Over the past few years satellite systems have
been replacing the traditional FM or FSK
transmission systems with more complex digital
modulations formats such as BPSK and QPSK.
These digital forms of modulation enable the
satellites to deliver more information in the
same satellite capacity that was used to deliver
the older analogue formats and with an
improvement in the quality of the delivered
signal. To say it another way, digital modulated
signals can deliver a greater amount of data,
with fewer errors, and using less of the
satellites capacity than previous analog
modulation systems.
In order to take full advantage of the benefits of
the more efficient digital modulation systems the
LNB used in the receiver terminal must be
matched to the digital signal characteristics.
From a technical perspective there are more
than fifty individual parameters that should be
considered when making an LNB selection. RF
leakage, rejection of transmit signals, in-band
spurious performance, out-of-band spurious
performance, long term aging effects, vibration
effects, corrosion resistance, connector types,
intermodulation performance, dynamic range
considerations, environmental effects, reliability
concerns and the list goes on. There are
however a few key specifications that need to
be addressed before getting into the finer
details of an LNB.
Noise Figure
The noise figure of the LNB is a measurement of
how sensitive the LNB is or how much noise the
LNB will add to the signal you may be intending
to receive. The lower the noise figure of the
LNB the better the LNB will be able to receive
weaker signals. For a C-band LNB that cover the
frequency range of 3.4 to 4.2 GHz the noise
figure is expressed in Kelvin or K. Kelvin is a
scientific unit of measurement that relates
absolute “ZERO” or
the level of molecular activity. Many people
refer to degrees Kelvin but that is technically
incorrect. Kelvin is a unit of mea- surement on
it’s own and is not related to degrees on it’s
own. “0” Kelvin represents the level of no
molecular activity or no noise in a system or
substance. A very good number for a C-band
LNB Oscillator Type
Frequency
Stability
Application
DRO ± 1.0 MHz to ±
150 KHz
Frequency stability
Internal
Reference PLL ± 150 KHz
to ± 5 KHz
External
Reference PLL 0
± 1 KHz
Broadcast Television
Wideband Data Broadcast
SCPC audio
New gathering VSAT
Satellite paging
Narrowband data
Table Of LNB applications and typical
frequency stability
LNB would be 15 Kelvin a more typical number
30 Kelvin.
Unlike C-band, the noise figure of Ku-band
(10.7 to 12.7 GHz) LNBs are expressed in
decibels or “dB.” It is possible to convert
between Kelvin and dB using a set of formulas
for comparison purposes if need be. A good
point of reference however is 35 Kelvin = 0.5
dB. A very good noise figure for a Ku-band LNB
would be 0.6 dB but a more typical value would
be 0.8 dB.
Gain The gain of an LNB is amount the LNB will
amplify the input signal which is expressed in
dB. The input signal is very weak when it
arrives at the receiving antenna and must be
amplified many time before it can be
transported down a coaxial cable. If the signal is
not amplified the signal would be absorbed by
the losses in the coaxial cable and never
reaches the receiver. When selecting an LNB for
a digital system it is important that the gain
does not change significantly with temperature
or over the received frequency range as digital
systems are much more sensitive to these
changes than previous analogue systems.
Digital systems typically require an LNB gain to
be 55 dB to 65 dB under all conditions. Gain
flatness across a
500 or 800 MHz band should be better than ±5.0
dB and less than ±1.0 dB in 27 MHz segments.
Variations greater than this can introduce gain
distortion onto the incoming signals resulting in
reduced receiver performance.
Local Oscillator Frequency Stability
There are three main types of frequency
conversion oscillators used in LNBs:
1. Dielectric Resonant Oscillator (DRO) Types –
The LNBs conversion oscillator frequency is
determined by a free running oscillator
whose frequency determining element is a
piece of feroceramic material refered to as a
puck.
2. Phase Locked Loop (PLL)Types – The LNBs
conversion oscillator frequency is
determined by an internal located
temperature compensated crystal oscillator
and a digital phase locking circuit.
External Referenced Phase Locked Types – The
LNBs conversion oscillators frequency is
determined by a reference oscillator located
outside of the LNB and is usually provide over
the center conductor of the coaxial cable that
connects the LNB to the receiver. It is usually
the responsibility of the satellite receiver to
provide this reference signal to the LNB. The
reference frequency in most cases is 10MHz.
Different types and bandwidths of digital signals
will require LNBs with different frequency
stability in order to provide optimum receiver
performance.
A wideband signal such as an MPEG II television
broadcast will require an LNB with low frequency
selectivity because the transmitted signal
occupies quite a wide bandwidth and the
receiver tuning can be wider. A narrow band
SCPC radio broadcast uses a very narrow signal
and will require a high stability PLL type so that
the receiver is able to track the signal.
