Cum măsuram semnalele DVB-T2 și DVB-S/S2 în sistemele TV.

DVB-T2 digital terrestrial TV signal measurement technique is not very different from DVB-T signal measurement. This is a direct result of the frequency characteristics of both signal types. Due to the fact that a COFDM digital signal is actually several thousand orthogonal subcarriers modulated in QAM, in order to properly measure its level, you need to measure the power density of the entire channel width of 7 MHz in the VHF band or 8 MHz in the UHF band. The H.265/HEVC compression standard was adopted for audio/video compression in the DVB-T2 signal. The DVB-T system used the MPEG-4 (H.264/AVC) standard. This basic difference is crucial when you decide to measure a digital signal with a meter that allows you to preview a given channel. In the case when the meter does not have a built-in HEVC(H.265) video decoder, but it enables DVB-T2 measurement, you will not see a preview of a given channel on the meter's display, but only the measurement itself.

On the market of TV systems, you can find different quality meters. From the cheapest and simplest indicators showing successively lit LEDs indicating the strength of a given signal to meters measuring the most important parameters of a digital signal without/with a preview of a given channel. In addition, a big advantage of using a professional measuring device is the calibration certificate issued by the manufacturer. This document, issued during the manufacturing process of the meter, confirms the maintenance of measurement consistency. It is issued only for brand new measuring instruments.

The RF Level expressed in dBμV is not the only parameter to consider when taking measurements of digital terrestrial as well as satellite TV. Equally important, if not more important, are: Modulation Error Ratio MER (Modulation Error Ratio), Link Margin and Bit Error Rate (BER).

Modulation Error Ratio MER – MER can be considered not as a measure of signal quality, but as a measure of the margin left by the system for total signal loss. Without knowing this parameter, the antenna installer cannot guarantee stable reception in adverse weather conditions or other situations when the signal can fluctuate (e.g. interfered by a passing truck). Providing a margin of safety (a sufficiently large distance from the cliff) by measuring the MER, will avoid this type of situation. Typical and minimum values of the MER parameter for proper reception depend on modulation technique. For 256-QAM modulation, 28.5 dB should be taken as the minimum value, while typical values are 31.5 dB and above.

Link Margin – Link Margin is a measure that indicates the margin of safety (a sufficiently large cliff gap) by measuring the MER. When Link Margin is at or near zero, the decoder is unable to recover lost data, resulting in a loss of signal. A high Link Margin is important because it ensures signal tolerance to degrading factors such as interference or system aging. When measuring terrestrial or satellite TV, pay attention not only to MER, but also to Link Margin. The boundary between an ideal signal and its complete absence is very narrow. Therefore, even in the case of slight signal degradation, you can see image artifacts and freezes on the TV screen. The higher its value, the greater the tolerance of the signal to degrading elements. A properly set amplifier, high-quality cabling, a minimum number of intermediate connections and other good practices will ensure Link Margin, which will protect the system from future signal loss.

Bit error rate BER – the fourth important parameter to consider when measuring digital terrestrial TV signals is BER. This parameter, well known to all installers of satellite dishes, illustrates the quality of the received signal. It tells what part of the bits that reach the receiver has been distorted due to interference and interference in the transmission channel. There are several types of BER ratio. Although only one of them (bBER) is of interest from the point of view of the system contractor, it is worth being aware of what the meter is capable of measuring (most measuring devices offer measurement of two types of bit error rate BER). The digital signal undergoes redundant coding before transmission. This means that, in addition to useful information, the transmitter also sends data that is useless from the receiver's point of view, allowing it to fix the bits that have been corrupted along the way. The number of these bits is determined by the FEC (Forward Error Correction) parameter. Most often it is 3/4 or 5/6, which means respectively 25% or 17% of redundant data in relation to the entire transmitted signal. Obviously, the larger the FEC, the fewer errors you will receive on the receiver side. However, it should also be borne in mind that too large a value of this parameter can cause unnecessary restrictions on the bandwidth of a given transmission channel. First in the receiver, the signal undergoes a decoding process, followed by Viterbi error correction, in which redundant data is used to eliminate errors. The signal, after correction, is characterized by significantly fewer errors. Signals before and after Viterbi correction are two different signals from the point of view of the number of errors present in them.

