Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-09-08
2003-01-21
Urban, Edward F. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S313000, C455S325000, C455S333000, C455S323000, C455S327000, C455S330000
Reexamination Certificate
active
06510314
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wireless communications devices and, in particular, relates to mixer circuits on integrated circuits that are used to convert between radio frequency (RF) signals and intermediate frequency (IF) signals in the wireless communications devices.
BACKGROUND OF THE INVENTION
Wireless communications devices are ubiquitous in modern life. In recent times, there has been an explosion in the usage of cellular telephones and other types of remote communications devices, as well as increasing usage of wireless communications devices in other applications, including automotive applications. One of the reasons for the recent explosion in the use of wireless communications devices has been the significant improvements in integrated circuit technologies, which have allowed the electronics necessary for wireless communications devices to be significantly reduced in size and price. For example, cellular telephones have become practical in large part because the reduction in size of the electrical components has allowed manufacturers to design smaller, more convenient handsets. Further reduction in the size and cost of the electrical components of wireless communications devices remains an imperative for the manufacturers of the wireless communications devices. In particular, the replacement of circuit elements that are not implementable on an integrated circuit with circuit elements that can be implemented on an integrated circuit remains an important goal.
A key component of the typical wireless communications device is the mixer circuit, which allows for frequency conversion between the radio frequency (RF) or even microwave signals received by the wireless communications device, which travel through the atmosphere, and intermediate frequency (IF) signals which are processed by the wireless communications device. The process of converting RF signals into IF signals is critical for isolating the information carried on a desired RF signal from all of the other information received by the wireless device which is carried on other RF signals. A conventional mixer circuit converts a RF signal into an IF signal by multiplying the RF signal by a local oscillator (LO) signal provided by a LO circuit. Because several signals including the desired IF signal are typically produced by this multiplication process, the mixer circuit typically includes one or more filters that filter out all signals other than the desired IF signal so that solely the desired IF signal is output by the mixer.
One common embodiment of a mixer circuit that is employed within many wireless communications devices is a double balanced mixer. Referring to
FIG. 1
(Prior Art), a double balanced mixer
10
includes a RF input port
20
, a LO input port
22
, and a supply voltage port (V
ref
)
24
. The RF input port
20
receives two RF signals from an antenna (not shown), while the LO input port
22
receives two LO signals provided by a LO circuit (not shown) that is within the wireless communications device. The two RF signals are inverted (i.e., 180 degrees out of phase) with respect to one another, as are the two LO signals. Typically, baluns (not shown) are employed to convert the single RF and LO signals that are respectively provided by the antenna and LO circuit into, respectively, the pair of RF signals that are inverted with respect to one another and the pair of LO signals that are inverted with respect to one another. The double balanced mixer
10
effectively multiplies the RF and LO signals provided at the RF and LO input ports
20
,
22
, respectively, to produce first and second intermediate signals at a pair of intermediate ports
26
,
28
, respectively. The intermediate signals at intermediate ports
26
,
28
include IF components that are inverted with respect to one another such that, when the IF component of the first intermediate signal at intermediate port
26
is high, the IF component of the second intermediate signal at intermediate port
28
is low, and vice-versa. The operation of the double balanced mixer
10
is discussed further below with respect to FIG.
2
.
The double balanced mixer
10
further includes (or is coupled to, depending upon how the mixer circuit is defined) an output stage
30
, which processes the two intermediate signals at the intermediate ports
26
,
28
to derive an IF output signal that is output at an IF output port
40
. As shown in
FIG. 1
, it is known for the output stage
30
to include a transformer
32
having a first coil
34
a
and a second coil
34
b
, and to further include a capacitor
36
that is coupled in parallel with the first coil. The parallel combination of the first coil
34
a
and the capacitor
36
is connected between the two intermediate ports
26
,
28
, while the second coil
34
b
is coupled between ground and the IF output port
40
. A tap
38
connected to the middle of the first coil
34
a
is coupled to an additional supply voltage port (V
+
)
39
. The output stage
30
operates to combine or add the IF components of the first and second intermediate signals at intermediate ports
26
,
28
. Thus, the IF output signal provided at IF output port
40
has approximately double the amplitude of the IF components of the first and second intermediate signals at intermediate ports
26
,
28
(assuming that the transformer
32
has an approximately 1:1 conversion ratio between coils
34
a
and
34
b
). For example, when the IF components of the first intermediate signal at intermediate port
26
is at its lowest value, the IF component of the second intermediate signal at intermediate port
28
is at its highest value such that the IF output signal at IF output port
40
is double the peak value of each of the IF components of the first and second intermediate signals.
In addition to deriving the IF output signal at IF output port
40
from the first and second intermediate signals at intermediate ports
26
,
28
, the output stage
30
has several additional purposes. First, the output stage
30
supplies DC power to each of the intermediate ports
26
,
28
of the mixer
10
from supply voltage port
39
through tap
38
and first coil
34
a
. The DC power is necessary to properly bias the mixer
10
for its operation and to determine the mixer's gain. Second, the output stage
30
provides desired AC impedance between the first and second intermediate ports
26
,
28
and the IF output port
40
. The AC impedance separates the desired IF signal component produced by the mixing operation of mixer
10
from the other signal components produced by the mixing operation, by filtering out those other signal components. Third, the transformer
32
of the output stage
30
buffers the IF output port
40
from the remainder of the mixer
10
.
Although the conventional output stage
30
shown in
FIG. 1
effectively filters the first and second intermediate signals and generates the IF output signal, and additionally provides the desired DC power to the mixer
10
and the desired buffering, the design of the conventional output stage is not conducive for implementation on an integrated circuit. In particular, the transformer
32
with its two coils
34
a
,
34
b
cannot be implemented effectively on an integrated circuit. Consequently, the conventional mixer
10
can only be partially incorporated on an integrated circuit insofar as at least part of the output stage
30
must be constructed from discrete circuit elements that are connected to the integrated circuit. Thus, the size and price of the conventional mixer
10
are greater than would might otherwise be the case if the entire output stage
30
was implementable on an integrated circuit.
Given that, as discussed above, it is generally desirable to reduce the size of the electronic circuitry of wireless communications devices and further desirable, in particular, to implement as much of the electronic circuitry on integrated circuits as possible, it would be desirable if a new double-balanced mixer could be designed that was complet
Kajander John E.
Milord Marceau
Quarles & Brady LLP
Urban Edward F.
Visteon Global Technologies Inc.
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