Wideband, millimeter wave frequency Gunn oscillator

A tunable millimeter wave frequency Gunn oscillator operates efficiently beyond the normal Gunn oscillator frequency range through the use of a second harmonic enhancement circuit. The enhancement circuit is tuned to the fundamental frequency of the oscillator and is tracked with a second harmonic circuit to provide a wide tuning range. Lumped element circuitry is used to aid in providing the wide tuning range and to facilitate adjustment and fabrication.

BACKGROUND 
1. Field 
This invention relates to improvements in millimeter wave frequency 
circuitry and in particular, to solid state millimeter wave oscillators. 
2. Prior Art 
GaAs Gunn diode oscillators currently operate over the frequency range of 3 
to 100 GHz. The theoretical high frequency limit of the Gunn diode 
oscillator range is just above 100 GHz and is set by the intervalley 
relaxation time of the Gunn diode. Second harmonic output from such 
oscillators has been obtained, but it is usually 20 dB below the 
fundamental power. Typically, circuits in these frequency ranges employ 
distributed parameter elements, such as multiwavelength transmission lines 
which limit the tuning range of an oscillator because of the rapid phase 
change of these lines with changes in operating frequency. 
SUMMARY 
It is an object of the present invention to provide a Gunn diode oscillator 
with a wide tuning range. 
It is another object to provide an oscillator capable of efficiently 
producing a signal in a frequency range beyond the normal operating range 
of a Gunn diode oscillator. 
It is another object to provide a millimeter wave Gunn diode oscillator 
with miniature, low cost and easily adjusted circuitry. 
The oscillator circuitry in the present invention comprises principally two 
series circuits placed across the Gunn diode. The first of these circuits 
presents a resonant high impedance to the diode at both the fundamental 
and the second harmonic frequencies, while the second presents a parallel 
resonance to the diode at the fundamental frequency and is used to enhance 
the output of the second harmonic signal. 
Both of these circuits employ lumped element components, including chip 
capacitors and varactors, as well as fractional wavelength air dielectric 
transmission lines.

