Motorola BT100 - Manual (Page 47)

BT100 Installation and Operation Manual
Section 6
Operating Tips
This section describes using amplifiers in lower frequency systems and in lower gain systems.
Using Amplifiers in Lower Frequency Systems
When using the BT100 in 870 MHz or 750 MHz systems, you must consider the best method for
handling the reduced bandwidth and channel-loading requirement. The following information
helps you determine the best approach.
For distribution systems designed and installed as 1 GHz systems, but carrying a reduced
channel load, there are no further concerns. You can add or remove channels at your discretion.
If the system operates with ADUs/QADUs, the pilot channel cannot be disturbed. Reduced
channel loading improves distortion.
For lower-frequency systems, such as 870 MHz or 750 MHz, you will need to take into account
the reduced gain from 1 GHz. Due to the amplifier tilt, there will be some loss in gain from the
published operational specification at 1 GHz. For example, a 42 dB 1 GHz BT100, with 14 dB of
output tilt, will lose approximately 1.3 dB of gain at 870 MHz, resulting in approximately
40.7 dB of gain at 870 MHz. If you need to adjust tilt, you may accommodate by changing the
forward equalizer (SFE) value. Also, to avoid any additional loss in gain, it is optimal to use the
equalizers that match the system frequency. For an 870 MHz system, the SFE-87-*s are the
best choice, even in a 1 GHz amplifier. Note also, that the equalizer cuts off at the frequency
value for which it is designed. For example, an SFE-87-* cuts off frequencies above 870 MHz.
Therefore, if you have a 1 GHz system and need the equalizer to perform to 1 GHz, you must
use an SFE-100-*.
Using Amplifiers in Lower Gain Systems
There are three pad facilities in the forward path: (1) is the pad location (JXP IN) at the input to
the amplifier, (2) is the midstage pad (JXP) located between the pre-amplifier and the interstage
hybrid, and (3) are the output pads (JXP OUT, JXP 1, JXP 3, and JXP 4) located between the
interstage hybrid and the output hybrids.
The input pad (JXP IN) is normally changed to accommodate excessive input levels. When
operating at the same output levels, a BT100 with an input pad has the same carrier-to-noise
(c/n) and distortion performance as a BT100 without the input pad. Because it only attenuates
excess signal, it has no effect on the overall performance of the BT100.
If necessary, to achieve the amplifier gain specification, you can use the midstage pad (JXP) or
output pads (JXP OUT, JXP 1, JXP 3, and JXP 4) to reduce the gain of the BT100. However,
this will affect amplifier performance. Output padding degrades the station distortion
performance while having minimal effect on the carrier-to-noise performance. Midstage padding
is the opposite and therefore has little effect on distortions, but degrades the carrier-to-noise
performance. In general, we recommend you pad to the amplifier gain specification on the
output. However, if you are unsure about the optimal padding technique for your system, we
recommend you split the required padding evenly between the midstage and output pads.
It is recommended that you contact Motorola’s TRC or your account representative for more
specific information regarding use of the midstage and output pads.