| Connection from the indoor equipment to the
outdoor equipment at the antenna normally involves two inter-facility (IFL)
cables.
For L band, 75 Ω impedance cables with F connectors are common for
consumer grade equipment. For L band BUCs, 50 Ω cable is sometimes used
and with 70 MHz systems 50 Ω cables are normal.
There is more dB attenuation loss on L band cables, particularly on very
long lengths, but you can compensate with slope amps. You can
alternatively use fiber optic to transmit either 70 MHz or L band.
Distances of several km are possible.
When clients use standard LMR400 and RG6 cables we suggest that they
limit their cable runs to less than 60 meters in order to minimize cable
losses.
For both BUCs and LNBs there is often a DC supply via the cable. This
has implications for earth (ground) loops and corrosion, cables with
very thin centre conductors or high DC resistance film screen/outers are
not suitable.
With L-Band, a wider range of frequencies can cause interference, so
be careful if you are locate near high power transmitters. At 70
MHz watch out for local FM radio stations at 88-108 MHz to avoid these
problems good screening is essential.
The best cables for any cross site application are foam filled Heliax,
with solid copper outers, but cost rules this out for consumer
installations. The more practical cable is LMR400 for the transmit
portion and something similar to RG6 on the receive side.
If you are considering multi-carrier operation with a BUC make sure
it is suitable. You don't want unexpected interaction between the
carriers. As an example it was noted that the DC power current taken by
a 2 watt BUC varied according to the carrier on/off. Operation of
multiple carriers through this type of BUC would be doubtful as TDMA
interruptions of one carrier may well cause amplitude/phase/frequency
hits on the other.
A client recently asked why BUC's and LNB's use a Local Oscillator
(LO) frequency.
The Local Oscillator (LO) frequency is what drives the mixer. The
output frequency is different from the input frequency. The difference
is the LO frequency. In the case of a Ku band BUC, the LO is normally
13.05 GHz so an L band input at 1 GHz comes out at 14.050 GHz. In the
case of a C band BUC, LO frequencies of 4.9 and 7.375 GHz are used. In
the case of 7.375 GHz the output is inverted so you need to set the
modem modulation for inverted spectrum.
Some BUCs are manufactured with different LO frequencies and so be
sure to check with the BUC supplier to be sure that you have the correct
value for the BUC that you are installing.
In the case of BUCs, the LO frequencies are stabilised by a 10 or 50
MHz reference which you need to transmit up through the cable to the BUC.
This reference frequency must be accurate and with low phase noise.
Check with a spectrum analyzer that you are not sending spurious low
frequencies into the cable (between say 10 kHz and 2 MHz), for example
from a noisy switch mode power supply or voltage regulator that is
faulty and oscillating. This applies to both LNBs as well as BUCs.
If you have very long cable run and erratic BUC current, consider
putting the DC supply close to the BUC. Chirp on the front end of TDMA
bursts is a possibility. There are also problems for the LNB as the
return outer conductor drops varying voltages and these are superimposed
on the LNB supply volts. The BUC and LNB are often partially connected
to one another and to the earth ground at the antenna, so there is scope
for strange voltages. Lightning and safety rules come first and the
consequences of this may mean that you need an extra very thick earth
(ground) cable between the antenna and indoors.
Note that 70 MHz transceivers can only transmit and receive over a
small part of the satellite bandwidth, so if you want to transmit
carriers 200 MHz apart you need two TX cross site cables and two
up-converters. Alternatively you can get transceivers with 140 MHz
centre frequency for a much wider range of transmission frequencies.
If you get a transceiver make sure you understand how the frequencies
are calculated. Sometimes a carrier at 70 MHz counts as zero frequency
when added to the up-converter front panel readout setting, or sometimes
70 MHz.
Check first so that you don't transmit on the wrong frequency.
Back to Explanation
of L-Band BUCs compared with 70 MHz Transceivers |
