Low noise I-Q Mixer

Abstract: The present invention relates to a low-noise quadrature phase I-Q modulator for applications in radio frequency signal receivers, of the type comprising a pair of Gilbert cell input stages (4, 5) driven by a feed voltage line (Vcc) and receiving in input respective square wave command signals (W LO,I , V LO,Q ) coming from a local oscillator (LO). The modulator comprises a circuital block (2) with transistors (Q1a, Q2a; Q1b, Q2b) connected to each cell (4,5) and destined to carry out a voltage-current conversion of a signal (V RF ) in radio frequency received from the block (2) itself; such block (2) further comprises a single degeneration resistance (R E1 ).

Description: Field of application The present invention as it is defined in the appended claims relates to a low noise I-Q phase-quadrature modulator. More in particular, the invention relates to a low noise I-Q phase-quadrature for applications in radio frequency signal receivers, of the type comprising a couple of Gilber cell input stages driven by a voltage feed line receiving respective square wave command signals in input coming from a local oscillator. Prior art As is well known, in recent years we have seen notable progress and changes in the world of telecommunications. In particular, the development of new integrated electronic devices on semi-conductors has contributed to a good number of these changes. Amongst the improvements which have been seen, is without any doubt the reduction of power consumption in a lot of portable electronic equipment. This reduction of energy consumption has permitted a reduction in weight and dimensions of this portable equipment, lengthening at the same time their useful life. All this has also meant a large decrease in costs, something which has made this type of portable electronic equipment accessible to a large part of the population as for example mobile cellular telephones or portable computers. The efforts made in trying to reduce the size of electronic circuitry more and more are still continuing. For example, various studies are going on at present for a new type of a radio frequency receiver RF which could be very compact. At the moment, many designers are working on receivers which are based on an architecture which includes a rejection-type image mixer which permits the reduction of the stringent specifications of a band-pass filter RF, downstream, therefore making it possible to have a significant decrease of the relative costs. An architecture of reception-type mixer of the image band of the known type has been proposed by Hatley and is represented in figure 1. This architecture allows the conversion to the frequency RF of a desired band, Sw(f) , of the RF signal, suppressing the image band, Su(f). GB-A-2 187 860 discloses an example of such a mixer. A signal 20 coming from a source LO of a local oscillator is applied to a network 21 of quadrature producing two signals 22, 23 out of phase shifted the one to the other by 90°. Such signals are applied to an I-Q modulator 24. The modulator 24 produces two output signals 25, 26 which are further cut of phase in order to be applied to the input of a combiner node 27 obtaining useful signals in phase and image signals in counter-phase. In the absence of errors between the two distinct signal pathways, the image signal of the band IS at intermediate frequency is totally eliminated. In a real mixer, nevertheless, the presence of module and phase errors in gains of the two signal pathways determines an incomplete suppression of the image at intermediate frequency. For example, the rejection requisite of image of 30 dBc requires at the same time a phase error which is less than 3° and an amplitude error which is less than 0,3 dB (3,5 %). The technical problem which is the basis of the present invention is that of trying to find a new type of I-Q modulator having structural and functional characteristics which would allow the use of an image rejection-type mixer with a very low level of introduction of noise compared with the types of the prior art. Summary of the invention. The resolutive idea which is at the basis of this invention is that of producing a new type of modulator which can operate in reception as if there was a "fusion" of the input stages formed by the Gilbert cells, therefore reducing at the same time the polarisation noise of each input stage. The characteristics and the advantages of the modulator according to the invention will become clear from the description hereinbelow, of an example of embodiment reported as indication and not limitation thereto of the invention, which makes reference to the attached drawings. Brief description of the drawings. figure 1 shows a schematic view of an I-Q modulator realised according to the prior art figure 2 shows a schematic view in circuital detail of the modulator of figure 1; figure 3 shows a schematic view of an I-Q modulator according to the present invention. Detailed description With reference to such figure, and in particular to the example in figure 3, with 1 a low-noise modulator is wholely and schematically indicated, which is realised according to the present invention. The phase-quadrature modulator 1 according to the invention structurally presents a mirrored symmetry which is highlighted by the letters A and B, which respectively indicate circuital stages relative to signals in phase, indexed I, and in quadrature, indexed Q. The structure of the modulator 1 can, though, be