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FIELD OF THE INVENTION
This invention relates to oscillators and more particularly to
electronically controlled oscillators operating in the rf/microwave
frequency range of several hundred MHz or more, and including operation
into the GHz range.
BACKGROUND OF THE INVENTION
Thin film resonators (TFR's) have been and are currently being investigated
as feedback elements in rf/microwave frequency oscillators. (See, for
example, "UHF Oscillator Performance Using Thin Film Resonator Based
Topologies", by Burns et al., 41st Annual Frequency Control
Symposium--1987, pp. 382-387.) One form of thin film resonator is the
overmoded stacked crystal filter (SCF) which has been investigated as the
feedback element in an oscillator to produce an oscillator output in the
form of a comb of frequencies normally associated with the overmoded
stacked crystal filter itself. Such a device is described in "Design and
Performance of Oscillators Using Semiconductor Delay Lines" by Burns et
al., 1987 Ultrasonics Symposium, pp. 369-373. Insofar as applicants are
aware, investigation of thin film resonators in oscillators, and
particularly in stacked crystal filter oscillators, has not heretofore
progressed to the stage where automatic electronic tuning of individual
frequencies has been accomplished.
In other contexts, however, voltage control of oscillators has proven to be
useful. Voltage control is accomplished by utilizing means such as a
varactor in conjunction with an oscillator of the type normally considered
tunable, with the varactor being the control element which serves to tune
the oscillator to a frequency related to the voltage applied to the
varactor.
Insofar as applicants are aware, and at least in large part because of the
many complexities involved, the voltage control techniques have not been
applied to thin film resonators particularly those of the stacked crystal
filter variety.
SUMMARY OF THE INVENTION
In view of the foregoing, it is a general aim of the present invention, at
least in one embodiment thereof, to provide an electronically controlled
oscillator capable of operating at rf/microwave frequencies which is
miniaturized, monolithically integratable, highly reliable and utilizes as
the frequency selective element a stacked crystal filter (SCF) operating
on a single fundamental or overtone mode.
In that regard, it is an object of the invention to utilize a stacked
crystal filter in the feedback path of an oscillator, and to associate
with such oscillator, electronic control elements for controllably tuning
the loop which contains the stacked crystal filter to predetermined
frequencies within the passband of the SCF.
In a particular aspect of the invention, it is a further object to provide
a frequency agile electronically controlled oscillator which exploits the
comb filter characteristics of an overmoded stacked crystal filter (or the
characteristics of other forms of thin film resonator capable of producing
a comb filter characteristic) to produce a frequency output readily
tunable to the individual responses of the comb.
Accordingly, there is provided in accordance with the invention an
electronically controlled oscillator comprising a non-linear element
including an amplifier, a thin film stacked crystal filter, and an
electronically variable impedance connected in a closed loop, the stacked
crystal filter and the electronically variable impedance being connected
in the feedback path of the amplifier. The amplifier provides a loop gain
of at least one to meet one aspect of the Barkhausen criteria. The thin
film stacked crystal filter has a resonant frequency in the rf/microwave
frequency range, the range in which the oscillator is intended to operate.
The electronically variable impedance is associated with the thin film
stacked crystal filter in the amplifier feedback path for introducing an
adjustment in the loop phase thereby to tune the frequency of the
oscillator within the desired bandpass of the stacked crystal filter.
Electronically variable means are coupled to the electronically variable
impedance for establishing the impedance of the latter and thereby
controllably adjusting the operating frequency of the oscillator.
The electronically variable impedance can be responsive to a number of
characteristics and can comprise a voltage variable impedance for
producing a VCO, a current variable impedance, or an impedance which is
ultimately responsive to temperature to produce a TCXO. Preferably,
however, the variable impedance is a voltage variable impedance such as a
varactor and is connected in series in the feedback path for maintaining a
filter Q which is independent of the tuning point of the oscillator.
