how to suppress spurious signals in oscillator designhk0460/data/dokumente/las_palmas_2007... · 1...

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


Uni Duisburg-Essen


Overview

Introduction

The Oscillator Circuit

CAD of the Oscillator

Experiments on the Oscillator

Spectral Lines

Improving of the Circuit and Results

Discussion

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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.


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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.

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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.


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The Oscillator Circuit

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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.


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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.

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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


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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 )

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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



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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


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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


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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.

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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.


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CAD of the Oscillator(Simulation of s11 at 4.0 Vdc Biasing)

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Experiments on the Oscillator Circuit

The Measurement Set Up


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Experiments on the Oscillator Circuit

The Measurement at a Voltage of 3.9 VDCat a Voltage of 3.9 VDC

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Experiments on the Oscillator CircuitThe Measurement at a Voltage of 3.9 VDCat a Voltage of 3.9 VDC


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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

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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


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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,

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Improving of the Circuit and

Results

in MHz


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Results

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Results

in MHz


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Results

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Results


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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.

how to suppress spurious signals in oscillator designhk0460/data/dokumente/las_palmas_2007... · 1...

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