COMBINED LOW POWER CF CMOS MIXER AND VCO WITH CURRENT REUSE

Abstract: An MOS-technology oscillator device comprises a self-controlled oscillator stage provided with oscillator means, cross-coupled gate means, and output means. Furthermore it comprises a mixer device with a first stage and a second stage that have respective separate sets of signal inputs for connecting to the output means of said oscillator device, and which also has mixer signal output facilities. In particular, the oscillator stage output connects through current reuse paths of the first stage.

Description: Combined low power CF CMOS mixer and VCO with current reuse. BACKGROUND OF THE INVENTION The invention relates to a mixer-oscillator device comprising an oscillator for generating a local oscillator signal, said oscillator having an oscillator supply terminal, the arrangement further comprises a mixer having a mixer supply terminal, the local oscillator being coupled to the mixer, the mixer further having a signal input for receiving a signal to be mixed with the local oscillator signal. The invention relates also to a RF receiver circuit module and to a portable telecommunications device. A device according to the preamble is known from the paper sp 23.7 read at the ISSCC97 in session 23 and published at pages 390-391 of the conference proceedings. Such circuitry is often used in various portable electronic telecommunication devices such as telephones and pagers. The functionality of the circuit in particular is generally to reduce the frequency of an input signal to a lower value that may even be dc. The current consumption of such circuitry is in general relatively large, as being dictated by the requirements of low noise values of the constituent sub-devices. The current drain represented thereby tends to keep battery lifetime and/or stand-alone interval between recharges all too short. SUMMARY TO THE INVENTION In consequence, amongst other things, it is an object of the present invention to provide a device according to the preamble of which the current consumption has been reduced. Now therefore, according to one of its aspects the invention is characterized in that the oscillator supply terminal is arranged for carrying a local oscillator signal current and a supply current, and in that mixer supply terminal is coupled to the oscillator supply terminal for receiving a supply current and a local oscillator current from said oscillator supply terminal. By coupling the oscillator supply terminal and the mixer supply terminal, it is provided that the supply current of the oscillator also flows through the mixer. This makes that the mixer does not need a separate supply current. This results in a reduction of the current consumption of the mixer oscillator device. An embodiment of the invention is characterized in that said oscillator supply terminal and said mixer supply terminal are coupled via a main current path of a transistor having a control electrode coupled to a reference voltage input. By coupling the oscillator supply terminal and the mixer supply terminal via a transistor having its control electrode coupled to a reference voltage, it is obtained that the voltage on the oscillator supply terminal is independent from the behavior of the mixer. This prevents the occurring of unwanted effects such as changing the frequency of the local oscillator by the applied RF signal. A further embodiment of the invention is characterized in that the oscillator comprises a further oscillator supply terminal being for carrying a local oscillator signal current and a supply current, and in that mixer comprises a further mixer supply terminal being coupled to the further oscillator terminal for receiving a supply current and a local oscillator current from the oscillator supply terminal, and wherein the oscillator signals provided by the first oscillator terminal and the second oscillator terminal have opposite phases. By using two oscillator supply terminals carrying local oscillator signals with opposite phases, it is obtained that the device draws a constant current from the supply source. This results in a decreased parasitic coupling via the supply lines to other circuits. This decreased parasitic coupling is quite important for RF circuits. BRIEF DESCRIPTION OF THE DRAWING These and further aspects and advantages of the invention will be discussed more in detail hereinafter with reference to the disclosure of preferred embodiments, and in particular with reference to the appended Figures that show: Figure 1, a one-chip transceiver concept with mixer & VCO in its front-end section; Figure 2, a CMOS VCO implementation; Figure 3 a Gilbert cell mixer; Figure 4 a low-detail schematic of Figures 1 and 2 combined; Figure 5 a comprehensive VCOMX implementation; Figure 6 a varactor tuning circuit; Figure 7 a low-detail schematic of Figure 5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 represents a one-chip transceiver concept with mixer & VCO in its RF-front-end section 24. Such realization will be useful in an environment for low-cost, wireless, and battery-powered transceivers, such as could be used in portable communication devices such as cellular and cordless telephones, pagers, and other. The schematic of Figure 1 has an RF front end 22+24, an RF back end 26 and baseband 28 subsystems. The signals picked up by antenna 20 are first amplified by an LNA circuit 22, and then down-convened with one or more mixers MDΩ block 24. Blocks 