In a further implementation of the invention, the frequency establishing
element of the oscillator is in the form of a thin film resonator
(preferably an overmoded stacked crystal filter) which produces a high Q
comb of resonant frequencies at predetermined intervals. The feedback loop
further includes a delay stacked crystal filter which operates in
combination with an electronically variable impedance, as in the prior
embodiment, to adjust total loop phase thereby to select individual
responses among the comb of filter responses to serve as the oscillator
output.
Other objects and advantages will become apparent from the following
detailed description when taken in conjunction with the drawings, in which
:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating a first embodiment of the present
invention utilizing a stacked crystal filter in association with an
electronically controlled impedance for controlling the output frequency
of the oscillator;
FIG. 2 is a diagram showing the amplitude and phase characteristics of the
SCF used in the electronically controlled oscillator of FIG. 1;
FIG. 3 is a block diagram illustrating an electronically controlled
oscillator utilizing an overmoded stacked crystal filter in a feedback
loop along with a voltage controlled impedance for selecting a particular
one of the responses of the overmoded stacked crystal filter to be
utilized as the oscillator output;
FIG. 4 is a diagram illustrating the amplitude and phase characteristics of
the overmoded stacked crystal filter of the electronically controlled
oscillator of FIG. 3;
FIG. 5 is a diagram illustrating the amplitude and phase characteristics of
the overmoded stacked crystal filter of FIG. 4 in tandem with a phase
delay stacked crystal filter; and
FIG. 6 is a diagram illustrating a microstrip implementation of the
electronically controlled oscillator of FIG. 3.
While the invention will be describe connection with certain preferred
embodiments, there is no intent to limit it to those embodiments. On the
contrary, the intent is to cover all alternatives, modifications and
equivalents included within the spirit and scope of the invention as
defined by the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the foregoing and also in the following, reference is made to
rf/microwave frequencies at which the oscillator according to the present
invention is operable. That band is intended to encompass the frequencies
beginning at about 200 MHz and extending well into the GHz range. When
utilizing the stacked crystal filter, GHz operation is particularly
appropriate since the stacked crystal filter is capable of achieving high
Q and low insertion loss characteristics at such operating frequencies due
to the relatively thin and very controllable piezoelectric films which are
deposited to form the resonators. However, certain advantages of the
invention can be realized with thicker films in the hundreds of MHz range,
and thus such range is encompassed within the broadest aspects of the
present invention.
In the following, there will be two general forms (with several variations)
of electronically controlled oscillators described which fall within the
scope of the present invention. The first, using a single mode or primary
mode thin film resonator, employs a particular form of thin film
resonator, i.e., the stacked crystal filter, in order to obtain adequate Q
and a well-behaved phase vs. frequency characteristic to yield a useful
electronically controlled oscillator. A second form of oscillator uses an
overmoded thin film resonator which produces a comb of frequency
responses, and such overmoded filter while preferably constructed as a
stacked crystal filter, can be configured in other forms of thin film
resonator such as the monolithic resonator. In the second form, a stacked
crystal filter is utilized in conjunction with the overmoded thin film
resonator to provide a composite phase characteristic, and an
electronically variable impedance is provided for tuning within that
composite characteristic to select individual ones among the responses in
the comb response of the overmoded thin film resonator. Since the
overmoded thin film resonator can produce adequate Q even when not
constructed as a stacked crystal filter, the frequency selective device in
that embodiment can be implemented from a broader group of devices.
In both cases, voltage control is the preferred mode of electronic tuning,
which is conveniently provided by use of a varactor as the tuning element.
In addition, however, electronic tuning can be accomplished by current
control such as a YIG (yttrium iron garnet) resonator which functions as a
current controlled device to vary the impedance where a temperature
compensated oscillator is desired, the control element (be it voltage or
current responsive) is driven by a temperature responsive device, such as
thermistor or sensistor which is configured, for example, to respond to
the temperature to which the oscillator is subjected and to produce an
electronic signal which adjusts the phase of the electronically controlled
device to maintain the oscillator output frequency constant over a desired
temperature range.
