A Selective RF Low Noise Amplifier
Kin-Keung Lee (sx07kl2@student.lth.se)
Yan Lu (sx07yl5@student.lth.se)
Radio project 2009
Department of Electrical and Information Technology,
Lund University
Supervisor: Göran Jönsson
Report of Radio Project 2009, By K. Lee and Y. Lu
Abstract
Low Noise Amplifier plays a key role in the front-end circuit of RF receiver. The usual
requirements are high gain, low noise as well as good input and output matching. In this
project, we provide a selective LNA works in FM broadcast band (88MHz – 108MHz). It
successfully achieves 25dB in-band transducer gain and 18dB mirror frequency rejection.
Table of Contents
Preface ....................................................................................................................................2
1. Specification ....................................................................................................................3
2. Circuit Design .................................................................................................................3
2.1
2.2
2.3
2.4
System Design .......................................................................................................3
Transistor Design ..................................................................................................3
Amplifier Topology ...............................................................................................4
Filter Design ..........................................................................................................5
2.5 DC Biasing Circuit Design....................................................................................6
2.6 AC Simulation Result ...........................................................................................7
2.7 PCB design ............................................................................................................7
3. Measurement results........................................................................................................8
4. Discussion ..................................................................................................................... 11
5. Conclusion ....................................................................................................................12
Acknowledgements ..............................................................................................................13
Reference..............................................................................................................................13
1
Report of Radio Project 2009, By K. Lee and Y. Lu
Preface
In analog super-heterodyne receiver, the selective RF amplifier was usually placed in the
front-end of the circuit, just after the antenna receiving block. The main task of this stage is
to amplifier the received weak radio signals for further processing. Besides that, with an
adjustable band-pass filter working together with the local oscillator, the wanted radio
frequency tone could be easily converted to a fixed IF.
According to the Friis’ Formula, the front-end stages will provide more contributions to the
total system output noise. As a result, the noise figure (NF) of this RF amplifier stage
should be kept as low as possible.
Meanwhile, the frequency rejection capacity is also quite important for analog
super-heterodyne receiver. A normal rejection requirement between signal tone and its
mirror frequency part is around 20dB.
2
Report of Radio Project 2009, By K. Lee and Y. Lu
1.
Specification
–
–
Operating frequency: 88 – 108MHz
Noise figure: F ≤ Fmin +3dB
–
Gain: G ≥ |S21|2
–
–
Image rejection ≥ 20dB
Vcc: 12V
–
–
Source impedance: 50Ω
Load impedance: 50Ω
2.
Circuit Design
2.1 System Design
In order to achieve good mirror frequency rejection performance, we introduced a LC
band-pass filter after the amplifier. The system is depicted in Figure 1.
Figure 1. System overview
2.2 Transistor Design
BFG520X [1] was chosen in this project because of its good balance between noise and
gain performance (see Figure 2) and it was recommended in this course.
Figure 2. Noise and gain performance of BFG520X
Because most of the parameters in the datasheet were measured under the condition that Ic
3
Report of Radio Project 2009, By K. Lee and Y. Lu
= 20mA and VCE = 6V, we also used these values in our project to get rid of the parameter
(except S-parameters) extraction.
2.3 Amplifier Topology
Since the gain requirement was not very severe, the single stage common-emitter structure
was adapted. The measured S-parameters at 98MHz were:
S11 = 0.648598∠–52.7598˚
S21 = 31.4684∠143.533˚
S12 = 0.0175707∠66.135˚
S22 = 0.776362∠–30.3839˚
And ∆ = 0.586 and K = 0.289, the transistor was conditional stable and matching networks
were needed to guarantee the stability. In our design, the output was perfectly matched to
the load, so that we could use the input matching network to achieve wanted NF.
