English
Polski
Česky
Deutsch
Irish
Magyar
Português
Română
Slovenský
 |  |  |  |
Parameters of signals, i.e. field strength and input levels of useful and unwanted signals, then their levels in subscriber outlets, levels of noise and interferences caused by subscriber receivers and nonlinear distortions, as well as actual impedances of the pieces of equipment, parameters of amplifiers and channel converters, together with reliability of components, and many other parameters - all that influences the total quality of antenna system. Knowledge of the parameters is the basis for optimum design of antenna installation.
Most of them can be measured using selective signal level meters. Performing such measurements is not a waste of time but essential stage for proper design, and the next - even more important - of verification of the work done. The measurements allow to verify correctness of the design and its implementation, as well as quickly find the errors arising both at concept and realization stages.
 |  |  |  |
Main parameters of signals distributed in antenna/cable installations:
Power level in given point of an installation. Power level of the signal is described in dBuV (terrestrial TV), and in dBm (satellite TV). Both units show the ratio of the actual signal level to reference points. For 1 dBuV the reference point is the power dissipating in 75 ohm resistance at 1uV voltage, while for 1 dBm it is 1mW.
Every level meter allows for measurements of average power of digital signal, too. To measure it, one should tune the meter to the middle frequency of the examined channel and add to the reading the correction coefficient given in the correction table provided with the device. The coefficient depends on the bandwidth of the channel. With meters that have separate mode for measurements of digital signals it is sufficient to choose this option.
Minimum (PSmin) and maximum (PSmax) output level at subscriber outlets. The measurements of signal levels at subscriber outlets are basic, simple and easy to interpret operations. The readings should to be within the limits 62 dBuV and 80 dBuV. The minimum value is the result of good enough S/N parameter (noise margin), the maximum level takes into consideration possibility of overdriving IF amplifier in a TV set. Practically, levels of 55 dBuV and 90 dBuV, adequately, are intransgressible, although they may not be suitable for some receivers - it is highly recommended the former range.
Range of output levels at subscriber outlets
Minimum level at the input of the first amplifier in the network (PINmin). Similarly to other levels in the network the main factor of it is proper noise margin. As the minimum value for good quality it is widely assumed 53 - 56 dBuV level (depending on the band). It is highly important to ensure good performance of the installation predicting some fluctuations of the input level and allowing to compensate the changes that can reach 2 - 4 dB. This margin should be within tolerance of well designed system, to protect it from overdriving in any stage.
Noise margin - Signal to Noise ratio (S/N). It says how many times power of the signal is higher than the power of the noise generated in an amplifier. The ratio depends on the level of the signal at active element/s of the amplifier and its own noise factor that should be given in the specifications, together with the information whether it is an average or maximal figure. Too small S/N value shows as "snow" in the image and further possible disappearance of color, audio distortions, problems with synchronization. Example norms give the minimum values from 43 to 45 dB in the case of TV and 55 dB in the case of FM radio. Most of signal level meters provide direct measurements of S/N ratio, others allow for indirect measurements described in their user's manuals.
Illustration of S/N definition
Measurement of S/N ratio is possible even when the meter is not equipped with this function. First, we measure the level of video carrier, reading the value from the scale and adding the correction from the systematic error correction table (it depends on the range). The measurement is to be made in the interesting channel, without filtration of other available signals.
In the next stage we disconnect the input circuit from the installation (lack of any useful signal), tune the meter by 2 MHz from the video carrier, and read the second value. The ratio (in logarithmic scale - the difference) is the wanted S/N parameter.
Similar measurements are performed in the first satellite IF band (950 -2150 MHz).
Carrier to Noise ratio (C/N). This parameter describes the quotient of the power of a carrier frequency to the power of noise. It is essential parameter in satellite systems, both with frequency modulation (analog satellite installations) and phase modulation (e.g. QPSK, used in digital DVB-S systems).
It is possible to calculate S/N ratio knowing C/N value and the formula describing characteristic of FM modulator. Proper reception of analog programs with FM modulation is possible for the C/N ratio higher than 5 to 10 dB, whereas for digital programs with QPSK modulation adequate minimum ratio is 7 to 12 dB.
