J
Jamie Morken
- Jan 1, 1970
- 0
For this crystal:
"http://www.rocketresearch.org/new/crystal/crystal2.jpg"
The crystal is on the right side of the board vertically
between the two load capacitors, the bottom load capacitors
trace is highlighted red. The board thickness is 1/16" and
the trace width is 10mils.
datasheet:
"http://www.eea.epson.com/go/Prod_Ad...35&categoryId=EEA.QD.Crystals.kHzSMD_Crystals"
(digikey part#: SE2413CT-ND)
(manufacturer part#: FC-135 32.7680KA-A3)
frequency: 32.768kHz
load capacitance: 12.5pF
For selecting the load capacitors to use for the crystal oscillator
circuit I am using the formula:
CL = [(CL1 * CL2) / (CL1 + CL2)] + Cstray
I am wondering what the stray capacitance Cstray is. If I estimate
2pF for the above circuit then I get:
[(21*21) / (21+21)] + 2 = 12.5pF
Also for this crystal:
"http://www.rocketresearch.org/new/crystal/crystal1.jpg"
C12 and C13 are the load capacitors and the trace width is 10mil
and the board thickness is 1/16"
datasheet:
"http://www.abracon.com/Resonators/abm3b.pdf"
(digikey part#: 300-8206-1-ND)
(manufacturer part#: ABM3B-16.000MHZ-10-1-U-T)
frequency: 16MHz
load capacitance: 10pF
If Cstray is 2pF, then:
[(16*16) / (16+16)] + 2 = 10pF
Would the two above crystal oscillator circuits have about 2pF Cstray
given the board layout?
Also if the actual load capacitance is high, the frequency will be
lower, and if the actual load capacitance is low, the frequency will be
higher. Is there a formula to give the percent change in frequency for
a given load capacitance mismatch?
What kind of error in frequency can I expect if I have an actual load
capacitance that is 15pF instead of the specified 10pF for the 16MHz
crystal?
cheers,
Jamie
"http://www.rocketresearch.org/new/crystal/crystal2.jpg"
The crystal is on the right side of the board vertically
between the two load capacitors, the bottom load capacitors
trace is highlighted red. The board thickness is 1/16" and
the trace width is 10mils.
datasheet:
"http://www.eea.epson.com/go/Prod_Ad...35&categoryId=EEA.QD.Crystals.kHzSMD_Crystals"
(digikey part#: SE2413CT-ND)
(manufacturer part#: FC-135 32.7680KA-A3)
frequency: 32.768kHz
load capacitance: 12.5pF
For selecting the load capacitors to use for the crystal oscillator
circuit I am using the formula:
CL = [(CL1 * CL2) / (CL1 + CL2)] + Cstray
I am wondering what the stray capacitance Cstray is. If I estimate
2pF for the above circuit then I get:
[(21*21) / (21+21)] + 2 = 12.5pF
Also for this crystal:
"http://www.rocketresearch.org/new/crystal/crystal1.jpg"
C12 and C13 are the load capacitors and the trace width is 10mil
and the board thickness is 1/16"
datasheet:
"http://www.abracon.com/Resonators/abm3b.pdf"
(digikey part#: 300-8206-1-ND)
(manufacturer part#: ABM3B-16.000MHZ-10-1-U-T)
frequency: 16MHz
load capacitance: 10pF
If Cstray is 2pF, then:
[(16*16) / (16+16)] + 2 = 10pF
Would the two above crystal oscillator circuits have about 2pF Cstray
given the board layout?
Also if the actual load capacitance is high, the frequency will be
lower, and if the actual load capacitance is low, the frequency will be
higher. Is there a formula to give the percent change in frequency for
a given load capacitance mismatch?
What kind of error in frequency can I expect if I have an actual load
capacitance that is 15pF instead of the specified 10pF for the 16MHz
crystal?
cheers,
Jamie