With that in mind, turning now to the drawings, FIG. 1 shows a first
embodiment of the present invention utilizing a single mode of a stacked
crystal filter as the frequency selective element in the feedback path of
an amplifier which also includes an electronically controllable device for
adjusting the output frequency of the oscillator. More particularly, in
FIG. 1 there is shown an oscillator generally indicated at 20 which has a
non-linear element including an amplifier 21 having a feedback path
generally indicated at 22 providing a closed loop circuit having an output
23 driving a load 24 at a frequency determined by the oscillator 20. The
amplifier 21 provides a system gain to meet one aspect of the Barkhausen
criteria, that is, that the loop gain which includes the amplifier 21 and
the elements in the feedback path 22 to be at least 1.
In practicing the invention, the feedback path 22 contains a thin film
element, in the FIG. 1 embodiment, a thin film stacked crystal filter 25
for establishing the basic frequency and frequency range of the oscillator
20. More particularly, the stacked crystal 25 has a resonant frequency
which is in the range at which the oscillator 20 is intended to operate.
Typically, the resonant frequency of the stacked crystal filter 25 will be
the primary resonant frequency, but as is well known to those skilled in
this art, the filter element 25 can be modified to select other low but
significant overtones for use as the primary filter bandpass
characteristic.
Associated with the SCF 25 in the feedback path 22 is an electronically
variable impedance 26 for adjusting the total loop phase to meet the
second aspect of the Barkhausen criteria, i.e., that the total loop phase
be zero degrees or 2.pi.n radians, where n is an integer, at the operating
frequency of the oscillator. Phase control means 28 are connected by means
of control line 29 to the variable impedance 26 for adjusting the actual
impedance thereof and thereby the phase of the feedback path 22. Thus, as
the impedance of the element 26 is adjusted, the total loop phase would
tend to change except that the stacked crystal filter 25 is driven to a
different point in its phase characteristic thereby to tune the frequency
of the loop and thereby the oscillator in a limited band about the
resonant frequency of the stacked crystal filter 25.
In the preferred practice of the invention, the amplifier 21 is a
commercially available high frequency amplifier having adequate gain to
overcome the insertion loss of the elements in the feedback path 22 and a
linear or relatively linear phase frequency characteristic to allow linear
frequency tuning of the oscillator. Commercially available amplifiers can
be provided having a typical 50 ohm output to allow the oscillator 20 to
drive a standard load of 50 ohms represented by the load impedance 24. Use
of a stacked crystal filter as the frequency selective device in such an
arrangement is very desirable since the stacked crystal filter is easily
configured to have a real impedance in a desired range such as a 50 ohm
input or output impedance.
The stacked crystal filter 25 is a known thin film device having three
metallic electrodes separated by two thin film piezoelectric resonator
elements. In the preferred embodiment, the electrodes of the SCF 25 are
aluminum and the piezoelectric films are A1N. All of the layers are grown
on a substrate which can be relieved to suspend the operative portion of
the resonator as a membrane. Typically, the aluminum electrodes are
deposited by electron beam evaporation and the A1N piezoelectric films by
sputtering. The configuration of the various layers of the SCF 25, and
most particularly the thickness of the A1N piezoelectric films are
carefully controlled, the thickness being maintained in the micron range,
to provide the SCF 25 with a resonant frequency peaked in the desired
range at which the oscillator 20 is intended to operate. As noted above,
the manner in which the SCF 25 is configured will allow the selection of
the primary or a low harmonic as the primary operating frequency of the
SCF 25, and that selected mode will in turn establish the operating
frequency of the oscillator 20. The insertion loss of the SCF 25 is also
of concern, as is the Q of the device. If high Q is a desired
characteristic, a number of individual SCF's can be connected in cascade,
although increasing the number of SCF's in cascade will tend to increase
the insertion loss. If minimum insertion loss is desired and a relatively
lower Q can be tolerated, only a single SCF need be used as the frequency
selective element.
