XE1200 External RSSI

XE1200 External RSSI
AN1200.01
Application Note
XE1200 External RSSI
Rev 1 August 2005
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XE1200 External RSSI
Table of Contents
1
2
3
4
5
Introduction ...................................................................................................................................................... 3
RSSI algorithm ................................................................................................................................................. 4
External RSSI usage........................................................................................................................................ 5
Hardware connections ..................................................................................................................................... 7
Software implementation ................................................................................................................................. 7
5.1
XE1203Driver.c......................................................................................................................................... 8
5.2
Initialisation.c ............................................................................................................................................ 9
5.3
Uart.c ........................................................................................................................................................ 9
5.4
Measure Rssi.c ....................................................................................................................................... 10
5.5
Considerations for the MeasureRssi implementation............................................................................. 10
6 Performances and trade-off ........................................................................................................................... 13
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XE1200 External RSSI
1
INTRODUCTION
VSS
VSSA
IAMP
QAMP
VDD
VDDA
VSSP
VDDF
VDDD
VDDP
VSSF
VSSD
The purpose of this application note is to develop a high dynamic and good accuracy external RSSI for the
XE1203F. This RSSI is made using I (IAMP) and Q (QAMP) analog signals. These signals are at the output of
the low pass filter in the XE1203F Rx path:
IAMP
QAMP
SI
RSSI
FAMP
LPF
BBAMP
SO
LIM
CONTROL
DATA
RFA
MATCHING
NETWORK
EN
LNA
FEI
SWITCH
RFB
FAMP
LPF
BBAMP
LIM
PATTERN
MATCHING
LO_BUF
IREF
DEMOD
BITSYNC
MMOD
DIVIDER
LOGIC
CONTROL
DATAIN
DATA
11 Bits
BARKER
ENCODER
modulator
/n
Synthesizer
DIVCTL
CLKXTAL
XE1203 TrueRF™
XE1203
RFOUT
PA
PATTERN
DCLK
PHASE
SHIFTER
11 Bits
BARKER
DECODER
MATCHING
NETWORK
SCK
CH PUMP
PFD
VCO
OSCILLATOR
/n
CLOCK OUT
IREF
POR
TKA
VCO
TANK
TKB
XTA
LFB
LOOP
FILTER
XTB
CLKOUT
TSUPP TMOD(3:0)
POR
XTAL
The RSSI algorithm will evaluate the power on I or Q signal. It represents the received signal’s power. It can be
implemented on any of the XE1200 transceivers that have I and Q outputs.
In a first section, we will explain the algorithm, its hardware and software implementation and finally present its
performances and trade-off. This algorithm is developed in C in order to be easily portable.
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XE1200 External RSSI
2
RSSI ALGORITHM
The RF received signal’s power can be evaluated at the I and Q level. Knowing the gain in the Rx path and the
power on I or Q, it is possible to determinate the received signal’s power.
The RSSI algorithm will evaluate the power on the I signal. This algorithm can be divided in two parts:
First, the estimation of the DC voltage on I and then the estimation of the RSSI.
The algorithm will be the following:
Step 2: RSSI Measurement
A/D converter
I or Q
1/A
A/D
ABS
z
−1
EstRssi
I_DC
z −1
Step 1: DC voltage estimation
Hardware
Software
Figure 1.
EstRssi relates to the level of the signal at the antenna, so depending on the application; one can calculate the
real power of the signal by applying the formula or works in relative.
To calculate the power of the received signal in dBm, first it is necessary to calculate the power of EstRssi with
a 50Ω load, to substract the gain of the Rx path, to transform dB to dBm and then to take into account that the
algorithm introduces a constant of 3.9. The formula is the following:
Formula1:
PRe alDbm = 10 * log( PEstRssi ) − G + 30 + 3.9 = 10 * log(
EstRssi 2
) − G + 30 + 3.9
50
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XE1200 External RSSI
3
EXTERNAL RSSI USAGE
This application can be easily set up. It only requires an RF signal generator or a XM1203F to transmit a RF
signal, a XM1203F to receive this signal, a MSP430, a serial cable and a PC to receive the data.
