Transcript LC Tunable Oscillator

LC Tunable Oscillator
ELG4135 Electronics III Project
Team Members:
Hubert Mamba
Fu Jingyi
Wang Jian
Introduction
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Tuned oscillators are largely used in data transmissions
and radio communications.
In RF circuits, mainly LC oscillators are used and their
frequency is, most of the time, controlled using a
variable-capacitance (varicap) diode . This method is a
little bit complicated and costly .
We present a single grounded resistance tunable
sinusoidal oscillator that
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requires only three transistors, some passive components and no
variable-capacitance and
the oscillating frequency is controlled through a grounded
resistor. Another grounded resistor independently controls the
condition of oscillation.
Main Objective:
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The main objective of this
project is to investigate the
performance of the new
method (Single grounded
resistance tunable Sinusoidal
Oscillator).
The usual method (VCO)
requires the use of variablecapacitance diode to control
the oscillating frequency
value. This VCO is expensive
and complicated to fabricate.
Differential VCO (Collpitt)
Oscillator. (from Lucio Carlo
Rodoni
“http://n.ethz.ch/student/rodonil/da/beri
cht/node28.html”
Main Objective (cont’d)
The alternative (new)
method requires no
variable-capacitance, it
is not expensive and is
easy to implement. Its
circuit diagram is shown
in the figure bellow.
VCC
12V
C1
150pF
VCC
12V
VCC
2
VCC
R6
10k
VCC
U1
VCC
6
12V
R2
10k
MU1*
Q2
5
PN2907
Q3
Q1
2N2222A
8
1
2N2222A
R3
560
R1
R5
3
330
10k
0
0
0
R4
3.3k
11
Background Knowledge Review
Many people have proposed different
models to produce and control
Oscillating frequency for different
purposes. Bellow is a circuit analysis of a
simple LC oscillator. We can see that the
oscillating frequency depends on the value
of L and C.
LC circuit oscillating frequency calculation:
Circuit analysis result
Theoretical calculation:
The above formulas are copied from
our report.
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1
LC 1   
1
LC 1   
k 1

k  1
R1 // R2 
R3
Theoretical Results
The
continuous
curves are
theoretical
plot from
Matlab
The doted curves are got
From the Multisim
simulation.
The simulation on Multisim 8 shows that for k=-1, the oscillating frequency is
increasing as R3 increases, and for k=1, the oscillating frequency decreases as
R3 decreases, it proves that this single resistor control LC circuit is working.
The theoretical result is very close to the simulation ones.
Simulation in Multisim
The simulation shows that the
circuit can oscillate and the
oscillating frequency agrees
with the theoretical result.
This oscillating simulation plot
is at R3=0.56K ohm, and
oscillating frequency is 300k
Hz at K=1.
Simulation in Multisim (cont’d)
Performance of the circuit
From the simulation
result we can see that
the theoretical
calculation is very
close to the simulation
result, especially when
R3 is bigger than 40K
ohm, the oscillating
frequency tends to
merge into one value
of different k values.
It shows that the
design of the circuit on
this paper is excellent.
Data recorded from the simulation, for each value of R3 and K, we found a
corresponding frequency f.
K=1
K=-1
R3(kohm)
f (kHz)
R3(kohm)
f ( kHz)
5.1
852
5.1
288
10
435
10
323
20
373
20
343
30
366
30
356
39
365
39
362
51
359
51
359
Conclusion
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The circuit was tested for positive and negative values of k.
A good quality sinusoidal signal was obtained for the negative value
of k,
but the shape of the signal is worse when k is positive,
especially for the low values of R3.
The simulation results are in agreement with the theory, particularly
for negative values of k.
Things to be improved
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It is not easy to get higher frequency values (Giga hurts) as we would in
practice.
The transformer ferromagnetic losses, the lag phase brought by Q1, Q2 and
Q3, the output impedance of Q3 and the stray capacity in
parallel with R1//R2 limit this high frequency value.
Designing more powerful amplitude limitation device, based on nonlinear
voltage-controlled voltage source
Further research to increase the quality factor (by decreasing the value of
resistors R1 and R2) should allow the
improvement of the signal shape for the positive values of k.
Higher frequency values should also be obtained.
References
1. J. Bayard ‘ Single grounded resistance tunable sinusoidal
oscillator’, IEEE Proc.-Circuits Devices Syst., Vol 151,No.2,April
2004.
2. Chen, JJ., Chen, C.C., Tsao, H.W., and Liu, S.I. :’Current mode
oscillators using single current follower’, Electron Lett.,
1991,27,(22), pp.2056-2059
3. Tao, Y., and Fidler, J.K.:’ Electronically tunable dual OTA second
order sinusoidal oscillators/filters with non-interacting controls: a
systematic synthesis approach’, IEEE Trans. Circuits Syst. I,
Fundam, Theory AppL, 2000,47,(2), pp.117-129
4. Cam,U., Kuntman, H. ,and Acar, C. :’On the realization of OTA-C
oscillators’,Int.J.Electron,1998,84,(3),pp.313-326
Special Words
Special thanks to the Prof. Dr.
Habbash
And
The teaching assistants