AN593: Temperature Sensor Usage and Frequency Calibration for the Si4x3x

AN593
TEMPERATURE S ENSOR U S A G E A N D F R E QUE N C Y
C ALIBRATION FOR T H E S i 4 X 3 X
1. Introduction
This document describes how to use and calibrate the built-in temperature sensor of EZRadioPRO radios. It also
describes how to calibrate the frequency offset caused by crystal temperature dependency.
2. Temperature Sensor
An integrated on-chip analog temperature sensor is available in the radio. The temperature sensor is automatically
enabled when it is selected as the input of the internal General Purpose ADC or when the analog voltage output of
the temperature sensor is selected to be on the analog test bus. If the General Purpose ADC is used, the
temperature value can be read over the SPI bus through register, “11h—ADC Value”.
2.1. Registers for Temperature Sensor
Table 1 lists the registers used to operate and calibrate the temperature sensor. Tsrange[1:0] defines the
temperature sensor range and operating mode, and tstrim[3:0] provides calibration options for the temperature
sensor. Tvoffs[7:0] can be used to automatically compensate for the offset of the measured temperature value.
The range of the temperature sensor is configurable. Table 2 lists the settings for different temperature ranges and
performance.
Table 1. Temperature Sensor Registers
Add
R/W
Function
D7
12
R/W
Temperature Sensor
Calibration
13
R/W
Temperature Value Offset
D6
tsrange[1:0]
D5
D4
entsoff
entstrim
D3
D2
D1
D0
tstrim[3:0]
tvoffs[7:0]
Table 2. Temperature Sensor Range
entoff
tsrange[1]
tsrange[0]
Temperature
Range
Unit
Slope
ADC8 LSB
1
0
0
–64 to 64
°C
8 mV/°C
0.5 °C
1
0
1
–64 to 192
°C
4 mV/°C
1 °C
1
1
0
0 to 128
°C
8 mV/°C
0.5 °C
1
1
1
–40 to 216
°F
4 mV/°F
1 °F
0*
1
0
0 to 341
°K
3 mV/°K
1.333 °K
*Note: Absolute temperature mode; no temperature shift. This mode is only for test purposes. POR value of EN_TOFF is 1.
Rev. 0.1 3/11
Copyright © 2011 by Silicon Laboratories
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2.2. Operation of the Temperature Sensor
To use the temperature sensor, perform the following steps:
1. Select the temperature sensor as the input of the general purpose ADC, “Register 0Fh. ADCconfiguration”,
adcsel[2:0] = 000
2. Set the reference for ADC, “Register 0Fh. ADC Configuration”, adcref[1:0] = 00
3. Set the temperature range for ADC, "Register 12h. Temperature Sensor Calibration", tsrange[1:0]
4. Measure and set the offset of measured temperature, “Register 12h. Temperature Sensor Calibration”,
entsoffs = 1
5. Start the measurement by triggering the ADC, “Register 0Fh. ADC Configuration”, adcstart = 1
6. Read temperature value, read contents of “Register 11h. ADC Value” after 350 µs.
The result measured by the General Purpose ADC can be translated to temperature by the following equation:
T = ADC x ADC LSB + Lowest Temperature in the temp range.
When the chip is in receiving or transmitting mode, the temperature of the die may rise. To avoid measurement
error, it is highly recommended to measure the temperature just before transmitting or receiving a packet.
2.3. Calibration of Temperature Sensor
The slope of the temperature sensor is linear and monotonic. However, for better than 10 °C absolute accuracy,
each chip has to be calibrated. This document describes two different calibration methods. After using one of the
two or both calibrations, better than ±3 °C absolute accuracy can be achieved.
2.3.1. Calibration by the Offset Register
The temperature offset error has to be measured for all parts individually. The offset can be written into the
Temperature Sensor Offset register. After enabling the automatic temperature calibration (by setting the entsoffs bit
in the “Register 12h.Temperature Sensor Control” register), the radio automatically adjusts the offset error for each
temperature value read.
In Register 12h, the MSB (tvoffs[7]) is the sign bit, and the remaining bits hold the offset value. The measured
temperature value will increase if the offset register value changes from –64 to 63 (0xFF to 0x7F).
2.3.2. Calibration by Trimming
The temperature measurement is realized as an analog circuit. The reference voltage of the circuit can be
calibrated by the tstrim bits if the entstrim bit is set.
