graph is very high, while the total energy of the 8 M bandwidth digital TV signal is evenly distributed in a frequency band of about 6.4 M, the spectrum energy is dispersed, the peak amplitude is relatively low, and the effective energy bandwidth is about 20 times the bandwidth of the analog TV signal, which is approximately 11~14 dB. There is a little error caused by the intermediate frequency scanning bandwidth of the field strength meter, the waveform coefficient of the intermediate frequency filter, the roll-off rate, etc., so when using an analog field strength meter to estimate the digital signal level, the simple conversion formula between the actual level (power) and the displayed level (power) is as follows:

  S = SO + 10 lg (RBW /B ) +K

  Where: S——actual level value;

  SO ——the level value displayed by the field strength meter;

  RBW ——Equivalent intermediate frequency scanning bandwidth, i.e. -3 dB signal bandwidth, usually *00 kHz

  

(In theory, the bandwidth of digital TV signals is related to the order of QAM modulation, symbol rate and roll-off factor. The specific formula is: BW (actual spectrum bandwidth) = F (symbol rate) * [1 + a (roll-off factor)]. my country adopts the European standard, 8 MHz bandwidth, generally uses 64 QAM modulation, 6.875MB aud symbol rate, and a roll-off factor of 0.15. Substituting into the above formula, the actual signal spectrum bandwidth is 7.90625MHz. Here, the -3 dB signal bandwidth is 6.4MHz. The bandwidth of the digital TV signal calculated theoretically is not necessarily consistent with the actual digital TV signal bandwidth. The signal bandwidth of each brand of QAM modulator will be slightly different.)

  B—— The resolution bandwidth set by the field strength meter (in kHz), generally 300 kHz.

  K - Correction coefficient. This correction value is different for different instruments and is generally 1.7 dB.

  Substituting the above values: S = S0 + 10 lg ( *00 /300) + 1.7 = SO + 15

  

3. Determination of digital signal level

  

In CATV systems, digital channels are usually arranged at the high-end frequency of the system, while analog channels are arranged at the low-end frequency. When the input level of the CATV system is too high, the nonlinear distortion index (CSO, CTB) of the system will deteriorate. For example, if the input level is increased by 1dB, the CTB will deteriorate by 2dB. These distortion products will fall into the digital channel, making the bit error rate BER of the digital signal higher, thus affecting the reception effect of digital TV. In the HFC network, the dynamic range of optical equipment is much smaller than that of the amplifier. In the general analog signal transmission system, if the transmission of digital signals is added, the excitation level of the laser will be increased, and the average power of the input laser will increase, thus causing the laser to clip, which will result in a rapid increase in the BER value, which will cause the digital demodulator to lose synchronization instantly. The distortion of digital signals will affect the C/N value of analog signals. At the same time, digital signals will be affected by the clipping of CSO, CTB and optical equipment. Therefore, the setting of digital signals must compromise between the two: (1) reducing the interference generated by analog channels; (2) increasing the anti-interference ability of digital signals. Test results show that when the power of digital signals increases, it will affect the laser. Experience tells us that the level of 64QAM signals should be set 10 dB lower than that of analog signals. If 256QAM modulation is used, the level value may need to be increased.

  

The national standard recommends that in a network with digital and analog compatible transmission, the level of the digital channel should be 10 dB lower than that of the analog channel. The purpose is to reduce the nonlinear distortion of the digital signal. After measurement, the minimum threshold level for digital TV users to watch digital TV can reach 35 dB.

Reference address:Test and Analysis of Digital Signal Level

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