Application Report
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
Khanh Nguyen
.............................................................................. High Speed and RF - RF Applications
ABSTRACT
The TRF6903 single-chip solution is an integrated circuit intended for use as a low-cost
FSK/OOK transceiver to establish a frequency-programmable, half-duplex, bidirectional
RF link. The TRF6903 transceiver is, therefore, well suited for many different wireless
systems and applications within the unlicensed bands worldwide.
This application report presents an example of designing the TRF6903 transceiver to
meet the requirements per the Konnex–RF System definition.
Detailed information of the TRF6903 is available from the TI-ISMRF Web site at
www.ti.com/ismrf.
1
2
3
4
5
6
Contents
Introduction .......................................................................................... 1
Quick View of the TRF6903 ....................................................................... 2
Configuring the TRF6903 .......................................................................... 3
Measured Data...................................................................................... 9
Conclusion ......................................................................................... 11
References ......................................................................................... 11
List of Figures
1
2
3
4
5
6
7
8
9
10
General Functional Block Diagram of the TRF6903 ........................................... 2
Configuration of the Crystal Oscillator of the TRF6903 ....................................... 5
Data Slicer and Bit Synchronizer Functional Block Diagram of the TRF6903 .............. 5
The Low-Pass Filter in the IF Amplifier Chain of the TRF6903 .............................. 6
Typical External Third-Order Passive Low-Pass Filter for the TRF6903 .................... 6
Calculation Using the SimpleTRF6903 Software Tool ......................................... 8
Carrier and Reference Spur ....................................................................... 9
Low-Bit Frequency and High-Bit Frequency .................................................... 9
Modulation Result at Data Rate of 8.192 kHz ................................................. 10
Modulation Result at Data Rate of 16.384 kHz ............................................... 10
List of Tables
1
1
Specification for the Spurious Response of the Crystal ....................................... 3
Introduction
The Konnex–Radio Frequency System is a specific RF system defined by the European Konnex–RF
Working Group. The physical layer parameters of this system specify that the transmitter frequency is
868.3 MHz with a transmitter frequency tolerance of ±35 ppm. The modulation is FSK with a typical
frequency deviation of ±50 kHz—minimum is ±40 kHz and maximum is ±80 kHz. The bit rate is
16.384 kbps using a Manchester coding technique. The minimum sensitivity of the receiver is specified to
be –95 dBm at a BER of 10e–4.
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
1
www.ti.com
Quick View of the TRF6903
This example of transceiver design details the configuration and the calculation of component values for a
system using the TRF6903 to meet the requirements as per the Konnex standard.
2
Quick View of the TRF6903
A simple block diagram of the TRF6903 is shown in Figure 1.
When the TRF6903 functions as a transmitter, the enabled blocks include the power amplifier (PA), the
phase-locked loop that includes the voltage-controlled oscillator, the frequency synthesizer, and the
reference oscillator and reference divider. The reference oscillator circuitry has dual functionalities. First,
the reference oscillator provides the reference signal to the phase-locked loop. Second, the reference
oscillator functions as a modulator. By switching the load capacitance of the crystal between the two
defined values that represent the low (0) and the high (1) data bits, the transmit frequency becomes an
FSK signal. The XTAL switch is normally closed when the TX_DATA is absent or the TX_DATA presents
a low-bit. The XTAL switch is opened when the TX_DATA is a high-bit.
1, 2
SLC_CAP
37
LEARN/HOLD
35
LPF_OUT
IF_IN1,2
LPF_IN
MIX_OUT
44, 43
36
39
34
LPF Amplifier
Limiter
Mixer
LNA_IN1, LNA_IN2
RFIN
47
CER_DIS
Ceramic
Discriminator
10.7-MHz Ceramic
or Discrete IF FIlter
LNA
Data Slicer
Quadrature
Demodulator
OOK
Switch
RSSI
33
Bit Synchronizer
and
Data Clock
27
41
23
Band Gap
8
/ACounter
/Div. CTRL
6
DET_OUT
45
Brownout
Detector
/N Prescaler
18
20
Serial
Interface
19
26
21
/BCounter
32/33
RX_DATA
DCLK
RSSI_OUT
RX_FLAG
CLOCK
DATA
STROBE
STDBY
MODE
22
Lock
Detect
LOCK_DETECT
PA
Output
Divider
1, 2, 3
PFD
CPs
VCO
VCO_TUNE
13
15 CP_OUT
/Ref
2...255
32
XTAL
Switch
30
XTAL_SW
4
XTAL
PA_OUT
TX_DATA
31
Loop Filter
Figure 1. General Functional Block Diagram of the TRF6903
2
TRF6903 Transceiver for Konnex–RF System
SWRA042 – May 2005
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Configuring the TRF6903
When the TRF6903 functions as a receiver, the enabled blocks include the LNA, the mixer, the
phase-locked loop, the limiter, the quadrature demodulator, the LPF amplifier, the data slicer, the bit
synchronizer, and the data clock. The local oscillator to the mixer consists of the phase-locked loop that
includes the voltage-controlled oscillator, the frequency synthesizer, and the reference oscillator and
reference divider. The reference oscillator is normally configured with the XTAL switch in the closed
position.
