Download Application Note No. 099

A p p l i c a t i o n N o t e , R e v . 2 . 0 , F e b . 2 00 7
A p p li c a t i o n N o t e N o . 0 9 9
A discrete based 315 MHz Oscillator Solution for
R e m o t e K e y l es s E n tr y S y s t e m u s i n g B F R 1 8 2 R F
B i p ol a r T r a ns i s t o r
BDTIC
R F & P r o t e c ti o n D e v i c e s
www.BDTIC.com/infineon
BDTIC
Edition 2007-02-12
Published by
Infineon Technologies AG
81726 München, Germany
© Infineon Technologies AG 2009.
All Rights Reserved.
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THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION
OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY
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ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY
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THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE.
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Application Note No. 099
Application Note No. 099
Revision History: 2007-02-12, Rev. 2.0
Previous Version: 2006-09-20
Page
Subjects (major changes since last revision)
9 – 14
Added typical characteristic vs. both supply voltage and temperature
BDTIC
Application Note
3
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Rev. 2.0, 2007-02-12
Application Note No. 099
Introduction
1
Introduction
This application note gives an introduction on how one can make a simple oscillator for low-cost applications like
remote keyless entry (RKE). For demonstration purposes, the oscillator is designed for a frequency of 315 MHz,
a commonly used frequency for remote keyless entry (RKE) and tire pressure monitoring systems (TPMS). The
oscillator is designed as a Colpitts oscillator, which is stabilized with a SAW-resonator and allows for a simple
design with only a few components besides the transistor and SAW-resonator. The modulation used for such a
design is amplitude shift keying (ASK) or simple on-off keying (OOK). Since the frequency of oscillation is fixed,
frequency shift keying (FSK) is not possible.
The transition frequency fT of the transistor should be several Gigahertz in order to ensure oscillator start-up.
However, using a transistor with too high of a fT will also increase the harmonic levels, and therefore it is not
recommended to use state-of-the-art transistors with transition frequencies far beyond 10 GHz. Furthermore, it
holds for silicon bipolar transistors that the phase noise gets smaller as the transition frequency decreases. Thus,
it appears that using Infineon’s RF transistor BFR182 with a transition frequency of 8 GHz is a good compromise.
It should be noted that phase noise depends not only on the flicker noise of the transistor itself but also on the
flicker noise and loaded Q-factor of the SAW-resonator. In fact, phase noise decreases quadratically with the
loaded Q-factor of the resonator.
BDTIC
Within every transistor family there exists several versions with different emitter areas that provide for different
collector currents. Since the loop gain of the oscillator must be greater than 1 in order to sustain oscillation, a
transistor that provides sufficient gain at the desired DC operating point and frequency of oscillation must be
selected. Infineon’s RF transistor BFR182 is a perfect fit, with enough loop gain, to sustain oscillation.
2
Principles
A principle schematic of a Colpitts oscillator in common-base configuration is shown in Figure 1. The frequency
of oscillation is determined by the resonance frequency of the parallel resonant circuit consisting of L1 and the
serial connection of C1 and C2, thus giving the resonance frequency as follows:
1 f0 = ---------------------,
2π L 1 C
(1)
where C is the combined capacitance of C1 and C2 and is expressed as follows:
C1 C 2
-.
C = -------------------C1 + C2
(2)
The serial connection of C1 and C2 acts as a voltage divider, so that not the entire output power of the transistor
is fed back to the input. By this means the harmonics will be kept low. The lower the ratio C1/C2, the higher the
voltage drop across C1 and therefore the lower the power that is fed back to the input. Furthermore, the ratio L1/C
is a figure of merit for the selectivity of the parallel resonance circuit. The higher the ratio L1/C, the lower the
selectivity and therefore the higher the second harmonic. On the other hand, to get the maximum power out of the
transistor, power matching must occur and therefore the output power of the oscillator changes with the
inductance value of L1 for a fixed ratio of L1/C and C1/C2.
T
R1
L1
C1
C2
AN099_Colpitts_Oscillator.vsd
Figure 1
Small Signal Equivalent Circuit of Colpitts Oscillator
Application Note
4
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Rev. 2.0, 2007-02-12
Application Note No. 099
The Application Board
3
The Application Board
For this application board ease of use and test has been the main consideration and therefore a SMA-connector
was used for measuring the output power directly into a 50 Ω load instead of using a loop antenna to make fieldstrength measurements. However, in the actual application the inductance L1 will be realized partially or entirely
with a loop antenna. Antenna design is a complex issue which goes beyond the scope of this application note. For
details on antenna design, please refer to [1].
