Motorola M12 Oncore User's Guide download pdf (Page 18)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

1PPS SIGNAL DEFINITION

0 to 5 V live pulse (0 to 3 V for M12+)

1 PPS time mark is synchronous with the mid point of the rising edge of the pulse

rising from 0 V to 5 V

Rise time is approximately 20 to 30 ns

5 V pulse width is approximately 200 ms ± 1 ms

The falling edge will occur approximately 200 ms after the rising edge

Accurate to < 500 ns (1 sigma) in stand alone mode (with SA on)

UT Oncore accurate to < 130 ns (1 sigma) in stand alone mode (with SA on)

UT Oncore accurate to < 50 ns (1 sigma) in position-hold mode (with SA on)

RF JAMMING IMMUNITY (UT MODEL ONLY)

Many precise timing GPS installations require locating the GPS antenna at close

range to radiating antennas such as cellular telephone, paging, or other wireless

communications systems. Some of these transmitters may randomly cause the GPS

receivers to lose lock on tracked satellites. This can be very disconcerting to the

timing user since the system must rely on clock coasting' until the satellite signals

are reacquired. Long coasting times require more expensive oscillators for the timing

electronics in order to meet system specifications for holdover capability.

The GPS signal is broadcast at 1575.42 MHz with a bandwidth of +/- 1 MHz.

Experience has shown that receiver selectivity, or the ability to select only the GPS

band of information and reject all other signals, is an important feature for GPS

receivers, especially in cases such as those often encountered in timing applications.

To reduce the risk of unintentional jamming from high power out-of-band signals

causing dropouts, additional filtering has been added to the UT Oncore. The desired

result was achieved by working with various GPS L-band filter suppliers to develop

filters that were small economical and had the desired characteristics.

The VP Oncore (the predecessor to the UT Oncore) with the best selectivity (B8

model) uses two L-band filters and a five pole first IF filter. Experience from this

model was used to design the improved UT Oncore. Although the B8 design is

effective, the bandwidth of the input filter on this model is comparatively wide and the

low side roll off is not very steep. The image filter and the first IF filter are very

effective and have been retained for the R5 model of the UT Oncore. The first L-band

filter has been replaced with one with a narrower bandwidth and steeper low side

roll off. In addition, a third L-band filter was added between the first filter and the

image filter. The second IF filter has also been improved. The result is a GPS receiver

with greatly improved selectivity, which is to say, better immunity to jamming signals.

Revision 5.0 08/30/02

3.18

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Motorola M12 Oncore User's Guide Page 18