User's Manual

Acutime 2000 Synchronization Kit User Guide 5-7
5 System Operation
5.2.4 PPS Quantization Error
The Acutime 2000 uses a high-precision, fixed-frequency oscillator
as the timing source to down-convert and decode the GPS signal and
to generate the PPS output signal. Since a fixed-frequency oscillator
is used, the Acutime 2000 must place the PPS output on the clock
edge that it determines is closest to UTC or GPS. This situation
results in a quantization error on the placement of the PPS whose
magnitude is equal to one-half the period of the fixed frequency
oscillator. The oscillator frequency is 12.504 MHz, which is
equivalent to a period just under 80 nanoseconds; therefore, the
quantization error on the PPS output is between +/-40ns.
Figure 5-1 illustrates quantization error.
The top waveform represents the 12.504 MHz clock.
The Acutime 2000 PPS output must be placed on one of the rising
edges of this clock. The middle waveform represents the UTC/GPS
on-time mark as determined by the Acutime 2000’s GPS receiver
electronics.
The bottom waveform represents the Acutime 2000’s PPS output,
which is output on the clock edge closest to the actual UTC/GPS on-
time mark.
Figure 5-1 PPS Quantization Error
The amount of quantization error present on each PPS output pulse is
reported in packet 0x8F-AC. This quantization error information can
be used to reduce the effective amount of jitter on the PPS pulse.
12.504 Mhz Clock
UTC or GPS PPS
Acutime 2000 PPS