Figure 2 shows the schematic of the application board and in comparison with Figure 1 additional components
are necessary for proper function. First of all, some resistors for biasing the transistor are required. The voltage
divider consisting of R2 and R3 is designed for a control voltage VON of 3 V, but different voltage levels require
different voltage dividers. Since the Q-factor of a LC-resonator is limited and the resonance frequency can change
by several percent due to tolerances, a SAW-resonator (SAWR) for frequency stabilization is required. The
matching network consisting of L1 and C3 transforms the 50 Ω load to an inductive impedance value and C4 is
simply a DC block. However, the matching network is also part of the LC-resonator, and therefore a clear
separation between matching network and LC-resonator is not possible. As already mentioned in the previous
chapter, output power changes with the inductance value of L1. It has been shown that the maximum output power
is achieved with an inductance value of 40 nH to 50 nH for L1. The RF chokes L2 and L3 as well as the RF bypass
capacitors C5 and C6 are optional and will not be required in the final, battery-powered application. The complete
bill of materials for the application board can be found in Table 1 on the next page.
BDTIC
The frequency of oscillation of the unstabilized oscillator, that is with the SAW-resonator replaced by a 560 pF
capacitor having a series resonant frequency of approximately 300 MHz, shall be roughly the desired frequency
of oscillation. Otherwise, one run the risk of causing a pseudo-oscillation at the unstabilized frequency of
oscillation. This is because of the SAW-resonator’s high Q-factor, which will result in a long settling time that gives
the oscillator enough time to start oscillation at the unstabilized frequency. Furthermore, the oscillator will not
oscillate exactly at the resonant frequency of the unloaded SAW-resonator, but the frequency of oscillation will be
shifted towards the unstabilized one. This is another reason why the unstabilized frequency of oscillation should
be close to the desired one. For this application board the frequency of oscillation of the unstabilized oscillator is
approximately 320 MHz, which results in a frequency shift of approximately 30 kHz compared to the resonant
frequency of the unloaded SAW-resonator.
V CC
C5
V ON
L3
C6
C2
C1
R1
T
L2
C4
L1
R3
RFOUT
C3
SAWR
R2
AN099_Schematic.vsd
Figure 2
Schematic of Application Board
Application Note
5
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
The Application Board
BDTIC
Figure 3
Photo of Application Board
Table 1
Bill of Materials
Designator
Value
Package
C1
C2
C3
C4
C5, C6
L1
L2, L3
R1
R2
R3
5 pF
0402
LC resonator
33 pF
0402
LC resonator
15 pF
0402
Matching
560 pF
0402
DC block
100 nF
0402
RF bypass (optional)
47 nH
0402
LC resonator
1000 nH
0805
RF choke (optional)
100 Ω
0402
Biasing
1.8 kΩ
0402
Biasing
2.7 kΩ
0402
SAWR
R961
DCC6E
EPCOS
SAW resonator, 315 MHz
T
BFR182
SOT23
Infineon
NPN Silicon RF transistor
Application Note
Vendor
Function
Biasing
6
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Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
4
Measurement Results
The (constant) collector voltage is provided through the VCC pin, while the (amplitude-modulated) control voltage
is provided through the VON pin. This makes it easy to measure collector current and control current independently.
All measurements for this application note were done with an unmodulated control voltage, that is in continuous
wave mode. Please note that for start-up time measurements of the oscillator, capacitor C6 has to be removed
first, if the function generator used to supply the control voltage has a source impedance of 50 Ω, rather than
Milliohms. Otherwise, one measures the low-pass filter consisting of capacitor C6 and the generator’s source
impedance.
Table 2 summarizes important electrical parameters of the discrete oscillator. The given values are an average of
six measurements on nominally identical boards. The oscillator is optimized for maximum output power at a low
collector current of only 6 mA. Along with the control current of 0.7 mA, the total DC current consumption of only
6.7 mA results in a high DC-RF conversion efficiency of 35 %.
BDTIC
Table 2
Electrical Characteristics at TA = 25 °C
Parameter
Symbol
Values
Min.
