View detail for ATAN0142: ATA8520D Crystal Calibration

ATAN0142
ATA8520D Crystal Calibration
APPLICATION NOTE
Features
•
•
•
•
Calibration for production and EOL test for ATA8520-EK1-E/-EK2-E/EK3-E evaluation kits [5], [6]
Crystal offset adjustment at room temperature (24°C)
Crystal temperature drift adjustment
For ATA8520 and ATA8520D device with internal temperature
measurement and calibration functions [1], [2]
Description
This application note explains the crystal behavior over temperature and
how to compensate the frequency drift for the SIGFOX application. The
ATA8520 and ATA8520D devices have a built-in temperature sensor and drift
compensation for the crystal frequency. This can be used to adjust the
crystal frequency for
•
•
The frequency offset at room temperature (24°C)
The frequency drift over temperature
The activation of the temperature measurement and the compensation of the
rx and tx frequency is accomplished with the SPI command “Trigger
Measurement (0x14)” before starting a tx or tx-rx operation or a test function.
During production testing the crystal coefficients for the correction are
programmed into EEPROM. These coefficients are provided by the crystal
vendor [3], [4] and are corrected by the frequency offset value at room
temperature.
The actual crystal coefficients must have an accuracy of < ±15ppm over the
temperature already considered in the device by the SIGFOX protocol
software. The static offset must be < ±10ppm but can easily be adjusted to <
±1ppm by an offset compensation at room temperature (25°C). The
temperature drift can be further optimized during end-of-line (EOL) testing,
for example, by additionally measuring the temperature at 40°C. This
compensates for the crystal variation of the temperature drift.
Atmel-9406A-ATAN0142_Application Note-10/2015
Table of Contents
Features.......................................................................................................................... 1
Description.......................................................................................................................1
1. Crystal Accuracy........................................................................................................3
2. Crystal Frequency Compensation............................................................................. 4
3. Results.......................................................................................................................6
4. Examples................................................................................................................... 8
5. Conclusion............................................................................................................... 10
6. References...............................................................................................................11
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
Atmel-9406A-ATAN0142_Application Note-10/2015
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1.
Crystal Accuracy
The ATA8520 and ATA8520D device uses a crystal with a nominal frequency of 24.305MHz. The
requirements for the accuracy of the frequency regarding offset drift and aging are defined by SIGFOX
and are already considered in the implementation of the SIGFOX protocol in the devices. To achieve
further accuracy, such as for CE and ETSI requirements, the offset and drift can be compensated during
production and EOL testing.
The accuracy for the crystal for an SIGFOX application is defined as follows:
•
•
•
±10ppm frequency tolerance at 25°C
±15ppm frequency drift between –40°C and +85°C
±5ppm frequency drift due to aging (5 years at 25°C)
Typical 24.305MHz crystals ([4], [5]) have a frequency drift over temperature shown in Figure1.
Figure 1-1 Crystal Frequency Drift Over Temperature
The variation of the crystal is mainly a tilt of the center curve (gray) represented by the lines (orange,
blue). All lines cross the zero point at 25°C room temperature. The effect of aging is not compensated but
considered in the SIGFOX protocol library as a guard band for the frequency range used in the 868MHz
ISM band.
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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2.
Crystal Frequency Compensation
The ATA8520 and ATA8520D devices have built-in temperature drift compensation for the crystal
frequency. The nominal correction coefficients are provided by the crystal vendor as shown in Figure 1-1.
The ATA8520 and ATA8520D can store these coefficients in the internal EEPROM and use them together
with the internal temperature sensor to compensate the crystal frequency. The nominal correction values
use the center values (gray) in Figure 1-1 as listed in the following Table 2-1.
Table 2-1 Nominal Crystal Temperature Compensation EEPROM Values
Compensatio −15
n value CO[i]
in [ppm]
Temperature
T[i] in [°C]
−8
−1
3
6
−48 −40 −32 −24 −16
7
7
5
3
0
−3
−6
−8
−10 −10
−9
−7
−3
−8
0
8
16
24
32
40
48
56
72
80
88
64
During production testing, a compensation of the frequency offset at 25°C can be performed and is
recommended for maintaining max. static offset of < ±10ppm. This is done at the center frequency of
868.13MHz. The coefficient offset value dC25 in [ppm] of the measured peak frequency fmeas25 in [Hz]
to this center frequency is
dC25 = (fmeas25 [Hz] – 868130000Hz)/868.13Hz
(1)
With this offset value, the corrected coefficients C25[i] for the center curve are
C25[i] = CO[i] + dC25
(2)
with CO[i] crystal correction value from datasheet.
This corrects the coefficients for the offset at room temperature, i.e., the zero crossing point in Figure 1-1.
