| |
|
|
|
 |
What is PPM? How to determine PPM? |
|
 |
 |
PPM stand for parts per million. PPM is the ratio of frequency deviation with respect to normal frequency.
Ex.:
Nominal Frequency = 13.000000 MHz
Actually Frequency = 12.999852 MHz
So:
(A.F. - N.F.) / N.F. = (12.999974-13.000000) / 13.000000
= - 0.000002
= - 0.000002 x 106 (parts per million)
= - 2 PPM |
|
|
What are the motional and shunt capacitances of a crystal unit? |
|
|
|
The shunt capacitance (C0) is the capacitance resulting from the presence of the electrodes on the quartz plate plus the capacitance inherent in the crystal holder. The motional capacitance is a parameter of the equivalent circuit. It is used as a means of describing the elasticity or "stiffness" of the quartz resonator. |
|
|
The load capacitance (CL) of my resonant crystal is rated at 16pF. How do I calculate the value of the load capacitors used in my parallel resonant oscillator circuit? |
|
|
|
Use this formula to approximate the value of capacitors needed:
CL=((Ci x Co) / (Ci + Co)) + Cstray
Cstray is the stray capacitance in the circuit, typically 2-5pF. If the oscillation frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the oscillation frequency. When CL =16pF, Ci and Co will be approximately 22-30pF each, plus the additional stray capacitance.
|
|
|
What is load capacitance (CL)? |
|
|
|
CL stands for Load Capacitance. Load capacitance is defined as being the total capacitance present in an oscillator circuit as measured or calculated across the pins of the crystal. In order for the quartz crystal unit to function in a normal manner, there has to be some defined capacitance in oscillation circuit to match the crystal. |
|
|
