Oscillators Theory and Practice - QSL.net

Oscill
Theory and

Ed Messer

October 20

First prese

Meeting Septe

There is no magic in RF: The
(the reason may

lators
d Practice

r KI4NNA

009 Rev B

ented at the

ember 10, 2009

ere is a reason for everything
y not be obvious)

OSCILLATOR

• Feedback Oscillators
– Most prevalent, many im
by recognizing compone

• Negative Resistance Osci
– Less popular, mostly mic
often a single active devi

• Fixed Oscillators: single freq
– Oscillators by resonator:
oscillators (XO, TCXO, o
DROs, Cavity

• Tunable Oscillators
– VCOs, VCXOs, mechani

RS BY TYPE

mplementations, understand
ent parts; RC, LC, other
illators
crowave implementations,
ice

quency operation
: L-C, (R-C), Crystal
ovenized), SAW, CROs,

ically tunable VFOs

Specifications an

• Operating Frequency •

• Frequency Drift (ppm,..) •

• Tuning Range •
•
• Pulling (verses VSWR
&/or transient)

• Pushing (kHz/volt) •

• Noise (Phase noise &
residual FM, AM noise) •

• Temperature effects •

• Reliable start-up and
spurious modes

nd Considerations

Harmonic performance
Frequency effects due to shock
and vibration
Injection locking
Repeatability
Power requirements including
noise
Mechanical considerations
(size, weight, connectors)
Environmental considerations
(temperature, moisture, drop)

Feedback O

Gain A 10 • Act
1 and

1/5 • Fee
2 Feedback
• Bot
β gai

Loop Gain = Axβ • Bot
pha

• Usu
sele

• Conditions for oscillation are (
– Loop gain greater than on
• Often want 3-4 dB marg
– Net phase shift around loo

• Oscillations build up from nois
the frequency that satisfies th

Oscillators

tive amplification provides output
d gain expressed as a ratio or in dB
edback is generally passive
th blocks have individual transfer
in vs. frequency characteristics
th blocks have individual transfer
ase vs. freq. characteristics
ually need one or more frequency
ective elements

(Barkhausen criteria):
ne, so Axβ >1 (sum>0 dB)
gin, or controlled for waveform

op = zero degrees (really Nx360)

se/transient into saturation at
he Barkhausen conditions

Active Devices

• Op-amps and discrete for a
• Bipolar transistors for low ph

– Like low parasitics, good
• FETs for high Q VCOs and
• Special purpose ICs includin
• RF amps for general purpos
• Microwave diodes above UH

Impatts…)
• Tubes for higher power, leg

– Magnetrons, conventiona

s for Oscillators

audio oscillators
hase noise RF
d switching, low 1/F & NF
general purpose
ng crystal oscillators
se and higher power
HF (Gunn, Tunnel (Back),

gacy and sometimes low cost
al, special applications

Explanation of Phase
Mechanical Osc

• Pendu

• Consi
of pha
(tappi
excita
(motio

• Consi
exam

B A C arm m
correc
with th

• Phase
some

e: Pendulum Example
cillator Analogy

ulum has a “natural freq.

ider in-phase verses out
ase or random excitation
ing): only in-phase
ation sustains oscillation
on)

ider rocking chair
mple with arm movement:
must be in sync, i.e.

ct frequency and in-phase
he rocking

e is always relative to
e reference

Sine Wave Phase S
Compo

Reference

In-Phase: zero
degrees, non-
inverted

Inverted, 180
degrees out

Lag, late peak
Lagging voltage

Lead, early peak

To

Shift and Effects of
onents

Increasing
Time
Note: steady state
relationships

(Phase shift often
accompanied with attenuation)

Capacitive Divider Reso
(Colpitts)

Basic Tank Split C

High Z Hi Z, Hi V,
@ Resonance Low Current

C1

L
L
H
C2

• All ports in-phase and resistive
• Capacitors are small, inexpens
• Clapp added C in series with th

onator/Matching Circuits

Seiler Modification

Higher Q in Tank

1 Small Higher Z,
value Higher V,
Low Z Lower current
Low V
High I Low Z,
Low voltage
High current

e at resonance (F=1/2Pi Root LC)
sive, precise
he inductor

Classic LC Feedback O

Armstrong

• Many different configurations of
depending on amplifier, bias, …
• Other architectures as well such
Clapp, Seiler, Miller, Butler, Frankli

Oscillator Architectures

Colpitts

each Hartley

as
in …

Wein (Bridge) Oscillator

Want G+=2+1=3
Lamp GE or Elde
1869
Rf = 1K
F= 1E6/2PiRC
LM741+ Op Amp with balanced positiv

r with Non-linear Device

• Audio sine-wave RC
Oscillator

• Upper circuit + amp
constitutes a 0 degree
amplifier

• Lower RCs implement
the feedback path, get
0 degrees at Fosc (lag
cancels lead)

3 • Lamp resistance
ema increases with signal

amplitude & reduces
gain preventing
amplifier saturation

ve and negative supplies, high Z inputs

Common Base Osc

5.6 pF?

