PSM-2100L Satellite Modem Addendum
Page L-Band - 7
required here are very good stability (probably an OCXO) and low phase noise. The BUC
manufacturer should specify the requirements, but it is not difficult to figure out some minimum
capabilities. First, to achieve an Intelsat specified transmit signal uncertainty of 50 Hz per kbps. A
C-Band reference for a 32 kbps carrier would require approximately 2 parts in 10
7
stability
minimum. This is +/- 1200 Hz at 6 GHz transmit frequency. A Ku-Band BUC would require 1 part
in 10
7
stability for a 32 kbps data rate.
The typical BUC level requirement for the reference input might by +3 to –3 or –5 dBm from a
sine wave oscillator. Since the typical OCXO type oscillator will usually have an appro7
dBm output this should not be a problem unless the signal is split many ways.
The reference oscillator phase noise is multiplied when phase locking the BUC Local Oscillator.
Thus the phase noise on the oscillator must be extremely low and probably cannot be viewed
directly on a spectrum analyzer. Its effect however will be visible on the BUC output with a known
clean carrier input.
3.5
Station Gain Budgets
Below is a block and level diagram of a typical station showing the levels of all relevant signals at
each point in the transmit and receive chain.
3.6
Cable Selection
Knowing what approximate levels are required at each point in the station block diagram permits
specification of required cable size and type. Several other factors enter here:
1. The transmit cable must also carry a heavy current on the order of 1 to 5 Amps to power
the BUC/PA combination. The DC resistance and cable voltage drop must allow this gear
to receive their minimum voltage plus enough margin for variation with time and
temperature.
2. At L-Band frequencies the loss variation with temperature can be extreme. For example a
200 foot length of RG214 cable (double shielded, ½ in class) has approximately 20 dB of
loss and a variation vs. Temperature of 0.2% (of dB) per degree Centigrade. If operating
in an exposed environment (like a desert) where the temperature may vary approximately
20 deg. C from day to night that could represent a variation of almost 1 dB over a 12 hour
period. In a 20 dB loss cable the attenuation change is then approximately .04 dB per
deg. C, or 0.8 dB over the full 20 deg. C change. First, this probably says that the cables
must be either buried in conduit or shielded from the sun if run on a cable rack to
minimize variations.
3. The transmit and receive cables must be separated and definitely not tied directly
together with “tie-wraps”, especially on longer runs. This is because of the tremendous
difference between the transmit and receive levels possible. This is made worse on long
cable runs because the modem end will have higher transmit levels and the receive will
have lower levels than on a short run. The better cables in this regard have double
shielding (two braids or a braid/foil combination) and a shielding efficiency of 100 dB or
better. A good note here is that with the typical LO frequencies as shown in the example
above, the transmit and receive L-Band frequencies are widely separated. If the signals
were within the LNB stability/drift frequency limits there might be a tendency for the
receive to attempt locking to its own transmit signal.
4. Considering the L-Band IF range is 700 MHz spanning close to an octave, the variation in
loss between the high and low ends of the IF range may be significant.
A nominal design point may be to allow for 10 to 15 dB of total cable losses and select cable that
will reliably achieve this. A more accurate “rule of thumb” would be to design for a total gain from
the modem to the antenna in transmit, or antenna to modem in receive of 35 dB. For example if