Figure 3. Gain, noise, stability circles and reflection coefficients
The MATLAB toolbox DESLIB was used to design the amplifier. The gain, noise and
stability cycles were plotted and we also used them to calculated required reflection
coefficients (see Figure 3). ΓS was selected to 0.447∠–63.4˚ (i.e. ZS = 50 – j50Ω) so that
the input matching network could be utilized by a single capacitor (it also provided the DC
4
Report of Radio Project 2009, By K. Lee and Y. Lu
coupling function). Because of the perfect matching at the output, an L-network was added
to the output and ΓL =Γout*. The schematic is shown in Figure 4. The values inside the
brackets are the real components values. The capacitor at the output is a variable capacitor
to provide selective filtering function.
Figure 4. Schematic of the amplifier
The simulated transducer gain (with real and calculated values) of the amplifier is shown in
Figure 5.
Figure 5. Transducer gain vs. frequency
2.4 Filter Design
An LC Band-pass filter was added to filter-out the image signal and, at the same time,
minimize the in-band attenuation. The brief specification of the filter was as following:
In-band frequency: 88 – 108MHz
In-band attenuation: < 2dB
Transition band: 11MHz
Stop-band attenuation: > 20dB
By using the design procedure in [2], the filter order was calculated to be at least 3.5. Since
the matching networks also provided some sort of filtering, we used a third-order
5
Report of Radio Project 2009, By K. Lee and Y. Lu
band-pass filter. The schematic of the filter is shown in Figure 6. The transfer function of
the filter is shown in Figure 7. With real component values, the in-band attenuation was
about 3 dB and the attenuation at 119MHz was about 10dB.
Figure 6. Schematic of the third-order LC band-pass filter
Figure 7. Transfer function of the LC filter
2.5 DC Biasing Circuit Design
The DC biasing circuit is depicted in Figure 8, the most important advantage is its
insensitivity to temperature and current gain variation [2]. C1 and C2 were 470pF and acted
as decoupling capacitor. The RF choke prevented the AC signal disturb DC biasing circuit.
The calculation of the component values was simple.
First, if we assume · and · (where β = 120 according to the
V − VCE
datasheet). Then, RC = CC
= 273Ω .
IC + I D + I B
We can further assume VD = 2 ⋅ VB = 4V , then,
-
RB1 =
VCC − VD
= 1.034 kΩ
ID + IB
6
Report of Radio Project 2009, By K. Lee and Y. Lu
-
RB 2 =
VD
= 2.19kΩ
ID
and
-
RB 3 =
VD − VB
= 11.98kΩ
IB
The real values of RC, RB1, RB2 and RB are 270Ω, 1kΩ, 2.2kΩ and 12kΩ respectively.
Figure 8. Schematic of DC biasing circuit
2.6 AC Simulation Result
After the design of the amplifier and filter were done, an AC simulation was done using
Agilent ADS. The result is shown in Figure 9. The maximum gain was 29.5dB and the
image rejection was 19.9dB.
Figure 9. AC simulation result
2.7 PCB design
The PCB layout is shown in Figure 10. The vias were used to make connections between
the top and bottom planes
7
Report of Radio Project 2009, By K. Lee and Y. Lu
Figure 10. PCB layout
3.
Measurement results
From the measured S-parameters (see Figure 12) of the whole system, we could find that
the transducer gain (S21) had a flat performance around 25dB during the FM radio
bandwidth, which was larger than |S21|2 and full filled the specification. The peak frequency
was around 99MHz, not far from what we expected. The feedback (S12) was kept as low as
-35dB for the entire measured frequency range (50-150MHz). However, there was an
unstable region at the input. It may be because ΓL (was set to Γout* in the design) was too
close to the output stable circle.
The 3dB bandwidth for this amplifier was from 87MHz to 111MHz, with a peak value of
25dB at 98MHz. The transducer gain was flat during the FM band.
Figure 11. Measured transducer gain
8
Report of Radio Project 2009, By K. Lee and Y. Lu
Figure 12. Resultant Smith Chart
By turning the variable capacitor, we successfully moved the peak frequency to 92MHz,
with a 3dB bandwidth form 84 to 102MHz.