In Out

510 CT L
ohms
6.5K
3.3K
Bypass
0.001 uF Caps

• Emitter capacitance and devi
• Can add varactor or just mod

cillator: Basic circuit

• Easy VHF feedback
oscillator

• Emitter & collector in-
phase at lower freq.

• Base at AC ground
LT as well as lower

-9VDC portion of tank circuit

0.1 uF • CT - LT tank
determine frequency

• Feedback cap in
blue, sometimes
unnecessary

ice lag correct feedback phase
dulate base to get some FM

Varactor Provides F

FM/VCO operation

• VHF voltage controlled
oscillator

• Common base oscillator
drawn “upside down”

• D1 varactor substituted
for normal tank
capacitor

• Can replace C1 with a
second varactor

• Negative supply

Common Colle
Vcc
•

C1 •
L1
Pout
C2
•

RE

Colpitts version shown, •
Clapp (and Seiler?) versions •
are popular. •

ector Oscillator

Popular “emitter follower”
oscillator, basic circuit, bias
and DC block circuitry not
shown.

High base input impedance
allows good Q

Split C tank steps up voltage
and matches low emitter
output impedance to high
base input impedance

Can take harmonic output
from a collector circuit

FETs with diode work for
higher noise applications

Replace inductor with ( )…

Crystal O

Ed Me
2nd Half of P

Oscillators

esser
Presentation

Crystal Oscillato

• Center Frequency: 30 kHz to 200 M
• Frequency accuracy: (initial & PTD
• Frequency accuracy vs. temperatur
• Output power and tolerance
• Phase noise
• Set-ability: Range, to within…
• Electronic tuning: maybe 50- 200 P

– Linearity consideration
• Start-up time: can be seconds

– Can improve somewhat with int
• Warm up time: 15 seconds to man
• Power Requirements: initially more
• Mechanical: size, weight, form facto
• Environmental

– Shock: crystals don’t like drop o
consideration

– Vibration

or Specifications

MHz
D) ±10 PPM or other, 5 PPM first year

re: ±10 PPM or other (2.5 PPM)

PPM, maybe 500+ fundamental

troduced transient
ny minutes
e if ovenized, more cold, regulator…
or

onto hard surfaces, also retrace

Partial List of Cr

• Mtronpti.com: Formerly PTI
• Statek.com: miniature reson
• Greenrayindustries.com: T5
• Crystekcrystal.com: Florida
• ICMFG.com (Kansas maybe
• Bomarcrystal.com (New Jer
• ESCXTAL.com
• OSCILENT.com (China, Ko
• CTS (lower performance?, U
• FILTRO.com (USA, website
• Quartztek: richard24@mind
• Suppliers: Mauser, Digikey,
• Audience participation ….

rystal Suppliers

I, High shock, high-rel, lesser
nators, USA
53 0.28 PPM Oscillator

e or… )
rsey & offshore)

orea)
USA)
e unimpressive, but...)
dspring.com
, other

Crystal Operation

and Construction

• Thickness-shear vibration
at crystals fundamental
and third overtone
– See next slide for
modes

• Bottom figure shows
crystal internal
construction
– Metalized surfaces for
connections
– Disk is typically
supported by the leads

Artists Rendition of Ov

• Rendition of reactance vers
• Shows fundamental, third a
• Spurious show up on high s
• An actual S21 sweep would

because responses are so n

vertones and Spurious

ses frequency
and fifth overtones
side (crystal filters also)
d probably show nothing
narrow ( i.e. high Q)

Crystal Temperature D

Blanks

• Maybe choose cut
based on temp range

• Compensation can be
done with TC caps, or
varactors coupled to
thermistors or D/A & uP

• Limit drive levels to 1
mW or less as specified

Drift Dependent on Cut

Flexure Mode

es In Crystals

Co/Cm(?)

Lower
Better for
Pulling

N2 ??

Thousands
For third OT?

BT cut
apparently
thicker for
fundamental

Crystal Equivalent Ci

In Lm Cm

or
out

FREQ. L Co

Cm Co

(MHz) (mH) (pF) (pF

1.0 3500 0.00724 3

10.0 12.7 0.01997 5.7

100.0 8.6 0.00029 2.1

ircuit & Some Values

m Rm In
or
out

o Q Delta F

oR

F) (Ohms) (Hz)

340 64,647 2415

7 8 99,695 35027

1 56 96,443 14,040

Plot of Crystal React

• Pl
• Cr
• Se
• Al

tance vs. Frequency

lot repeats at odd harmonic frequencies
rystals etched for series resonance
et for a defined condition
lso responses at spurious frequencies

100 MHz Crystal Simula

• Just the crystal, plot shows se
• Zero phase is actually 30 Hz h
• Parallel resonance (“anti-reso

ation: S21 Gain & Phase

eries and parallel resonances
high of 100 MHz for values shown
onance”) is 7 kHz higher

CRYSTAL WITH HIGH Q PARAL

• In close the circuit acts like a pu
• Zero phase pretty much right at

LLEL RESONATING INDUCTOR

ure series tank resonator
100 MHz

CRYSTAL WITH LOW Q P
INDUC

• Q=30 inductor does not hurt “ph

PARALLEL RESONATING
CTOR

hase rate”, i.e. loaded Q