RF cable
RF signal
Generator
Serial cable
XM1203 F
MSP430 :
RSSI
software
embedded
Or
Serial cable
XM1203 F
XM1203 F
MSP430 :
RSSI
software
embedded
The configuration of the UART is the following:
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XE1200 External RSSI
On the PC, the received data will be in the following format:
The return value is related to the 12 bit ADC. To get the received signal power in dBm, we should apply the
formula 1 from section 2.
So for a return value of 0x0303, it corresponds to x = 303*2.5/FFF = 0.47 V and if we apply the formula
PRe alDbm = 10 * log(
EstRssi 2
) − G + 30 + 3.9 , we find that the level is -73.6 dBm for G = 84 dB.
50
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XE1200 External RSSI
4
HARDWARE CONNECTIONS
This algorithm is developed in C and has been implemented on a MSP430. It can be easily ported to another
microcontroller that has an ADC.
Connections between the XM1203 and the MSP430:
XM1203
SCK
SI
SO
EN
TX
RX
DATAIN
P1.0
P1.1
P1.2
P1.3
P1.4
P1.5
P1.6
DCLK
DATA
PATTERN
P2.0
P2.1
P2.2
Pad I or Q
MSP430
P6.3: ADC Input
Required for the RSSI application
Optional
5
SOFTWARE IMPLEMENTATION
For the software implementation, the main function calls sub functions for the initialization of the MSP430, the
initialization of the XM1203F, to set the XM1203F into receive mode, to calculate the RSSI and to send the
result to the UART.
The sub functions related to the XM1203F are located in the file XE1203Driver.c. The ones related to the
initialization of the MSP430 are in the file Initialisation.c. Uart.c contains the sub functions to handle the UART
and MeasureRssi.c to measure the RSSI.
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XE1200 External RSSI
5.1
XE1203DRIVER.C
The file XE1203Driver.c includes functions to drive the XE1203. They have the same prototypes as the ones
developed for the API TN8000.18 “XE8000 driving XE1200 transceivers”. The same conventions and global
variables are used.
For the different data types, the same conventions are used but adapted to the MSP430 compiler:
Standard type name
unsigned char
signed char
unsigned short
short
long
unsigned long
double
Defined type name
_U8
_S8
_U16
_S16
_S32
_U32
_F32
Comment
8 bit quantity
8 bit signed quantity
16 bit quantity
16 bit signed quantity
32 bit signed quantity
32 bit quantity
32 bit float quantity
To configure the XE1203F (baud rate, frequency deviation, mode A or B), the variable RegistersCfg[] is used
( see chap 5.4 “Initializing transceiver parameters” from TN8000.18 [2]).
Most of the functions in the XE1203Driver.c file of this application can be found in the XE1203Driver.c file of
TN8000.18 [2]. They are here implemented on a MSP430 using the same syntax. Utility functions have been
specially developed. For further information see chap. 6 “API Functions” of TN8000.18 [2].
Configuration functions:
InitRFChip : This routine initializes the RFChip registers, Using Pre Initialized variable
SetRFMode : Sets the XE1203F operating mode (Sleep, Receiver, Transmitter)
WriteRegister : Writes the register value at the given address on the XE1203F
ReadRegister : Reads the register value at the given address on the XE1203F
Communication functions:
SendRfFrame : Sends a RF frame
ReceiveRfFrame : Receives a RF frame
SendByte : Send a data of 8 bits to the transceiver LSB first
ReceiveByte : Receives a data of 8 bits from the transceiver LSB first
Utility functions:
Wait : This routine uses the timer A to create a delay using the ACLK clock
InvertByte : Inverts a byte. MSB -> LSB, LSB -> MSB
Functions developed specifically for the MSP430:
WaitSCK : This routine use a time out loop to have SCK lower than 1 MHz
TxInterruptOn : Initializes the timers and the interruptions related to the TX routines
TxInterruptOff : Disables the timers and the interruptions related to the TX routines
RxInterruptOn : Initializes the timers and the interrupt related to the RX routines
RxInterruptOff : Disables the timers and the interrupt related to the RX routines
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XE1200 External RSSI
5.2
INITIALISATION.C
The file Initilisation.c contains all the functions used to initialize the MSP430.
The MSP430 will be clocked by an 8 MHz external quartz.