3
Configuring the TRF6903
3.1
Selecting the Crystal for the Reference Frequency of the Phase-Locked Loop
In order to fully use all features of the TRF6903, such as the data clock recovery, a certain crystal
frequency is preferred at a specific transmit-receive bit rate. Referring to the Konnex–RF System
specification, the bit rate of the system is 16.384 kbps or equivalently, the data rate is 16.384 kHz. The
preferred crystal frequency is 9.8304 MHz, 15.72864 MHz, or 19.6608 MHz per Table 2 of the TRF6903
Single-Chip Multiband RF Transceiver data sheet (SWRS022). A 19.6608-MHz crystal, CRYSTEK part
number 017119, is recommended for this application. This particular crystal is chosen for its low spurious
emission and harmonics distortion to prevent excessive distortion of the transmit data. Table 1 details the
spurious level of the crystal in this example.
Table 1. Specification for the Spurious Response of the Crystal
FREQUENCY OFFSET RELATIVE TO FUNDAMENTAL CRYSTAL RESPONSE
3.2
MAXIMUM SPURIOUS LEVEL
Frequency offset ≤ 30 kHz
–40 dBc
30 kHz < frequency offset ≤ 100 kHz
–35 dBC
100 kHz < frequency offset ≤ 200 kHz
–30 dBC
200 kHz < frequency offset ≤ 500 kHz
–25 dBC
500 kHz < frequency offset ≤ 1 MHz
–15 dBC
1 MHz < frequency offset
–10 dBC
Searching for the Reference Frequency
In general, the required –3-dB bandwidth of the loop filter of the phase-locked loop is about two times the
maximum data rate to minimize the distortion as well as to avoid the differences in frequency deviation of
the modulated signal at various data rates. Consequently, the reference of the phase-locked loop is about
10 times the –3-dB loop bandwidth to ensure the stability of the loop. Therefore, the minimum –3-dB
bandwidth of the loop is
LoopBWmin = 2 × DataRate = 2 × 16.384 kHz = 32.768 kHz
(1)
and the minimum reference frequency for this example is
F ref_min 10 LoopBW min 10 32.768 kHz 327.68 kHz
(2)
If the reference frequency of 327.68 kHz is chosen as a starting point, the reference divider is
ref_div 19.6608 MHz 60
327.68 kHz
(3)
With the center frequency of 868.3 MHz and the frequency deviation of ±50 kHz, the frequency of the
low-bit is
FLOW-BIT = FCENTER – FDEVIATION = 868.3 MHz – 50 kHz = 868.25 MHz; expected value
(4)
Consequently, the main divider for the transmitter is
tx_main_div 868.25 Mhz 2649.624
327.68 kHz
(5)
The value of the tx_main_div must be an integer; choose tx_main_div = 2650.
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
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Configuring the TRF6903
The actual frequency that represents the low-bit is
FLOW-BIT = tx_main_div × Fref = 2650 × 327.68 kHz = 868.352 MHz
(6)
The calculated value of Equation 6 that represents the low-bit is about 100 kHz higher than the value
defined by Equation 4.
Furthermore, the calculation of the receiver VCO frequency shows that
FVCO_RX = FCENTER + FIF = 868.3 MHz + 10.7 MHz = 879 MHz; expected value
(7)
and the main divider of the receiver VCO is
rx_main_div 879 MHz 2682.495
327.68 kHz
(8)
The value of rx_main_div must be an integer; choose rx_main_div = 2683.
The actual frequency of the receiver VCO is
FVCO_RX = rx_main_div × Fref = 2683 × 327.68 kHz = 879.16544 MHz
(9)
The calculated value of Equation 9 is about 160 kHz higher than the value defined by Equation 7.
