QPSK modulator or demodulator using subharmonic pump carrier signals

The present invention relates to a QPSK modulator or demodulator for modulating two different input bit streams with a subharmonic pump carrier signal to produce an appropriately encoded QPSK output signal. The subharmonic pump carrier signal used is a submultiple of a predetermined microwave or millimeter-wave carrier frequency. The QPSK modulator or demodulator is capable of being fabricated on a planar substrate using appropriate stripline filters and a mixer diode in each of an in-phase and quadrature signal path. A fin line arrangement, also capable of fabrication on the substrate, can be used to introduce an appropriately phased subharmonic pump carrier signal into each of the in-phase and quadrature signal paths before each mixer diode. Each mixer diode mixes the associated data bit stream and one of the two appropriately phased pump carrier signals to produce separate output signals which modulate along separate orthogonal axes, which output signals are combined to produce the QPSK modulator output signal.

TECHNICAL FIELD 
The present invention relates to a QPSK modulator or demodulator and, more 
particularly, to a QPSK modulator or demodulator comprising mixer diodes 
fabricated on a single substrate, and an arrangement for introducing a 
subharmonic local oscillator signal into the mixer sections of the 
modulator or demodulator. 
DESCRIPTION OF THE PRIOR ART 
Many modulation techniques are known for use in transmitting information in 
communication systems. One of such modulation techniques is known as 
Quadrature Phase Shift Keying (QPSK). Various forms of QPSK modulators are 
known, one of which is disclosed in the article "Pulsed Offset QPSK 
Modulator" by R. J. Giannini et al in IBM Technical Disclosure Bulletin, 
Vol. 21, No. 1, June 1978 at pages 123-124. There, a single stream bit 
pattern is applied at an input to a combination demultiplexer and pulse 
expander including bit timing to produce predetermined waveforms in 
associated I and Q channels. More particularly, the QPSK modulator is 
operated with the control of the ON/OFF timing such that the I and Q 
channels are alternately gated off for 0.5T seconds with spacing of 2T 
seconds, or pulsed for 1.5T seconds with a spacing of 2T seconds. 
Another form of a QPSK demodulator, or modulator, is disclosed in U.S. Pat. 
No. 4,352,071 issued to W. H. Childs et al. on Sept. 28, 1982. There, the 
demodulator includes an arrangement of couplers to permit the ports to be 
adjacent one another such that in mixer applications the circuitry need 
not leave the plane of the integrated circuit. A similar arrangement using 
couplers for a single input signal QPSK modulator is disclosed in FIG. 4 
of the article by R. K. Shoho et al. in Microwave Journal, Vol. 25, No. 9, 
September, 1982, at pages 131-138. 
As disclosed in U.S. Pat. No. 4,480,336 issued to J. S. Wong et al. on Oct. 
30, 1984, it is known to make an orthogonal hybrid fin-line mixer which 
includes a two-piece housing with a fin-line mounted within the waveguide 
housing. There, one side of the substrate is disposed within the top half 
of the waveguide housing and the other half of the substrate is disposed 
within the bottom half of the waveguide housing. Such mixer arrangement, 
however, is not disclosed as concerning itself with QPSK modulation. 
It is to be understood that the switching time of diodes in conventional 
path-length modulators has a significant impact on the data rate which a 
modulator can produce from an applied carrier. Such diodes necessarily 
operate on the minority carrier storage principle and, therefore, tend to 
be slow. For a data rate of 3 Gbit/sec, the bit interval is 0.3 ns, and 
20% of the bit interval is equivalent to 60 picoseconds, which is the 
maximum tolerated rise time of the switching diode. Such short switching 
times are difficult to achieve with PIN diodes, dual-gate FETs and bipolar 
transistors (minority storage principle switches). Even bipolar 
transistors and dual-gate FETs, which operate on different principles, are 
still too slow. It should be further noted that PIN diodes used in 
path-length modulators have impedance characteristics which, as a function 
of bias voltage, cross from one point on the Smith chart to a 
corresponding point 180 degrees across the chart. During a fraction of 
this crossing time, the carrier is approximately matched to the switching 
diode. This high-loss resistive switching interval will result in 
"notching" of the carrier power during the transition which may last for 
several cycles of the pump. Additionally, with QPSK, the local oscillator 
used generally generates the same frequency as the frequency desired at 
the output. Therefore, mixing at millimeter waves requires a high 
frequency oscillator, which is very expensive, and the mixing process of 
the oscillator carrier frequency and the input signals provides more loss 
at the higher frequency. The problem, therefore, remaining in the prior 
art is to eliminate, as much as possible, the above-mentioned problems of 
rise time and notching and higher loss in the mixing process at the high 
frequencies. 