19 are band-pass filters, each with a suitable characteπstic. The function of the mixer in question is to convert the input signal into an LF signal with a reduced frequency which even can be zero. A conventional mixer topology is the Gilbert cell that has one input port for the RF signal from the LNA, and the other from a VCO or local oscillator that has been tuned exactly to the desired frequency. Depending on the chosen architecture, the LO frequency may be higher than (high side injection), lower than (low side injection), or exactly equal to the input RF frequency. Generally, the current consumption by both the VCO and the mixer is in the milliamps range, as being dictated by the required phase noise/spectrum puπty of the VCO, and the noise figure of the mixer stage, respectively. The invention solves to a great extent the disadvantage caused by these relatively large current values The inventive circuitry to be descπbed more in detail hereinafter forms part of comprehensive circuit module 22 that effects nearly all of the necessary receiver functionality of the portable telecommunications device 36 and which may be realized as a single integrated circuit package The telecommunications device will generally take up the output signal from baseband stage 28 in processing stage 32 for further conversion, such as to speech and control signals Item 34 is a human interface module that compπses keyboard, display, speech I/O and possibly further functionality. Item 30 is a transmitter functionality module that receives signals from processing module 32 for therefrom constructing a feasible transmission signal Generally, this needs reusing vaπous further elements of the device, such as power supply and antenna For brevity, these further aspects are not recited in detail. Figure 2 shows without intended limitation a typical CMOS VCO implementation In this stand-alone circuit block, an LCR oscillator circuit connects via resistors R to an appropπate power source Vdd. Certain other oscillator circuit configurations are feasible as well Cross-coupled transistors Ml and M2 connect via current source Iss to ground, thereby providing an appropπate current level. Figure 3 shows without intended limitation a CMOS-implemenfed Gilbert-cell mixer. In this stand-alone circuit block, the mixer circuit connects via impedances Z to an appropriate power source Vdd. A first input mixer stage comprises paired transistors M7 and M8 that are controlled by first complementary input signals RF/VCO and connect via current source Iss to ground, thereby enabling transmission of an appropriate current level, as being further controlled by a second input mixer stage. This second input mixer stage comprises four paired transistors M3, M4, M5, M6, each respective pair being arranged in series with one of the transistors of the first stage and each pair being controlled by second complementary input signals VCO RF. In this manner, the output current level on terminals MXout represents the mixing result of the two input signals. Now, the technique according to the invention for saving power is current reuse, which has been proven very effective. For detailed explanation, reference is first had to Figure 4, which is a low-detail schematic of Figures 2 and 3 combined, the thick lines indicating the principal current flows. Now, the arrows connected to the West and East sides of the mixer stage MX indicate input and output signal directions, respectively. Furthermore, there are clearly two current paths IMX and IVCO, respectively, draining parallel currents from supply to ground, to wit separately for each one of the two partial circuits VCO and MX. Now, a schematic according to the invention has been shown in Figure 7, that is a low-detail schematic of Figure 5. Herein, the currents that have again been indicated by thick lines, flow first through the mixer MX and next through the oscillator VCO, before reaching the ground terminal. Consequently, the supply current is reused. If both partial circuits require indeed identical levels of current, the overall power saving could amount to 50%, by putting the two circuits properly stacked on each other, and designed properly with respect to each other. In practice, the current saving may be less than 50%, such as when one of the two items MX , VCO could in practice do with less current than the other. Figure 5 shows a comprehensive VCOMX implementation according to the invention, as being realized in nMOS transistors. By itself, other MOS-type implementations could be feasible as well. Now, transistors Ml, M2 are in positive feedback to create a negative small-signal resistance to ensure initial oscillation start-up and subsequently to provide large-signal amplitude regulation. The oscillation frequency can be tuned by virtue of varactors. Transistors M3, M4, M5 and M6 form again the double-balanced Gilbert cell stage of Figure 2, together with transistors Ml, M2, M7, M8, which at the same time constitute an LO oscillator. The oscillator and the Gilbert cell have now been merged in an inventive manner. The RF signals are differentially applied to transistors M3-M6 with their output cross-coupled in similar manner as in Figure 2. The oscillator is coupled to the mixer via the transistors M7 and M8. Therefor the source of the transistor M7 is connected to the oscillator supply terminal and the drain of the transistor M7 is connected to the mixer supply terminal. The source of the transistor M8 