UBX-G7020 - Hardware Integration Manual
Design-in
GPS.G7-HW-10003
Objective Specification
Page 27 of 74
2.4.4
Crystal oscillator
The UBX-G7020 chip comes with a Pierce oscillator. It supports 26MHz crystals with 19pF load capacitance.
Figure 9: Crystal circuit
2.4.4.1
Crystal oscillator frequency tuning
The actual frequency of a crystal oscillator depends on its load capacitance which is mainly formed by the two
capacitors C14 and C15 shown in Figure 9. Each capacitor has an additional parasitic capacitance to GND in
parallel formed by the circuit traces on the printed circuit board and the capacitance of the UBX-G7020 input-
and output (XTAL_I and XTAL_O). The crystal oscillator will oscillate on its specified frequency only if the crystal
“sees” the specified load capacitance into the oscillator circuit (item 2.1 in Table 15). Figure 10 shows the crystal
oscillator including its parasitic capacitors. Generally C14 and C15 should be equal in size, but sometimes one or
two E-series steps difference in value may be required to approach the desired load capacitance.
Figure 10: Crystal oscillator and parasitic capacitors
The load capacitance CL “seen” by the crystal can now be calculated as follows:
The relation between actual frequency offset and load capacitance offset is given by the Pullability of a
crystal (item 2.3 in Table 15). Generally it can be said that a crystal designed for high load capacitance will have a
lower Pullability value than one designed for a lower load capacitance. However, high load capacitance values
require higher power for the oscillator to operate. From the Pullability specification of the crystal used it can
immediately be concluded that even a 1 pF error in load capacitance will result in a significant frequency error.
Only high-quality, low-tolerance COG capacitors must be used for C14 and C15.
As it is usually quite difficult to predict circuit board parasitics accurately, it is recommended to use a tuning
approach based on measurement for determination of the correct values of C14 and C15 for any given board
design. The goal of the tuning is to achieve a frequency error of less than +/- 12 ppm (+/- 19 kHz Doppler) at
room temperature for all produced devices.
A conventional temperature compensated crystal will also exhibit a frequency/temperature dependency. Luckily
this offset is minimal at room temperature. Thus, a measurement at room temperature can be used to determine
the initial offset of the oscillator circuit caused by any load capacitance error.
As it happens a GPS/GNSS receiver is also a very precise tool to measure frequencies, so not even a frequency
counter is needed for this task. See the
u-blox 7 Receiver Description including Protocol Specification
[3] for
more information about protocol messages dealing with local oscillator frequency measurements. Two options
exist for the frequency measurement:
Confidential