SUMMARY OF THE INVENTION 
The foregoing problem in the prior art has been solved in accordance with 
the present invention which relates to a QPSK modulator or demodulator 
comprising mixer diodes fabricated on a single substrate in quadrature 
signal paths with means for introducing a separate one of an in-phase and 
90.degree./n subharmonic local oscillator pump carrier signal plus a 
separate first and second associated input signal data stream, 
respectively, to the mixer diodes in each of the respective quadrature 
signal paths of the modulator or demodulator. 
It is an aspect of the present invention to provide a QPSK modulator or 
demodulator as described above wherein the mixer diode is switched from a 
conducting to a non-conducting state at some multiple of the pump cycle, 
as, for example, twice per pump cycle which corresponds to once per 
carrier cycle. This mode of diode operation allows the use of Schottky 
barrier diodes with vastly greater speed compared to the PINs, bipolar and 
FETs. 
Other and further aspects of the present invention will become apparent 
during the course of the following description and by reference to the 
accompanying drawings.

DETAILED DESCRIPTION 
The present invention relates to a high-speed QPSK modulator or demodulator 
for modulating a first and second appropriately phased subharmonic pump 
carrier frequency with first and second data streams, respectively, which 
data streams can be in the multigigabit range. The present QPSK modulator 
or demodulator is applicable for transmitting a sequence of pseudo-random 
pulses, comprising digitally encoded information, by means of a microwave 
or millimeter-wave carrier. It is to be understood that the present QPSK 
modulator or demodulator is applicable for use in satellite or terrestrial 
communication systems requiring the transmission of data at high bit 
rates. The advantages presented by the present QPSK modulator or 
demodulator arrangement are that (1) the modulator or demodulator is 
inherently broad-band, i.e., the circuit does not contain narrow-band 
couplers, shorting stubs or PIN switching diodes, (2) the QPSK modulator 
or demodulator is pumped with a local oscillator source at a submultiple 
of the microwave or millimeter-wave carrier frequency, and (3) the 
modulator or demodulator can readily be fabricated on a single substrate 
using conventional photolithographic pattern generation techniques. 
FIG. 1 is a block diagram of a QPSK modulator or demodulator in accordance 
with the present invention. The discussion which follows is primarily 
directed at the QPSK modulator. A first and second bit stream, comprising 
a separator first and second input signal, respectively, are received and 
propagated via a respective first and second rail 10 and 11 through the 
QPSK modulator. For purposes of description, the first signal on rail 10 
will also be known as the quadrature signal and the second signal on rail 
11 will also be known as the in-phase signal. It is to be understood that 
the first and second signal on rails 10 and 11, respectively, are received 
as separate input signals which are not in quadrature with one another, 
but that these input signals will be modulated into quadrature signals 
within the QPSK modulator prior to being transmitted from the output. 
The first and second input signals, propagation on rails 10 and 11, pass 
through a first and a second low-pass filter 12 and 13, respectively. 
Low-pass filters 12 and 13 function to pass the input signal frequency but 
prevent a subharmonic pump carrier signal, introduced into rails 10 and 11 
after filters 12 and 13, from reaching the input signal sources connected 
to the modulator. A local oscillator 14 generates a subharmonic pump 
carrier signal which carrier signal is a submultiple of a microwave or 
millimeter-wave carrier frequency. An in-phase subharmonic pump carrier 
signal from local oscillator 14 is introduced into rail 11 at the output 
of low-pass filter 13. The output signal from oscillator 14 is also sent 
through a 45 degree phase shifting means 15 to obtain a 45 degree 
subharmonic pump carrier signal which is introduced into rail 10 at the 
output of low-pass filter 12. The 45 degree subharmonic pump carrier 
signal produces a quadrature phase shift in the mixer output at twice the 
pump frequency. The combination of the first input signal and the 45 
degree subharmonic pump carrier signal on rail 10 is sent through a third 
low-pass filter 16. Similarly, the combination of the second input signal 
and the in-phase subharmonic pump carrier signal on rail 11 is sent 
through a fourth low-pass filter 17. Low-pass filters 16 and 17 function 
to prevent the signals generated by diode mixers 18 and 19, on rails 10 
and 11, respectively, from propagating back toward the associated input 
signal source but permit the associated digital bit stream input signal 
and the subharmonic pump carrier signal to pass therethrough. 