Figure 13. Mirror frequency rejection
The task of mirror frequency rejection was achieved by a 3rd order Butterworth band-pass
filter. It can be seen in Figure 13, an 18dB mirror frequency rejection was achieved the
9
Report of Radio Project 2009, By K. Lee and Y. Lu
designed center frequency 98MHz. Actually, there were trade-offs between circuit
complexity and performance. If using high Q value components or hiring higher order
filter, we could get an even better mirror frequency rejection performance.
The 1dB compressing point was measured using VNA. The worst output compression
point in-band was 2.4dBm, and that will make an input limit at around -23dBm during this
power supply option.
Figure 14. Result of 1dB compression point measurement
The 3rd order interception point was measured by Spectrum Analyzer. The trend was also
plotted based on the measured results. We can speculate a 3rd order interception point for
98MHz is around +16dBm.
IP3 Measurements
30
Outputt Power (dBm)
20
10
0
-80
-60
-40
-20
-10 0
-20
Fundumental
3-rd Harmonic
-30
-40
-50
Input Power (dBm)
-60
Figure 15. Result of IP3 measurement
10
Report of Radio Project 2009, By K. Lee and Y. Lu
The noise performance is shown in Figure 16. The blue and red curves represent the gain
and the NF of the amplifier respectively. We can find the NF was between 2 to 3dB for the
whole bandwidth, which fulfills the specification. The peaks in the noise curve were due to
outside-world radio interference. They could be eliminated if the measurement was carried
out in a shield room.
Figure 16. Result of noise measurement
4.
Discussion
To perform frequency selection is a challenge in this design. We had to make trade-offs
between circuit complexity and performance. Meanwhile, with the mirror frequency
rejection filter, implementing the frequency selection by a LC tank became even more
difficult. Because the frequency response of band-pass filter strongly affected the turning
circuit. Actually, turning the shunt capacitor in the filter was more effective according to
the MATLAB simulation, the transfer function is shown in Figure 17, however, such big
variable capacitors were not available in our lab.
Another trick came from the bias circuit. Because the current gain of the transistor was
different to the expected, the value of RB2 was increased to 2kΩ.
11
Report of Radio Project 2009, By K. Lee and Y. Lu
Figure 17. Transfer function of the filter under different shunt capacitance
Finally, because of the on-board parasitic capacitance, the operating frequency range was
different from our calculated value. As a result, we changed the shunt capacitor in the
band-pass filter to 180pF (originally 330pF).
The final schematic is shown in Figure 18.
Figure 18. Schematic of the whole circuit
5.
Conclusion
We successfully designed a Selective Low Noise RF amplifier in this project. Most of the
specifications are met. The 3dB bandwidth of this amplifier covers the whole FM radio
frequency range; in-band transducer gain is around 25dB, the mirror frequency rejection is
18dB. In addition, by turning the capacitor, this circuit also shows a frequency selective
12
Report of Radio Project 2009, By K. Lee and Y. Lu
characteristic. The measured 1dB compressing point and IP3 point are around normal
values and will not be a problem for FM receiver.
The future work may include a better band-pass filter which could combine output
matching and frequency selection together. That will also save components.
Acknowledgements
We would like to express our great thanks to Göran Jönsson, Dept. of Electrical and
Information Technology of Lund University. He gave us many pieces of precious advice
during the whole work and always be patient to our mistakes. We also like to thank Lars
Hedenstjerna, who made this so beautiful PCB for us. Special thanks to Joakim Ericsson,
Wang Jing and Anders Dahlström from Sony Ericsson for giving us technical opinions and
sharing their industrial experience with us throughout the course.
Reference
[1]
“BFG520X Product Specification”,
Website: http://www.nxp.com/acrobat/datasheets/BFG520XR_N_4.pdf
[2]
L. Sundström, G. Jönsson and H. Börjeson, “Radio Electronics”, 2004
13