BETS-4 - Technical Standards and Requirements for Television Broadcasting Transmitters - Spectrum Management and Telecommunications

Spectrum Management and Telecommunications

2.7 Non-Linear Distortion

2.7.1 Audio Harmonic Distortion

2.7.1.1 Definition

This form of non-linear distortion is expressed in terms of the output voltage of the harmonics produced from a sine wave input signal.

Total harmonic distortion (THD) is defined as the ratio of the RMS output voltage of the total harmonics to the total RMS output voltage.

Harmonic distortion of audio modulating frequency signals in the range 30 Hz to 15 kHz is specified within the 15 kHz channel and above the 15 kHz channel. The former produces distortion within the main audio channel and the latter produces spurious components above the main audio channel.

2.7.1.2 Method of Measurement

In-channel THD measurements require the use of an aural demodulator with an inherent THD of less than 0.1 %, which is fed from an RF monitoring connection in the aural transmitter output transmission line. A distortion analyser with THD measurement capability within a 30 Hz to 15 kHz range is required.

A 75 μs de-emphasis network in the demodulator is used when testing the audio input which contains a 75 μs pre-emphasis.

The audio output of the aural demodulator is connected through a 30 kHz low pass filter with a minimum slope of 12 dB/octave to the input of a distortion analyser of low residual THD, 0.05 % or less.

Out-of-channel harmonic measurements require the use of a spectrum analyser to observe the output of a demodulator exhibiting flat response from 30 Hz to 120 kHz as specified in paragraph 2.2.4.2.

The audio input signal to the transmitter shall be supplied from a source having less than 0.05 % THD.

2.7.1.3 Standard

2.7.1.3.1 In-Channel

For modulating signals from 30 Hz to 7500 Hz, the total harmonic distortion, including harmonics up to 15 kHz, shall not exceed 0.5 % with deviations up to ±25 kHz.

2.7.1.3.2 Out-of-Channel

For modulating signals from 30 Hz to 15 kHz any individual harmonic component measured in the frequency range 15 kHz to 120 kHz shall not exceed 0.2% with carrier deviations up to ±25 kHz.

These requirements apply to both the audio input and the composite input modulation terminals.

2.7.2 Audio Difference Frequency Distortion

2.7.2.1 Definition

This form of non-linear distortion is expressed in terms of the output voltage of the difference frequency signal produced when two equal amplitude high frequency sine wave signals are passed through a device containing non-linearities.

For this standard, difference frequency distortion is defined as the ratio of the RMS voltage of the difference frequency signal to the RMS voltage of one of the high frequency output signals.

2.7.2.2 Method of Measurement

Two equal amplitude sine wave signals, a 14 kHz signal and a 15 kHz signal are applied to the audio input through a suitable mixing network either from separate audio oscillators or from a combined unit designed for difference frequency measurement. Each signal shall contain less than 0.05 % total harmonic distortion.

The measurement requires the use of a low frequency spectrum analyser and an aural demodulator with an inherent difference frequency distortion of less than 0.1% which is fed from an RF monitoring connection at the aural transmitter output.

Measurements shall be made with ±25 kHz deviation at inputs to the audio and composite terminals.

The demodulator output response shall be ±0.5 dB from 30 Hz to 30 kHz (i.e, de-emphasis shall not be used).

The level of the difference component shall be measured with respect to either of the two input signals. For this measurement, pre-and de-emphasis shall not be used.

2.7.2.3 Standard

Audio difference frequency distortion shall not exceed 1.0% with a deviation of ±25 kHz. This requirement applies to the audio input and the composite input modulation terminals.

2.7.3 Composite Harmonic and Intermodulation Distortion

2.7.3.1 Definition

This form of multichannel non-linear distortion spurious products, including harmonics, sum and difference, and other intermodulation products result from amplitude and phase non-linearities in the aural FM transmission system.

Equivalent deviation of spurious levels in the appropriate frequency bands is given by the ratio of spurious product RMS voltage to a standard reference level RMS voltage.

2.7.3.2 Method of Measurement

Four low distortion (0.05% THD) signals are combined in a summing circuit, which minimizes signal interaction, to drive the composite input terminals at the proper levels to achieve specified deviations.

The aural carrier shall be demodulated with a suitable wideband linear FM discriminator providing a minimum baseband bandwidth of 30 Hz to 120 kHz (preferably 200 kHz). The baseband output shall be displayed on a suitable spectrum analyser, and the spurious signal levels measured referenced to the Fl, 12 kHz signal.

2.7.3.3 Standard

Multichannel harmonic and intermodulation products are measured with the following channel deviations and frequencies:

Channel	Channel Frequency	Deviation
TOTAL:		±75 kHz
F₁	12 kHz	±27 kHz
F₂	31.5 kHz	±30 kHz
F₃	78.6 kHz	±15 kHz
F₄	102.3 kHz	±3 kHz

Maximum spurious product levels, referenced to ±25 kHz deviation shall be as follows:

Frequency Band	I.M.D.	Deviation
30 Hz to 94 kHz	-54 dB	50 Hz
94 kHz to 120 kHz	-48 dB	100 Hz
Above 120 kHz	-40 dB	250 Hz

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3. RF Performance Standards

3.1 RF Input Impedance - Translator Standard

The standard RF input impedance of the television translator shall be 50 or 75 ohms unbalanced. The return loss over the entire input channel shall not be less than 18 dB. This value shall be maintained for input signal levels of 1 mV ±16 dB.

3.2 Input Noise Figure - Translator

3.2.1 Definition

The noise figure is a measure of the noise contribution of the television translator to the overall video and audio signal to noise ratio.

3.2.2 Method of Measurement

The television translator shall be fed with a standard test signal of 1 mV amplitude and the gain control adjusted to obtain rated power output. The input signal shall then be replaced by a signal from a noise figure metre.

3.2.3 Standard

The noise figure at 1 mV input signal level shall be as follows:

for channels 2 to 13

7 dB maximum

for channels 14 to 69

9 dB maximum

3.3 Automatic Gain Control Performance - Translator

3.3.1 Definition

The AGC performance is the ability of a television translator to maintain a given output level while being fed an input signal of varying levels.

3.3.2 Method of Measurement

The translator shall be fed with the standard test input signal of 1 mV (0 dBmV). Vary the level between 0 dBmV and ±16 dBmV. Observe the power output variation.

3.3.3 Standard

The peak power output level of the television translator shall remain within ±0.5 dB of its rating.

3.4 Visual to Aural Cross Modulation - Translator

3.4.1 Definition

The visual to aural cross-modulation is the extent to which a signal modulating the visual carrier also amplitude modulates the aural carrier when both signals are simultaneously passed through the amplifier stages of the television translator.

3.4.2 Method of Measurement

The visual carrier shall be modulated with the standard test signal. The aural carrier shall be unmodulated. An amplitude modulation monitor shall be used to measure the percent amplitude modulation on the aural carrier.

3.4.3 Standard

The visual to aural cross-modulation shall not exceed 10% peak, i.e. the video information shall not amplitude modulate the aural carrier more than 10% peak in a 15 kHz bandwidth.

3.5 Aural to Visual Cross Modulation - Translator

3.5.1 Definition

The aural to aural cross-modulation is the extent to which the audio information modulating the aural carrier also modulates the visual carrier.

3.5.2 Method of Measurement

The visual carrier shall be modulated with a staircase signal. The aural carrier shall be unmodulated with 4000 Hz signal at a deviation of ±25 kHz. Observe the detected video signal on a waveform monitor at a field rate. Measure the presence of audio on the staircase steps.