Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2000-08-28
2003-07-15
Chin, Vivian (Department: 2681)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S333000, C455S313000, C330S254000
Reexamination Certificate
active
06594504
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to frequency transposition and, more particularly, to radio frequency transposition such as in a mobile telephone. In mobile telephones, radio-frequency circuits make wide use of frequency transposition devices (mixers), both on transmission and reception of information signals.
BACKGROUND OF THE INVENTION
During transmission of an information signal, the purpose of a frequency mixer, which in this instance is a frequency raising circuit, is to transpose the information signal into a baseband signal around a transmission carrier signal. On reception of the information signal, the frequency mixer is a frequency lowering arrangement.
The frequency transposition function is critical, both on account of its conventional limitations (e.g., linearity, power consumption, noise factor), and also because of the isolation of the signal originating from the local oscillator towards the output signal from the mixer. The residual quantity of a local oscillator signal in the output signal from the mixer must be minimized to make it easier to recover the carrier signal.
As an illustration, if the input signal is at a frequency between 100 and 400 MHz, and to produce an output signal around 1 or 2 GHz, the local oscillator signal, which is locked onto a frequency of about 1 or 2 GHz, is close to the synthesized carrier signal at the output. The subsequent filtering of the local oscillator signal is often difficult, at least at an acceptable cost.
The leakage of the local oscillator signal to the output signal, which is to be minimized, is called a structural leakage, i.e., it is intrinsic to the mixer. Another cause of leakage that is conventionally encountered occurs between the input of the local oscillator on the chip containing the mixer, and the output of the mixer by way of the parasitic elements of the package. This leakage can be easily addressed with modern packaging.
FIG. 1
illustrates the structure customarily used for frequency transposition devices. In the top part of
FIG. 1
, the reference DTF designates a frequency transposition device, or mixer, which is also referred to herein as a frequency raiser. The frequency raiser includes an input terminal BE for receiving an input signal at an intermediate frequency IF, which may be 200 MHz, for example, and an input terminal BO for receiving the local oscillator signal LO, which may be 2 GHz, for example. An output terminal BS delivers the output signal whose frequency spectrum exhibits a line at the frequency LO−IF and a line at the frequency LO+IF. The dashed arrow illustrates the structural leakage of the oscillator signal LO to the output signal.
The structure customarily used for these mixers is a differential GILBERT type, as illustrated in the bottom part of FIG.
1
. More precisely, such a structure comprises a differential transducer block BTC for converting the input signal (voltage) present on the terminals BE into a differential current. This block BTC also comprises a stage forming the output stage, and includes a differential pair of transistors T
1
and T
2
, whose respective bases are linked to the input terminals BE by two capacitors C
6
and C
7
. The collectors of the two transistors T
1
and T
2
of the output stage form the output terminals of this transducer block BTC. Alternatively, the block BTC may comprise several stages.
A resistor RP, contributing to defining the transductance value of the block, is connected between the emitters of the transistors T
1
and T
2
. The transistors T
1
and T
2
are biased by biasing means MPL which includes two current sources SC
1
and SC
2
connected respectively between ground and the two terminals of the resistor RP. The biasing means MPL also comprises two base resistors RB
1
and RB
2
, both connected to a voltage source ST. This connection forms a return path for the base currents of the transistors T
1
and T
2
.
At the output of the transconductor block BTC, i.e., the collectors of the transistors T
1
and T
2
of the output stage, a current switching block BCC is connected for shunting the current alternately to one of the two output terminals BS at the frequency of the local oscillator signal LO received at the terminals BO. This block BCC conventionally comprises two pairs of transistors Q
3
, Q
4
and Q
5
, Q
6
.
Each resistor ZL, connected between the output terminals BS of the block BCC and the supply Vcc, represents the output load of the mixer DTF. The transconductor BTC, which is formed by transistors T
1
and T
2
and resistor RP, and which is also used to define the transconductance of the block BTC, converts the power or the voltage applied to the input BE into a differential current. This differential current is an image, assumed linear, of the input signal. This linear signal is then chopped by a nonlinear square function (+1, −1, +1, −1 . . . ) carried out by the double switch BCC at the frequency of the signal LO. The double switch acts as a dynamic shunter of the current. The output signal is gathered at the terminals of the differential load
2
ZL.
In the first instance, it is shown that the output is a balanced structure that generates an output signal free of the local oscillator signal residual. However, this absence of the local oscillator signal in the output signal relies on a perfectly differential structure. In practice, a residual quantity of the local oscillator signal exists in the output signal by reason of an imperfectly differential structure. Stated otherwise, the symmetric elements of the structure do not posses identical characteristics after fabrication on silicon, i.e, they are not matched.
A common cause is poor matching of the parasitic capacitances of the current switch and/or of the two output load impedances ZL. The present invention provides an approach to this problem by adopting a radically different approach from that of the prior art, which reduced the lack of dynamic matching of the structure, i.e., the lack of capacitive and resistive matching.
The dominant cause of the lack of matching of the differential structure in not from a lack of dynamic matching, but from a lack of static matching, and more particularly, from poor matching of the quiescent currents of the differential structure.
SUMMARY OF THE INVENTION
An object of the present invention is to better match the quiescent or bias currents of the differential pair of transistors of the output stage of the transconductor block BTC. In other words, the object of the present invention is to better match the emitter currents of these transistors.
The biasing means conventionally used, such as illustrated in
FIG. 1
under the reference MPL, exhibit numerous causes leading to an absence of matching. One of these causes results from the poor matching of the two current sources SC
1
and SC
2
. Moreover, when the transistors T
1
and T
2
are biased to high current levels (a few mA to 10 mA) they easily develop offset voltages from 5 to 10 mV. The matching of the bias currents depend not only on the matching of the respective factors &bgr; of the transistors, but also on the matching of the respective offset voltages of these transistors.
Moreover, the base bias resistors RB
1
and RB
2
induce a lack of matching, which is very sensitive on account of the base current offset (poor matching of the DC gains). This is found in bipolar transistors, particularly in high speed devices. The orders of magnitude are such that the lack of matching of the output currents is from 5 to 10% if one assumes a lack of matching of the gains of the transistors of 10%. The situation may be worse when the matching of the gains is yet further degraded to around 20% or 30%, as is sometimes the case in high speed devices.
The invention therefore matches the quiescent currents of the structure to reduce the residual quantity of the local oscillator signal in the output signal from such a mixer. This is done by performing differential slaving of the quiescent currents of the output stage of the transconduct
Afshar Kamran
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Chin Vivian
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