AN TDA5225 35 40 Sensitivity improvement usingAFC V1.3

S m a r t L E W I S TM R X +
TDA524 0/35 /25
Sensitivity Improvement by using the Automatic
Frequency Control (AFC) for FSK
App lication No te
V1.3, 2013-10-11
Wireless Sense & Control
Edition 2013-10-11
Published by Infineon Technologies AG
Am Campeon 1 - 12, 85579 Neubiberg, Germany
© 2013-10-11 Infineon Technologies AG
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Sensitivity Improvement using AFC
TDA5240 Enhanced Sensitivity Receiver TDA5240
Revision History: 2013-10-11, V1.3
Previous Revision: V1.1
Page
Subjects (major changes since last revision)
---
Keywords added in document
Page 8
Min values for analog bandwidth added
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Application Note
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Sensitivity Improvement using AFC
Table of Contents
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Receiver and FSK Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3
Bandwidth considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
4.1
AFC configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
AFC frequency overshoot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
Sensitivity Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6
Blocking Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Application Note
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List of Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Double down conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Single down conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
System BW measurement using RSSI voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Input power = -80 dBm; K1 = K2; 25% overshoot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Input power = -80 dBm; K2 = K1/10; no overshoot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Input power at sensitivity level; K1 = K2; 25% overshoot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Input power at sensitivity level; K2 = K1/10; no overshoot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Sensitivity Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Blocking Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Application Note
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Introduction
1
Introduction
The Automatic Frequency Control (AFC) can be used to decrease the used Channel Filters to the given theoretical
occupied bandwidth of the transmitted signal. By doing this, the sensitivity of the receiver system which is defined
by the front end noise figure and the used receive channel bandwidth can be increased significantly.
This application note describes the technical background of the AFC and the techniques how to setup the channel
filters and AFC loop constants of the receiver. In general it can be stated that the described techniques are only
valid for FSK modulation.
In the Chapter 5 and Chapter 6 some measurements shows the significant sensitivity improvement by using the
AFC in combination with wide possible frequency offsets of the transmitter counterpart.
2
Receiver and FSK Demodulation
The receiver is realized as a superheterodyne architecture with single or double down conversion to the 274kHz
IF frequency. The pictures below shows the principle of single and double down conversion:
match
+
SAW
CERF
MIX1
fc=10.7MHz
fo=10.7MHz
150~300kHz
BPF
fc=274Hz
MIX2
50, 80, 125,
fo=274kHz 200, 300kHz
LIM
fc=274kHz
DIGMIX
fo=0Hz
PDF
33~283kHz
LNA
FSK
FSK
10.974
MHz
274kHz
SDPLL
Figure 1
AFC
Double down conversion
match
+
SAW
MIX1
fo=274kHz
BPF
fc=274Hz
50, 80, 125,
200, 300kHz
LIM
fc=274kHz
LNA
DIGMIX
fo=0Hz
PDF
33~283kHz
FSK
FSK
274kHz
SDPLL
Figure 2
AFC
Single down conversion
The local oscillator frequency of the receiver is derived out of the on-chip fully integrated Sigma-Delta Fractional N
PLL Synthesizer which enables a high frequency resolution. The functionality of the digital programmable
Application Note
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Sensitivity Improvement using AFC
Bandwidth considerations
Synthesizer is used by the AFC circuitry to adjust the receiver local oscillator to the transmitted center frequency.
In reality all transmitters and receivers have a frequency drift over temperature and supply voltage. Due to this
frequency drift of the system, receivers without AFC need to take this frequency drift into account by selecting the
appropriate receiver channel filter (occupied bandwidth of Signal + receiver / transmitter frequency inaccuracy).
The architecture of the IFX receiver which uses a analog bandpass filter after the down conversion and a additional
digital predemodulation filter where the bandwidth of both filters can be selected independently, enables a wide
frequency offset in combination with good jammer behavior and outstanding sensitivity performance.
In parallel to the predemodulation filter (PDF) a second FSK demodulator is used to derive the control signal for
the Automatic Frequency Control Unit, which is actually the DC value of the FSK demodulated signal. This makes
the AFC loop independent from signal path filtering and allow a wider frequency capture range of the AFC.
Since the digital FSK demodulator determines the exact frequency offset between the received input frequency
and the programmed input center frequency of the receiver, this offset can be corrected through the sigma delta
control of the PLL. As shown in Figure 1 and Figure 2, for AFC purposes a parallel demodulation path is
implemented. This path does not contain the digital low pass filter (PDF). The entire IF bandwidth, filtered by the
analog bandpass filter only, is processed by the AFC demodulator.