LNB PHASE NOISE PERFORMANCE
The phase noise specification of an LNB is an
indication of the level of noise introduced on to
the received signal at various frequency
distances from the converted carrier. This noise
is generated by the conversion oscillator within
the LNB and is a direct function of the quality of
that oscillator. The phase noise specification of
an LNB is defined at 100Hz,
1.0kHz, 10kHz, 100kHz and 1.0MHz distances
from the center frequency of the converted
frequency.
In a digital system the bit error rate (BER) of
the receiver will be directly affected by the
level of the phase noise in the received signal.
The higher the phase noise level the more
errors there will be in the received signal.
SUSCEPTIBILITY TO MICROPHONICS
When an LNB is installed on an antenna it will
be subjected to environmental factors such as
wind, rain, and hail. Rain or hail hitting the
LNB will cause small disturbances in the
electrical performance of the LNB. Wind will
move or vibrate the antenna which causes a
similar effect. These disturbances are then
superimposed or modulated onto the incoming
signal.
It is not uncommon for these disturbances to
distort the incoming signal such that the
incoming signal cannot be received. The local
oscillator in the LNB is the circuit most
commonly affected by these disturbances. Great
care must be taken in the mechanical and
electrical design of an LNB to minimize this
effect. In the early days of radio, unwanted
vibrations applied to the receiving equipment
would show up in the demodulated audio as
sounds, and were thus referred to as
Microphonics because they behaved in much the
same way as a microphone would. Today this
effect is still referred to as Microphonics.
There are no standards or units of measurement
associated with evaluating an LNB’s sensitivity to
Microphonics. Some people use simulated rain
drops, some use a specialized tool they have
developed, some use very elaborate shock table
setups; while others just use a screw driver to
tap on the LNB to check how the received signal
is affected. The method used is dictated by the
individual system designer.
INPUT VSWR
VSWR is an abbreviation for Voltage Standing
Wave Ratio which can also be referred to as
Return Loss. The technical description of VSWR
is the ratio of incident voltage or primary wave
of voltage present on a transmission line or
waveguide versus any reflected voltage on that
line that may be present as a result of a
mismatch condition.
Offset
From
Carrier
Analogue
DRO
Digital
DRO
PLL
PLL
Internal
External
Reference
Reference
100 Hz Not
Specified
Not
Specified -70 dBc/Hz -65 dBc
1.0
KHz - 55
dBc/Hz -65
dBc/Hz -75 dBc/Hz -75 dBc
10 KHz - 70
dBc/Hz
-80
dBc/Hz -80 dBc/Hz -85 dBc
100
KHz
- 85
dBc/Hz
-100
dBc/Hz -85 dBc/Hz -95 dBc
1.0
MHz
- 95
dBc/Hz
- 100
dBc/Hz -95 dBc/Hz -105 dB
Table Of Typical phase noise
specifications for different types of
Ku-band LNBs.
In a perfect situation where the transmission
line (feed) is absolutely matched to the load
(LNB) there would be no reflected voltage and
the VSWR would be stated as being 1:1 or a
perfect match. As with most things this is not
the case in the real world. Variations of
electrical and physical parameters on the
transmission line and the load are seldom
perfectly matched. This mismatch will result in
some of the energy contained in the primary
wave (the received signal) being reflected back
from the load (LNB) and lost. To make things
worse the reflected wave will also interfere with
the incident (incoming) wave causing the signal
to be reduced as well.
It is most important to maintain a good match
between the feed and the LNB in order to
ensure that the maximum amount of signal is
transferred to the LNB. The chart below shows
approximate effects of VSWR on measured noise
figures or temperatures of an LNB. An LNB with
a measured C band noise figure (NF) of 30 K is
used as an example.
As you can see, the effect of VSWR on the noise
figure of an LNB can be substantial. Therefore it
is most important to consider the effects of
VSWR when making your LNB selection.
SOME EXAMPLES OF LNB APPLICATIONS
There are many applications where selecting the
correct LNB will make the difference between a
system operating to it’s full potential and
providing far less then satisfactory
performance. Listed below are examples of some
applications and the types of LNB which will
provide the best performance:
Satellite digital paging networks require a
high stability PLL or even External
Reference PLL LNB such as the Norsat 1000
or 3000 series LNB.
MPEG II digital video applications require
high stability DRO LNBs such as the Norsat
4000 series.
VSAT and Point of Sale (POS) systems may
use a DRO LNB but most users prefer a PLL
to ensure the highest possible system
reliability.
Radio and TV broadcast stations use PLL
types to ensure the most reliable
performance of their station.
Satellite News Gathering (SNG) trucks use
Norsat 1000 Series PLL LNBs for the most
reliable performance in the worst
conditions.
by ROTMANLUV 2023-07-08, 17:34
