LO cancelling mixer

Details LO cancelling mixer LO cancelling mixer

1998-12-22

2002-01-29

Maung, Nay (Department: 2744)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S305000, C455S310000, C455S326000

Reexamination Certificate

active

06343211

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process and a system for reducing leakage of LO (LLO) in a system which converts the frequency of a first signal by mixing the first signal with a second supplied signal to a third signal of a frequency which is different from the frequency for the first and second signal. More specifically the invention relates to a process and a system which removes LLO in non-ideally balanced transistor mixers.
2. Description of the Related Art
In modern radio communication there is used a conversion from a carrier wave frequency to a second. For this conversion a frequency converter/mixer is employed. Ideally this mixer functions so that it receives the signal which is to be converted—IF—and a local oscillator signal—LO—and emits only one signal—RF—which has a frequency equal to the sum or the difference of the frequencies for IF and LO. Practical limitations cause several other undesired signals also to be present together with RF. A so-called “image”—IM—will be present (both “the sum signal” and “the difference signal” will come to the RF-gate—the one is desired, while the other is IM and consequently undesired). LO will leak out. In addition a series of other undesired signals will arise in the mixer. These can be made small and insignificant by employing as far as possible a linear mixing element and by allowing LO to be strong and dominating relative to IF and RF. The level of leaked LO—LLO—is proportional to the level of the supplied LO—TLO. The two dominating undesired signals will therefore be IM and LLO.
There are described some solutions for reducing LLO.
FIG. 2
a
shows a balanced solution according to the state of the art which is employed for attenuating LLO. There are employed two equal mixing elements which are supplied each with its half of LO and IF. The halves must be in counter-phase. At the end that which comes out from the two mixing elements is added up. RF and IM will be added separately in phase, while LLO will be added in counter-phase and be phased out. Practical limitations in the components will however limit the effect of the phasing out so that LLO becomes more than 1 per thousand of LO (−30 dB). For example the coupler will in practice deviate and will not be exactly equal to 180 degrees (often in a region from 175-185), and the phase path in the transistors is often also somewhat different (deviation of up to 5 degrees). The requirement is often that LLO shall be less than 0.01 per thousand (−50 dB). The solution according to the state of the art is therefore not satisfactory.
A more general way to realise a balanced mixer is by allowing the coupler at the input to produce a phase difference of AB
1
(0 to 180 degrees) and to replace the adding coupler at the output with a coupler which produces a phase difference of AB2 (180 degrees−AB
1
).
Furthermore it is known that IM can be attenuated by employing a so-called “image reject” solution (see
FIG. 1
b
). That is to say to employ two equal mixers (prefereably, each of the mixers can generally be balanced mixers) which are supplied each with its half of LO and IF. The halves must be 90° out of phase. At the output RF is added in phase, and IM in counter-phase. In this system however LLO is added 90° out of phase and is little reduced. A more general way to realise an image reject mixer is by allowing the coupler at the input to produce a phase difference of AQ
1
(0 to 90°) and to replace the adding coupler at the output with a coupler which produces a phase difference of AQ
2
(90 degrees−AQ
1
).
On the basis of the conditions indicated, one of the main problems with mixers is therefore that a compromise must be made between the following requirements:
1. The mixers must as far as possible be clean/linear in the conversion. This implies a strong TLO.
2. The mixers must not have too high LLO. This involves a weaker TLO.
This relation is outlined in
FIG. 1
b.
The distance “3” in
FIG. 1
b
is usually critical in a mixer, that is to say the relationship between the RF-signal and LLO. This relationship can be increased in two ways; (a) in that “1” in
FIG. 1
b
is reduced, for example by phasing out as shown in
FIG. 3
or by other LLO—reducing methods, or by (b) increasing “2” which can be achieved by increasing TIF (supplied IF) so that RF increases, but this requires that a more linear element is employed. The transistor is generally more linear than the diode, particularly when it operates in the passive mode. With a passive mode mixer “2” it is possible to raise “2” in
FIG. 1
b
considerably.
In order to obtain a satisfactory solution there will be a need for a mixer element which is as far as possible linear. Thus less LO will be required in order to manage the same linearity. Simultaneously there is a need for developing better methods than those which are known within the state of the art for reducing LLO.
The mixer element which has been most usually employed up to to-day is a Schottky—diode. An example of the removal of LLO is present in a diode mixer ( Wolfgang Schiller, “Broadband Linear SSB Upconverter with Electronically Controlled LO Suppression for 16-QAM Applications at 4 GHz ”, 13th EuMC, pp. 585-589, September 1983).
Publications do not show clearly how the circuit operates, but it appears as if the diode mixer in Schiller's article employs an adjustment of the reflection in order to reducing the LLO.
The use of transistors in mixers has however many clear advantages compared with the use of diodes, such as improved linearity, lower costs for production, mounting and the like and increased working life. For a comparison of advantages and disadvantages with diodes relative to transistors reference is made to M. J. et al.: “A Comparison of GaAs Transistors as Passive Mode Mixers”, 1994 IEEE MTT-S Digest, pp 937-940 and Stephen A. Maas, “Microwave Mixers—Second Edition”, Artech House, 1993, page 313.
The use of transistors as mixing elements is therefore increasing. It is therefore crucial to develop a good method for removing LLO in mixers which employ transistors.
The element which to-day is considered to be the most linear is a transistor in so-called passive mode, that is to say without supply voltage, that is to say with only biasing (voltage signal) at the input (the Gate). This is so far tested on three transistors; MOSFET, MESFET and HEMT.
The methods which exist for reducing LLO in mixers which employ transistors are few. The most obvious method is filtering. But if IF has low frequency relative to LO and RF, the filtering will often not be practically realisable.
An alternative aproach is described in U.S. Pat. No. 4,355,420 (Ishihara), and is a so-called phasing out method (see FIG.
3
). A small portion of TLO is decoupled before the rest is supplied to a mixer (this can well be both balanced and image reject). The portion which is dicoupled is adjusted in amplitude and phase before it is supplied at the output after the mixer. By correct adjustment the adjusted LO portion (JLO) will have the same level as LLO, but be in counter-phase and thus phase out LLO. This method ensures at LLO is sufficiently low if the correct amplitude and phase are adjusted in. However the method is dependent on a so-called phase shifter which will be able to adjust JLO in phase and amplitude. This increases the price of and complicates the mixer. Further this method involves a loss of TLO in first coupler and loss of RF in last coupler. Both leading to increased loss in the mixer and the linearity suffers. Furthermore it is a fact that the elements which determine level and amplitude of LLO and JLO are vastly different, and they will consequently vary differently with respect to temperature. Thus the phasing out or the LO-cancellation will be very sensitive to swings in temperature. In worst cases the method will lead to increased LLO, if JLO is added in close phase.
These limitations of known solutions and systems aim to be minimised by providing a system and a process where transist

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