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1
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 1
How to Suppress Spurious
Signals in Oscillator Designby
Norbert H.L. Koster
Bettina J. Koster
Adalbert Beyer, Fellow IEEE
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 2
Overview
Introduction
The Oscillator Circuit
CAD of the Oscillator
Experiments on the Oscillator
Spectral Lines
Improving of the Circuit and Results
Discussion
2
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 3
IntroductionA technique entitled
Fast Approximation Formulas Describingthe Non-linear Intrinsic Transistor
Equivalent Circuit Elements Speed Up andImprove the CAD of Oscillator Circuits
was introduced in frame of the 2003 IEEE MTTIMS workshop marked by WSI in Philadelphia.That contribution has treated an important topicof oscillator design.During the last three years, new developments
on this field justify a continuation of the
presentation mentioned above.
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 4
IntroductionSome Facts:
Unwanted spurious signals appear in the
oscillator’s spectrum when the oscillator circuit
produces at least one additional signal than
only the designed one.
These additional signals interfere with the
projected oscillation frequency or with its
harmonics and produce a number of disturbing
spurious signals.
3
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 5
Introduction
What is to do?
To avoid this problem it is necessary to analyzethe reason for these additional oscillations andto take such measures as inhibiting theoscillator’ s circuit capability to produceadditional parasitic oscillations.
This presentation examines the problem ofexciting spurious signal in an oscillator circuitand demonstrates, how an oscillator can beCAD designed and assembled, producing thedesired frequency signal, only.
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 6
The Oscillator Circuit
4
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 7
CAD of the OscillatorIn order to analyze the oscillator circuit, we use a programpackage from Stephen A. Maas and Arthur Nichols entitled
„C/NL 2 for Windows 95, NTT and 3.1: Version 1.2 – Linearand Nonlinear Circuit Analysis and Optimization, ArtechHouse Publishers, London, 1996. – ISBN 0-89006-899-2.”
Although, this CAD tool is very compact and easy to use, itallows a very fast and effective implementation of all thenecessary equations to calculate the voltage-dependentvalues of the intrinsic elements of both the JFETs during thetuning and analyzing sequence.
Additionally, it is possible to insert easily a sufficient numberof own equations directly into the CAD program.
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 8
CAD of the Oscillator
The program uses nodal analysis similar to SPICE.
The first task is to determine the variation of small-signal parameters of the active elements as afunction of dc operating (quiescent) point.Furthermore, it is necessary to predict the feasibilityfor oscillation.
These problems were solved in the Philadelphia talkby using simple algebraic expressions forcalculating those quantities.
5
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 9
CAD of the OscillatorDC Analysis (Source Coupled JFETs)
( )d2d1SsIIRV +=
sgs2gs1 VVV ==
( )d2d1Scds1IIRVV +=
( )d2dd2d1Scds2IRIIRVV +=
RD Vr
RSVS
VC
T1 T2
Id1 Id2
Vds1 Vds2
Vgs1 Vgs2
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 10
CAD of the OscillatorDrain-Source-Voltage of JFET 1
Rd Ur
RSUs
UbT1 T2
Id1 Id2
Uds1 Uds2
Ugs1 Ugs2
0
1
2
3
4
5
1 2 3 4 5
Ub / V
Ud
s1
/ V
Rs
10
15
22
33
47
56
68
82
100
f01
= 0.0303
RS
f02
= 1.385
VC
Vquantitiesthewith
Vds1
V= 0.9613
Vc
V1.0246 (1 e
f01)(1 e
f02 )
6
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 11
Small Signal Equivalent Circuit of a JFET
Cgd
ugs Cgs
RgsRds
Cds
ids
ids = ugs gm e-j
RGLG RD LD
RS
LS
CPG CPD
CPGD
ZL e , e , l e ZL a , a , l a
1
1'
2
2'
G D
S
g d
s
CAD of the Oscillator
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 12
CAD of the Oscillator
Drain-Source-Capacitor
Cgd
ugs Cgs
Rgs Rds
Cds
ids
ids = ugs gm e-j
g d
s
abbreviation:
6010631)e1(732285 13f
fF
ds,,
C=
V
ds
V
gs13
4831
15691f
U
,U
,
+
=
7
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 13
CAD of the Oscillator
Drain-Source-Capacitor
Cgd
ugs Cgs
Rgs Rds
Cds
ids
ids = ugs gm e-j
g d
s
0
100
200
300
0 1 2 3 4 5
Uds / V
Cd
s /
fF
Ugs / V
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
-1,2
-1,4
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 14
CAD of the Oscillator
Remember, the JFETs used as active devices in
this oscillator are the type of
CFY30
from INFINEON Technologies AG.
