BS EN 60728-6-2011 pdf download.Cable networks for television signals, sound signals and interactive services
Part 6: Optical equipment.
4.13.2 EquIpment required
For this test method the following pieces of equipment are needed.
a) Signal generators covering the appropriate vision carrier frequencies as listed in Annex C of lEG 60728-3. all having the required modulation facilities, and linearity at the depth of modulation to be used.
NOTE It is recommended that the modulation frequency approximate the line scan frequency of the TV signals In order to include effects which may be caused by the low frequency circuits (e.g. decoupling) In the equipment to be tested. The modulation frequency should not be a multiple of the power supply frequency. Any symmetrical modulation waveform (excluding pulse modulation) may be used providing the same signal generator is used for both calibration and measurement, and the modulation depth and waveform remain the same
b) A modulating voltage generator of sufficient output to provide common modulation of the signal generators in item a).
c) A combiner, matching device, attenuators, filters. etc. to obtain the correct signal levels, matching and reduction of spurious signals.
d) A spectrum analyzer with I kHz IF bandwidth and 10 Hz video bandwidth capability.
e) A bandpass filter for each channel to be tested or a tuneable bandpass filter. This filter shall attenuate the other channels present on the system to be tested sufficiently to ensure that the products generated by non-linearity in the spectrum analyzer itself do not contribute significantly to the crossmoduiation products to be measured. The passband of this filter shall be flat at least to within 1 dB over the frequency range of interest, and shall be well-matched over the complete frequency band. If necessary, a fixed attenuator shall be connected to the input of the filter.
f) An optical receiver with high linearity.
g) A length of fibre for connecting the transmitter to the receiver.
h) If the optical output power of the transmitter is higher than the specified input power of the receiver, an optical attenuator shall be used to reduce the power.
4.13.3 Procedure
For the measurement proceed as follows.
a) Set the supply voltage(s) and any control input signal(s) to the specified value(s).
b) Connect the equipment as shown in Figure 12.
p) Repeat steps 9) to 0) of this procedure, each time selecting a different wanted signal. until all channels used in this test have been selected.
q) To make sure that the distortion of the receiver can be neglected, a second measurement shall be carried out with a different attenuation between the optical transmitter and the optical receiver. If the result changes, it Indicates that the receiver distortion is too high.
r) The worst case maximum output level giving the required signal to composite total crossmodulation ratio shall be noted for publication.
4.13.4 Potential sources of error
The figure measured is the composite crossmodulation of the whole optical system. The influence of the optical receiver can be neglected only if its crossmodulation is much better than that of the transmitter, but there is no direct way of measuring the crossmodulation of an optical receiver. The only way to make sure that the receiver has no influence on the result is to repeat the measurement several times with different optical levels at the receiver’s Input.
4.14 ReceIver Intermodulation
4.14.1 Purpose
This method is applicable to the measurement of the carrier to second and third-order intermodulation products and triple beats produced in optical receivers with high linearity. The method described is not applicable to coherent receivers. The intermodulation shall be expressed in dB.
4.14.2 Equipment required
For this test method the following pieces of equipment are needed.
a) Two signal generators for second and third-order intermodulation and three signal generators for triple beats covering the frequencies at which the tests are to be carried out.
b) Two transmitters with similar optical output power but slightly different wavelengths. The difference of the frequencies of the emitted light shall be greater than the bandwidth of the receiver to be tested.
C) An optical coupler with similar loss in both paths.
d) Two variable optical attenuators with a range great enough to cover the range of the optical input power of the receiver to be tested.
e) A variable electrical attenuator with a range greater than the signal-to-intermodulation ratio expected.
1) A selective voltmeter covering the frequency range of the receiver to be tested.BS EN 60728-6-2011 pdf download.