In addition to having relatively high Q for selectivity (while still
allowing tuning over the desired passband) and a tolerable insertion loss,
the stacked crystal filter 25 desirably also possesses a well-behaved
phase-frequency characteristic. Not all thin film resonators possess such
a characteristic. For example, a conventional monolithic thin film
resonator, in contrast to the desired linear frequency-phase
characteristic, may have a number of minor but potentially significant
perturbations in the phase frequency characteristic as illustrated in the
diagram at page 385 of the aforementioned Burns et al. article. By way of
contrast, a stacked crystal filter when used as the frequency selective
element for the oscillator 20 can be configured with a more well-behaved
phase characteristic, such that when coupled with an electronically
controlled impedance having a linear characteristic, the oscillator 20 is
provided with a linear control characteristic without points of
non-linearity which might occur when using a monolithic thin film
resonator in the circuit of FIG. 1.
In further practicing the invention, the feedback loop 22, in addition to
the stacked crystal filter 25, includes an electronically controllable
impedance 26 for adjusting the split of total loop phase among the loop
elements, to select the point on the frequency phase characteristic of the
stacked crystal filter 25 at which the oscillator 20 will oscillate. In
the preferred embodiment, the electronically variable impedance 26
comprises a voltage controllable device, such as a varactor; when used
with such a voltage controlled device, the oscillator 20 functions as a
voltage controlled oscillator (VCO). VCO operation is achieved by
adjusting the reverse bias voltage of the varactor utilized as
electronically adjustable impedance 26. In a preferred implementation of
the invention, a hyperabrupt junction varactor diode is used for the
control element 26; such element exhibited a phase characteristic which
varied in a relatively linear fashion from approximately 3 to
approximately 9 volts bias with a bias center of approximately 6 volts.
The oscillator was designed such that the center of the diode varactor
tuning range was aligned with the center of the filter passband at about
1.033 GHz. The frequency tunability was determined to be about 0.335 MHz
per volt for a phase slope of 9.75.degree./MHz for the filter and a
phase-voltage slope of about 3.27.degree./volt for the series varactor
diode. It is preferred to use a hyperabrupt junction varactor diode, as
compared, for example, to an abrupt or linear graded junction varactor
diode because the hyperabrupt device allows for the greatest variation in
capacitance for a given bias voltage change.
The phase control 28 is not illustrated in detail since it is typically a
conventional element depending upon the use for which the voltage
controlled oscillator 20 is intended. For example, if the oscillator 20 is
intended for use in a phase locked loop, the phase control 28 will include
a phase detector which compares the oscillator operating frequency at the
output 23 with the phase of an external oscillator to which the oscillator
20 is to be locked. The phase control 28 will produce a voltage signal on
an output line 29 which controls the impedance of the element 26 (and
thereby the phase of the SCF) to cause the phase produced by the
oscillator 20 to match that of the external oscillator to which it is
phase locked. Those familiar with other forms of oscillator control will
appreciate other means of utilizing the voltage controlled oscillator of
FIG. 1, such as the well-known frequency modulated oscillator in which the
phase control 28 would produce a modulating signal on its output 29 for
modulating the output of the oscillator 20.
In the voltage controlled embodiment (as well as in others), it is
important to appropriately connect the electronically controllable
impedance 26 in the feedback loop 22. In many types of oscillators used
for voltage control, a varactor 26 is typically connected in parallel with
the frequency determining elements of the loop for establishing the
electronic control of frequency. However, in the oscillator 20, if the
varactor 26 were connected in parallel with the stacked crystal filter 25,
that would result in degradation of oscillator performance based on a
change in the composite network feedback phase slope with operating
frequency. In a simple resonator feedback circuit, the oscillator can be
"pulled" by adding a capacitance in parallel with the SCF, but the Q
related to the resonator phase slope is a rapid function of frequency and
consequently noise performance will degrade. However, when the varactor
diode represented by electronically adjustable impedance 26 is connected
in series in the feedback path 22, the stacked crystal filter Q is not
significantly affected by tuning of the controllable impedance 26, and the
spectral purity of the VCO output as compared with prior fixed frequency
SCF based oscillators is maintained. When connected in series, the
varactor acts as a variable RC phase modulator having an amplitude
response which remains substantially equal to unity over the entire range
of controllable phase adjustment.