If tstrim[3] = 1, the measured temperature value will increase when the tstrim[2:0] value increases.
If tstrim[3] = 0, the measured temperature value will decrease when the tstrim[2:0] value increases.
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3. Frequency Calibration
Usually there are frequency differences between the transmitter and receiver reference frequencies due to the
inaccuracy of the crystal. Worldwide regulatory standards control the transmit frequency accuracy; however, there
are no such rules for the receiver. The frequency offset between transmit and receive frequencies degrades the
quality of the wireless RF link.
3.1. Automatic Frequency Calibration Mechanism
There are internal autonomous algorithms that can help cancel the offset (Internal modem compensation and
Automatic Frequency Calibration).
The internal modem compensation circuit is part of the digital modem. That circuit works when the radio is set to
receive mode. It can tolerate a frequency offset up to 0.25 times the IF bandwidth.
There is another circuit called Auto Frequency Compensation (AFC), which also works in receive mode. The AFC
slightly modifies the Synthesizer Local Oscillator frequency and tries to determine the transmit frequency. When
AFC is enabled, the received signal will be centered in the passband of the IF filter, providing optimal sensitivity
and selectivity over a wider range of frequency offsets up to 0.35 times the IF bandwidth.
AFC works during preamble reception. One of the drawbacks of AFC is that it requires one additional byte of
preamble.
The circuit can be enabled by setting enafc=1 in register 0x1D.
Note: If AFC is enabled, the measured offset can be read from the Frequency Offset registers. This information can be used
later for that particular transmitter board by disabling AFC and entering this value into the Frequency Offset register.
However, these mechanisms are sometimes not suitable for the application, and it can happen that the offset has
to be canceled for each board manually. For this purpose, the Frequency Offset register and the Crystal Oscillator
Load Capacitance register can be used.
If the AFC is disabled and the frequency error is known, the synthesizer frequency may be adjusted by
programming the Frequency Offset register fo[9:0] in “Register 73h. Frequency Offset 1” and “Register 74h.
Frequency Offset 2”.
The frequency offset adjustment is implemented by shifting the Synthesizer Local Oscillator frequency. This
register is a signed register. In order to get a negative offset, it is necessary to take the 2s complement of the
positive offset number. The offset can be calculated by the following formula:
Desired Offset = 156.25 Hz x (hbsel +1) x fo[9:0], where hbsel is high band select in Register 75h.
Note: AFC and Frequency Offset register cannot be used simultaneously, since they share the same registers.
The following sections are a summary about the registers involved in different methods.
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3.1.1. Frequency Offset Registers
Register 73h. Frequency Offset 1
Bit
D7
D6
D5
D4
Name
fo[7:0]
Type
R/W
D3
D2
D1
D0
D3
D2
D1
D0
Register 74h. Frequency Offset 2
Bit
D7
D6
D5
D4
Name
fo[9:8]
Type
R
R/W
3.1.2. Crystal Load Capacitance Register
Register 09h. 30 MHz Crystal Oscillator Load Capacitance
Bit
D7
D6
D5
D4
D3
Name
xtalshft
xlc[6:0]
Type
R/W
R/W
D2
D1
D0
D2
D1
D0
3.1.3. AFC Control
Register 1Dh. AFC Loop Gearshift Override
Bit
D7
D6
Name
afcbd
enafc
afcgearh[2:0]
afcgearl[2:0]
Type
R/W
R/W
R/W
R/W
4
D5
D4
Rev. 0.1
D3
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3.2. Manual Calibration of Frequency
The accuracy of the crystal oscillator determines the accuracy of the carrier frequency. The crystal accuracy is the
sum of the following factors:
1. Initial (production) tolerance
2. Crystal load capacitance mismatch
3. Temperature dependency
4. Aging
All these numbers are on the order of 1–100 ppm for a typical 30 MHz crystal.
The offset coming from the initial tolerance and the crystal load capacitance mismatch can be compensated for
during manufacturing. However, the temperature dependency and aging can be compensated for only during
runtime.
3.2.1. Calibration of Frequency Offset Caused by Initial Tolerance and Load Capacitance
The easiest way to compensate the frequency error from the initial tolerance and the crystal load capacitance
mismatch is to tune the Crystal Oscillator Load capacitance value as long as the center frequency is accurate and
there is not more than ±1 kHz offset.
The method described in the next section can be used for initial calibration purposes.
