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Fig. 2: Setup for testing cable TV receivers in line with SCTE 40.
Linearity of amplifiers
Testing amplifiers for cable networks presents similar
challenges for test equipment. The challenge here is to
generate a signal as close to ideal as possible. It must
be ensured that any detrimental effect that the amplifier
might have on the signal is kept as small as possible. Two
types of measurements are performed for this: The first
type requires feeding a full channel load to the amplifier.
At the output, a test receiver determines the bit error ratio
(BER). The deterioration of the BER provides a measure
of the extent to which the amplifier distorts the signal.
The second type of measurement test intermodulation
behaviour. Again a full load is again fed to the amplifier. In
this case the channel that is to be measured remains free.
The input signal is a full load with a gap of one channel (Fig.
5). The intermodulation is measured at the amplifier output
in the unassigned channel. When the full load that is being
applied at the amplifier’s input consists exclusively of analog
TV signals or CW carriers, aggregations of discrete noise
Since the second and third order products dominate, these
noise lines are referred to as composite second-order /
composite triple-beat (CSO/CTB).
Fig. 3: Block diagram for an FPGA-based multichannel signal generator for
simulating full channel load.
Fig. 4: Complex cable network loading with 7 MHz channels and
8 MHz channels, PAL and DVB-C.
In networks with digital QAM signals this parameter
is increasingly losing its significance. As with all digital
modulation signals, a QAM signal is noise-like. In a
completely digital network intermodulation expresses
itself as an increase in the noise in the measurement
channel which is measured with the aid of a spectrum
analyser. A cable TV amplifier must meet stringent linearity
requirements. In order to measure the intermodulations the
signal bearing a full channel load can only have very low
intermodulations of its own. For this reason, it is necessary
to reduce the signal source’s intermodulations in the useful
channel by applying a bandstop filter at the amplifier’s input
Using generators to simulate a cable network
When performing simulations the number of channels
within the network presents the largest challenge. State-
of-the-art signal generators that are capable of producing
many signals simultaneously are able to significantly reduce
the amount of equipment required. That saves space in
the lab, and lowers the cost of the devices. The generated
signals are highly precise and provide reliable simulations.
Signal generators such as the CLG help substantially
reduce risks when developing cable TV components. With
them, engineers can perform development projects quickly
Fig. 5: Full channel load with a one-channel gap
Cable Load Generator
... Cable TV Amplifier
(D.U.T.) Spectrum Analyzer /
TV Test Receiver
multiple QAM signals with deep notch
... additional noise introduced by DUT
Fig 6: Setup for testing the linearity of cable TV
TV-BAY MAGAZINE: ISSUE 79 JULY 2013 | 51