Oscillators – Plural oscillators – Parallel connected
Reexamination Certificate
1999-09-30
2001-12-04
Grimm, Siegfried H. (Department: 2817)
Oscillators
Plural oscillators
Parallel connected
C331S034000, C331S102000, C331S114000, C331S115000, C331S1170FE, C331S17700V, C333S206000, C333S219100, C333S222000
Reexamination Certificate
active
06326854
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to coaxial resonators and more specifically to modifications within coaxial resonator oscillation circuit designs.
BACKGROUND OF THE INVENTION
Oscillators are required within many different technology areas, especially within the expanding communication industry. In communication applications, oscillators are commonly used to generate carrier signals at specific frequencies on which information signals are subsequently modulated. For instance, a Voltage Controlled Oscillator (VCO) within a Personal Communication System (PCS) would typically be tuned around 1900 MHz.
FIG. 1
illustrates a typical Phase Locked Loop-Frequency Synthesizer (PLL-FS) that is a standard implementation for a VCO within a communication apparatus. In the case shown in
FIG. 1
, the PLL-FS includes a crystal reference oscillator
20
, in this case operating at 8 MHz, coupled in series with a first frequency divider
22
, a phase detector
24
, a loop filter
26
, a VCO in the form of a Voltage Controlled-Coaxial Resonator Oscillator (VC-CRO)
28
, a coupler
30
that generates a sample of the signal output from the VC-CRO
28
, and an amplifier
32
that outputs a signal S
OUT
(t). Further, the PLL-FS includes a phase feedback path comprising a second frequency divider
36
coupled between the coupler
30
and the phase detector
24
.
Within the circuit of
FIG. 1
, the crystal reference oscillator
20
outputs a crystal reference signal at 8 MHz that is subsequently frequency divided down to 160 KHz by the first frequency divider
22
. The phase detector
24
receives the divided crystal reference signal and compares its phase with a feedback signal, the generation of the feedback signal being described herein below. The output of the phase detector
24
is a baseband signal, the amplitude of which is proportional to the phase difference between the two signals input to the phase detector
24
, along with comparison frequency spurs at integer multiples of 160 KHz. The loop filter
26
(that could be either passive or active) receives the output from the phase detector
24
and removes the spurs within the signal by rejecting the components at multiples (n×160 KHz) of the comparison frequency (160 KHz), leaving only the baseband signal. This filtered result is fed as a control voltage into a tuning port
34
of the VC-CRO
28
, the frequency of which is controlled with a varactor diode arrangement (not shown). The VC-CRO
28
in this case comprises a Colpitts oscillator stabilized with a ceramic coaxial resonator that creates a signal at an oscillation frequency based upon the frequency of resonance of the particular resonator used and the control voltage applied at the tuning port
34
. The oscillation frequency is normally slightly less than that of the frequency of resonance (typically between 200 MHz and 5 GHz). The high frequency signal output from the VC-CRO
28
is sampled by the coupler
30
and frequency divided by the second frequency divider
36
to generate the feedback signal input to the phase detector
24
. One should understand that the amount the frequency of the feedback signal is divided within the second frequency divider
36
determines the control voltage output from the phase detector
26
. This voltage level subsequently determines the oscillation frequency at which the VC-CRO
28
is tuned, with changes in the division factor allowing for step changes in the oscillation frequency. As depicted in
FIG. 1
, the output from the VC-CRO
28
is received at the amplifier
32
which amplifies the signal and outputs the amplified result as the signal S
OUT
(t). Overall, the PLL synthesizer architecture enables digital control over the VC-CRO frequency, and also locks the VC-CRO to the reference crystal oscillator which ensures the frequency stability of the source over all system conditions such as temperature, ageing, and mechanical stress.
There are a number of advantages of using a ceramic coaxial resonator to stabilize a VC-CRO within a PLL-FS. These advantages relate to the physical design of a ceramic coaxial resonator. Typically, a ceramic coaxial resonator comprises a ceramic dielectric material formed as a rectangular prism with a coaxial hole running lengthwise through the prism and a electrical connector connected to one end. The outer and inner surfaces of the prism, with the exception of the end connected to the electrical connector and possibly the opposite end, are coated in a metal such as copper or silver. A device formed in this manner essentially forms a resonant RF circuit, including capacitance, inductance, and resistance, that oscillates when in the Transverse Electromagnetic (TEM) mode (as is the case when stabilizing a Colpitts oscillator). The advantages gained with this design include a high Q value (typically approx. 800) and therefore low noise oscillations associated with the resonator as well as temperature stability and resistance to microphonics that characterize a ceramic coaxial resonator. These advantages result in a further important advantage, that being a low cost; currently approximately 65 cents per resonator.
Unfortunately, there is a significant problem with the use of ceramic coaxial resonators as currently designed. The frequency of resonance for a ceramic coaxial resonator has a maximum frequency that can be output due to physical limitations. The frequency of resonance for a ceramic coaxial resonator is based upon the physical size and shape of the particular resonator. Generally, the smaller the size of the resonator, the higher is the frequency of resonance and vice versa. The problem is that ceramic coaxial resonators have a minimum size at which they can be manufactured that limits the frequency of resonance equal to or below a maximum value. This is a physical limit that, as currently designed, limits the output of a typical Coaxial Resonator Oscillator (CRO) using a ceramic coaxial resonator to approximately 5 GHz, whether the CRO is voltage controlled or not.
Up until recently, this 5 GHz limit has not significantly affected the use of ceramic coaxial resonators within VC-CROs or CROs since the frequency of operation of previous communication equipment was typically below this level. For example, PCS equipment operate at approximately 1900 MHz. Currently there are a number of different communication standards that require VCOs with oscillation frequencies higher than 5 GHz. For instance, OC-192 fiber optic signals are transmitted at approximately 10 GHz and the newly developed Local Multipoint Distribution System (LMDS), slated to be used for the Internet over wireless, is set to operate between 28 to 30 GHz. It can be assumed that further developments and standards will be designed that require yet higher oscillation frequencies.
One well-known technique to increase the oscillation frequency of signals within a system using a standard VCO as depicted in
FIG. 1
is to use a subharmonically pumped mixer that doubles the oscillation frequency at a stage after the VCO. Unfortunately, even with the use of a subharmonically pumped mixer, a system using the standard VCO that operates with a ceramic coaxial resonator is still limited to a maximum oscillation frequency of 10 GHz which is insufficient for LMDS applications. Hence, techniques are required to increase the oscillation frequency within the actual VCOs.
One technique that has been tried to increase the oscillation frequency output from a PLL-FS as depicted in
FIG. 1
beyond the 5 GHz limit is to add a frequency multiplication stage after the amplifier
32
. An example of such a multiplication stage is illustrated within FIG.
2
. As can be seen, a frequency multiplier
38
is coupled to the output of the amplifier
32
and further coupled in series with a first filter
40
, an amplifier
42
, and a second filter
44
. In this design, the multiplier
38
increases the oscillation frequency of the signal by three times that of the frequency output from the amplifier
32
. Hence, if the original frequency of the VCO was 5
Grundlingh Johan M.
Nicholls Charles Tremlett
Grimm Siegfried H.
Haszko Dennis R.
Nortel Networks Limited
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