Oscillator having feedback isolated from its output

An oscillator including an active gain device and feedback between the output of the the device and its input for sustaining oscillations that incorporates a power splitter between the output of the device, the output of the oscillator and a feedback path for isolating the feedback path from the output of the oscillator for improving power and frequency pushing.

BACKGROUND OF THE INVENTION 
The present invention relates to oscillators and, more particularly, 
circuitry and method for providing isolation between the oscillator output 
and the feedback path to the input of the gain producing device of the 
oscillator to improve the operating characteristics thereof. 
In general, most, if not all, oscillators include a two port gain producing 
device having an input and an output which produces an output signal at 
the output of the oscillator and feedback between the output and the input 
of the device of proper phase and gain to induce and sustain oscillation. 
There are a myriad of structures and methods for providing a feedback path 
to couple a portion of the output signal from the gain producing device 
back to its input. However, in all known oscillators the feedback path is 
not inherently isolated from the output of the oscillator wherein the 
phase of the feedback signal becomes a function of the characteristics of 
the load coupled to the output of the oscillator. This lack of isolation 
is critical in that large changes in the frequency and output power of the 
oscillator can be caused by a change in the impedance of the oscillator 
load. In the extreme, oscillation may not be sustained and the device 
could be destroyed due to extreme load mismatching. 
Hence, a need exists for an improved oscillator in which isolation is 
provided between the output of the oscillator and the feedback path to the 
input of the gain producing device thereof for improving the frequency and 
power pulling characteristics of the oscillator. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved oscillator. 
It is another object of the invention to provide an oscillator having 
improved frequency and power pulling characteristics. 
Still another object of the present invention is to provide an improved 
oscillator having isolation between the output of the oscillator and the 
feedback path coupled between the output and input of the gain producing 
device of the oscillator. 
Yet another oject of the present invention is to provide a microwave 
oscillator having coupler circuitry for splitting the power between the 
output of the oscillator and a feedback path while providing isolation 
therebetween. 
In accordance with the above and other objects there is provided an 
oscillator comprised of a two port or three-terminal active device having 
feedback between the output and input thereof of the correct phase and 
magnitude to sustain oscillations and coupler circuitry for splitting the 
power from the output of the active device between the output of the 
oscillator and the feedback path while isolating the output of the 
oscillator from the feedback path. 
It is a feature of the invention that the coupler circuitry is suited to be 
realized in many known microstrip or other known transmission line 
configurations such as a coupled line directional coupler, a Lange 
coupler, a Wilkinson power splitter, a branch line coupler, and a rat race 
coupler to name a few. However, only couplers or power splitters that 
provide isolation between the two output ports of the power splitter may 
be used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is to be understood that although the following description of the 
oscillator of the preferred embodiments is made using microwave microstrip 
oscillators as examples, the novel concept of isolating the output of the 
oscillator from the feedback path is useful in any type of oscillator. For 
instance, waveguide oscillators as well as any other oscillator having a 
two-port or three terminal active device are readily able to use the 
present invention. It is well within the scope of this invention that the 
two port active device might be any type of amplifier such as an 
monolithic microwave integrated circuit amplifier or any active three 
terminal device such as a FET, bipolar transistor or heterojunction 
transistor. Additionally, it is well within the scope of the present 
invention for one skilled in the art to realize that the novel concepts 
described herein also apply to voltage tuned oscillators which use 
varactors or other tuning devices, YIG oscillators and cavity oscillators. 
Turning now to FIG. 1, there is shown the microstrip circuitry of a 
microwave oscillator 10. Oscillator 10 is enclosed within metal housing 12 
and includes a substrate 14 (FIG. 3) upon which the microstrip circuitry 
is disposed as is known. Substrate 14 is divided by the metal rib 16 of 
housing 10. A three terminal active device 18, for example, a field effect 
transistor is disposed on rib 16 having its drain electrode coupled to 
conductor 20 by a bonding wire, its gate electrode coupled to conductor 22 
by a bonding wire and its source electrode coupled through RF bypass 
capacitors 24 and 26 to RF ground potential via rib 16. A source of 
positive DC operating potential is supplied from feedthrough terminal 28 
and lead 30 to conductive pad 32. The operating potential is coupled to 
the drain electrode of FET 18 through a conventional RF choke comprised of 
alternating quarter wavelength high impedance lines 34, 36 and 38 and 
quarter wavelength low impedance stubs 40 and 42. Hence, the DC potential 
is applied to the drain of FET 18 while at the operating frequency of the 
oscillator the RF choke presents a high impedance. The drain is coupled 
through DC block and matching transformer 44 to the output of the 
oscillator via center conductor 46. As understood in the art, power 
splitter 66, comprised of electro-magnetically coupled lines 68 and 70, 
should be properly power impedance matched for power flow from the output 
of the gain producing device 18 to both the oscillator output 48 and the 
feedback path from coupler 66 to the gate terminal of FET 18. A connector 
48 provides external connection to oscillator 10 and is connected to 
conductor 46 via tab 50. The source electrode of FET 18 is RF bypassed to 
ground at the operating frequency via capacitors 24 and 26 while a DC 
conduction path is provided between the source and gate electrodes to 
establish the proper DC difference potential therebetwee via resistor 27 
and RF choke or stub 52. Resistor 27 is coupled between the source 
electrode and ground. At the operating frequency, stub 52, being a quarter 
wavelength high impedance line, presents a high impedance to line 22 which 
electrically removes the DC bias circuit from the RF circuit. 