The illustration of controllable impedance 26 as a general block is
intentional since, although voltage controlled operation by means of a
series connected varactor is preferred, other forms of electronically
variable impedance can also be employed. Among those form is a current
controlled device such as that known commercial as YIG (yttrium iron
garnet) resonator which can be used as a current controlled phase shift
device when connected in the feedback path 22. In that configuration, the
phase control device 28 would provide a current output on its output line
29 to control the YIG device positioned in the feedback path 22.
As a further important alternative, temperature controlled crystal
oscillators (TXCO's) can be an important application of the present
invention. In such devices, the electronically variable impedance 26 can
be either current or voltage responsive, but the phase control element 28
itself is temperature responsive (for example, responsive to the ambient
temperature to which the oscillator 20 is subjected) for operating on the
phase of the controllable element 26 to maintain the oscillator output
constant over the intended temperature operating range of the oscillator.
In such an application, the phase control element 28 contains a thermistor
or sensistor for measuring the ambient temperature and producing a signal
which ultimately controls the electronically variable impedance 26 to
maintain the output frequency of the oscillator 20 at its desired level.
As an alternative, the varactor which might be used as the electronically
adjustable impedance 26 can be replaced with a temperature variable
capacitor for more direct control of the phase within the feedback loop 22
and the resulting operating frequency of the oscillator 20.
The operation of the oscillator loop including the amplifier 21 and its
feedback path 22, in tuning the output frequency of the oscillator will be
better understood by reference to FIG. 2 which shows the amplitude
transfer function and phase function of the stacked crystal filter 25 as a
function of filter frequency. The upper plot of FIG. 2 shows a
characteristic curve 30 representing the filter transfer function
otherwise known as S.sub.21, the S parameter of the filter which defines
the forward transfer function as a function of frequency. It is seen that
the function 30 has a sharp peak at point 32 at the resonant frequency of
the SCF 25, in the illustrated embodiment at about 1.033 GHz. The
relatively sharp skirts 33, 34 of the characteristic 30 illustrate that
the Q of the filter is relatively high; in the illustrated embodiment the
open loop Q of the SCF is about 80. It will thus be appreciated that the
filter can exhibit a passband of at least about 5 MHz when tuned about its
center frequency along the sharp but not overly peaked response
characteristic at the point 32.
The lower plot 31 of FIG. 2 illustrates the phase characteristic with
respect to frequency and it is seen that a substantially linear
characteristic 40 is provided in the tuning range of the oscillator
centered at about 1.033 GHz and having a passband of about 5 MHz
controlled by the total loop phase which causes the SCF 25 to operate at a
particular point along the linear portion of its characteristic 40. More
particularly, if the total phase around the oscillator loop were such that
the SCF were operating at the frequency established by operating point 42
along the characteristic 40, and if the phase control 28 were to produce a
signal to cause the phase of electronically variable impedance 26 to
decrease, in order to continue to satisfy the phase aspect of the
Barkhausen criteria, the operating point of the SCF would shift to a
second point 43. As a result, the operating frequency of the oscillator
would accordingly shift to the frequency established by operating point
43, thereby electronically adjusting the oscillator operating frequency
within the relatively narrow passband centered about 1.033 GHz center
frequency of the SCF. Similarly, if the electronically adjustable
impedance 26 were adjusted to increase the total loop phase, the operating
point of the SCF would switch to a point 44 to maintain total loop phase
equal to 2.pi.n radians (where n is an integer), and cause a shift in the
oscillator operating point to the frequency associated with operating
point 44.
With the foregoing description in mind, those skilled in the art of
oscillator and particularly microwave oscillator design will now be
capable of implementing an oscillator according to the present invention
for a desired frequency and frequency tuning range. Among the criteria to
be used by such skilled artisans in configuring a particular
implementation of the inventive oscillator, there will be utilized
modeling of both the stacked crystal filter and the associated amplifier,
preferably to produce S matrices which are manipulated to satisfy the
Barkhausen criteria at the desired frequency operating point and tuning
range.
Oscillator design requires measuring the S parameters of the SCF as a
function of frequency in and near the desired passband, and converting
these data to a Butterworth VanDyke equivalent circuit, derived from the
Mason model. Since the SCF dimensions exhibit a large lateral dimension to
thickness ratio, a one-dimensional plane wave analysis can be utilized.
