User`s guide

Chapter 4 - Antenna Descriptions
Antenna System RF Parameter Considerations
Both the gain and the noise of the overall system affect the performance of the A/D converter in
the Oncore GPS receiver. The illustration below illustrates typical values for the M12+ receiver
when used with the Hawk antenna and the standard length of 5 meters of cable. The thresholds
and ranges listed should be considered to have a tolerance of 2 to 3 dB. Figure 4.7 below details
a typical configuration.
System Constraints:
• The gain in decibels is cumulative through all stages (i.e. G = G1+ G2 + G3 .. ). The
optimal gain of the antenna, cabling and any in-line amplifiers and splitters for the M12+
receiver is between 18 and 36 dB. The M12+ may operate outside of the optimal gain
range but performance will degrade. Therefore, Motorola does not recommend operating
outside of the optimal gain range as indicated above. For the system illustrated below,
the external gain is approximately 24 dB in front of the receiver.
• System noise (F) is not to exceed 4dB. The cascaded system noise figure
formula is:
.........,
11
1
21
3
1
2
gg
f
g
f
ff
×
−−
++=
(=1.9dB for the system shown below)
.
where ƒ
1
is the noise figure for stage one and g
1
is the gain for stage one. Note that all of the
values used in this equation are absolute. The resulting number must be converted back to
decibels in order ascertain if it is less than 4dB and to compare it with other antenna systems
configurations.
Recall the formula for converting absolute values to decibels, and decibels to absolute values:
10logƒ = ƒ(dB).
Stage 1
antenna with low noise amplifier (LNA)
g = 29dB,
ƒ = 1.8dB
Stage 2
5m of RG-174 Cable
g = -5dB,
ƒ = -5dB
Stage 3
core receiver
,
ƒ = 6.5dB
M12+ On
G =85dB
Hawk
Figure 4.7: Typical System Gain/Noise Figure Calculations
Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05
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