considered also from another point of view, and in particular as a circuital device essentially comprising two functional blocks 2, 3. The first block 2 is destined to carry out a voltage-current conversion, while the other block 3 is destined to carry out a frequency conversion. The voltage-current conversion block 2 comprises a double couple of transistors Q1a, Q2a and Q1b, Q2b; a resistance RE1 of degeneration and a pair of current generators I B1 . The transistors Q1a, Q2a and Q1b, Q2b are all of the NMOS type, even if there is nothing preventing the realisation of a dual circuit circuit incorporating PMOS transistors. The transistors Q1a and Q1b have respective source terminals. Amongst such source terminals and a reference of voltage Vee there is one of the current generators I B1 . The transistors Q2a and Q2b also have respective gate terminals in common, as they also have respective source terminals. Between such source terminals and the Vee voltage reference is the other current generator I B1 . The resistance RE1 of degeneration is connected to the source terminals of transistors Q1a, Q1b and Q2a, Q2b. Block 3 which carries out the frequency conversion comprises a pair of Gilbert cells 4 and 5. The Gilbert cells are essentially differential cells with a particular structure, but are of known type. For example, Gilbert cells similar to those of the modulator according to the invention are also incorporated in the known type of modulator illustrated in figure 2. The first cell 4 comprises the transistors from Q3a and Q6a to Q6b. Each cell 4, 5 receives in input a respective square wave signal V LO,I ; V LO,Q produced by a local oscillator LO, as the one shown in figure 1. Cells 4 and 5 are fed by a line 6 of Vcc feed voltage. Furthermore, charge resistancess Rc are foreseen between the line 6 and each branch of the cells 4, 5 in order to correctly polarise the transistors incorporated in the cells themselves. The first cell 4 comprises two circuital nodes A1, A2 of connection to the voltage-current conversion block 2. The first node A1 is connected to a conduction terminal, that is to say that one of drain, of the transistor Q1a, while the second node A2 is connected to the terminal of drain of the transistor Q2a. The second cell 5 comprises two circuital nodes B1, B2 of connection to the voltage -current conversion block 2. The first node B1 of the second cell is connected to a conduction terminal, that is to say that one of drain, of the transistor Q1b, while the second node 32 is connected to the drain terminal of the transistor Q2b. Advantageously, according to the invention, the modulator 1 also comprises some resistances of sinks. In particular a resistance R B is foreseen between each of the nodes A1, A2, B1, B2 and the Vcc feed voltage reference. For ease of illustration. the operation of a modulator according to the invention will be now described, although only making reference to section A, as sections A and B are completely specular. A voltage radio frequency signal VRF is converted into current by the voltage- current converter 2 comprising the transistors Q1a., Q2a and of the resistance RE1. The current so generated concerns the Gilbert cell 4 comprising the transistors Q3a-Q6a which invert or not the phase of signal according to the command in voltage V LO,I received from the local oscillator. The signal of out-going current from cell 4 is therefore converted into a voltage signal via the charge resistances Rc. The operation which results, is an analogic multiplication of the radio frequency signals V RF for the square wave signal V LO,I . The resulting signal V IF,1 is formed by variuos harmonic components and is generally filtered in order to extract the intermediate frequency component desired, which is the difference of the fundamental frequencies of the signals V RF and V LO,I . The presence of the resistances of sink R 3 allows, as will be explained later, the reduction of the polarisation current of the cell 4 with the objective of decreasing the noise it makes. In order to highlight the advantages of the proposed solution it is useful to compare it with that of a traditional I-Q modulator with separate voltage-current converters, as illustrated in figure 3. For this reason it is convenient to carry out this comparison on equal power dissipation, that is imposing that the current generators I B1 = 2 I B2 . In such conditions, the relations of respective conversion gains for the circuits presented in figures 2 and 3 are respectively G 1 ≡ V IF V RF = 2 π R C R E 1 g m R E 1 1 + g m R E 1 G 2 ≡ V IF V RF = 2 π R C R E 2 g m R E 2 1 + g m R E 2 2 Each of the relations (1) and (2) shows a term, dependent on transconductance g m which takes into account a not ideal transferral of the voltage-current converter. The requirement of linearity of the circuit depends upon such term. In fact, in conditions of ample signal, the transconductance g m varies considerably because of the strong variation of the current in the transistors, so generating a dependence of the conversion gain from the amplitude of the signal itself. As can be inferred by the expressions (1) and (2), with the condition R E1 = (1/2) R E2 the two modulators show the same requirement of linearity and the same conversion gain. Therefore, at equal dissipation of power and with a degeneration resistance R E1 = (1/2)R E2 the modulator proposed by the present invention has the same linearity and the same gain of voltage as the traditional