Group
Delay Variation
Group
delay refers to the time shift experienced by signals of differing frequencies.
If high frequency signal components were delayed by significant amounts of time
compared with low frequency signal components, then the demodulator in your
receiver is going to have a hard time.
It's
hard to think of a good analogy to this, perhaps this one will do.
Imagine
a set of traffic lights at a road junction, but the speed of propagation
of the light is dependent on the light's colour. Imagine that the speed
of propagation of the green light is normal, but the red light is only
propagated at a speed of one metre per second.
Sitting
in your car at the lights you would see the green light go off, but
the illumination from the red light would take a few seconds to reach
you, so you would see no light at all for a few seconds.
After
a while the red light would arrive and all seem to be well again.
But
then, when the red light goes off and the red green light comes on,
you would see both lights at once for a while - until all of the red
light still on its way to you had passed you by.
Pedestrians
and car drivers, trying to demodulate the on/off digital signals from
the traffic lights, would quite often get it all wrong - there would
certainly be a lot of accidents at the junction!
But
reduce the difference in light's propagation speeds, or the group
delay, and the number of accidents will reduce too.
I
have never seen a lnb spec that even mentions group delay. If the gain flatness
criterion is met, we can but hope that the group delay one will be met too.
Multi-carrier
intermodulation ratio
Earlier
I said that it was possible that a big dish looking at a high power cluster
of satellites like Hotbird or Astra1 could cause the receiver to be overloaded
with signals. One effect of this overload would be for the transponders to interfere
with each other inside the lnb's amplifier, resulting in signals that weren't
in the original broadcasts magically appearing at the lnb output as distortion
products. This would be fine if there was no legitimate signals at the point
where the interference pops up, because you could just ignore these 'ghost signals'.
But this isn't going to happen on a cluster like Hotbird, as there is going
to be a legitimate signal on every frequency in the KU band. If these
spurious signals are too large then they are going to severely spoil your fun.
The Eutelsat specification recommends that such signals should be at least 35dB
down when compared with the wanted signals.
Once
again, I have never seen a lnb specification that quotes this parameter. Some
lnb specifications do mention such things as the 'third order intercept point'
or the '1 dB gain compression point', which at least shows that the manufacturer
has considered the problems of handling high power signals. If you see such
details in the specification then high numbers are good to see, look for a third
order intercept point of about 15 dBm, or a 1dB gain compression point of
about 5 dBm.
If
these values are not quoted, then this criterion has a better chance of being
met in a lower gain lnb. Yet another reason not to look for a super high gain
lnb.
Local
Oscillator intermodulation products
There
is another source of interfering signals built right into the lnb itself - the
local oscillator. Using a properly balanced mixer, unwanted intermodulation
products should be at least 60dB down. There is, of course a wanted intermodulation
product too, that’s the frequency-translated signal fed out to the coaxial output
connector!
Local
oscillator phase noise
Fact:
digital satellite signals are phase modulated.
What
a lnb should do is frequency shift the incoming signals down to the IF output
frequency and amplify them without altering the vital phase modulation in the
signal.
The
worst thing your lnb could possibly do is to add further phase modulation to
the signals. It doesn’t matter how big your dish is, or how'
quiet' your lnb is if you let the local oscillator in the lnb wantonly add its
own phase modulation and 'scribble' over your precious signals.
The
phase noise values suggested by Eutelsat here are the recommended phase noise
levels for wideband MCPC (multi-channel per carrier) signals. If you are looking
for SCPC (single channel per carrier) or other narrow bandwidth signals, then
you will be better off looking for a specification that is better than that
shown.
It
is remarkable how few lnb manufacturers mention phase noise. Some, not very
helpfully, say that it is low. Phase noise is one of the most important parameters
by which we can judge a digital lnb.
By
comparison, the Eutelsat spec makes no mention of the noise factor of the lnb,
held to be so important by advertisers, dealers, magazines, and the man in the
pub!
Hopefully
a low-noise lnb will probably have a low phase noise too. But a low noise factor
lnb, by itself, is no guarantee of good reception. Indeed, there comes a point
where the further reduction of white noise within the lnb is pointless, because
the noise picked up by the antenna and feedhorn will be many times more significant
than the noise contribution of the lnb.
Its
time for another analogy!
Imagine
that you have slight hearing problems and have to wear a hearing aid,
it's a good hearing aid but it makes a hissing noise in your ear. Despite
this, it is good enough to understand people talking up to 20 metres away
from you in a quiet room.
One
day you go to a bar with friends, there are a lot of other people talking,
laughing, and there is music playing too. You find it hard to understand
your friends under these environmental conditions, so you buy a more expensive
less hissy, hearing aid and can now understand people 100 metres away
in the quiet room. But in the bar you still can't understand your friend's
conversations.
This
is hardly surprising, as the noise in the bar is much greater than the
hiss made by the hearing aid.
You
later notice that your friends are also having trouble hearing each other
and occasionally resort to cupping their hand behind their ear to hear
better. They are making their ears more effective by making their hearing
more directional and sensitive.
The
moral of this tale is that a bigger, more directional antenna is often
a better solution than a quieter lnb!
Microwave
noise figures are a notoriously difficult thing to measure, because there
is no such thing as a noise figure meter to measure them with. All noise factor
measurements are inferred from running an experiment, which attempts to distinguish
between internally generated noise and noise naturally present in the testing
environment and the measuring instruments. When people start worrying about
fractions of a dB it is time to start querying how these measurements are
made and how repeatable the test results are.
Spurious
components at LNB output
Just
in case we haven't covered all the possible sources of interfering signals,
there's another catch-all category - simply labelled spurious components. Once
again, the bigger the negative number the better things will be.
LNB
isolation
This parameter applies to
multiple lnb set-ups, including monoblock lnbs, and specifies the maximum amount
of signal that should leak through from one lnb to the other. The value shown
in the specification seems a very low target to aim for compared with some of
the other specifications here. Serious hobbyists, looking for the ultimate
in reception ability, will probably not be using such a set-up anyway.
The
other stuff
This specifies the connectors,
voltages currents and interfaces to the outside world - it also, not unsurprisingly,
incorporates a further recommendation of the DiSeQc standards.
The only mysterious feature
here is the lnb output return loss. This loss should be as high as possible,
and is a measurement of by how much spurious signals coming up the lnb
coaxial cable and re-entering the lnb are reduced before re-emerging from the
socket. These signals can originate from the receiver and/or from signal reflections
caused by discontinuities in the coaxial cable run. These rogue signals can
cause no end of bother and can even be responsible for the mysterious inability
to receive certain transponders from a satellite. A high return loss will help
reduce the level of these unwanted signals.
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