Assume that it is possible to correct the frequency that represents the transmit low-bit by adjusting the
crystal frequency to be within the expected frequency tolerance. The error at the receiver VCO still
remains at least 35 kHz higher than the expected frequency. Thus, another reference frequency should be
tried.
Choose Fref = 357.469 kHz. Then, ref_div is 55 and tx_main_div is 2428.88. Again, the chosen integer
value of tx_main_div is 2429.
Using Equation 6, the actual frequency that represents the transmit low-bit is
FLOW-BIT = tx_main_div × Fref = 2429 × 357.47 kHz = 868.29463 MHz
and the receiver VCO main divider is 2458.9538 or the selected integer value is 2459. The actual
frequency of the receiver VCO is
FVCO_RX = rx_main_div × Fref = 2459 × 357.469 kHz = 879.0165 MHz
Again, if the frequency of the transmit low-bit is adjusted to be within the tolerance of 35 ppm or 30.34 kHz
allowed per Konnex specifications, the actual frequency of the receiver VCO is also corrected to be close
to the expected frequency. Therefore, the final reference frequency used is 357.47 kHz.
3.3
Switching Capacitors and the Crystal Oscillator Circuit
The frequency deviation during FSK operation is accomplished by pulling the crystal off-frequency. The
configuration of the crystal oscillator of the TRF6903 is shown in Figure 2 and the procedure of calculating
the capacitors Cext1 and Cext2 is detailed in the TRF6903 Design Guide (SWRU009). In this example,
the calculated value of capacitor Cext1 is 33.6 pF and capacitor Cext2 is 20 pF.
4
TRF6903 Transceiver for Konnex–RF System
SWRA042 – May 2005
www.ti.com
Configuring the TRF6903
TRF6903
CRYSTAL
Oscillator
XTAL
Ca
Cint
Programmable
Capacitor
Bank
Cb
Cext1
XTAL_SW
Switch
Cext
Switching
Capacitors
Cext2
TX_DATA
Figure 2. Configuration of the Crystal Oscillator of the TRF6903
3.4
Calculating the Sample-and-Hold Capacitor
The data slicer and bit synchronizer functional block diagram of the TRF6903 is shown in Figure 3. The
operation of the data slicer circuitry is detailed in the TRF6903 Design Guide (SWRU009).
Low-Pass Amplifier
LPF Out
39
51 kΩ
Comparator
Bit Synchronizer
and
Data Clock
33
27
23
34
Csh
LEARN/HOLD
External
Sample-and-Hold Capacitor
Figure 3. Data Slicer and Bit Synchronizer Functional Block Diagram of the TRF6903
The value of Csh, the sample-and-hold capacitor, can be calculated with the following equation:
Number of training bits
Csh ; Farad
5 51 k DataRate (Hz)
(10)
where the data rate in Hz is the fundamental frequency (in Hz) of the baseband waveform. With
Manchester encoding, the data rate equals the bit rate and with NRZ, the data rate equals half the bit rate.
The TRF6903 allows for 3 to 127 training bits, depending on the value of field TWO (in steps of 4). In
solving Equation 10, the calculated value should be rounded to the nearest standard value of capacitor.
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
5
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Configuring the TRF6903
In this example, given a system with a training sequence of 7 bits and a data rate (fundamental frequency
of the baseband) of 16.384 kHz, the sample-and-hold capacitor can be calculated as:
7
Csh 3350.94 pF
5 51 k 16.384e3
Choose the nearest standard value, which is 3300 pF, for the sample-and-hold capacitor.
3.5
Calculating the Low-Pass Filter for the Post-Detection Circuitry
The low-pass filter in the IF amplifier chain (see Figure 4) should have a bandwidth that is about twice the
data rate.
R2
C2
External Low-Pass Filter
37
36
C1
2 pF
Internal Low-Pass Amplifier
Figure 4. The Low-Pass Filter in the IF Amplifier Chain of the TRF6903
The typical value of resistor R2 in Figure 4 is 220 kΩ and capacitor C2 is calculated as:
1
C2 2 (2 DataRate) R2
(11)
In this example, the value of the capacitor is C2 = 22 pF.
3.6
Calculating the Loop Filter for the Phase-Locked Loop
The local oscillator of the TRF6903 is a phase-locked loop (PLL) that consists of an internal
voltage-controlled oscillator (VCO), an internal frequency synthesizer, and an external passive loop filter.