The maximum frequency offset removed by the AFC can be limited to avoid the tracking of unwanted signal too
far away from the selected channel.
3
Bandwidth considerations
The minimal bandwidth for the expected signal is defined by the carson rule:
(1)
BW min = 2 × ( f DRmax + f deviation )
•
•
fDRmax = max frequency in baseband data
fdeviation = deviation of FSK modulation
To achieve the best sensitivity the reception bandwidth of the receiver should be equal or just slightly bigger then
the minimal bandwidth. That means that the digital predemodulation filter (PDF) of the receiver should be set to
the minimum bandwidth as calculated by the carson rule.
As mentioned before, due to tolerances on the receiver and transmitter side the caption range of the AFC must be
large enough to cover those frequency offsets. With the two filter approach the analog bandpass filter (BPF) can
be adjusted to cover the frequency tolerances and the PDF can be set to the minimal required occupied carson
bandwidth. With such a technique the receiver is setup for highest possible sensitivity in combination with wide
frequency tolerances of the transmitter / receiver.
The best way to define the analog system bandwidth is to measure the RSSI voltage by applying a continuous
wave (CW) signal at the RF input of the receiver. The measurement shall be done by changing the frequency of
the CW and collect at each point a RSSI measurement value. The 3dB reference value should be taken at the
center frequency by decreasing the CW signal power by 3dB. In the following picture such a measurement was
done and the resulting system bandwidth is calculated.
Application Note
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Sensitivity Improvement using AFC
AFC configuration
RSSI [V]
System Bandwidth measured at analog RSSI
1.6
1.55
1.5
1.45
1.4
1.35
1.3
1.25
1.2
1.15
1.1
1.05
1
-300
-200
-100
0
0.95
200
300
Thousands
100
Signal frequency offset [kHz]
Signal -90dBm - BPF50kHz
Figure 3
Signal -90dBm - BPF125kHz
Signal -90dBm - BPF300kHz
System BW measurement using RSSI voltage
The analog system bandwidth for different bandpass filter settings are shown in the table below:
Table 1
Analog system bandwidth (related to a CW input signal)
BPF setting
BWana (typ.)
BWana (min.)
50 kHz
50 kHz (DDC/SDC)
20 kHz (DDC/SDC)
80 kHz
80 kHz (DDC/SDC)
47 kHz (DDC/SDC)
125 kHz
120 kHz (DDC/SDC)
88 kHz (DDC/SDC)
200 kHz
180 kHz (DDC/SDC)
140 kHz (DDC/SDC)
1)
300 kHz
230 kHz (DDC )
180 kHz (DDC)
240 kHz (SDC2))
200 kHz (SDC)
1) DDC .. Double Down Conversion
2) SDC .. Single Down Conversion
4
AFC configuration
A general rule to setup the right AFC limit for Run Mode Slave (always on) is:
(2)
BW ana
f AFClimit ≤ ----------------- + f deviation
4
This rule maintains that the frequency change done by the AFC will never push the center frequency out of the
analog frontend system bandwidth.
Application Note
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V1.3, 2013-10-11
Sensitivity Improvement using AFC
AFC configuration
This has the advantage that the receiver functionality at the presence of a possible interferer which is out of band
is not significantly degraded due to the AFC frequency de tuning. (See Figure 9 “Blocking Comparison” on
Page 14)
The receiver has also the functionality of Self Polling which means autonomous changing from Sleep Mode to Run
Mode. If the receiver is configured in self polling mode, the AFC is initialized after each change from Sleep Mode
to Run Mode. This means also, that a possible interferer is not able to de tune the AFC which enables a even
higher AFC limit.
A general rule to setup the right AFC limit for Self Polling Mode is:
(3)
BW ana
f AFClimit ≤ ----------------- + f deviation
2
The AFC bandwidth (and thus the settling time) of the AFC loop is programmed by means of the integrator gain
coefficients K1 and K2 (x_AFCK1CFG and x_AFCK2CFG register)
K1 mainly determines the AFC bandwidth (AFCBW). K2 influences the dynamics/damping (overshoot) - smaller K2
means smaller overshoot, but slower dynamics.
(4)
AFC BW = f DR ⋅ K1 ⋅ 1, 3
To avoid residual FM, limiting the AFCBW to 1/20 ~ 1/80 of the data rate is suggested, therefore K1 must be set to
approximately 1/25 ~ 1/100 of the data rate.