3.2.1.1. Using a Spectrum Analyzer (Works only in TX Mode, not for RX)
Turn the radio on in continuous wave transmit mode and measure the center frequency of the transmitted signal.
Make sure that a small resolution bandwidth is used so that the frequency can be measured accurately and so the
Crystal Oscillator Load Capacitance register can be tuned. This tuning can happen as long as the frequency
accuracy is better than 1 kHz.
3.2.1.2. Using Frequency Counter
The microcontroller clock output can be multiplexed onto GPIO2 of the radio. Since that clock is divided from the
XTAL frequency, measuring it accurately with a frequency counter provides the accuracy of the crystal. This allows
for tuning of the Crystal Oscillator Load Capacitance register as long as the exact microcontroller clock output is
observed.
Note: An oscilloscope does not provide enough accuracy for this test, and it is necessary to use a frequency counter that can
measure the clock with ppm accuracy.
The crystal load capacitance can be digitally programmed to accommodate crystals with various load capacitance
requirements and to adjust the frequency of the crystal oscillator. The tuning of the crystal load capacitance is
programmed through the xlc[6:0] field of “Register 09h. 30 MHz Crystal Oscillator Load Capacitance”. The total
internal capacitance is 16.3 pF and is adjustable in approximately 127 steps (97 fF/step). The xtalshift bit provides
a coarse shift in frequency but is not binary with xlc[6:0].
The typical value of the total on-chip capacitance Cint can be calculated as follows:
Cint = 1.8 pF + 0.085 pF x xlc[6:0] + 3.7 pF x xtalshift
It is important to provide the required amount of load capacitance specified by the crystal manufacturer. The total
load capacitance, Cload, seen by the crystal can be calculated by adding the sum of all external parasitic PCB
capacitances to Cint.
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Figure 1. Connected Capacitance
The resulting load capacitance will be:
Cload = Cint + CXTD + CXTI x CXTO / (CXTI + CXTO)
CXTD, CXTI, and CXTO are present as PCB parasitics.
The internal load capacitance register value for the customer boards should be decided by measurement under the
operating frequency.
The diagram in Figure 1 can be used in deciding the load capacitance value.
Initialized the transmitter
RF parameter registers
Set the load capacitance
register to Min value
Start transmitting
Adjust the load capacitance
register value
Measure the transmitted
frequency and calculate the
frequency error
Frequency
error <
tolerance
No
YES
Record the load capacitance
register setting
Figure 2. Determining the Load Capacitance Value
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3.2.2. Calibration of Frequency Offset Caused by Temperature Change
In several narrow-band systems, the accuracy requirements cannot be achieved without compensating for the
temperature dependency of the crystal during runtime.
A cost-effective method is to utilize the on-chip temperature sensor and digitally adjust the frequency offset register
with a suitable software control loop.
Figure 3 shows the temperature characteristics of the reference frequency, measured for recommended crystals
from various vendors.
Figure 3. Crystal Frequencies vs. Temperature
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Set the test temperature
Set the radio in Idle mode
and Measure the temperature
using on-chip sensor
Initialized the transmitter
RF parameter registers
Set the frequency offset
register to Min value
Start transmitting
Adjust the frequency offset
register
Measure the transmitted
frequency and calculate the
frequency error
Frequency
error <
tolerance
No
YES
Record the frequency offset
register value under current
temperature setting
Figure 4. Measuring the Frequency Offset Value at Given Temperature
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The temperature dependency comes from the crystal itself, and the curves shown are typical for AT-cut crystals.
Other cuts have parabolic frequency error vs. temperature curves and a very small frequency error gradient around
room temperature, but, compared to the AT-cut, significantly larger overall frequency variation across extended
temperature ranges.
The actual frequency of the crystal doesn't change linearly with temperature variation as shown on Figure 3. The
actual temperature has to be periodically measured for accurate compensation. In this case, a temperature vs.
frequency offset register value table should be measured and stored in the MCU in advance. After measuring the
ambient temperature using the internal temperature sensor, the frequency offset value can be easily determined
from the lookup table according to the measured temperature and used to compensate for frequency drift.
Each chip or board should be frequency-calibrated separately because there is a temperature measurement
variation from chip to chip. To achieve 3 °C accuracy, each chip should use its own compensation offset value
because, under the same temperature, the measured temperature can spread up to ±10 degrees. It is not practical
to find a common compensation value for a batch of boards/chips.
The flowchart in Figure 4 can be used to help determine the frequency offset value at different temperature
settings.
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