Typ.
Unit
Note / Test Condition
Max.
DC Characteristics (verified by 6 samples)
Supply Voltage
Control Voltage
Collector Current
Control Current
Collector Cutoff
Current
VCC
VON
IC
ION
IC,OFF
3
V
3
V
6
mA
0.7
mA
2
nA
Unmodulated
VCC = 3.2 V, VON = 0
AC Characteristics (verified by 6 samples)
Oscillation
Frequency
fOSC
315
MHz
Output Power
POUT
POUT,2
POUT,3
L(∆f)
tON,3 dB
tON,1 dB
tON,½ dB
8.3
dBm
315 MHz
-34
dBc
630 MHz
-56
dBc
945 MHz
-110
dBc/Hz ∆f = 1 kHz
15
µs
3 dB down (50% output power)
22
µs
1 dB down (80% output power)
µs
½ dB down (90% output power)
Second Harmonic
Third Harmonic
SSB Phase Noise
1)
Start-up Time
28
1) For start-up time measurements, the capacitor C6 was removed.
Figure 4 shows the harmonic suppression of three out of six boards, one with the lowest collector current
(5.76 mA), one with a typical collector current (6.01 mA) and one with the highest collector current (6.36 mA). The
second harmonic suppression of 34 dBc and even the third and subsequent harmonic suppressions of more than
50 dBc are much greater than the mandatory 20 dBc. A plot of the oscillator’s single sideband (SSB) phase noise
is shown in Figure 5. For comparison reasons, the noise floor of the source signal analyzer (SSA) is also shown.
For phase noise measurements with the SSA a 10 times correlation was used, which improves the SSA’s SSB
phase noise sensitivity by 5 dB. As shown in Figure 5, this simple SAW-resonator based oscillator achieves
exceptional low phase noise levels, much lower than PLL-based oscillators would ever achieve. On Page 9 to
Page 14 average-value curves of important electrical parameters are shown versus supply voltage as well as
versus temperature. The supply voltage for these measurements was varied between 2.5 V and 3.2 V, the typical
battery voltage of a nominal 3 V battery during its lifetime, and the temperature was varied between -40 °C and
85 °C. The shift in frequency of oscillation shown on Page 9 and Page 12 relates to the typical frequency of
Application Note
7
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
oscillation at a temperature of 25 °C and a supply voltage of 3 V. Please note that the shift in frequency of
oscillation is only a few kilohertz and this shift depends strongly on the performance of the SAW-resonator used
in the application.
Harmonic Suppression
V =3V, V =3V, T =25°C
CC
ON
A
20
Sample 1 (5.76 mA)
Sample 2 (6.01 mA)
Sample 3 (6.36 mA)
Harmonic Suppression (dBc)
30
H2
BDTIC
40
50
H3
H
4
H
5
H6
H7
H
8
H9
H
10
60
H
H
11
12
H
13
70
80
315
Figure 4
630
945
1260
1575
1890
2205
2520
Frequency (MHz)
2835
3150
3465
3780
4095
4410
Harmonic Suppression from Second to Thirteenth Harmonic
SSB Phase noise
V =3V, V =3V, T =25°C
CC
ON
A
-100
RKE Oscillator
Noise floor
-110
Phase Noise (dBc/Hz)
-120
-130
-140
-150
-160
-170
-180
3
10
Figure 5
10
4
5
10
Offset Frequency (Hz)
10
6
10
7
SSB Phase Noise of RKE Oscillator and Noise Floor of Signal Source Analyzer
Application Note
8
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Characteristic vs. Supply Voltage (verified by 6 samples)
Collector Current IC = f(VCC)
V = 3.0V, T = Parameter
ON
Output Power POUT = f(VCC)
V = 3.0V, T = Parameter
A
ON
A
7.5
9.5
7
max
9
max
85°C
8.5
85°C
25°C
Power (dBm)
Current (mA)
6.5
BDTIC
25°C
6
8
7.5
-40°C
5.5
7
-40°C
5
4.5
2.4
min
min
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
6.5
6
2.4
3.3
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
3.3
Oscillation Frequency Shift ∆fOSC = f(VCC)
VON = 3.0V, TA = Parameter
20
max
10
25°C
Frequency Shift (kHz)
0
-10
-20
-30
-40°C
-40
85°C
-50
min
-60
-70
2.4
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
3.3
Note:
1. The min/max curves show the minimum/maximum measured values and must not be taken to mean
guaranteed minimum/maximum limits.