Table 2
settings available in the Explorer tooling:
AFC tool setting
K1 factor
AFC bandwidth
ultra fast
Data Rate / 10
Data Rate / 7.7
fast
Data Rate / 25
Data Rate / 20
normal
Data Rate / 50
Data Rate / 38.5
slow
Data Rate / 75
Data Rate / 57.7
very slow
Data Rate / 100
Data Rate / 80
Application Note
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Sensitivity Improvement using AFC
AFC configuration
4.1
AFC frequency overshoot
The overshoot of the frequency which is mainly influenced by the K2 factor should be reduced as much as
possible. Due to the fact that the digital filter is very tight, a overshoot in the frequency domain would result in a
extended settling time of the receiver at low input power levels.
In the picture below the output of the FSK demodulator (magenta curve) and the corresponding baseband data of
the transmitter (blue curve) are shown at -80dBm input level and 40kHz center frequency offset. In addition the
digital channel D1 shows the clock recovered data (CHIP_DATA) and the digital channel D0 shows the analog
slicer output (DATA).
Figure 4
Input power = -80 dBm; K1 = K2; 25% overshoot
Application Note
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Sensitivity Improvement using AFC
AFC configuration
Figure 5
Input power = -80 dBm; K2 = K1/10; no overshoot
It can be seen that at high input level the overshoot effect has no influence on the settling time.
The next pictures were taken with input power levels at the sensitivity boarder and 40kHz center frequency offset.
Figure 6
Input power at sensitivity level; K1 = K2; 25% overshoot
Application Note
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Sensitivity Improvement using AFC
AFC configuration
This figure shows that the overshoot results in a degradation of the FSK output signal and corresponding to that
also a degradation of the sensitivity by a given constant settling time of the receiver.
Figure 7
Input power at sensitivity level; K2 = K1/10; no overshoot
By avoiding the overshoot the signal stays always within the digital PDF filter and therefore no degradation of the
sensitivity is expected.
Application Note
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Sensitivity Improvement using AFC
Sensitivity Comparison
5
Sensitivity Comparison
In this section of the application note we will show, how the sensitivity of the system can be improved by using the
AFC and the tight predemodulation filter.
The measurement was packet based using the on chip FIFO and the message error rate limit was set to 10%.
2.4kBit, FSK, 5k Dev, Message Error Rate
-116
-114
Sen s [d B m ]
-112
-110
-108
-106
-104
Thousands
-102
-100
-100
-80
-60
-40
-20
0
20
40
60
80
100
frequency offset [kHz]
AFC ON 125kBPF 33kPDF 42kAFCLIM
Figure 8
AFC ON 300kBPF 33kPDF 64kAFCLIM
AFC OFF 125kBPF 132kPDF
AFC OFF 300kBPF 282kPDF
Sensitivity Comparison
It can be seen by using the AFC a sensitivity improvement of ~5dB can be achieved with 125kHz BPF and ~7dB
improvement by using the 300kHz BPF setting.
The following formula shows the theoretical sensitivity improvement by reducing the PDF filter bandwidth.
(5)
Sens diff = 10 × log ( BW diff )
Example:
(6)
120kHz
Sens diff = 10 × log ( BW diff ) = 10 × log ⎛ --------------------⎞ = 5, 6dB
⎝ 33kHz ⎠
(7)
230kHz
Sens diff = 10 × log ( BW diff ) = 10 × log ⎛⎝ --------------------⎞⎠ = 8, 4dB
33kHz
As can be seen the calculation follows roughly the measurements.
Application Note
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Sensitivity Improvement using AFC
Blocking Comparison
6
Blocking Comparison
The setup for the blocking measurement: Two Signal Generators where used and combined to the RF input of the
receiver. The wanted signal was set to 3 dB above the sensitivity limit at the center of the channel and the jammer
power level was increased as long the message error rate of 10% was reached again. The difference between
jammer level and wanted signal power level for different BPF bandwidth settings and AFC configurations are
plotted in the figure below.
Blocking
20
10
0
S/J [dB]
-10
-20
-30
-40
-50
-60
-500
-400
-300
-200
-100
0
100
200
Jam m er frequency offset [kHz]
125kBPF 33kPDF AFCON
Figure 9
Blocking Comparison
7
Conclusion
300kBPF 33kPDF AFCON
125k BPF 132kPDF AFCOFF
300
400
500
Thousands
300kBPF 283kPDF AFCOFF
It can be seen that usage of the AFC has a slight impact on the blocking performance. The curves using the AFC
are wider according to the used AFC limit. A trade off between the wider blocking curve and the enhanced
sensitivity needs to be done.
In general it can be seen, by using the AFC a significant sensitivity improvement can be achieved which
approximately doubles the range of the system.
Application Note
14
V1.3, 2013-10-11
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