Abstract:
A silicon bipolar VCO implementing a double-coupled transformer is disclosed. The VCO circuit, which is suitable in the field of integrated RF circuits, has been integrated into a universal LNB having a down-converter block and PLL merged into a single die and implemented in silicon bipolar technology. The integrated transformer is formed by three turns of stacked metal layers, where the topmost metal layer is employed for the resonator inductance. The VCO is missing conventional biasing resistors and decoupling capacitors, thus improving phase noise and tuning range performance.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to integrated Voltage Controlled Oscillators (VCO) for RF applications. In particular, the invention relates to a transformer-based VCO for both phase noise and tuning range improvement in silicon bipolar technology. 
   The invention further relates to a Digital Video Broadcast-Satellite (DVB-S) receiver including a low noise block down-converter (LNB) for translating RF satellite signals picked up by a parabolic dish from a Ku-band to an intermediate frequency IF. 
   2. Description of the Related Art 
   In the last few years digital video broadcast services supported by geostationary satellites (DVB-S) have grown rapidly. In a DVB-S receiver the low noise block down-converter (LNB) translates the RF satellite signals picked up by a parabolic dish from the Ku-band (10.7–12.75 GHz) to an intermediate frequency IF ranging in the L-band (0.95–2.15 GHz). 
   The converted IF band is then sent to a decoder for tuning and digital demodulation. 
   LNB specifications are very challenging. See for instance the Technical Guide: EUTELSAT—“Information for Installers of HOT BIRD™ DVB-S Systems”—Vol. 1. 
   Indeed, an LO phase noise as low as −95 dBc/Hz at 100 kHz frequency offset from the carrier at 10 GHz has to be guaranteed. 
   In addition, an amplitude gain variation and an output P1 dB respectively within 8 dB and +5 dBm over the 2.05 GHz RF band must also be met. 
   Up to now, commercial LNBs have all been manufactured using discrete components. Usually, GaAs high electron mobility transistors (HEMT) and FET devices are used for the down-converter block, while dielectric resonator oscillators (DRO) are employed to generate the LO signal. 
   An integrated approach to LNB implementation would drastically reduce manufacturing costs. Moreover, the use of a PLL to generate the LO signal would eliminate the need for a testing phase to guarantee the accuracy of the oscillation frequency. 
   However, some technical problems are still to be solved to provide fully integrated solutions. 
   As is well know for those skilled in this specific technical art, voltage controlled oscillators (VCO) are circuit blocks of fundamental importance in radio-frequency integrated circuits. 
   The basic parameters for evaluating the performances of a VCO are: phase noise and tuning range. 
   The phase noise is an indicator of the spectrum purity of the generated oscillation; while the tuning range is the band in which it may be possible to control the oscillation frequency of the VCO. 
   In modern telecommunication systems the VCOs are requested to comply to more and more specific requirements in terms of phase noise and tuning range. 
   A conventional VCO structure realized by bipolar technology is shown in  FIG. 1 . The oscillation frequency of such a VCO is given by a resonator element LC wherein the capacitance C is the equivalent capacitance seen from the collector of each bipolar transistor Q 1 , Q 2 . 
   The capacitors CD and the resistors RB are used for providing a proper inverse biasing to the varactor diodes C v . 
   For this biasing configuration, the parasitic capacitances CPk at the cathode terminals of the varactor diodes are connected to a virtual ground for the AC signals, so that they do not modify the oscillation frequency of the VCO structure. 
   While being advantageous under many points of view, this structure still suffers for some drawbacks. 
   Unfortunately, the capacitors CD, which are used to decouple the anode of the varactor diodes C v , as well as the decoupling capacitors CDD, reduce the tuning range. 
   Moreover, the so-called thermal noise that is generated by the resistors RB increases the phase noise as well. 
   The biasing resistors RBB introduce a further contribution to the phase noise. 
   In this respect reference can be made to an article by: C. Samori, A. L. Laicata, A. Zanchi, S. Levantino, G. Cali, “Phase Noise Degradation At High Oscillation Amplitudes In LC-tuned VCOs,” IEEE Journal of S—S Circuits, vol. 35, no. 1, pp. 96–99, January 2000. 
   In this article it is disclosed the Single Sideband to Carrier Ratio (SSCR) dependence on the oscillation amplitude of a fully integrated LC-tuned voltage-controlled oscillator, manufactured in high-speed bipolar technology. 
   As the oscillation amplitude increases, the SSCR reaches a minimum and then steeply rises, setting a limit to the range where better performance can be traded against higher power dissipation. 
   This dependence is fully explained by taking into account that noise and disturbances modulate the phase delay due to the active elements. 
   Known prior art solutions still don&#39;t provide a fully integrated circuit 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the present invention provides a transformer-based VCO in bipolar technology which eliminates biasing resistors and decoupling capacitors, thus improving both phase noise and tuning range. 
   An embodiment of the present invention is that of providing a silicon bipolar VCO implementing a differential integrated transformer which is double-coupled, and lacks conventional biasing resistors and decoupling capacitors, thus improving phase noise and tuning range performance. 
