R&S FSL Cable TV Measurements (Option K20)
1300.2519.12 4.211 E-11
Signal level
The signal level represents the expected RMS (digital TV) or peak (analog TV) channel power. This
power is only valid for one single channel, which is the current measurement channel. The signal level
is not identical to the level of the signal fed into the R&S FSL's RF input. This signal may consist of
many TV channels and hence have a much higher total power than the current channel to be
investigated.
In most measurements the signal level determines the upper horizontal display line. The signal level
value is related to the reference level by a constant offset. The reference level value is the maximum
value the AD converter can handle without distortion of the measured value. The signal level can either
be entered via the Signal Lvl key or by using the auto scale function (e.g. Auto Range softkey in the
Tilt measurement).
Attenuation adjustment
To get valid measurement results it is very important to set the R&S FSL's hardware, i.e. the attenuator
and the preamplifier, in a reasonable way. This can either be done manually or by relying on the Adjust
Attenuation softkey. Apart from the hardware setting, the displayed grid depends on the signal level of
the signal present in the current measurement channel.
Preamplifier
IF Stages...
RF Overload
Detector
IF Overload
Detector
Attenuator ADC
RF Input
Fig. 4-23 R&S FSL hardware overview (only relevant parts for attenuation adjustment)
Fig. 4-23 shows the R&S FSL's hardware. It only comprises blocks that effect the level settings
described in this section. The signal from the device under test, e.g. a single cable TV transmitter or a
test point of a cable TV network with many channels, is plugged into the R&S FSL's RF input. The level
of the signal is reduced by the electronic attenuator or increased by the preamplifier (option B22
required). Then the signal is fed into the first mixer.
The level of this signal is typically referred to as mixer level. The mixer level is a very crucial parameter.
If the mixer level is too high the mixer will be overloaded. As a result, non–linear effects are produced,
e.g. intermodulation products, and the intermodulation products (e.g. CTB or CSO) of the device under
test can not be distinguished from those of the R&S FSL's mixer. To reveal this, the R&S FSL's
hardware has two overload detectors. The RF overload detector reports a mixer overload and the IF
overload detector indicates an overload of the analog to digital converter (ADC). The combination of the
results of both detectors control the overload message OVL displayed on the R&S FSL's screen.
Increasing the attenuation reduces the signal to noise ratio but also reduces the mixer's non–linear
effects. For cable TV measurements such as CTB, CSO, CCDF, and APD that measure non–linearities,
it is recommended to chose a higher attenuation than for other measurements.
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