From there, all results given in this presentationrefers solely to this type of JFET, but may be similarif comparable types are used.
8
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 15
CAD of the Oscillator
The simulated magnitude of the complex S-
parameter s11 is considered for a wide
frequency range from dc up to 10 GHz.
The analysis of the oscillator circuit gives
some hints (parasitic elements) for additional
oscillations in the upper GHz region.
This seems to be the reason for the spurious
oscillation. Next view graph shows the
calculated course of the magnitude of the
reflection coefficient s11 from dc to 10 GHz.
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 16
CAD of the Oscillator(Simulation of s11 at 4.0 Vdc Biasing)
9
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 17
Experiments on the Oscillator Circuit
The Measurement Set Up
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 18
Experiments on the Oscillator Circuit
The Measurement at a Voltage of 3.9 VDCat a Voltage of 3.9 VDC
10
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 19
Experiments on the Oscillator CircuitThe Measurement at a Voltage of 3.9 VDCat a Voltage of 3.9 VDC
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 20
Spectral Lines
To give some explanation for the number of the
resulting spectral lines, we will regard the simple
mixture of two sinusoidal voltages v1(t) and v
2(t)
with different frequencies f1 and f2.
The non-linear context between the current i(t)
and the signal voltage v(t) at an active element
can be described approximately by a polynomial
with the order of n = N
[ ] .)()(1
=
=
=Nn
n
n
ntvkti
11
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 21
Spectral Lines
If the signal voltage v(t) is a superposition of two
sinusoidal voltages with different frequencies and
different amplitudes, it is valid:
=
=
=
2
1
)()(i
i
itvtv
with
( ) .2,12sinˆ == ifortfvv ii
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 22
Spectral Lines
After a little algebraic manipulation a result for the signal voltage
v(t) having the following components may be obtained:
1) the two frequencies f1 and f2,
2) the harmonics 2 f1 , 3 f1, 4 f1, 2 f2, 3 f2 and 4 f2,
3) the mixtures of second grade f1 ± f24) the mixtures of third grade 2 f1 ± f2 and f1 ± 2 f2and
6) the mixtures of fourth grade 3 f1 ± f2, 2 f1 ± 2 f2, f1 ± 3 f2.
These are already the resulting frequencies assuming N = 4.
More realistic is an amount of N of above 18,
12
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 23
Improving of the Circuit and
Results
in MHz
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 24
Improving of the Circuit and
Results
13
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 25
Improving of the Circuit and
Results
in MHz
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 26
Improving of the Circuit and
Results
14
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 27
Improving of the Circuit and
Results
Prof. Dr.-Ing. A. Beyer
Uni Duisburg-Essen
Uni Gran Canaria, Las Palmas, 2007 28
Discussion This paper has been shown that a number of additional
signals can be excited by the oscillator’s active kernel dueto parasitic effects. These interfere with the projectedoscillation frequency or with its harmonics and produce anumber of disturbing spurious signals.
By means of a suitable CAD program, such as C/NL2, itis possible to avoid this problem. The CAD tool can analyzethe reason for these additional oscillations and comfortablygive advice how to take measures in order to inhibit theoscillator’s circuit capability to produce these unwantedadditional parasitic oscillations.
This talk examined the problem of exciting spurioussignals in an oscillator circuit and proves that an oscillatorcan be easily CAD designed and assembled, producing thedesigned frequency signal, only.
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