The following tables give examples of the measurement values of the MER parameter for both DVB-T2 digital terrestrial TV and DVB-S/S2 satellite TV.

Modulation	Parameter	MER FEC 2/3	MER FEC 3/4	MER FEC 5/6	Link Margin
DVB-T2 256 QAM	Min	25	26.5	28.5	3
DVB-T2 256 QAM	Typical	28	29.5	31.5	6

The above table shows the minimum and typical values of the MER parameter depending on the modulation type for terrestrial digital TV DVB-T2.

Modulation	Parameter	MER FEC 2/3	MER FEC 3/4	MER FEC 5/6	Link Margin
DVB-S QPSK	Min	9	10	11	3
DVB-S QPSK	Typical	12	13	14	6
DVB-S2 8PSK	Min	11	12	13	3
DVB-S2 8PSK	Typical	14	15	16	6

The above table shows minimum and typical values of MER parameter depending on modulation type for digital satellite TV DVB-S/S2.

In addition to the measurement of individual parameters of digital TV signals, which have a significant impact on the correct implementation and operation of RF/SAT systems, one of the main criteria in selecting a meter will be its ease of use and functionality. Certainly, a much more convenient device will be a meter that has one input common for DVB-T2 and DVB-S/S2 signals. This solution eliminates the cumbersome switching or rearranging of the cable when measuring directly from the multiswitch. Another important feature is the measurement of RF signals over a very wide range, which eliminates the problem of signal overdriving on individual elements in the system as well as the meter itself.

The screen shoots above show the measurement of DVB-T2 digital terrestrial TV signal and DVB-S2 satellite TV signal (HotBird 13.0E Satellite, Transponder 11508 V). The main parameters are the following: POWER (RF signal level), noise margin, modulation error rate (MER), errors before Viterbi correction (CBER) and after the correction (VBER). All measurements are presented on a single screen. The spectrum distribution analyzer, especially for terrestrial TV, allows you to view strong interfering LTE signals (function for real-time comparison) which can adversely affect the quality of DVB-T2 signal causing interference and interfering with proper reception of terrestrial digital TV.

Channel amplifiers perform an important function in any SMATV system. They are mounted at the input of the system to amplify and equalize all DVB-T2 multiplexes. The configuration of the channel amplifier itself is simple and quite intuitive. Typically, channel amplifiers are configured using a knob (buttons) located on the front casing of the device or using any mobile device equipped with Android (possibly a Windows computer). The choice of configuration method depends on the manufacturer of the product. A good installation practice first recommends measuring the DVB-T2 terrestrial TV signal measured directly from the antenna, and then measure this signal for each multiplex at the output of the amplifier. Ideally, you should use a certified ALPSAT AS07STCA-4K R10206 TV signal meter.

The above picture shows the measurement of terrestrial digital DVB-T2/HEVC with the ALPSAT AS07STCA-4K R10206 meter at the output of the PROFINO Revolution 6711 R89062 channel amplifier for the example channel 23 in the UHF band. An equivalent measurement should be made for the other MUXes in the system. If this is the case, the best solution is to use a signal spectrum analyzer or the Tilt Limit function, which allows bar measurement of up to 12 MUX of digital terrestrial TV. The amplifier has been set to an RF output level of 90 dBuV for each of the 5 multiplexes. This level of RF signal is completely sufficient for distribution and correct reception to dozens of receivers in the system.

Comparison of key features of COMBO-type meters for DVB-T2 digital terrestrial and DVB-S/S2 satellite measurements.


Code	R10155	R10206	R10210
Calibration certificate	NO	YES	NO
Spectrum analyzer	NO	YES	YES
Real-time signal spectrum analyzer for detecting interference from mobile signals	NO	5G, LTE 4G, GSM, DCS, UMTS 3G	NO
Measuring parameters	RF Level, MER, aBER	RF Level, MER,Link Margin, bBER, aBER	RF Level, MER, aBER
Export measurements to .csv file	NO	YES	NO
Firmware upgrade	File	Automatically via RJ-45 port	File
RF – signal strength	44...85 dBμV	25...100 dBμV	30...90 dBμV
Battery – capacity	2600 mAh	7000 mAh	5000 mAh