Trimble Outdoors AcutimeTM2000 Synchronization Kit manual PPS Quantization Error

5 System Operation

5.2.4PPS 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.

12.504 Mhz Clock

UTC or GPS PPS

Acutime 2000 PPS

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.

Acutime 2000 Synchronization Kit User Guide 5-7