InitMicro : Initializes the MicroController peripherals, it calls the following sub functions:
InitPort1 : Initializes the Port 1 as an I/O and set the pins as outputs
InitPort2 : Initializes the Port 2 as an I/O and set the pins as inputs
InitPort3 : Initializes the Port 3 and set P3.4 for the UART
InitPort6 : Initializes the Port 6 and set P6.3 = A3 = Input used by the ADC
InitClockModule : Initializes the clock module. MCLK = LFXT1 = 8MHz and ACLK = LFXT1 = 8MHz,
SMCLK=DCOCLK (Res=0, DCO=0 -> freq=74.1 KHz)
InitTimerA : Initializes the Timer A
InitTimerB : Initializes the Timer B
InitADC : Initializes the ADC. The internal reference voltage 2.5 V is used for Vr+ and Vr- = AVss.
Continuous sampling and conversion mode is programmed.
ADC12CLK = ACLK = 8 MHz, tsample = 4* ADC12CLK, tconvert = 13* ADC12CLK
InitUART: Initialize the UART0 to 8 bits data, 1 stop bit, even parity, Baud Rate = 19200 bps
5.3
UART.C
This contains functions to transmit data to the UART.
Hex2Char: Converts an hexadecimal number
SendChar: Sends a character to the UART
SendString: Send a string on UART Tx using SendChar function
SendCRLF: Sends carriage return and line feed characters
TransmitRssiUart: It transmits the result of the RSSI to the UART
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XE1200 External RSSI
5.4
MEASURE RSSI.C
The file MeasureRssi.c contains the functions to measure the RSSI.
MeasureRssi: It first estimates the DC on I and calculates EstRssi
MeasureRssi function: is a function that is in the fileMeasureRssi.c
5.5 CONSIDERATIONS FOR THE MEASURERSSI IMPLEMENTATION
With the 12 bit MSP430 ADC, it takes 17 clock cycles to sample and convert a signal. So the maximum
sampling frequency is 470 KHz. Between two samplings, it is necessary to process them. To estimate the DC or
to calculate the RSSI, it requires more than 17 clock cycles. So the real sampling frequency is fixed by the time
execution of the code which is 250 KHz in the two processes. The goal is to get the maximum sampling
frequency in order to deal with higher frequency deviation. When a sample is ready, a flag is activated, the
sample is processed and we wait for the next sample.
In order to fill the Shannon theory and avoid overlapping, the sampling frequency will be at least two times the
frequency of I or Q. The maximum sampling frequency obtained is 250 KHz, so the maximum frequency
deviation that can be used is 125 KHz. When changing the frequency deviation, the variable FREQ_DEV
located in MeasureRssi.h should be updated and set to the new frequency deviation. The variable
FREQ_SAMPLE is fixed to 250000.
This code is optimized for high frequency deviation; the goal has been to set as less instructions and cases as
possible.
The measurement of the RSSI will be done in two steps as followed:
DC Estimation
RSSI calculation
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XE1200 External RSSI
DC Estimation
no
Number of samples <
NB_SAMPLE_I_DC
yes
no
Sample ready
yes
Read sample
Algorithm
see fig 1.
Number of
samples ++
I_DC is evaluated
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XE1200 External RSSI
EstRssi Calculation
no
Number of samples <
NB_SAMPLE_I_RSSI
yes
no
Sample ready
yes
Read sample
Algorithm
see fig 1.
Number of
samples ++
RSSI is evaluated
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XE1200 External RSSI
6
PERFORMANCES AND TRADE-OFF
6.1
PERFORMANCES
In order to get a greater dynamic, we use mode A and mode B of the XE1203F. In mode A, the gain of the
XE1203F is the greatest, it is about 84 dB. In mode B, the linearity of the receiver is increased and the gain is
about 67 dB. The RSSI dynamic obtained is 60 dB, going from -111 dBm to -51 dBm. The standard deviation
obtained is:
- 0.3 dB from -51 dBm to -70 dBm in mode B
- 0.2 dB from -70 dBm to -95 dBm in mode A
- 0.3 dB to 0.6 dB from -95 dBm to -111 dBm in mode A
Following is the linearity of the RSSI in mode A and mode B:
Linarity of the RSSI in mode A
-55
Mode A, Gain Rx= 84 dB
Dev = 55 KHz,
BR=4.8 Kps
-65
-75
RSSI Out
-85
average mesure
theorical measure
min measure
max measure
-95
-105
-115
-125
-125
-115
-105
-95
-85
-75
-65
-55
Level at the antenna ( dBm )
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XE1200 External RSSI
Linearity of the RSSI in mode B
-40
Mode B, Gain Rx = 67 dB
Dev = 55 KHz,
BR=4.8 Kps
-50
-60
RSSI Out
-70
average measure
theorical measure
min measure
max measure
-80
-90
-100
-110
-110
-100
-90
-80
-70
-60
-50
-40
Level at the antenna
6.2
TRADE-OFF
To get this dynamic, this accuracy and this linearity, it takes first 680 µs to estimate the DC and 500 µs to
calculate EstRssi. This time can be optimized by changing some parameters in the DC estimation or in the
EstRssi calculation.