To determine the slope of the line for the variation of the crystal coefficients, a second measurement at a
different temperature has to be performed. This results in an equation for the correction dC[i] of the
coefficients over temperature T[i] as follows:
dC[i] = A × T[i] + B
(3)
Using a second measurement, for example, at a temperature of 40°C, results in the correction value in
[ppm]
dC40 = (fmeas40 [Hz] – 868130000Hz) / 868.13Hz
(4)
The coefficients for equation (3) can then be derived using
A = ( (dC40 – dC25) – (CO[T40] – CO[T25]) )/(40°C – 25°C)
(5)
B = –A × 25°C
(6)
Finally, the corrected crystal coefficients in [ppm] are
C[i] = C25[i] + dC[i]
(7a)
or
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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C[i] = C25[i] + A × (T[i] – 25°C)
(7b)
C[i] = CO[i] + dC25 + A × (T[i] – 25°C)
(7c)
or
These final crystal compensation coefficients compensate for the crystal offset at room temperature and
for variations in the crystals. Ultimately, precision depends on the accuracy of the temperature
measurement and calculation.
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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3.
Results
The following calculations take the crystal coefficients shown in Figure 1-1 as examples for the accuracy
of the compensation achieved.
Table 3-1 lists the coefficients for a typical crystal for some temperature values with high and low limits
and center values from Figure 1-1.
Table 3-1 Crystal Compensation Coefficients from Figure 1-1
Temperature T[i] in[°C]
–40
–20
0
25
40
60
80
High Coefficients CO_h[i] in [ppm]
–1.9
9.8
9.6
0
–7.1
–14
–13.5
Low Coefficients CO_l[i] in [ppm]
–15
0.5
4.3
0
–3.8
–6
–0.7
Center Coefficients CO[i] in [ppm]
–8.4
5.1
6.9
0
–5.5
–10
–7.1
The following examples use the calculation schemes from Section Crystal Frequency Compensation on
page 4 and crystal parameters typically occurring for some temperature values from Table 3-1. A typical
offset value is additionally assumed.
Table 3-2 shows the calculations for a crystal with coefficients equal to the center values CC[i]=CO[i] in
Table 3-1 with
dC25 = –6ppm, CO[T25] = 0ppm
dC40 = –11.5ppm, CO[T40] = –5.5ppm
A = 0.0, B = 0
Table 3-2 Calculation Example for Center Values with Offset of –6ppm
Temperature T[i] in[°C]
–40
–20
0
25
40
60
80
Crystal Coefficients CC[i] in [ppm]
–14.4
–0.9
0.9
–6
–11.5
–16
–13.1
Center Coefficients CO[i] in [ppm]
–8.4
5.1
6.9
0
–5.5
–10
–7.1
C25[i] (see 2)
–14.4
–0.9
0.9
–6
–11.5
–16
–13.1
0
0
0
0
0
0
0
–14.4
–0.9
0.9
–6
–11.5
–16
–13.1
0
0
0
0
0
0
0
dC1[i] = CC[i] – C25[i]
C[i] (see 7)
dC2[i] = CC[i] – C[i]
The final deviation dC1[i] and dC2[i] are zero as expected, because the crystal coefficients used for the
compensation are identical to the center coefficients of the crystal.
Table 3-3 shows the calculations for a crystal with coefficients equal to the high values CC[i]=CO_h[i] in
Table 3-1 with
dC25 = –9ppm, CO[T25] = 0ppm
dC40 = –16.1ppm, CO[T40] = –5.5ppm
A = –0.11, B = 2.75
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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Table 3-3 Calculation Example for High Values with Offset of –9ppm
Temperature T[i] in[°C]
–40
–20
0
25
40
60
80
Crystal Coefficients CC[i] in [ppm]
–10.9
0.8
0.6
–9
–16.1
–23
–22.5
Center Coefficients CO[i] in [ppm]
–8.4
5.1
6.9
0
–5.5
–10
–7.1
C25[i] (see 2)
–16.4
–3.9
–2.1
–9
–14.5
–19
–16.1
5.5
4.7
2.7
0
–1.6
–4
–6.4
C[i] (see 7)
–10.25
1.05
0.65
–9
–16.15
–22.85
–22.15
dC2[i] = CC[i] – C[i]
–0.65
–0.25
–0.05
0
0.05
–0.15
–0.35
dC1[i] = CC[i] – C25[i]
With crystal coefficients similar to the high coefficients, the residual difference dC1[i] for offset
compensation has a drift range of ±7ppm, which is not a significant improvement and is highly dependent
on the offset value at room temperature. Using the additional drift compensation with a second reference
value at 40°C improves the final temperature drift to < ±1ppm, which is observable for residual drift dC2[i].