2. The shift in frequency of oscillation relates to the typical frequency of oscillation at 25 °C and 3 V.
Application Note
9
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Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Start-up Time vs. Supply Voltage (verified by 6 samples)
tON,3dB
IC ==f(V
f(VCC
))
Start-up Time
Collector
Current
CC
V = 3.0V, T = Parameter
ON
= =f(Vf(V
) )
Start-upPower
Output
Time tPON,1dB
OUT
CCCC
V = 3.0V, T = Parameter
A
ON
A
7.5
25
9.5
40
7
max
max
9
35
85°C
8.5
max
max
85°C
25°C
Power
Time (dBm)
(µs)
Current
(mA)
Time (µ
s)
6.5
20
30
8
BDTIC
25°C
-40°C
6
25°C
5.5
15
85°C
4.5
10
2.4
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
-40°C
25°C
min
85°C
20
6.5
min
2.5
7.5
25
7
-40°C
5
-40°C
15
6
2.4
3.3
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
3.3
tON,1/2dB =Shift
f(VCC
Start-up
Time
Oscillation
Frequency
∆)fOSC = f(VCC)
V = 3.0V, T = Parameter
ON
A
20
50
max
10
45
max
25°C
Frequency
Time Shift
(µs) (kHz)
0
40
-10
35
-20
-40°C
-30
30
25°C
-40°C
-40
25
85°C
85°C
-50
min
20
-60
-70
15
2.4
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
3.3
Note: The max curve shows the maximum measured values and must not be taken to mean guaranteed maximum
limits.
Application Note
10
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Characteristic of Second and Third Harmonic vs. Supply Voltage (verified by 6 samples)
POUT,2
= f(VCC
= )f(VCC)
Second Harmonic
Collector
Current IC
V = 3.0V, T = Parameter
ON
POUT,3
= f(V=CC
f(V
) CC)
Output
Third
Harmonic
Power POUT
V = 3.0V, T = Parameter
A
ON
A
-30
7.5
-45
9.5
-31
7
-32
25°C
25°C
-34
Power (dBm)
(dBc)
Power
Power (dBc)
Current
(mA)
-50
8.5
-40°C
85°C
-33
6.5
max
9
max
max
85°C
-40°C
25°C
BDTIC
-35
6
85°C
-36
5.5
-37
-38
5
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
-40°C
7
-60
min
6.5
3.2
-65
6
2.4
3.3
25°C
7.5
-40°C
-39
-40
4.5
2.4
8
-55
85°C
min
2.5
2.6
2.7 2.8 2.9
3
Supply Voltage (V)
3.1
3.2
3.3
Note: The max curve shows the maximum measured values and must not be taken to mean guaranteed maximum
limits.
Application Note
11
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Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Characteristic vs. Temperature (verified by 6 samples)
tON,3dB
IC ==f(T
f(V
f(VACC
) ))
Start-up Time
Collector
Current
CC
T ==Parameter
Parameter
V = 3.0V, V
ON
tON,1dB
= =f(T
f(Vf(V
)) )
Start-upPower
Output
Time P
OUT
A
CCCC
T ==Parameter
Parameter
V = 3.0V, V
A
CC
ON
A
CC
7.5
25
9.5
40
7
85°C
3.2V
8.5
3.0V
25°C
-40°C
30
8
Power
Time (dBm)
(µs)
Current
(mA)
Time (µ
s)
6.5
20
max
max
max
3.2V
9
35
max
max
max
85°C
3.0V
25°C
BDTIC
2.8V
6
25°C
2.5V
85°C
min
-40°C
5.5
15
5
4.5
10
-40
2.4
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
7.5
25
-40°C
25°C
7
2.5V
min
85°C
20
6.5
min
2.5-20 2.6
2.8V
-40°C
15
6
-40
2.4
3.3
85
min
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
3.3
85
tON,1/2dB =Shift
f(VCC
Start-up Time
f(V ) )
Oscillation
Frequency
∆f)OSC = f(T
A
CC
V = 3.0V, T
VACC==Parameter
Parameter
ON
50
20
max
10
45
max
max
25°C
Frequency
Time Shift
(µs) (kHz)
0
40
-10
35
-20
3.2V
3.0V
-40°C
min
-30
30
25°C
-40°C
-40
25
85°C
85°C
2.8V
2.5V
min
-50
20
-60
-70
15
-40
2.4
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
3.3
85
Note:
3. The min/max curves show the minimum/maximum measured values and must not be taken to mean
guaranteed minimum/maximum limits.