Diode mixer 18 functions to mix the first digital data bit stream input 
signal with the 45 degree subharmonic pump carrier signal to produce a 
signal which is modulated along one axis between 90 and 270 degrees. Diode 
mixer 19 functions to mix the second digital data bit stream input signal 
with the in-phase subharmonic pump carrier signal to produce a signal 
which is modulated along a second axis between 0 and 180 degrees. Mixer 
diodes suitable for subharmonic mixing are GaAs or InP planar-doped 
barrier devices (PDB diodes), GaInAs structures, or two Schottky barrier 
diodes connected in anti-parallel as shown, for example, in the article 
"Harmonically Pumped Stripline Down-Converter" by M. V. Schneider et al. 
in IEEE Transactions On Microwave Theory and Tecniques, Vol. MTT-23, No. 
2, March 1975, at pages 271-275. The output signals generated by diode 
mixers 18 and 19 are sent through high-pass filters 20 and 21, 
respectively, which function to pass the data bit stream modulated carrier 
signals at the desired frequency while preventing the lower frequency 
subharmonic pump carrier signals and baseband frequency bit streams from 
passing therethrough. The output signal from high-pass filter 20, 
modulating between 90 and 270 degrees, and the output signal from 
high-pass filter 21, modulating between 0 and 180 degrees, are combined in 
a combining means 22 to generate an output signal comprising a QPSK signal 
vectored at 45, 135, 235 or 315 degrees. 
FIGS. 2 and 3 illustrate a preferred arrangement for forming the QPSK 
modulator arrangement of FIG. 1 on a substrate 24 of dielectric material 
in accordance with the present invention. More particularly, FIG. 2 
illustrates one major surface, e.g., a top surface, of dielectric 
substrate 24 including a first pattern of conductive material, shown in 
darkened form, disposed on the one major surface. FIG. 3 illustrates an 
opposing second major surface, e.g. a bottom surface, of the same 
substrate 24 of FIG. 2 including a second pattern of conductive material, 
shown in darkened form, disposed on the second major surface. The 
conductive material patterns on opposing sides of substrate 24 form the 
components of FIG. 1. It is to be understood that the conductive pattern 
on each side of the substrate 24 can be formed, for example, by well known 
photolithographic techniques. More particularly, a dielectric substrate 24 
with a layer of conductive material on both opposing sides can be 
appropriately etched, by chemical or laser means, to remove the conductive 
material at specific areas and form the desired pattern. 
In the arrangement of FIGS. 2 and 3, low-pass filters 12, 13, 16 and 17 are 
formed by (1) the alternating large and very narrow areas on the first 
side of substrate 24 as shown in FIG. 2, and (2) the layer of conductive 
material on corresponding areas of substrate 24 shown in FIG. 3 to form 
the appropriate L-C networks. High-pass filters 20 and 21 are formed by 
the spaced-apart reversed and mirrored step patterns on opposing edges of 
substrate 24. The outputs from high-pass filters 20 and 21 are combined in 
a combining means 22, which is achieved, as shown in FIG. 2, by merging 
the striplines from filters 20 and 21 at point 22. Mixers 18 and 19 can be 
formed using any suitable diode mixer as, for example, a planar doped 
barrier diode device which is well known in the art. 
The local oscillator generated subharmonic pump carrier signal is 
introduced via a waveguide into the area 26 of removed conductive material 
as shown in FIG. 3. The subharmonic pump carrier signal propagates in a 
fin-line arrangement 27 to an area 28 where it is divided by a novel 
fin-line divider into two oppositely directed fin-lines 29. At the point 
where fin-lines 29, on the second side of substrate 24, pass under the 
striplines, on the first side of substrate 24, interconnecting low-pass 
filters 12 and 16 and low-pass filters 13 and 17, the associated pump 
subharmonic carrier signal will be introduced into the stripline 
interconnections between the associated low-pass filters. It is to be 
noted that the two opposingly directed fin-lines 29 have different lengths 
before they cross under the stripline interconnections between the 
low-pass filters. This difference in length between the fin-lines 29 forms 
phase shift means 15 and functions to introduce a 45 degree phase shift in 
the pump carrier signal propagating in the longer of fin-lines 29 before 
crossing under the associated stripline interconnection between low-pass 
filters 12 and 16. The ends 30 of fin-lines 29, after crossing under the 
stripline interconnections between low-pass filters 12-16 and 13-17, form 
shorts for the pumped subharmonic carrier wave. 