When operating at microwave frequencies, it is also important to include
package parasitics in the model of the SCF. In certain cases, it is
difficult to measure directly certain circuit parameters which depend
strongly on k, the coupling coefficient, and Q. In those cases, model
parameters can be obtained, as is well known to those skilled in this art,
by ranging over k and Q value using microwave computerized analysis
techniques available to those skilled in the art, such as the Touchstone
circuit analysis and optimization routines, to provide a best fit of the
model parameters to the measured data.
For the purpose of characterizing the commercially available hybrid broad
band amplifier, such as a 2 GHz, 10 db gain, impedance matched unit
utilized for an embodiment of the invention, a perturbational method of
measuring large-signal, full-matrix, S-parameters can be used to
characterize the amplifier operating at the compression level present in
the closed loop configuration. The large-signal, S-parameters for the
amplifier can then be combined with the linear S-parameters for the
remainder of the oscillator circuit (including the SCF and electronically
controllable impedance) and the net-two-port S-parameters can be computed
using the aforementioned Touchstone trademark of EESOF computerized
modeling techniques. Since it is known that the Barkhausen criteria
requires a loop gain of at least 1 and a total loop phase of zero or
2.pi.n radians where n is an integer, applying those criteria to the model
derived from the foregoing analysis will predict the frequency at the
onset of oscillation and the oscillation bandwidth. For use in such
predictions, a further computerized technique widely used by those skilled
in this art, and known as SPICE circuit simulation techniques, is utilized
to determine the circuit conditions at which the Barkhausen criteria are
satisfied.
For ease of fabrication and prototyping, the oscillator circuits of FIG. 1
can be constructed on microstrip and, to the extent available,
commercially available devices such as the commercially available high
frequency amplifier and varactor can be utilized. However, when it is
desired to produce the device in higher quantities with maximum economics,
when utilizing a stacked crystal filter in the feedback path it will be
preferable to monolithically integrate all of the devices on a single
semiconductor substrate. Such integration can be accomplished on silicon
if the frequency requirements of the oscillator are such that silicon
technology for the amplifier 21 is adequate, or if higher frequency is
desired, and since SCF's have been constructed on GaAs and GaAs provides
enhanced high frequency operation, the device in that case is preferably
constructed on a monolithic GaAs substrate.
Turning now to FIG. 3, there is illustrated a further embodiment of the
present invention which has particular application as a frequency agile
oscillator. A frequency agile oscillator for purposes of this disclosure
is considered to be one which can be rapidly tuned at spaced points in the
frequency domain, the spacing between the points being significant as
compared to the bandwidth of a single lobe of the oscillator frequency.
For example, in an implementation of the present invention, a frequency
agile oscillator has been configured having an operating frequency of
about 1 GHz, and having discrete frequency shifts of about 4 MHz.
Preferably, the oscillator provides the option of fine tuning about the
operating points which are spaced at 4 MHz intervals. It will be apparent
that the center frequency and the spacing between responses can be
adjusted by appropriately configuring the frequency selective device in
the frequency agile oscillator.
FIG. 3 shows the frequency agile oscillator 50 based on a non-linear
element including an amplifier 51 and a feedback path 52 forming a closed
oscillator loop. The oscillator has an output port 53 driving a load
impedance illustrated at 54. Associated with the amplifier 51 is a
feedback loop 52 which includes a frequency selective element 55. In the
exemplary embodiment such frequency selective element is described
generically as a thin film resonator. The thin film resonator in many
instances is an overmoded stacked crystal filter, but can also be
configured in certain embodiments as an alternate form of thin film
resonator such a monolithic resonator. A common characteristic of all such
thin film resonators used in this embodiment of the invention is the
provision of an output response which has a high Q comb of frequency
responses separated by predetermined frequency intervals. As in the FIG. 1
embodiment, however, the element 55 is the main frequency selective
element of the oscillator 50 and establishes the center frequency of the
oscillator output as well as the bandwidth of the oscillator, although the
particular operating point in the bandwidth is controlled by other
elements in the feedback loop. More particularly, as in the FIG. 1
embodiment, the feedback loop 52 includes an electronically controllable
impedance Z.sub.cont 56 connected in the feedback path for introducing a
controllable phase delay thereby to shift the operating phase point of the
thin film resonator 55 and thereby the operating frequency of the
oscillator 50. The controllable impedance 56, as in the prior embodiment,
is preferably a hyperabrupt junction varactor diode, having a reverse bias
applied by phase control circuitry 58 by means of interconnection 59 so
that the phase control 58 is the direct mechanism for adjusting the split
of the overall loop phase and thereby selecting the operating point of the
thin film resonator 55 and thus the oscillator output frequency. As in the
prior embodiment, alternatives for the elements 56, 58 are available in
the form of current control devices or temperature responsive devices when
such operation is desired of the oscillator 50.
