Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-09-08
2003-08-12
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S10800D, C331S17700V, C331S1170FE, C331S1170FE
Reexamination Certificate
active
06606006
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9903219-5 filed in Sweden on Sep. 8, 1999; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to oscillators, specially a voltage-controlled oscillator, preferably an oscillator realized in MMIC (Monolithic Microwave Integrated Circuit) technique. The oscillator comprises a first substrate, on which a resonator circuit and an amplifier circuit are arranged. The resonator circuit comprises first set of components and said amplifier circuit comprising a second set of components and an amplifier transistor.
BACKGROUND
Some contradictory compromises must be made when designing broadband oscillators with low phase noise. Also, the limiting characteristics of the components constituting the oscillator must be taken into consideration.
The phase noise of an oscillator is given by the so-called Leeson's equation (1):
L
(
f
m
)
=
1
2
[
1
+
1
f
m
2
(
f
0
2
Q
)
2
]
F
k
T
P
(
1
+
f
c
f
m
)
[
dBc
/
Hz
]
Where
f
m
=
offset
frequency
from
the
oscillation
frequency
,
f
0
=
oscillation
frequency
,
F
=
phase
noise
of
the
reflection
amplifier
,
k
=
Boltzmann
constant
,
T
=
temperature
,
f
c
=
switching
frequency
for
1
/
f
noise
,
Q
=
Q
-
factor
for
resonator
circuit
,
and
P
=
input
power
of
the
reflection
amplifier
.
(
1
)
Every oscillator is a periodically time-varying system, and the time varying nature of it must be considered when modelling phase noise. The noise source in the circuit can be divided into two groups: device noise such as thermal, shot and flicker noise and interface such as substrate and supply noise.
When designing a resonator circuit for a voltage-controlled oscillator (VCO), usually varactor diodes are used as voltage controlled capacitors. Advantageously, these are realised on GaAs (Gallium Arsenide) or Si (Silicon) substrates. However, GaAs is preferred as a considerably better Q-value is obtained for the resonator circuit as a whole. This is due to the fact that both varactor diodes and planar inductor coils have superior performance on GaAs relative to Si. Especially the resonator circuit usually comprises an inductor, which is a metallic coil directly arranged on the semiconductor surface. Since it is advantageous to arrange the inductor coil on an insulating substrate, a GaAs substrate which is substantially insulating is more suitable than a Si substrate which is semi-insulating. The same applies to the case of a microstrip transmission line resonator. A resonator with variable resonance is described in the Swedish patent application No. 9900850-0, “Varactor Coupled High-Q Monolithic Resonator” (Resonator Application).
The differences between GaAs and Si are specially outstanding for frequencies above some GHz, which in fact prevents employment of Si for producing resonant LC structures thereon.
As mentioned above, the oscillator comprises a second amplifier part which preferably is a reflection amplifier. In a preferred embodiment which is based on a transistor, the reflection amplifier has an amount of amplification that is needed to overcome the losses of the resonator and thus obtain a self oscillation. An output is arranged on an appropriate point of the amplifier and connected to the signal chain.
Known transistor techniques on GaAs are, e.g. MESFET (Metal-Schottky Field Effect Transistor), PHEMT (Pseudomorphic High Electron Mobility Transistor) and HBT (Heterojunction Bipolar Transistor). Generally, PHEMT offers good amplification at high frequencies, MESFET has low cost of production and HBT has high efficiency, positive voltage supply and good linearity.
Generally, on Si CMOS and bipolar processes are used. The development of the Si transistors has resulted in applicable transistors for frequencies up to 10 GHz. The SiGe technique provides much higher cut off frequencies and it performance can seriously be compared to~the GaAs processes. Also, with regard to the price, the Si-based processes have considerable advantages.
A demand for a transistor to be used in an oscillator is that the transistor has a low I/f-noise. Consequently, this low frequency noise is converted through the non-linearity of the circuit to phase noise. Consequently, also the non-linear characteristics of the transistor which are of interest are effected. Mainly, the I/f noise is a surface phenomenon meaning that transistors having vertical structure, such as bipolar transistors, have a lower I/f noise than surface oriented transistors, such as MESFET and PHEMT. Typically, the switch frequency for I/f noise for the different transistor techniques range from >1 MHZ for GaAs PHEMT and MESFET, >100 kHz for GaAs HBT and <10 kHz for Si BJT. The result is that on GaAs usually HBT technique is preferred when producing oscillators. However, the yet lower switching frequency of the bipolar Si transistors could additionally reduce the phase noise if it was possible to use these types of transistors.
Among the GaAs HBTs, there are two main groups with different material in the emitter layer: AlGaAs/GaAs-HBT and InGaP/GaAs-HBT. Different manufacturers use different material. In HBTs with AlGaAs appear so-called deep electron traps, due to the aluminum content. The traps are actuated by heat and trap and release electrons with some certain time constants, which results in a disturbance in the uniform flow of the electrons. The disturbance appears as noise, which assumes Lorentz formed spectra in the frequency range of 10 kHz to 1 MHZ. This is an additional low frequency noise contribution which can be converted to the phase noise in an oscillator and it is normally called generation-recombination-noise (g-r-noise) or “burst noise”. The temperature dependency of the nose results in temperature variations in the phase burst in an oscillator made of said type of transistors. HBTs with InGaP in the emitter layer are normally free from mentioned type of electron traps. However, a manufacturer uses one of the processes, which means that it is not always possible to choose a specific transistor type.
The deep electron traps are absent in the silicon-based transistors thus making them free from the g-r-noise, which is an advantage when designing oscillators.
The known solutions to the problem of making broadband VCOs, specially in MMIC technique are:
As the realisation of hyper abrupt varactor diodes on an MMIC is not possible through any of the known standard processes, usually they are located outside the chip. Normally, the entire resonator circuit
110
(
FIG. 1
) is arranged outside the chip
100
to obtain a better Q factor. The production is more expansive owing to the additional costs for the varactor diode. The performance,does not improve compared to a construction with the entire resonator on a single chip. Neither it is possible to use the encapsulated varactors at the frequencies above about 5 GHz due to theparasite reactances of the capsule, as the varactor diodes in chip form are difficult to bond to.
It is also possible to arrange the entire VCO on a Si or SiGe, if so-called 3D technique is used, in which dielectric layers are arranged on top of the chip, and a new ground plane and above it a new conducting layer are provided. Thus, it is possible to realise inductors with low losses. Nevertheless, the varactor diodes are still in the silicon and accordingly they are inferior to diodes in GaAs. Consequently, the entire solution tends to become inferior.
EP 523 564 describes an improved
Kinkead Arnold
Telefonaktiebolaget LM Ericsson (publ)
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