   The features and advantages of the transformer-based VCO according to the present invention will be apparent from the following detailed description of the embodiment thereof, given as non-limiting examples with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  shows schematically a Transformer-Based VCO according to the prior art; 
       FIG. 2  shows a block diagram of a DVB-S receiver including a low noise block down-converter (LNB) translating the RF satellite signals picked up by a parabolic dish to an intermediate frequency IF; 
       FIG. 3  shows schematically a transformer-based VCO incorporated into the down converter block of  FIG. 2  and realized according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the drawings, and more specifically to the examples of  FIGS. 2 and 3 , with  1  is globally and schematically indicated a Transformer-Based VCO realized according to one embodiment of the present invention. 
   The VCO circuit  1  is incorporated in a DVB-S integrated receiver (IC)  2 . The IC  2  is included in a low noise block down-converter (LNB)  10  which translates the RF satellite signals picked up by a parabolic dish from a Ku-band (in the range: 10.7–12.75 GHz) to an intermediate frequency IF ranging in a L-band (in the range: 0.95–2.15 GHz). 
   The converted IF band is then sent to a decoder for tuning and digital demodulation. 
   The LNB  10  includes a down-converter block and a PLL  11  that are integrated into a single die IC and implemented in a low-cost silicon bipolar technology. The block diagram of the LNB  10  is shown in  FIG. 2 . 
   More specifically, the receiver  2  is based on a dual-conversion superheterodyne architecture. The dashed box IC shows the integrated circuit. A low-noise amplifier (LNA) block  8  provides the gain and low noise figure, a mixer MIX 1  converts the RF band into a first 7 GHz IF band, and a second mixer MIX 2  carries out a second conversion to the final IF band. 
   An output buffer  9  guarantees the 5 dBm output P1 dB required by the standard. A PLL block  11  implements the LO for both mixers MIX 1  and MIX 2 . 
   Conventionally, the RF band of the DVB-S application is split into a low-band (LB: 10.7–11.7 GHz) and a high-band (HB: 11.7–12.75 GHz). 
   The low-band LB and high-band HB are selected by locking the PLL oscillation frequency respectively to 4.875 GHz and 5.300 GHz. 
   As shown in  FIG. 2 , the LNB  10  includes external HEMTs for the low noise figure, an image rejection filter IRF between the first and the second mixer, and a control block  12  for the band and polarization selection. By enabling HEMT1H or HEMT1V branches either the horizontal (H) or the vertical (V) linear polarization can be selected. 
   The VCO circuit  1  is connected between the PLL block  11  and the two mixers MIX 1  and MIX 2 . 
   This circuit  1  has been manufactured with a 46-GHz-f T  bipolar technology. Details of the circuit  1  are shown in  FIG. 3 . 
   The VCO  1  is based on a three turns, double-coupled differential integrated transformer  3 . It allows to improve both performances in terms of tuning range and phase noise if compared with the prior art solutions. 
   In particular, the VCO integrated differential transformer  3  comprises three stacked, single turn of metal circular inductors, to obtain the highest value for the coupling factor k. The topmost metal layer is used to realize resonator inductance to obtain the best quality factor. 
   The couples of coil ( 4 – 4 ′), ( 5 – 5 ′) and ( 6 – 6 ′) represent the inductances of each turn of the differential integrated transformer  3 . 
   The coupling factor between the first and second turn is indicated with K 1 . 
   The coupling factor between the second and third turn is indicated with K 2 . 
   The coupling factor K 1  may equal to the coupling factor K 2  but may even be different from the coupling factor K 2 . 
   The three couples of coils ( 4 – 4 ′), ( 5 – 5 ′) and ( 6 – 6 ′) of the integrated transformer  3  realize an inductor for an LC resonator  13  that also includes first and second varactor diodes  14 ,  15  respectively coupled between the coils  4 ,  4 ′ and the tuned voltage V TUNE  provided by the PLL  11 . 
   A DC path is provided for biasing the base terminals of the bipolar transistors Q 1 –Q 2  of the cross-coupled pair  7  and the base terminals of the differential source follower buffer transistors Q 3 –Q 4 . 
   The transformer  3  provides: 
   the load for the cross-coupled pair  7 ; 
   the inductor for the LC resonator; 
   the feedback coupling between the resonator and the cross-coupled pair  7 . 
   The unique transformer  3  does not require the use of the biasing resistors RB and RBB of the prior art solutions. Even the capacitors CD and CDD are missing in the VCO circuit  1 . 
   The VCO  1  may thus be named as “resistor-less” and “capacitor-less”. This improves a lot both phase noise and tuning range. 
   The VCO  1  includes resistors  16 ,  17  respectively coupled between ground and the source terminals of buffer transistors Q 3  and Q 4 . In addition, the VCO  1  includes a resistor R EE  coupled between ground and the source terminals of the cross-coupled transistors Q 1 , Q 2 . 
   This original idea has been validated comparing the simulation performance of the VCO  1  versus the state-of-the-art VCO shown in  FIG. 1  using an oscillation frequency at 5 GHz; a voltage supply of 3.3 V and a biasing current of 8 mA. 
   Both circuits have been simulated by setting the same bias current and supply voltage and using a 46-GHz-fT silicon bipolar technology. The VCO  1  provides 16.7% tuning range increasing and 2.6 dB phase noise reduction. 
   In the following table 1 the simulation comparison parameters are reported: 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
           
