The Six-Port as a Communications Receiver
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 3, MARCH 2005 1039
The Six-Port as a Communications Receiver
Tim Hentschel
Abstract—Configurable radio terminals require receivers with
wide-band capabilities in order to support as many services
as possible at most different carrier frequencies. Conventional
well-known receiver architectures employing active circuitry are
limited in this respect. Therefore, alternative architectures are
investigated, such as the six-port, which has been introduced as
a very flexible and elegant means for microwave measurements
in the 1960s and 1970s. Later on, it has been used in radar appli-
cations. It was not until recently that communications receivers
have been built upon the six-port principle. However, in all publi-
cations, there is always a certain mystic about the six-port. It has
even been described as a “black box.” In order to help paving the
way for a wider application of the six-port technology, this paper
describes the basic six-port theory and sets it into relation with
the conventional receiver architectures such as the homodyne and
heterodyne receiver. Finally, the advantages and possible applica-
tions of receivers based on the six-port technology are discussed.
Index Terms—Communication terminals, frequency conversion,
heterodyning, homodyne detection, receivers.
I. INTRODUCTION
T
HE requirements on wireless communications receivers
are twofold. On one side, size and cost should be brought
down; on the other side, the receivers should be more and more
wide-band in order to meet the increasing demand for high data
rates. Conventional heterodyne receivers require filters at the RF
and IF, which can usually only be implemented by bulky sur-
face acoustic wave (SAW) or crystal filters. Hence, an integra-
tion on a single chip and, thus, small size and low cost, cannot
be achieved. For that reason, designers have been directing their
effort toward the homodyne receiver (better known as the direct
conversion receiver). The direct conversion receiver has gath-
ered much attention, particularly within the research commu-
nity since then. There are many problems due to analog impair-
ments to be solved when implementing a direct conversion re-
ceiver. Therefore, other receiver architectures are sought after.
One promising architecture is the six-port.
The application of the six-port to communications receivers
has been presented in [1]–[6]. The general problem with these
publications is the fact that the six-port itself is not explained.
This is not satisfying. For many engineers who are only familiar
with the above-mentioned conventional receiver architectures,
it would be extremely useful to know the relationship between
the six-port and conventional architectures. Instead of providing
this knowledge, there is a certain mystic about the six-port. In
Manuscript received February 4, 2004; revised September 27, 2004. This
work was supported by the German Ministry for Education and Research and
by Alcatel SEL AG.
The author is with Vodafone Chair Mobile Communications Sys-
tems, Technische Universität Dresden, 01062 Dresden, Germany (e-mail:
Digital Object Identifier 10.1109/TMTT.2005.843507
Fig. 1. Microwave network for the determination of the reflection coefficient
of the DUT through independent remote measurements.
[1] and [4], statements can be found such as “A six-port is a
black box with two inputs and four outputs.”
Among the first publications on the application of the six-port
to communications receivers was [5]. Li et al. proved the con-
cept of calculating the complex ratio of an incoming signal and a
known local oscillator (LO) signal by using the six-port. Origi-
nally, the six-port was found to be a very good means to measure
the complex reflection coefficient of a microwave device.
II. B
ACKGROUND OF THE SIX-PORT TECHNIQUE
The problem of microwave measurement is that connecting
a probe to the device-under-test (DUT) considerably changes
the very characteristics to be measured of the device, e.g., its
reflection coefficient. This can be circumvented by determining
the reflection coefficient of the DUT through a certain set of
independent remote observations from a linear network to which
the DUT is connected. Given the network of Fig. 1, it is possible
to relate all “reflected” waves to all “incident” waves by means
of the
-parameters
(1)
where
are complex parameters and and
are the complex amplitudes of the “incident” and
“reflected” waves, respectively.
Assuming that the relationship of the “reflected” and “in-
cident” waves at some of the ports (say, ports 1-
) can be
uniquely described by the respective reflection coefficient
,
it is
and (2)
Equations (1) and (2) form a system of
equations with
unknown variables and . Hence, unknown variables
can be described by the remaining
variables. In the case
0018-9480/$20.00 © 2005 IEEE
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