FIG. 4 illustrates an exemplary arrangement for mounting the QPSK modulator 
arrangement of FIGS. 2 and 3. In FIG. 4, a lower half 40 and an upper half 
41 of a waveguide housing, for mounting substrate 24 therein, comprise 
corresponding waveguide grooves which match the associated configuration 
of substrate 24. A recessed shelf 42 in the lower half 40 of the waveguide 
housing permits the appropriate positioning of substrate 24 in the 
waveguide housing. Once substrate 24 is positioned in lower half 40, the 
input terminals 43 and 44 on substrate 24 are electrically connected to, 
for example, coaxial cable connectors 45 and 46, respectively, via 
respective holes 47 and 48 in lower half 40 of the waveguide housing. 
Similarly, the output of combining means 22 of the QPSK modulator is 
electrically connected to, for example, a coaxial waveguide connector 49. 
A waveguide output pump oscillator 50, as, for example, a Varian model 
VSK-9004 Gunn oscillator, is shown mounted on lower half 40 for 
introducing the subharmonic pump carrier signal from local oscillator 14 
to the lower section of the center leg of substrate 24. It is to be 
understood that any other suitable arrangement for introducing the 
subharmonic pump carrier signal can be used. A step is shown in lower half 
40 at the output of pump oscillator 50 to provide appropriate matching 
between waveguides. Once substrate 24 is positioned on shelf 42 in lower 
half 40 and connected to the appropriate input and output connectors, the 
top half 41 of the waveguide housing is positioned on the bottom half 40 
and secured together for subsequent mounting in a transmitter or receiver. 
FIG. 5 illustrates a novel fin-line divider 28 forming part of the fin-line 
arrangement for introducing the subharmonic pump carrier signal to rails 
10 and 11. It is to be understood that such fin-line divider can be used 
for dividing any kind of signal propagating in a fin-line arrangement. In 
FIG. 5, the subharmonic pump carrier signal arriving in fin-line 27 
arrives at point 60 and divides equally into each of fin-lines 61 and 62 
via the angled separation. Paths 61 and 62 are extended into paths 29a and 
29b, respectively, which are angled outwards to continue on the fin-lines 
29 shown in FIG. 3. Each of the angled separations between fin-lines 61-62 
and 29a-29b form gradual separations to prevent mode changes and to 
provide appropriate cancellation of waves propagating in the fin-lines 
29a-29b and 61-62 back towards fin-line 27. A thin resistive fin-line 63, 
which can include a chip resistor 64, is disposed between the intersection 
of fin-lines 61 and 62 with fin-lines 29a and 29b, respectively. 
Therefore, waves propagating back along fin-lines 29a and 29b will be 
directed into resistive fin-line 63 and, if properly oriented with one 
another, will be canceled. Remaining waves propagating back along 
fin-lines 61 and 62, if properly oriented when arriving at point 60 will 
also be shorted and canceled. 
It is to be understood that the abovedescribed embodiments are simply 
illustrative of the principles of the invention. Various other 
modifications and changes may be made by those skilled in the art which 
will embody the principles of the invention and fall within the spirit and 
scope thereof. For example, the arrangements of FIGS. 2 and 3 could be 
modified to use other configurations or elements for the filters, the 
mixer, the combining means, and the means for introducing the subharmonic 
pump carrier signal to the mixer. Additionally, the waveguide housing of 
FIG. 4 could be replaced or modified with some other similar suitable 
arrangement. It is to be understood that the description hereinbefore for 
the QPSK modulator can also be used for an embodiment of a QPSK 
demodulator at a receiver, where the modulated carrier from a remote 
transmitter is received at combining means 22 and the subharmonic pump 
signal from local oscillator 14 is synchronized with the modulated carrier 
.