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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 Rx is placed in series to the crystal.

-Increase Rx 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= RXstart+ESRxTAL

.Safety margin:

The maximal crystal ESR(Rs1,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

.ESRTYP≤OA/ 5



Why a Safety Margin is Needed?

Crystal ESR vs Temperature


Effective Load Capacitance


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

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

·CLoadis equal to CG and CD in series + Cstray

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

CLoad=CD*CG/(CD+CG))+Cstray


Chart of△FL,Dependant on CL

Load Capacitance / pF

Load Capacitance / pF

Defining CLoad


FL: 32.768 kHz

CL:12.5 pF

Tol.:+/- 30 ppm

Always start with a known crystal!

R1
Fs
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 CL 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 3rd, 5th and 7th harmonics.

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

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



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