is connected to the further oscillator supply terminal and the drain of the transistor M8 is connected to the further mixer supply terminal. A particular function of transistors M7, M8 that now get appropriate biasing by voltage Vbias is to provide necessary isolation between the VCO part and the upper part of the mixer circuit. The networks Z may be frequency selective. The oscillator is voltage controlled through a varactor tuning circuit as shown in Figure 6. Note in particular the corresponding letters A, B in Figures 5 and 6, whereas further elements of Figure 5 have been suppressed for clarity. Since the control signal is a dc voltage, a resistor is required to provide necessary isolation. On the other hand, the control voltage over the varactor should neither make the varactor forward-biased nor affect the operation point at nodes A and B. This requires two series-capacitors to the inductor. Therefore, the oscillation frequency is determined by all elements but can be tuned through control voltage Vc. To extend the oscillation tuning range and to compensate for process variations, temperature variations, aging, etcetera, binary-weighted capacitor arrays may be added that have not been detailed for brevity. In consequence, the circuit as shown comprises a series arrangement of resistors 44, 46, capacitors 40, and oscillator circuit part L, in combination with branched varactors 40 interconnected to ground. In addition to power saving, a total of two connecting pads may be saved, as compared with the present stand-alone solution. Also, the total transistor count is nearly reduced to that of the Gilbert mixer alone, which will save silicon area. Computer tests have yielded the following results. With a 1 millivolt peak-to-peak input sinusoidal of 930 MHz, the oscillation frequency of the merged oscillator was 879.5 MHz, with low-side injection. The resulting output LF was 50.5 MHz. The oscillation stabilized 3 microsecs after applying supply voltage. The LC resonance network had a Q=60 with L=10 nH and C= 2.78 pF. Achieved mixer gain was 6 dB. Further parameters were Vdd = 2 V, current consumption 200 microAmps, and a CO75 CMOS process with a minimum gate width of 0.35 micron.

Claims: CLAIMS: 1. Mixer-oscillator device comprising an oscillator for generating a local oscillator signal, said oscillator having an oscillator supply terminal, the arrangement further comprises a mixer having a mixer supply terminal, the local oscillator being coupled to the mixer, the mixer further having a signal input for receiving a signal to be mixed with the local oscillator signal, characterized in that the oscillator supply terminal is arranged for carrying a local oscillator signal current and a supply current, and in that mixer supply terminal is coupled to the oscillator supply terminal for receiving a supply current and a local oscillator current from said oscillator supply terminal. 2. A device as claimed in Claim 1, characterized in that said oscillator supply terminal and said mixer supply terminal are coupled via a main current path of a transistor having a control electrode coupled to a reference voltage input. 3. A device as claimed in claim 1 or claim 2, characterized in that the oscillator comprises a further oscillator supply terminal for carrying a local oscillator signal current and a supply current, and in that mixer comprises a further mixer supply terminal being coupled to the further oscillator supply terminal for receiving a supply current and a local oscillator current from said further oscillator supply terminal, and wherein the oscillator signals provided by the first oscillator supply terminal and the second oscillator supply terminal have opposite phases. 4. A device as claimed in Claim 1, wherein said mixer device comprises a Gilbert cell. 5. A device as claimed in Claim 3. characterized in that said oscillator comprises a differential transistor pair with a first transistor and a second transistor, a first main electrode of the first transistor and a first main electrode of the second transistor being coupled to a current source, a second main electrode of the first transistor being coupled to the first oscillator supply terminal, a second main electrode of the second transistor being coupled to the second oscillator supply terminal, the second main electrode of the first transistor being coupled to a control electrode of the second transistor and the second main electrode of the second transistor being coupled to a control electrode of the first transistor. 6. A device according to claim 6, characterized in that the oscillator comprises a frequency determining element being coupled between the second main terminal of the first transistor and the second main electrode of the second transistor. 7. An RF receiver circuit module comprising an RF front end module based on a device as claimed in Claim 1, an output of the mixer being coupled to an input of a cascade connection of an LF module and a baseband module. 8. A portable telecommunications device comprising a receiver circuit module as claimed in Claim 7, and being interconnected to signal processing means, and furthermore comprising interconnected therewith human interface means and signal transmitter means.

Inventors: WANG ZHENHUA, WANG, ZHENHUA
Applicants: KONINKL PHILIPS ELECTRONICS NV, WANG ZHENHUA, KONINKLIJKE PHILIPS ELECTRONICS N.V., WANG, ZHENHUA
Application year: 1999
Main classification: H03D7
IPC codes: H03B1, H03B5, H03D7