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Want to know what lnb specifications
mean. Well read on!
An
lnb, or low-noise block downconverter, picks up the signals collected
by your satellite antenna, amplifies them, and converts them to a lower frequency
more suited to transmission via a co-axial cable to your receiver's input circuits.
Without this frequency down conversion and amplification the signal would have
to arrive at your receiver by means of microwave waveguides, a veritable plumber's
nightmare of pipework, which would do little to improve your domestic décor
and harmony.
The lnb should accomplish
this without adding too many spurious signals of its own.
The following table shows
the recommended specifications for lnbs, according to the Eutelsat web site,
for users of the Hotbird satellite cluster. The full document is available as
a pdf file here http://www.eutelsat.com/fr/satellites/pdf/dealers/info_installer_hot_bird_dvbs.pdf
RF frequency range:
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10.70 - 12.75 GHz
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Lower band range
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10.70 - 11.70 GHz
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Lower band local oscillator
frequency
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9.75 GHz ± 5 MHz
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Upper band range
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11.70 - 12.75 GHz
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Upper band local oscillator
frequency
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10.6 GHz ± 5 MHz
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IF frequency range
(minimum)
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950 - 2150 MHz
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Small signal gain
over RF frequency range
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40dB Gain 65dB
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Max. amplitude variation
over IF frequency range
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2.0 dB (within any
36 MHz bandwidth)
1.5 dB (within any
27 MHz bandwidth)
8.0 dB (over entire
IF range - DTH)
5.0 dB (over entire
IF range - SMATV)
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Max. group delay variation
over IF band
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20 ns (within any
36 MHz bandwidth)
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Multi-carrier intermodulation
ratio
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>=35 dB.
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Local oscillator intermodulation
products at lnb output
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<
-60 dBc
(in the frequency
band 950 - 2150 MHz)
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Local oscillator phase
noise
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|
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-50 dBc (1 kHz)
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-75 dBc (10 kHz)
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-95 dBc (100 kHz)
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Spurious components
at LNB output
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-60 dBc (in frequency
band fc ± 120 kHz)
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LNB isolation
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> 26 dB
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LNB input RF interface:
(optional, if waveguide
is employed):
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PBR 120 (rectangular),
or C120 (circular) with gasket groove
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Antenna feed RF interface:
(optional, if waveguide
is employed):
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UBR 120 (rectangular),
or C120 (circular) without gasket groove
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Output IF connector
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IEC 169-24 type F,
female
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IF output characteristic
impedance:
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75 Ohm
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LNB output return
loss (this is the recommended value in the ETSI BSS and FSS specifications)
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>=8 dB (over the frequency
range 950 - 2150 MHz)
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Recommended switching
control signals
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DiSEqC2.0 e
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DC supply voltage (assuming conventional
switching method)
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+11.5 to +19 V
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maximum current (assuming conventional
switching method)
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250 mA (single-band) 300 mA (dual-band)
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But what does it all
mean?
The frequency range
stuff
This
is all pretty self-evident. The lnb has to cover all of the frequencies that
the satellite broadcasts on, so a coverage of 10.70 - 12.75 GHz. is essential
if you are going to be able to see all the KU band programming. The local oscillator
frequency is shown with a tolerance of 5MHz either way. This tolerance is fine
for Hotbird, where all of the transponders are wideband ones. Searchers for
SCPC narrow bandwidth channels might be better off with a tighter frequency
tolerance.
The
local oscillator frequencies of 9.75 and 10.6 GHz are now acknowledged standards
for universal lnbs, and all digital satellite receivers with the DVB logo should
support them. Some (usually the more expensive) receivers support other local
oscillator frequencies, so strict adherence to these frequency values may not
be absolutely essential.
The
IF frequency range is the range of frequencies that your receiver should tune
to if you are going to be able to see all the frequency ranges covered by a
universal lnb.
The lnb amplifier details.
This bit describes how well
the lnb actually works, and requires a bit of interpretation.
Gain
The
lnb gain tells us how much the incoming signal is amplified before being sent
off down the coaxial cable to the receiver. The range of gain specified is between
40dB and 65dB (somewhere between 10,000 and 4,000,000 times the incoming signal
power). At first sight, the highest gain you can get would be the obvious thing
to look for; but that is not the only criterion when it comes to lnbs.
When
you have a big dish looking at a high power cluster of satellites like Hotbird
or Astra1 the gain can be so high that the receiver is overloaded with signals.
Yes, it is possible to get too much signal!
Even
if the receiver itself can handle a massive diet of signal, there can be problems
within the lnb itself when large amounts of amplification are employed. This
leads to the generation of spurious signals and distortion products, in much
the same way as turning up the volume on a transistor radio does. This distortion
will interfere with the reception of your signals - there's more on this later
further down the spec.
So
unless you have specific reasons to want an ultra high gain lnb, (perhaps you
are looking for a weak carrier and you can guarantee the absence of unwanted
high power signals on the same satellite) then look for a gain around 50-60dB.
Gain
Flatness
To let the demodulator in
the receiver work effectively, the gain at all frequencies in the within any
broadcast channel should be the same. This isn't a very difficult requirement
to meet, except perhaps at the edges of the band, as long as the lnb is constructed
properly. Notice that the DTH (direct to home) requirements for the flatness
across the entire band are less severe than for the SMATV (satellite
master antenna television- e.g. distribution around a block of flats), where
we might expect additional splitters, switches and amplifiers in the signal
path.
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