Quartz Crystal / Oscillator

PRECAUTIONS IN OSCILLATION CIRCUIT DESIGN

To utilize Crystal units characteristics fully, is necessary to design a oscillation circuit with optimal conditions.

Based on Citizen's experience accumulated over the year,Citizen offers such service as checking the oscillation circuit designed by a customer with regard to items mentioning following to ensure optimal matching between the crystal units and the oscillation circuit. The results of investigation with regard to items will be reported to the customer as Citizen's "Circuit Investigation Report".

1. MOS Fundamental Crystal Oscillation Circuit

CL	Load capacitance
-R	Negative resistance
Le	Effective Inductance
Re	Effective resistance

2. Load capacitance (CL)

The Load capacitance (CL) refers to an effective external series capacitance in an assuming oscillation circuit that is viewed from the crystal unit side.

Resonance frequency will be determined with the Load capacitance and the crystal unit.

For this reason, differences in the load capacitance of the oscillation circuit may cause the frequency deviations (result in a different resonance frequency from the required one).

3. Negative resistance (-R)

The Negative resistance reflects the allowance and margin for oscillation motility.

An insufficient negative resistance may cause unexpected trouble such as No-Oscillation or slow start-up time.

4. Drive Current (i)

The drive current refers to the current flows through a crystal unit. An excessive drive current that is applying a crystal unit may cause the following trouble and phenomenon.

1.	Increase in the electromagnetic wave noise.
2.	Crystal element of T/F breaking off
3.	Abnormality of the frequency temperature behavior occurs.
4.	Get worse the characteristics of the crystal unit such as frequency Tolerance, Frequency Tolerance over Operating Temperature Range, Motional resistance (Series resistance)

5. Frequency - Voltage coefficient

Represents the rate of frequency change against variations in the supply voltage.

The cause of frequency changing by the power supply variation attributes to factors on the oscillation circuit side (especially IC) than the Crystal unit.

It is normally desirable to keep within ±5ppm in the rate of frequency change against ±10% variation in the supply voltage.

6. Frequency tolerance over operating temperature range

Represents the rate of frequency change against various in operating temperature.

There may be a large difference between the oscillation circuit and the crystal unit in Frequency tolerance over operating temperature range.

7. How to Check a margin for oscillation motility

1.	To know a margin for oscillation motility should check the negative resistance (-R) of a oscillation circuit.
2.	Connect a variable resistor (VR) to the oscillation circuit in series with the crystal unit. To turn down the value resistor of VR from the NOOSCILLATION in abeyance to the oscillation START-UP. The value shall be the negative resistance (-R) at the oscillation starts up.
3.	Adjust value of C1, C2 and Rd as the negative resistance to be 5 times larger than the effective resistance in order to get enough margins for oscillation motility.And to get 10 times of -R to the effective resistance is recommended for equipment that is required the high quality and reliability such as auto-motive etc.