Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
1998-01-07
2001-10-23
Le, Thanh Cong (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S323000, C327S359000
Reexamination Certificate
active
06308058
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to image reject mixer circuits and in particular to image reject mixer circuits having a single ended input and two differential outputs. Radio frequency image reject mixers are very popular blocks of modern radio systems and are often used in preference to superheterodyne receivers, especially where frequency agility is required. The parameters of an image reject mixer determine the main characteristics of the system into which they are incorporated.
Prior art image reject mixers are generally based on the Gilbert cell or the micromixer circuit configuration. In the case of the Gilbert cell, an image reject mixer is compiled simply by connecting two Gilbert cell circuits in parallel. The input signal is split into two branches where they are mixed with an unshifted local oscillator signal and a 90° shifted local oscillator signal respectively.
A section of an image reject mixer circuit based on the micromixer configuration is shown in FIG.
1
.
In use, an input signal is applied to terminal
120
, a local oscillator signal is applied to terminals
130
,
131
and a 90° phase shifted local oscillator signal is applied to terminals
132
,
133
. A reference potential is applied to terminal
125
to bias transistors
103
and
104
. By virtue of resistors
150
,
151
, transistor
105
and biased transistor
103
, a signal input at terminal
120
will give rise to complementary output current signals at the collector electrodes of transistors
103
and
101
. Mixer core
160
, formed by transistors
106
-
109
, mixes the current signals from the collector electrodes of transistors
101
and
103
with the local oscillator signal applied to terminals
130
,
131
and outputs a current signal at terminals
140
,
141
. Transistors
102
and
104
produce at their collector electrodes substantially the same current signal as is produced by corresponding transistor
101
,
103
because these corresponding transistors are driven by the same input signal. Mixer core
170
mixes the current signal from the collector electrodes of transistors
102
,
104
with a 90° phase shifted local oscillator signal applied to terminals
132
,
133
and outputs a current signal at terminals
142
,
143
. Because an unphased oscillator signal is applied to terminals
130
,
131
and a 90° phase shifted signal is applied to terminals
132
,
133
, output terminals
140
,
141
will show an In-phase differential output and output terminals
142
,
143
will show a Quadrature differential output.
As will be appreciated, the mixer circuit shown in
FIG. 1
is incomplete. The full mixer circuit implementation would also have means for phase-shifting the output of mixer core
170
by 90° and summing the resultant signal with the output from mixer core
160
. This would result in either the image band signal or the signal band signal as the complete mixer circuit output, depending on the sign of the 90° phase-shift imposed on the signal output from mixer core
170
.
Whilst the image reject mixer circuit section of
FIG. 1
has a wide dynamic range and very linear operation, the presence of so many resistors gives the mixer circuit very poor noise properties.
Image reject mixers constructed from Gilbert cell circuits have poor noise properties due to resistors in the main current paths, current sources experiencing high frequency, large voltage swings and poor transistor arrangements. It is difficult also to design an image reject mixer using Gilbert cell circuits so that it has a particular input impedance. This can be a drawback when impedance matching with a pre-amp stage is necessary. There exists a need for an image reject mixer circuit with improved noise
SUMMARY OF THE INVENTION properties.
In accordance with the present invention, there is provided an image reject mixer circuit arrangement comprising an input amplifier connected to first and second phase-splitters, the phase-splitters each having two substantially complementary outputs, a first mixer core arranged to mix two of said phase-splitter outputs with a local oscillator signal and a second mixer core arranged to mix the other two of said phase splitter outputs with a phase shifted local oscillator signal.
REFERENCES:
patent: 4675911 (1987-06-01), Sokolov et al.
patent: 4994755 (1991-02-01), Titus et al.
patent: 5303417 (1994-04-01), Laws
patent: 5361408 (1994-11-01), Watanabe et al.
patent: 5448772 (1995-09-01), Grandfield
patent: 5625307 (1997-04-01), Scheinberg
patent: 5630228 (1997-05-01), Mittel
patent: 5847623 (1998-12-01), Hadjichristos
patent: 6029059 (2000-02-01), Bojer
patent: 2 239 143 A (1991-06-01), None
Chan Tak K
Souetinov Viatcheslav I
Cong Le Thanh
Kirschstein et al.
Mitel Semiconductor Limited
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