PAS-BC3WK Datasheet, PDF (1/3 Pages) Abracon Corporation – The majority of electronic designs have some form of timing associated with them

PIERCE ANALYZER SYSTEM (PAS)
ADVANCED BOARD CHARACTERIZATION SERVICE
PAS-BC3WK
INTRODUCTION:
The majority of electronic designs have some form of timing associated with them. Depending on the frequency accuracy
requirements, some employ oscillators while others use off-the-shelf crystals in conjunction with the built-in oscillator circuit;
embedded in most µcontrollers and µprocessors.
Most if not all embedded solutions use the Pierce Oscillator configuration, integrated as part of the SOC (system on chip).
The obvious advantages include cost, size and power compared to a stand-alone oscillator; while the key limitation is the
proper matching of the quartz crystal with the on-board Pierce Oscillator.
Figure (1) outlines the oscillator block and the key components that influence the overall performance of the timing loop.
Effective Loaded Capacitance, as seen by the Crystal = CL
CL = {((Cin+C1) x (C2+Cout)) / (Cin+C1+C2+Cout)} + Board Strays
For example; Let C1 = C2 = 27 pF; Cin = 5.0 pF & Cout = 10.0 pF and Board Strays = 0.50 pF then;
CL = { ((27+5) x (27+10)) / (5+27+27+10) } + 0.50 = 17.65 pF
Therefore specifying a crystal with 18.0 pF plating load capacitance would be the closest match for frequency accuracy.
Besides the frequency accuracy issue, while accounting for Quartz Crystal’s set tolerance; the reactive impedance (Xc) of the
loop capacitors, in combination with the Inverter Amplifier’s transconductance (gm), the current limiting resistor Rs and the
presence or absence of the AGC or ALC circuit as part of the oscillator loop – determines the boundary condition of the
design.
This boundary condition, commonly referred to as the Safety Factor (SF), is an important parameter to ensure that the
product design has sufficient margin to accommodate part-to-part and lot-to-lot variations; as well as eliminating product
performance uncertainty in production volume.
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