When you are planning to purchase a GPS NTP Time Server device, the GPS receiver and how it is connected to the antenna is an important decision criteria.
There are several concepts at the market, and i want to introduce to a few of them:
Normal L1 Antenna
If the receiver uses a normal L1 antenna, the frequency that has to be transmitted over the cable is the full
1.575 GHz and due to this very high frequency the length of the cable is very limited. Usually it is about 20 meter and can only increased by using expensive special low loss cable.
Receiver in the Antenna
This is an approach that several manufacturers like Galleon are trying. The receiver is in the antenna, and time information is transported via RS-422 or a similar protocol to the device. Due to the differential signal of RS-422 or RS-485 the cable can be much longer. The big disadvantage of this technique is that the receiver is inside the antenna and is influenced by all weather conditions like sun, ice or snow.
The precision of the timing signal will vary with the fluctuations of the temperature.
Up/Down Converted GPS signal
This is the technique, the guys at Meinberg are using, and it seems to be the best approach, because you can use standard RG58 coaxial cable up to 300 meter, and with RG213 even up to 600 meter.
And the receiver and the high-precision oscillator are in a regulated environment.
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