Feedback is provided from the output of FET 18 to the gate electrode 
thereof as a portion of the output power is coupled through a conventional 
stripline power splitter section 66. Coupler 66 couples power from the 
output of transistor 18 to the two output ports of the power splitter to 
lines 46 and and 72 while isolating output 48 from the the feedback path 
to the gate electrode of transistor 18. Hence, any power traveling from 
connector 48 towards coupler 66 is coupled to load 74 and the continuation 
of line 46 and is isolated from the feedback path, line 72. Hence, power 
splitter section 66 comprises a quarter-wave section 68 of conductor 46 
which electromagnetically couples power to section 70 of conductor 72. One 
end of the power splitter is terminated by a resistor to ground; resistor 
74 being typically 50 ohms to match the conductor line impedance. 
Conductor 72 is connected to conductor 78 by substrate plug 76 the latter 
of which rests on rib 16 with connection being made by wire bonding as 
shown. A resonator 80 is electromagnetically coupled between conductor 78 
and matching transformer 82. This resonator may or may not be present in 
the feedback path. When a resonator is used it may be of any type 
conceivable. A resonator is generally used to increase the Q of the 
oscillator and/or to electronically or machanically tune the frequency of 
the oscillator. Matching transofrmer 82 transforms the gate impedance of 
transistor 18 to properly load resonator 80 for correct insertion loss. 
Matching transformer 82 is not necessary if proper coupling is made 
between resonator 80 and line 22 to provide the correct insertion loss. 
Correct insertion loss is that value which will provide at least enough 
magnitude of signal to the gate electrode of transistor 18 to maintain 
oscillation and store energy in the resonator to raise the Q as is 
understood. Coupling between resonator 80 and line 78 should be adjusted 
for a power impedance match for the feedback signal. As is known to those 
skilled in the art, proper design of the oscillator requires the total 
phase shift from the gate terminal of FET 18 around the feedback path to 
the gate terminal to be zero degrees at the design frequency and the 
resonator to be resonant at or close to the design frequency. While no 
varactor is shown coupled to resonator 80 it is well understood and within 
the scope of the present invention for one skilled in the art to realize 
that the novel concepts described herein could include such a varactor 
coupled to resonator 80 to provide electronic tuning of the oscillator 
frequency. 
In operation, transistor 18 provides gain at the desired operating 
frequency as well as a predetermined phase shift from input to output. By 
feeding back a portion of the output power provided at the drain electrode 
of transistor 18 to its gate electrode of the correct phase and magnitude, 
oscillation can be sustained at the desired oscillation frequency. Hence, 
except for the use of power splitter 66 for providing isolation between 
the output of the oscillator and the feedback path, oscillator 10 is 
generally understood by those skilled in the art. 
A problem with oscillators of the type previously known is that the output 
power and frequency are directly affected by the load characteristics 
coupled to the output of the oscillator. As the load impedance is varied 
in phase and magnitude both the power and operating frequency of the 
oscillator varies. Power splitter 66 of the present invention minimizes 
the effect of the load impedance on power and frequency pulling of 
oscillator 10 by providing isolation between the oscillator load and the 
feedback path. By properly terminating power splitter 66 in matched 
impedances any reflected power from the oscillator load through terminal 
48 will be isolated from line 72 and hence reduced in the feedback path. 
In the preferred embodiment power splitter 66 typically provides at least 
20 dB of direct loss in the reverse direction, i.e., from output 48 to 
line 72 and hence the input of transistor 18 while splitting the power in 
the forward direction between the feedback path (line 72) and output 
terminal 48. Thus, power splitter 66 improves the performance of 
oscillator 10 by isolating the output of the oscillato from the feedback 
path thereof. 
Referring to FIG. 2 oscillator 90 is illsutrated which operates in the same 
manner as oscillator 10 described above and is shown with the components 
thereof corresponding to like components of FIG. 1 having the same 
reference numerals. Isolation between the output of oscillator 90 and the 
feedback path is provided by a Wilkinson type power splitter 92 and 
resistor 94. 
Turning to FIG. 4 there is shown dielectric resonator stabilized oscillator 
(DRSO) 100 including power splitter 66 (FIG. 1) of the present invention. 
DRSO 100 is illustrated as another example of an oscillator suited to 
utilize the isolated feedback of the preferred embodiment. Again, 
components of FIG. 3 which correspond to similar components of FIG. 1 are 
designated by the same reference numerals. DRSO 100 comprises a dielectric 
resonator puck 102 coupled between conductor 78 of the feedback path and 
conductor 79. Typically, resonator 102 is placed above striplines 78 and 
79 as described, for example, in U.S. Pat. No. 4,591,806, however, any 
suitable method may be employed as long as the correct coupling between 
the two lines is obtained. Resistor 108 may or may not be included for 
providing out of band stability. In all oscillators it is generally known 
that conditions for oscillation should not be present at any other 
frequency or frequencies than the desired frequency. This is called out of 
band stability. While not delineated herein, the oscillators 
representative of this invention must also be designed for out of band 
stability as is well known in the art. Tuning pads 110 and 112 may be used 
with wire bonds for changing the coupling of resonator 102 to stripline 
78. If resistor 108 is not used, the same type of tuning pads may be used 
for changing the coupling of resonator 102 to stripline 79. Hence, what 
has been described above are several examples of oscillators wherein the 
improvement comprises using a power splitter for providing feedback 
between the output and input of the gain producing device thereof while 
isolating the oscillator output from the feedback path. By isloating the 
feedback path from the oscillator output the frequency and power pulling 
characteristics of the oscillator are improved.