Output power of an analog TV transmitter


Output power of an analog TV transmitter

Output power of a TV transmitter is the electric power applied to antenna system. There are two definitions: nominal (or peak) and thermal.

Contents

Power defined in terms of voltage

The average power for a sinusoidal drive is [1]

 P = \frac{1}{T}\int_0^T i(t) \cdot e (t) dt\,\!.

For a system where the voltage and the current are in phase, the output power can be given as

 P = \frac{1}{T \cdot R}\int_0^T e(t)^2 dt\,\!.

In this equation R is the resistance and e(t) is the output voltage

Nominal power of a TV transmitter

Nominal power of a TV transmitter is given as the power during the sync interval. (For the sake of simplicity aural power is omitted) Since, the voltage during the sync interval is a fixed value,

 P_n = \frac{E_p^2}{2\cdot R}\,\!
 P_n = \frac{E^2}{R}\,\!

where E is the rms value of the output voltage.

To measure the nominal output power, measuring devices with time constants much greater than the line time are used. So the measuring equipments measure only the highest level (sync pulse) of a line waveform which is 100 %.

The thermal power

In analogue TV broadcasting, the video signal modulates a carrier by a kind of amplitude modulation (VSB modulation or C3F). The modulation polarity is negative . That means that the higher the level of the video signal the lower the power of the RF signal. The lowest possible modulating signal during the synchrone interval yields 100 % of the carrier. (The nominal power of the transmitter.) The blanking level (300 mV) yields 73 % (in an ideally linear transmitter)[2]. The highest modulating signal at white (1000 mV) yields only 10 % of the carrier. ( so called residual carrier)[3]. So the output power applied to the antenna system is considerably lower than the nominal power. The thermal power which can be measured by a microwave power meter depends on the program content as well as the residual carrier and sync depts.

Ratio of thermal power to nominal power

Since the program content is variable, the thermal power varies during the transmission. However, for testing purposes a standard line waveform can be applied to the transmitter. Usually line waveforms corresponding to 350 mV or 300 mV black image (and without field sync) are applied to the input of the transmitter.

For system B, the duration of the black level 300 mV (together with the front and back porches), is 59.3 μs and it corresponds to 73 % of maximum voltage level.The duration of the sync pulse is 4.7 μs. The total duration of the line is 64 μs.

 P_t = \frac{E^2}{64\cdot R}\cdot (4.7\cdot ( 100  %)^2 + 59.3\cdot ( 73  %)^2) \approx 57 % \cdot \frac{ E^2 }{R}\,\!.

So the maximum thermal power applied to the antenna system is 57 % of the nominal power, even in the black scene. In normal program content this ratio may be around 25 % or less.

References and notes

  1. ^ MIT lecture
  2. ^ Usually the figure 75 % is found to be acceptable.
  3. ^ Sometimes 12.5 % is used as the residual carrier

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