The internal VCO has a sensitivity level of Kvco ≈ 100 MHz/V and the internal frequency synthesizer has a
selectable charge-pump current of Icp = (0.25 mA, 0.5 mA, or 1 mA). The external loop filter is typically
designed with a third-order passive low-pass filter as shown in Figure 5.
VTUNE
R3
Icp
C1
C2
C3
R2
Figure 5. Typical External Third-Order Passive Low-Pass Filter for the TRF6903
Because the modulation is done within the loop bandwidth, the design of the loop filter requires:
• As flat a gain response as possible across the pass band to avoid a variation in amplitude of the
baseband signal. This requirement implies that the damping ratio of the loop must be about two or,
equivalently, exhibit a phase margin of about 67 degrees.
6
TRF6903 Transceiver for Konnex–RF System
SWRA042 – May 2005
www.ti.com
Configuring the TRF6903
•
The response of the loop must be as fast as the VCO changes from the frequency that represents a 1
to the frequency that represents a 0 and vice-versa. This requirement avoids over-distortion of the
baseband signal. An empirical result suggests that the required response time of the loop, as the VCO
changing from the frequency that represents a 1 to the frequency that represents a 0 and vice-versa,
can be defined based on the transmit data rate as:
1
response_time 0.8 DataRate (Hz)
(12)
where DataRate is the fundamental frequency (in Hz) of the baseband waveform.
Equivalently, the –3-dB bandwidth of the loop must be about 2 × DataRate.
Thorough analysis of the loop filter design is rather complicated; however, the calculation of the
components of the loop filter can be summarized as:
KPDK VCO
C 1 2.08e−3 ; Farad
N (DataRate)2
C 38.5 C ; Farad
2
1
1
R 2 0.65 ; Ohm
C DataRate
2
R 2 R ; Ohms
3
2
1
C3 ; Farad
50.5 DataRate R
3
(13)
(14)
(15)
(16)
(17)
Where:
I
: phase−detector gain, CP ; Arad
PD
2
ƒ
K VCO: VCO gain, 2
; radV
V
Operating_Frequency
N
: main divider constant
Reference_Frequency
K
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
7
www.ti.com
Configuring the TRF6903
3.7
Using the SimpleTRF6903 Software Tool
Figure 6 shows a screenshot of the SimpleTRF6903 software tool. Users simply enter the required
parameters and click the Calculate button to obtain the value of all needed components. This software is
available at www.ti.com/ismrf.
C001
Figure 6. Calculation Using the SimpleTRF6903 Software Tool
3.8
Selection of Component Values
The calculated values of components are not standard values; therefore, the following values were
chosen:
Crystal = 19.6608 MHz – CRYSTEK P/N 017119
Reference divider = 55 → reference frequency = 357.462 kHz
Data clock setup: D1 = 5; D2 = 16; D3 = 15
Loop filter select: C21 = 180 pF; C20 = 6800 pF; C19 = 100 pF; R4 = 6.8 kΩ; R3 = 15 kΩ
Sample-and-hold capacitor: C10 = 3300 pF
Low-pass filter: R1 = 220 kΩ; C7 = 22 pF
Crystal switching capacitors: C15 = 33 pF; C16 = 27 pF
8
TRF6903 Transceiver for Konnex–RF System
SWRA042 – May 2005
www.ti.com
Measured Data
4
Measured Data
Figure 7 through Figure 10 reveal the performance of the TRF6903 for the Konnex–RF System.
Figure 7 shows the frequency that represents the low-bit and the level of the reference spur in dBc.
D e lt a 1 [T 1 ]
− 5 6. 6 8 dB
3 5 9 .47 3 0 08 0 2 kH z
Re f L v l
20 dBm
RBW
VBW
SWT
3 kHz
10 0 H z
8. 4 s
RF A t t
50 d B
Un i t
dBm
20
1 [T1]
7 .9 3
8 6 8 .25 8 9 17 8 4
1 [T1]
− 5 6 .6 8
3 5 9 .47 3 0 08 0 2
1
10
0
d Bm
M Hz
dB
k Hz
A
−10
−20
−30
−40
1
−50
1V I E W
1AP
−60
−70
−80
C en t e r 8 68 . 3 M H z
1 00 k H z /
S p an 1 MH z
Figure 7. Carrier and Reference Spur
Figure 8 shows the frequencies that represent the low-bit and the high-bit transmit data.