I_DC Estimation:
The DC estimation is done on NB _ SAMPLE _ I _ DC = 25 *
FREQ _ SAMPLE
samples that is to say 25
FREQ _ DEV
periods of the frequency deviation.
For a frequency deviation of 55 KHz ( FREQ _ DEV = 55000) , it takes 680 µs to make the DC estimation but
we can decrease this time by setting NB _ SAMPLE _ I _ DC = 10 *
FREQ _ SAMPLE
. The DC estimation
FREQ _ DEV
will be done on 10 periods of the frequency deviation.
These two examples have been implemented. In the second case, the DC is not estimated with the best
accuracy. So, the lowest level of the RSSI is shifted and the dynamic decreased. The accuracy of the RSSI has
remained the same.
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XE1200 External RSSI
Linarity of the RSSI versus the time to estimate I_DC
-55
Mode A, Gain Rx= 84 dB
Dev = 55 KHz,
BR=4.8 Kps
-65
-75
RSSI Out
-85
average measure, Time
I_DC=680 us
theorical measure
average measure, Time
I_DC=358 us
-95
-105
-115
-125
-125
-115
-105
-95
-85
-75
-65
-55
Level at the antenna ( dBm )
As a general rule, the time to estimate I_DC is:
TimeI _ DC = 103 *
NB _ SAMPLE _ I _ DC
+ 140
200
EstRssi calculation: Integration of | I - I_DC |
The EstRssi calculation is done on
NB _ SAMPLE _ RSSI = 25 *
FREQ _ SAMPLE
samples.
FREQ _ DEV
It takes 500 µs to make this calculation. As for the DC estimation, this time can be reduced by setting:
NB _ SAMPLE _ RSSI = 10 *
FREQ _ SAMPLE
FREQ _ DEV
These two examples have also been implemented and we can see that the dynamic is the same but the
accuracy has been degraded (standard deviation goes from 0.3 to 0.8 dB)
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XE1200 External RSSI
Linarity of the RSSI versus the time to integrate | I - I_DC |
-55
Mode A, Gain Rx= 84 dB
Dev = 55 KHz,
BR=4.8 Kps
A=8
-65
average measure, Time
Est_Rssi=500 us
theorical measure
-75
min measure, Time
Est_Rssi=500 us
max measure, Time
Est_Rssi=500 us
RSSI Out
-85
average measure, Time
Est_Rssi=300 us
min measure, Time
Est_Rssi=300 us
max measure, Time
Est_Rssi=300 us
-95
-105
-115
-125
-125
-115
-105
-95
-85
-75
-65
-55
Level at the antenna ( dBm )
As a general rule the time to calculate the EstRssi is:
TimeEst Rssi = 103 *
NB _ SAMPLE _ I _ RSSI
+ 140
275
So depending on the application, we can decide either to improve the dynamic, the resolution or the time by
changing the parameter NB_SAMPLE_I_DC or NB _SAMPLE_RSSI located in the file MeasureRssi.h.
So finally, we will only have to change the parameter FREQ_DEV which corresponds to the frequency deviation
of the application and if necessary NB_SAMPLE_I_DC or NB _SAMPLE_RSSI in the file MeasureRssi.h.
REFERENCES
[1] XE1203F datasheet
[2] TN8000.18:XE8000 driving XE1200 transceivers standard API definitions
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XE1200 External RSSI
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