DETAILED DESCRIPTION OF THE INVENTION 
The operating frequency of a GaAs Gunn diode is limited to approximately 
100 GHz by the intervalley relaxation time. If a higher frequency is 
desired, such as 120 GHz, the second harmonic of a Gunn oscillator may be 
used to provide this signal. However, the second harmonic output power is 
usually well below that of the fundamental and if the second harmonic 
signal is to be usable, it must have sufficient power for an intended 
application. Therefore, the efficient generation of power at the second 
harmonic is important. An improvement of 10 dB in the second harmonic 
power output over that obtained from conventional circuitry is provided by 
the circuitry of the present invention. 
The elements of the present invention shown in FIG. 1 are a bias filter 
choke 2, a bias filter capacitor 3, a first variable capacitor 4, a first 
circuit choke 5, a Gunn diode 6, a second circuit choke 7, and a second 
variable capacitor 8. Drawing numerals 1 and 9 refer to a bias input 
terminal and a signal output port, respectively. 
For purposes of analysis, this circuit may be divided into three portions: 
the bias circuitry, the second harmonic frequency circuitry and the 
fundamental frequency circuitry. The bias circuitry consists of the filter 
capacitor 3, bias choke 2 and to a limited extent, choke 5. Capacitor 3 
and choke 2 form a simple low-pass filter designed to isolate the bias 
supply from the millimeter wave circuitry of the oscillator. Bias voltage 
is applied to a bias terminal 1 and then is supplied to the Gunn diode 
through chokes 2 and 5. Choke 5 serves to isolate the diode at the 
fundamental oscillator frequency from the bias circuitry. The complete 
function of choke 5 is explained in greater detail in connection with the 
second harmonic circuitry. 
As is common in the operation of Gunn diodes, the diode bias is set to a 
voltage which causes the diode to exhibit a negative resistance. Placing 
this negative resistance across a parallel resonant circuit satisfies the 
conditions necessary for oscillation. The fundamental frequency circuitry 
provides the required parallel resonant circuit for oscillation. 
The fundamental frequency circuitry comprises the choke 7 and the capacitor 
8. This circuit, in combination with the reactance of the diode and any 
residual reactance present at the fundamental frequency from other 
circuitry connected to the diode, constitute a resonant circuit at the 
fundamental frequency. This circuit may be tuned by variable capacitor 8. 
As shown in FIG. 3, signal output port 9 is a coaxial transmission line. A 
transition to waveguide, shown in FIG. 4A, is used at port 9 to provide an 
interface to conventional millimeter waveguide, which provides 
significantly lower transmission loss to the load over that provided by a 
purely coaxial line. In FIG. 4A, an extension 14 of the coaxial line at 
port 9 is shown with its center conductor 15 extending into a waveguide 
13. A short, 16, is located at one end of the waveguide, while the 
opposite end of the waveguide 17 serves as the output port. 
The transition is such that energy can propagate equally in two directions 
in the waveguide portions, as indicated by the arrows 22 and 23. The short 
circuit is positioned to reflect energy 24 at the fundamental frequency in 
the proper phase to suppress the fundamental from the output, while energy 
at the second harmonic is reinforced at the output. These effects at the 
fundamental and second harmonic are complementary. A high-pass filter may 
be used at the waveguide output port to provide increased suppression of 
the fundamental from the load, if required. 
It has been found experimentally that the position of the short does not 
have to be adjusted to provide satisfactory performance over a relatively 
wide band of frequencies; however, complete optimization can be achieved 
by fine tuning the position of the short. 
It is possible to electronically adjust the position of the short and to 
"gang" the control voltage for this electronic adjustment to the control 
voltages used for other varactors to be described later, making the short 
adjustment automatic. 
An embodiment shown in FIG. 4B which may be used to achieve this end 
consists of two series circuits, the first tuned near resonance at the 
fundamental frequency while the second is tuned near resonance at the 
second harmonic frequency. The first resonant circuit consists of choke 18 
and varactor 19 while the second consists of choke 20 and varactor 21. 
These circuits are placed a small fraction of a wavelength in front of the 
short, as shown by the position of choke 18 and varactor 19 in FIG. 4A. 
Biasing circuits for the varactors are conventional and are not shown to 
simplify the drawings. Adjustment of control voltages applied to the 
varactors in these circuits will tune them close to resonance at their 
respective operating frequencies, thereby affecting the phase of the 
reflected signals emanating from the region of the short to effectively 
electronically adjust the position of the short. 
By means of the adjustment of the short, the fundamental frequency power is 
reflected back to the diode in a phase relationship which increases the 
second harmonic output power and thus the oscillator efficiency at 
frequencies beyond the fundamental operating range of the Gunn diode. For 
the sake of brevity, this effect is referred to herein as second harmonic 
enhancement. 
The second harmonic circuitry comprises the capacitor 4 and the choke 5. 
The purpose of this circuit is to present a reactance to the Gunn diode 
which in combination with the reactance of the diode and associated 
circuitry forms a parallel resonant circuit at the second harmonic 
frequency. This circuit may be tuned by the variable capacitor 4. 
The capacitors 4 and 8 are ganged so that both the fundamental and second 
harmonic circuits may be simultaneously tuned over a relatively wide 
tuning range. 
In the more practical embodiment of the invention shown in FIG. 2, the 
tuning is performed electronically by means of varactors. The capacitors 4 
and 8 in FIG. 1 are replaced by capacitors 4A, 4B, 8A and 8B in FIG. 2. 
Capacitors 4A and 8A are varactors while capacitors 4B and 8B are fixed 
value capacitors which function as a dc block and rf bypass. The required 
variation in capacity is supplied entirely by the varactors. The varactors 
also may be "ganged" merely by supplying the bias for both from a common 
source. This approach can be used to electronically control the short as 
well. Usually, a small offset in the bias control voltage is all that is 
required to account for the different frequency at which varactor 4A and 
8A must operate. 
The bias voltage to the varactors is supplied through terminal 10 for 
varactor 4A and through terminal 11 for varactor 8A. Although not shown, a 
bias choke is normally added to the varactor bias terminals. This choke 
provides for increased bias filtering from that provided by capacitors 4B 
and 8B alone. 
Choke 5 is a one-quarter wave transmission line at the fundamental 
frequency and a one-half wavelength transmission line at the second 
harmonic frequency. The capacitors 4A and 4B represent a relatively low 
impedance at the fundamental frequency which is reflected through the 
one-quarter wavelength line of choke 5 to the Gunn diode as a high 
impedance, preventing the loss of power at the fundamental frequency 
through the second harmonic circuit. 
As a one-half wave transmission line at the second harmonic frequency, 
choke 5 reflects the reactance of the combination of capacitor 4A and 4B 
across the Gunn diode to tune the diode and its associated reactance to 
parallel resonance at the second harmonic frequency. Choke 5 performs the 
functions of isolating the diode from the bias network at terminal 1 at 
the fundamental frequency, isolating the diode from capacitors 4A and 4B 
at the fundamental frequency, supplying the bias voltage to the diode, and 
presenting the reactance of capacitors 4A and 4B to the diode at the 
second harmonic frequency. All these functions as well as the advantages 
of ease of tuning and wide bandwidth are produced simply and at low cost 
by a single lumped element component. The simplicity of this component can 
be seen in FIG. 3. 
FIG. 3 is a pictorial representation of a circuit embodying the present 
invention. All the components employ the same drawing numerals as were 
used in FIGS. 1 and 2. The only added drawing numeral is 12 which 
represents a ground plane and mounting surface. The actual size of this 
surface is only one-eighth inch by one-eighth inch in K.sub.a band (26 to 
40 GHz) and is sized to be cut off to waveguide modes. Although this 
circuit is designed to operate at millimeter wave frequencies, it uses 
lumped element components throughout. Capacitors 4B and 8B are MOS type 
chip capacitors, while varactors 4A and 8A are chip varactors. Chokes 5 
and 7 are air dielectric transmission lines formed of single conductors 
spaced above the ground plane. These lines are considered as lumped 
element devices because their lengths are only a small fraction of a 
wavelength. As such, these lines do not present the problem encountered 
with distributed parameter circuitry where rapidly varying phase as a 
function of frequency produced by multiwavelength lines prevent tuning the 
oscillator over wide frequency ranges. 
It is evident from the simplicity of the circuitry shown in FIG. 3 that 
small and easily fabricated circuits may be constructed using lumped 
circuit elements at frequencies in the millimeter wave frequency range. 
This type of construction also facilitates adjustment of the circuit. The 
effective length and impedance of the air dielectric transmission lines 
shown in FIG. 3 may be adjusted by arching the lines above the ground 
plane. This avoids the usual redesign and refabrication necessary with 
printed transmission lines to make the same adjustments. 
It should be noted that the construction of the lines shown in FIG. 3 is 
merely representative. Other types of transmission lines are contemplated 
as being within the spirit and scope of this invention. For example, there 
need be no ground plane or the ground plane need not be as well defined as 
it is in FIG. 3. The representation of a transmission line by a single 
line and ground plane will be used herein to represent transmission lines 
with single or multiple conductors and with or without a defined ground 
plane.