Tektronix TG700, Руководство пользователя

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GPS7 GPS Synchronization and Timecode Module
A GPS speci fi c antenna with ampli fi er provides suf fi cient gain to drive a
reasonable length of cable, and provides fi ltering to reject signals at other
frequencies. A typical example of this is the Trimble Bullet III, 35 dB, 5 V,
antenna.
In a simple system without the optional booster, Cable 1 connects the antenna to
the GPS7. The length of this cable is limited by its attenuation at the GPS carrier
frequency of 1575 MHz. The GPS7 should have a signal that is 18 dB or greater
above the ambient level. For example, for a 35 dB antenna, the allowed cable loss
is 35 – 18 = 17 dB. (See Figure 3-53.)
Cables.
Attenuation varies signi fi cantly depending on cable type. Cable loss is
about 13 dB/100 ft for a miniature coaxial cable like the Belden 1855, while for a
RG11 style like the Belden 7731, the loss is only 5.5 dB/100 ft. This correlates to
an allowable length of 130 ft for the small cable, to over 300 ft for the larger cable.
A booster ampli fi er can be added if more length is needed, as shown in the
optional block in the signal path system. (See Figure 3-53.)
If a 20 dB ampli fi er is added, then 20 dB more cable loss can be accommodated.
This equates to another 150 ft of small coax, or 360 ft of large coax.
Although the GPS7 input and most of the other components are 50 Ω , either
50 Ω or 75 Ω cables can be used in most installations. The re fl ections from the
impedance mismatch will not cause signi fi cant changes in the system because the
signal is narrow band and the cable loss is usually many dBs. However, you
should not mix short cable lengths of different impedances, as this might create
re
fl ections with the potential to cause signal degradation.
Ampli fi cation.
The GPS7 provides either 3.3 V or 5 V DC power to drive the
ampli fi ed antenna and booster ampli fi er. The power is carried on the same coax
as the GPS signal, and can be turned off if the antenna is powered by a separate
supply. When you are designing the antenna system, check the voltage and current
requirements of the components to insure compatibility.
The location of the booster ampli fi er is important. It needs to be before the
second length of cable shown in the simpli fi ed antenna system diagram. If the
booster ampli fi er is placed just before the GPS7, then the signal will have been
attenuated too far and the output may be noisy. If you cannot put the optional
booster ampli
fi er in the middle of a long run of cable, then put it near the antenna
rather than near the GPS7 end.
For more complex systems, a variety of booster ampli fi ers, powered and passive
splitters, DC blocks, and fi lters are available from a number of vendors.
Antenna location.
It is important that the GPS antenna location has a clear view of
a large part of the sky. Since GPS satellites are constantly orbiting the earth, they
may be in any direction at a given time. If part of the sky is blocked by buildings,
trees, mountains, etc., then fewer satellites will be visible. It is also possible to get
re
fl ected signals that will have come by a longer path than expected and thus may
3–102 TG700 TV Signal Generator Platform User Manual