In the frequency agile oscillator using a thin film resonator with a comb
frequency output in order to stabilize the oscillator loop and allow ready
selection between responses of the comb characteristic of the thin film
resonator, a further stacked crystal filter in the form of delay SCF 60 is
associated with thin film resonator 55 and connected in the feedback path
52 as illustrated. The delay SCF 60 has a center frequency which is very
near that of the center frequency of the thin film resonator 55, and
provides additional phase delay which allows individual selection from
among the comb of responses of the thin film resonator (as will be
described in greater detail below in connection with FIGS. 4 and 5).
Suffice it to say for the moment that the combination of the thin film
resonator 55 with its comb output and the additional delay introduced in
the passband by the delay SCF 60 adjusts the phase of the comb responses
one with respect to the other such that the controllable impedance 56 can
select individual responses from the comb and cause the oscillator 50 to
operate only at the selected response.
A further coarse phase adjustment element 62 is illustrated for the sake of
completeness. In the microstrip implementation of the prototype of the
FIG. 3 device, a relatively large additional phase delay is often required
in order to satisfy the phase aspect of the Barkhausen criteria and the
coarse phase adjust 62 in the form of a line stretcher is utilized so that
the impedance introduced by the electronically adjustable impedance 56 is
able to tune the combined characteristic of the thin film resonator 55 and
delay SCF 60 in the desired output frequency operating range.
Turning to FIG. 4, there is shown the phase and magnitude characteristics
of an overmoded SCF for use as thin film resonator 55 in the oscillator 50
of FIG. 3. FIG. 4 includes an upper plot 70 which illustrates the phase of
the overmoded SCF as a function of frequency, and lower plot 71 which
illustrates the magnitude (the S.sub.21 characteristic) as a similar
function of frequency. It will be appreciated that the showing of FIG. 4
illustrates only a very small portion of the frequency spectrum of the
overmoded stacked crystal filter since the spacing between peaks on the
plots 70, 71 is about 4 MHz, although the center frequency for the plot is
about 1 GHz.
The plots 70, 71 of the phase and magnitude characteristics, respectively
of the overmoded stacked crystal filter, illustrate the comb-like nature
of the overmoded oscillator (or also of other thin film oscillators
capable of operating as overmoded devices, such as the monolithic
resonator). The comb-like characteristic demonstrates a plurality of
responses at frequencies associated with magnitude peaks 72-75. The
"responses" of the overmoded filter referred to herein are intended to
refer to operation of the filter at the magnitude peaks and in the linear
portions of the phase characteristics at about the frequencies identified
by the magnitude peaks 72-75. It will be appreciated from the sharpness of
those peaks in FIG. 4 that the device is indeed high Q, an exemplary
embodiment providing unloaded Q's on the order of 2,000.
It will be appreciated from an examination of FIG. 4 that when overmoded
stacked crystal filters were used in oscillators in the past, the output
of such oscillators included the full comb characteristic of the frequency
selective device, as described, for example, in the Burns et al. paper
(see FIG. 5 at page 371). That will be appreciated, in part, because FIG.
4 demonstrates that even if selectivity were provided for such an
oscillator, there is little opportunity to select between the various
responses because the phase and magnitude characteristics are so sim | | |