Table 3-4 shows the calculations for a crystal with coefficients equal to the low values CC[i]=CO_l[i] in
Table 3-1 with
dC25 = –2ppm, CO[T25] = 0ppm
dC40 = –5.8ppm, CO[T40] = –5.5ppm
A = 0.11, B = –2.75
Table 3-4 Calculation Example for Low Values with Offset of –2ppm
Temperature T[i] in [°C]
–40
–20
0
25
40
60
80
Crystal Coefficients CC[i] in [ppm]
–17
–1.5
2.3
–2
–5.8
–8
–2.7
Center Coefficients CO[i] in [ppm]
–8.4
5.1
6.9
0
–5.5
–10
–7.1
C25[i] (see 2)
–10.4
3.1
4.9
–2
–7.5
–12
–9.1
dC1[i] = CC[i] – C25[i]
–6.6
–4.6
–2.6
0
1.7
4
6.4
–17.55
–1.85
2.15
–2
–5.85
–8.15
–3.05
0.55
0.35
0.15
0
0.05
0.15
0.35
C[i] (see 7)
dC2[i] = CC[i] – C[i]
As shown in Table 3-3, with crystal coefficients similar to the low coefficients, the residual difference
dC1[i] for offset compensation has a similar drift range of ±7ppm. Using the additional drift compensation
with a second reference value at 40°C improves the final temperature drift to < ±1ppm, which is
observable for residual drift dC2[i].
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
Atmel-9406A-ATAN0142_Application Note-10/2015
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4.
Examples
The compensation was tested on two evaluation kits ATA8520D-EK1-E. The following Figure 4-1 and
Figure 4-2 shows the final frequency drift of the RF TX frequency, which should be nominal at
868.044MHz. The results are summarized in Table 4-1.
The frequency drift was measured between −20°C and +70°C
Figure 4-1 Module1 Results after Compensation (7)
Figure 4-2 Module2 Results after Compensation (7)
Table 4-1 Drift Compensation for Module1 and Module2 for Temperature Range of −20°C to +70°C
Temperature T[i] in [°C]
Module 1
Module 2
RF Freq. at 24°C in [MHz]
868.04267
868.04150
dC25 in [ppm]
−12.67
−6.91
dC40 in [ppm]
−18.43
−10.37
Parameter A
−0.044
0.109
Parameter B
1.1
−2.75
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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Temperature T[i] in [°C]
Module 1
Module 2
Initial Drift in [ppm]
−24
−7
−14
−6
Drift after Offset Compensation (2) in [ppm]
−12
+6
−7
+1
Drift after Compensation (7) in [ppm]
−2.3
+1.2
−3.7
+2.1
The table compares the
•
•
•
initial drift over temperature without any compensation
drift after offset compensation according to (2)
drift after compensation according to (7)
The offset parameter dC25 in Table 4-1 are greater than the offset specified by the crystal vendor. This is
due to additional capacitance on the PCB resulting from the ground plane below the crystal and the
connection to the device. As the ATA8520 is using only internal capacitors for the crystal this effect has to
be reduced either by
•
•
Changing the ground plane in this area, i.e. increasing the distance to the ground layer
Compensating the offset according to (2)
The compensation according to (2) results in a final drift compensation which is in the limits defined by
the crystal vendor of ±15ppm as shown in Figure 1-1.
According to the SIGFOX requirements [7] the drifts as specified by the crystal vendor are taken into the
account for the usage of the frequency band of 868.13MHz ±96kHz. The SIGFOX requirements [7] are
fulfilled by using the compensation according to (2).
According to the ETSI requirements [8] for the adjacent power measurement over temperature the
smallest channel spacing of 12.5kHz has to be applied which specifies a maximum temperature drift of
12.5kHz / 2 = 6.25kHz. This will result in a maximum allowed temperature drift at 868.13MHz of 7.2ppm.
To fulfill this requirement a drift compensation according (7) has to be used.
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
Atmel-9406A-ATAN0142_Application Note-10/2015
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5.
Conclusion
A temperature drift compensation for the TX and RX RF frequencies of the ATA8520/20D device is
described to compensate the frequency offset at room temperature according to equation (2). This
compensation will satisfy the requirements defined by SIGFOX.
In addition a compensation using two measurements, at room temperature and at a second temperature.
i.e. 40°C, will satisfy the requirements according to the ETSI requirements for CE certification.
The differences in accuracy between theoretical compensation as shown in section Results on page 6
and the measurements in section Examples on page 8 are resulting from different factors:
•
•
•
Accuracy of the RF frequency measurement
Accuracy of the temperature measurement
Calculation accuracy
These effects results in an additional error of about ±3ppm which is mainly caused by the accuracy of the
temperature measurement.
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
Atmel-9406A-ATAN0142_Application Note-10/2015
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6.
References
[1] ATA8520 transmitter data sheet
[2] ATA8520D transceiver data sheet
[3] NDK crystal data sheet EXS10B-22817 for 24.305MHz (type EXS00A-CS08559)
[4] KDS crystal data sheet 1C324305AB0B for 24.305MHz (type DSX321G)
[5] ATAN0136 – ATA8520D Production and EOL Testing
[6] ATAN0136_ToolPack_V1.0.zip
[7] SIGFOX "Ultra narrow Band Transceiver Product requirements specifications"
[8] ETSI EN 300 220-1 Specification
Atmel ATA8520D Crystal Calibration [APPLICATION NOTE]
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