In the case of SMATV systems the minimum C/N figure is 16 dB. Most of signal level meters are equipped with direct function of C/N measurement or provide adequate indirect procedure.
Bit Error Ratio (BER). BER is the parameter indicating possibility and quality of reception of digital broadcasts (e.g. with QPSK modulation, used for Digital Video Broadcasting - Satellite: DVB-S).
Practically, the ratio has to be lower than 10-3 to enable any reception, whereas the level 10-9 guarantees ideal transmission (C/N - carrier-to-noise ratio - is adequately equal to 7 dB and 12 dB).
Practically, the ratio has to be lower than 10-3 to enable any reception, whereas the level 10-9 guarantees ideal transmission (C/N - carrier-to-noise ratio - is adequately equal to 7 dB and 12 dB).
Error rate 10-9 means that only one in a billion of transmitted bits may be misrepresented. Due to Forward Error Correction (FEC), the BER value ensuring correct reception is changeable.
Measurements of BER parameter can be performed with a meter having this function. BER is interdependent with C/N, i.e. for a given C/N it is possible to calculate BER. But in practice, so called implementation loss (actual characteristics of specific devices), being very difficult to estimate, makes such an assessment inaccurate.
Impact of C/N on BER and image quality
C/N[dB] | BER | Quality of transmission |
| 5 | 10-3 | unacceptable |
| 8 | 10-4 | inadequate |
| 10.5 | 10-5 | satisfactory |
| 11.7 | 10-6 | good |
| 12.3 | 10-7 | very good |
| 12.8 | 10-8 | excellent |
| 13.2 | 10-9 | ideal |
Differences in signal levels. The differences among levels of programs/channels in antenna installation shouldn't exceed 10 to 12 dB within whole frequency range. Due to limited selectivity of TV sets, VCRs etc. it is not advised to distribute signals on adjacent channels. It is impossible in bigger installations with a variety of programs, similar to cable TV systems - then the levels of any adjacent channels can't differ in levels more than 3 dB (up to 6 dB in VHF I). Alignment of channel levels is one of essential stages of final adjustment of any antenna installation. And it is the main job of signal level meters.
Differences of levels among channels
Video carrier to audio subcarrier ratio. This parameter shows another ratio, this time of carrier signals within a TV channel, significant for quality of images and sound, especially in antenna systems using adjacent channels. The ratio should be higher than 10 dB, although the optimal value is 16 dB. In the case of using second audio subcarrier the ratio has to rise to 18-20 dB. It is important to set proper value of this parameter due to its impact on sound quality (when too high) and interferences in the image on adjacent channel (when too low).
Differences of carrier levels in TV channel
SPECTRUM ANALYZERS. These devices are important category of measuring equipment, very suitable for variety of RF measurements and tests:
- looking for parasitic signals generated by nonlinear elements of amplifiers, those being harmonics of generators in modulators, or even coming from heterodynes of improperly working tuners in TV sets,
- testing separation of polarizations,
- adjustment of angular displacement of converters,
- testing multiswitches,
- very quick, although not so precise adjustment of channel levels for a set of channel amplifiers or satellite receivers,
- determining the actual source of interferences (amplifier or receiver).
Very useful function offered by high-class devices is possibility of observation of horizontal synchronizing pulses of TV signals. Shape of the pulse, i.e. difference from ideal, right-angled form shows the impact of multipath propagation. Watching this pulse it is possible to precisely set the antenna, minimizing unwanted distortions and maximizing the main signal.
All measurements should be put into the report covering all this activity. Such periodic reports are obligatory in many countries in case of larger systems (e.g. over 1000 subscribers), but even for smaller installations such information is very useful for maintaining and servicing the network.
All the figures are based on:
- The Polish norm PN-79/T-05210 [Community antenna installations. General requirements and testing] PKNMiJ Warszawa 1980;
- Annex no. 21 to the order of Minister of Communications issued on September 4, 1997: [Requirements for components of cable television] Warszawa 1997.)