one. Anyway, the proposed solution is more advantageous with respect to the traditional one in term of less noise. From this point of view, with the idea of comparing the two solutions, the mathematical relations for the equivalent resistance of input noise of the circuits represented in figures 3 and 2 are respectively shown hereinbelow wherein the following symbols are used r b base resistance of the transistors of the voltage-current converter g m transconductance of the transistors of the voltage-current converter RS resistance of the command source of the voltage-current converter current gain of the transistors of the voltage-current converter coefficient dependent on the commutational speed of the quads g mq transconductance of the transistors of the quads r bq base resistance of the transistors of the quads R sq command source resistance of the quads Comparing the two expressions, and having set R E1 = (1/2)R E2 so as to restore the same conditions of linearity, the circuit in figure 3 features, with respect to traditional topology, a contribution of noise due to the degeneration resistance of the voltage-current converter (R E1 ) divided by two and a contribution of noise due to the charge resistances (Rc) reduced by a factor ¼. Furthermore, thanks to the presence of the sink resistance R 3 which subtracts polarisation current from the Gilber cell and reduces the value of the transconductance G mc also the contribution of noise due to cell 4 is less than that of the circuit in figure 2. Clearly, the sink resistance R B itself generates a noise contribution which, in any case, given the values of R B commonly used (with R B >> R E1 ) is negligible . In order to better understand the advantages of the solution according to the invention, it might be useful to find out the numeric values obtained with the expressions (3) and (4) for a typical circuit. Having set: I B1 = 3mA; R B = 30Ω; R E1 = 50Ω; R B = 4KΩ; R C = 500Ω; R S =100Ω; R sq =30Ω; β = 70 and γ = 0, from the expressions (3) and (4) we obtain R EQ1 =376Ω and R EQ1 = 2,4KΩ Or equivalently in terms of noise figure F 1 =6,8 dB and F 2 = 14 dB. Therefore, at equal dissipation and linearity the modulator proposed introduces a notably lower noise level to the traditionally used circuit. The circuital solution proposed by the present invention finds a use in applications in which a low noise level is necessary, such as for example: receivers with a passive antenna and, in general, receivers with particularly stringent noise specifications. In fact, as has been described in detail, the proposed solution has the advantage of introducing, at work conditions being equal, a notably lower noise level with respect to that introduced by commonly used circuital solutions.

Claims: Low noise phase quadrature I-Q modulator for applications in radio frequency signal receivers, of the type comprising a pair of Gilbert cell (4, 5) input stages fed by a feed voltage Vcc line and receiving in input respective square wave command signals (V LO,I , V LO,Q ) coming from a local oscillator (LO), charaterised in that it comprises a circuital transistor (Q1a, Q2a; Q1b, Q2b) block (2) connected to each cell (4,5) and destined to carry out a voltage-current conversion of a signal (V RF ) in radio frequency received by the block (2) itself, said block (2) comprising a single degeneration resistance (R E1 ). Modulator (1) according to claim 1, characterised in that said block (2) of voltage-current converter comprises a double pair of transistors (Q1a, Q2a; Q1b, Q2b), said degeneration resistance R E1 and a pair of current generators (I B1 ). Modulator (1) according to claim 1, characterised in that the transistors (Q1a, Q2b; Q1b, Q2b) of said block (2) are of the NMOS type. Modulator (1) according to claim 3, characterised in that the transistors (Q1a, Q1b) of the first pair have their respective gate terminals as well as their terminals of source in common, and that between such terminals of source and a second reference of voltage (Vee) there is one of the current generators (I B1 ). Modulator (1) according to claim 3, characterised in that the transistors (Q2a, Q2b) of the second pair have respectively gate terminals as well as their terminals of source in common, and that between such terminals of source and a second reference of voltage (Vee) there is one of the current generators (I B1 ). Modulator (1) according to claim 1, characterised in that the degeneration resistance (R E1 ) is connected between the terminals of source of the first (Q1a, Q1b) and of the second (Q1a, Q1b) pair of transistors. Modulator (1) according to claim 1, characterised in that each cell (4,5) comprises two circuital nodes (A1, A2; B1, B2) of connection to the block (2) of voltage-current conversion, a first node (A1, B1) is connected to a conduction terminal of a first transistor (Q1a, Q1b) while the second node (A2, B2) is connected to the conduction terminal of a second transistor (Q2a, Q2b). Modulator (1) according to claim 7, characterised in that a sink resistance is foreseen (R B ) between each one of said nodes (A1, A2, B1, B2) and the feed voltage reference (Vcc) . Modulator (1) according to claim 8, characterised in that said sink resistance has a higher value that said degeneration resistance (R B , R E1 ).

Inventors: FILORAMO PIETRO, PALMISANO GIUSEPPE, SALERNO RAFFAELE, FILORAMO, PIETRO, PALMISANO, GIUSEPPE, SALERNO, RAFFAELE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H03D7
IPC codes: H03D7