Re f L v l
20 dBm
M a rk e r 1 [ T 2 ]
− 2 1. 7 3 dB m
8 6 8 .30 0 0 00 0 0 MH z
RBW
10 k H z
VBW
SWT
1 kHz
2 50 m s
20
1
2
10
0
−10
1
−20
1 [T2]
RF A t t
50 d B
Un i t
dBm
− 2 1 .7 3
8 6 8 .30 0 0 00 0 0
1 [T4]
2 9 .7 2
− 4 0 .00 0 0 00 0 0
2 [T2]
2 9 .7 2
4 0 .00 0 0 00 0 0
d Bm
M Hz
dB
k Hz
dB
k Hz
B
−30
−40
−50
−60
− 7 0 2V I E W
− 8 0 4V I E W
C en t e r 8 68 . 3 M H z
Date:
3 0. AUG.2 00 4
2AP
4AP
1 00 k H z /
S p an 1 MH z
15 :0 4:15
Figure 8. Low-Bit Frequency and High-Bit Frequency
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
9
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Measured Data
Figure 9 and Figure 10 present the modulated data signal and the modulation summary of the data rates
of 8.192 kHz and 16.384 kHz, respectively.
CF
Ref
Lvl
20
dBm
868.3
DEMOD
BW:
300
MHz
Real
kHz
AF−Signal
FM
Time
OFF
[Hz]
10 0 k
80k
60k
40k
20k
0
−2 0 k
−4 0 k
−6 0 k
−8 0 k
− 10 0 k
START
0
s
ST O P
CF
Ref
Lvl
20
dBm
868.3
DEMOD
BW:
300
Real
kHz
MOD
MODULATION
SINAD 1
kHz:
69.423
kHz +Pk
69.102
kHz
s
OFF
SUMMARY
FM
DEMOD
SUMM ARY FM
Pk/2
−6 8.780 kHz
−Pk
4 7.409 kHz
RMS
B
−−
AUDIO FR EQ:
8.19210
kHz
FREQ ERR OR:
−2.075
kHz
CARR PWR :
FILTER:
500
Time
ANALOG
FM:
8.22
HP
dBm
−−
LP
none
DEEMPH −−
PRE
AM:
3.82
PM:
10.061
Da te:
MHz
30.AUG.2004
%
DISP −−
Pk/2
rad
Pk/2
15:14:30
Figure 9. Modulation Result at Data Rate of 8.192 kHz
CF
Ref
Lvl
20
dBm
8 6 8. 3
DEMOD
BW:
30 0
MHz
Real
kHz
AF−Signal
FM
Time
OFF
[Hz]
100k
80k
60k
40k
20k
0
−20k
−40k
−60k
−80k
−100k
START
0
s
STOP
CF
Ref
Lvl
20
dBm
8 6 8. 3
DEMOD
BW:
30 0
MHz
Real
kHz
MOD
Time
500
SUMMARY
ANALOG
s
OFF
FM
DEMOD
MODULATIO N SUMMARY FM
F M:
67 .227 kHz +Pk
Pk /2
67 .619 kHz
S INAD 1
kHz:
kHz −Pk
46.174
kHz RMS
16. 3840 kHz
F REQ ERROR:
−2 .069 kHz
C ARR PWR:
8 .18 dBm
HP
−−
LP n one
DEEMPH −−
PRE DISP
Date :
B
−−
A UDIO FREQ:
F ILTER:
− 68.012
A M:
4 .03 %
P M:
4 .841
30.AU G.2004
−−
Pk/2
rad
P k/2
15:2 1:01
Figure 10. Modulation Result at Data Rate of 16.384 kHz
10
TRF6903 Transceiver for Konnex–RF System
SWRA042 – May 2005
www.ti.com
Conclusion
5
Conclusion
This application report presented the design process for a Konnex–RF System using the TRF6903. To
avoid distortion of the baseband data, the selected crystal must meet the requirements of Table 1.
Calculated values for the required components can be easily obtained by using the SimpleTRF6903
software tool available at www.ti.com/ismrf.
6
References
1. TRF6903 Single-Chip Multiband RF Transceiver Data Sheet (SWRS022)
2. TRF6903 Design Guide (SWRU009)
3. KNX – Radio Frequency Specification – Version 10 – Volume No. 22
SWRA042 – May 2005
TRF6903 Transceiver for Konnex–RF System
11
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