4. The shift in frequency of oscillation relates to the typical frequency of oscillation at 25 °C and 3 V.
Application Note
12
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Start-up Time vs. Temperature (verified by 6 samples)
tON,3dB
IC ==f(T
f(V
f(VACC
f(T
))))
CollectorTime
Start-up
Current
A
CC
T ==Parameter
Parameter
V = 3.0V, V
ON
= =f(T
f(Vf(T
f(V
)CC
)) )
Output Power
Start-up
Time tPON,1dB
OUT
A
CCA
T ==Parameter
Parameter
V = 3.0V, V
A
CC
ON
A
CC
7.5
25
9.5
40
35
7
85°C
3.2V
3.0V
25°C
-40°C
Power
Time (dBm)
(µs)
Current
(mA)
Time (µ
s)
6.5
20
max
max
max
3.2V
9
35
30
8.5
max
max
max
85°C
3.0V
25°C
30
8
BDTIC
2.8V
6
25°C
max
2.5V
85°C
min
3.0V
-40°C
2.5V
3.2V
2.8V
5.5
15
5
2.8V
-40°C
25
7.5
25
-40°C
25°C
max
7
20
20
6.5
min
3.2V
2.5V
min
3.0V
85°C
2.8V
min
2.5V
4.5
10
-40
2.4
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
15
6
-40
2.4
3.3
85
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
3.3
85
tON,1/2dB =Shift
f(VACC
f(T
)∆f)
Start-up
Time
= f(T
f(V ) )
Oscillation
Frequency
OSC
A
CC
T ==Parameter
Parameter
V = 3.0V, V
ON
A
CC
50
20
45
max
Frequency
Time Shift
(µs) (kHz)
10
45
40
0
max
max
25°C
40
-10
35
35
-20
30
-30
30
3.2V
3.0V
-40°C
min
25°C
max
-40°C
-40
25
25
3.2V
85°C
3.0V
85°C
2.8V
-50
20
20
-60
2.5V
2.8V
min
-70
15
-40
2.4
2.5V
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
3.3
85
Note: The max curve shows the maximum measured values and must not be taken to mean guaranteed maximum
limits.
Application Note
13
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12
Application Note No. 099
Measurement Results
Typical Characteristic of Second and Third Harmonic vs. Temperature (verified by 6 samples)
Second Harmonic POUT,2 = f(TA)
V = 3.0V, V = Parameter
ON
POUT,3
= =f(T
f(V=
f(V
f(T
f(T
)CC
))A))
Output
Start-up
Third
Harmonic
Power
Time tPON,1dB
OUT
A
CCA
T ==Parameter
Parameter
V = 3.0V, V
CC
ON
A
CC
-30
-45
9.5
40
35
-31
-32
-34
Power
Power
Time (dBm)
((dBc)
µs)
-33
Power (dBc)
max
max
max
3.2V
9
35
-50
30
8.5
85°C
3.0V
25°C
30
8
max
BDTIC
max
-35
3.2V
3.0V
-36
7.5
25
3.2V
-38
-40°C
25°C
max
3.0V
2.8V
-37
7
-60
20
20
6.5
2.5V
-39
-40
-40
2.8V
-40°C
-55
25
2.8V
3.2V
2.5V
min
3.0V
85°C
2.5V
2.8V
min
2.5V
-20
0
25
Temperature (°C)
55
-65
15
6
-40
2.4
85
2.5-20 2.6
2.7
0
2.8 252.9
3
Supply
Temperature
Voltage
(°C)
(V)
55
3.1
3.2
3.3
85
Note: The max curve shows the maximum measured values and must not be taken to mean guaranteed maximum
limits.
References
[1]
John Kraus, Ronald Marhefka, “Antennas for All Applications,” 3rd Edition, McGraw-Hill, Dec. 2001,
ISBN 0-071-12240-0
Application Note
14
www.BDTIC.com/infineon
Rev. 2.0, 2007-02-12