Micro Crystal Test Method To Measure Oscillator's Performances

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Test Method to Measure Oscillator's Performances

Mar 01, 2021

Oscillation Allowance (OA) from “MICRO CRYSTAL”

Test-Methode to measure the Oscillator-Performance

-Resistor R_x is placed in series to the crystal.

-Increase R_x until oscillation stops / starts.

Oscillation Allowance (OA) from “MICRO CRYSTAL”

. The OA value describes the performance of the active part of the oscillator (inverter).

.Rx value is increased until the oscillation starts / stops.

.OA= RX_start+ESR_xTAL

.Safety margin:

The maximal crystal ESR(R_s1，R₁) must be 5 times smaller than the OA, in order to have enough safety margin for lC's production tolerances and negative influence of increasing ambient temperature on lC and crystal

.ESR_TYP≤OA/ 5

Why a Safety Margin is Needed?

Crystal ESR vs Temperature

Effective Load Capacitance

·C_G and C_D are the load capacitors of the oscillator circuit (regarded in series). They are integrated into the lC or external as SMD components.

· C_strayis equal to the board layout stray(parasitic) capacitance.

·C_Loadis equal to C_G and C_Din series + C_stray

. C_Lof the crystal is the capacitance C_Load the crystal needs to“see”from the oscillator circuit, it is not the capacitance of the crystal itself.

C_Load=C_D*C_G/(C_D+C_G))+C_stray

Chart of△FL,Dependant on C_L

Load Capacitance / pF

Defining CLoad

FL: 32.768 kHz

CL:12.5 pF

Tol.:+/- 30 ppm

Always start with a known crystal!

R₁	F_s	C₁	∆ FL	C₀	P
㏀	KHz	fF	ppm	pF	㎼
39.684	32.76557	2.25	+9.9	0.86	1.037

Frequency measured on a buffered clock output.√
Frequency measured directly on the crystal:

Using a high impedance probe (0.1 pF).√

Using a standard probe (8-12 pF).Ⅹ

Frequency offset calculation from time

observation on a display over several days.√

The result represents the design offset + the crystal's tolerance (∆FL).

Frequency vs Temperature

Conventional Tuning Fork

T: 25+/-5℃

Temperature /℃

Frequency Accuracy

Time Deviation of a 32.768 kHz Crystal

	Deviation ppm	Per day seconds	Per 3days seconds	Per month seconds	Per year minutes
Specified Tolerance	+100	8.64	25.9	259	51.8
Specified Tolerance	+30	2.59	7.8	78	15.6
Specified Tolerance	+20	1.73	5.2	52	10.4
Specified Tolerance	+10	0.86	2.6	26	5.2
Nominal Frequency	0	0.00	0	0	0
Specified Tolerance	-10	-0.86	-2.6	-26	-5.2
Specified Tolerance	-20	-1.73	-5.2	-52	-10.4
Specified Tolerance	-30	-2.59	-7.8	-78	-15.6
Specified Tolerance	-100	-8.64	-25.9	-259	-51.8

Frequency Accuracy

Additionally to the specified tolerance,the following factors influence the total time deviation:

. The aging of the crystal as a permanent frequency shift.

. The design offset: mismatch between the crystal C_L specification and the effective load capacitance of the oscillator design.

- The frequency vs temperature characteristic as a temporary frequency shift.

Example of Calculation

Crystal Tolerance	+8.0 ppm
crystal Aging	-1.0 ppm
Design Offset	+5.8 ppm
Temperature Drift at 35℃	-3.5 ppm
Total	+9.3 ppm

Overtone Mode Suppression

. The overtone is the 1st harmonic mode. For Tuning Fork crystals, it's about 6.1 x higher than the fundamental mode.AT-Cut crystals have 3^r^d, 5^t^h and 7^t^h harmonics.

. Overtone suppression: the need of adding the R_D resistor depends on the phase shift characteristics of the used inverter at fundamental and overtone mode frequencies.

. The resistor R_D together with load capacitor C_D, forms a low pass filter. Usually 220KΩ is safe.

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Mother of Timing 32.768kHz Clock Crystal