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
             
             
               Parameter 
               State-of-the art VCO 
               VCO 1 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               VCC 
               3.3 
               V 
               3.3 
               V 
             
             
               IBIAS 
               8 
               mA 
               8 
               mA 
             
             
               fo 
               5 
               GHz 
               5 
               GHz 
             
             
               L 
               0.7 
               nH 
               0.7 
               nH 
             
           
        
         
             
               RB 
               2 
               kΩ 
               — 
             
             
               CD 
               5 
               pF 
               — 
             
             
               RBB 
               2 
               kΩ 
               — 
             
             
               CDD 
               5 
               pF 
               — 
             
           
        
         
             
               K 
               — 
               0.87 
                 
             
           
        
         
             
               Phase Noise 
               −102.7 
               dBc/Hz @ 
               −105.3 
               dBc/Hz @ 
             
             
                 
               100 
               kHz 
               100 
               kHz 
             
             
               Tuning Range 
               4.7–5.18 
               GHz (9.7%) 
               4.1–5.35 
               GHz (26.4%) 
             
             
                 
             
           
        
       
     
   
   Thus, the VCO circuit  1  allows to improve the tuning range of about 16.7% with respect to the known solution having better performances. 
   The phase noise has been reduced as well of about 2.6 dB. 
   The VCO  1  removes conventional biasing resistors and decoupling capacitors, thus improving phase noise and tuning range performance. 
   This original idea has been validated comparing the simulation performance of the proposed VCO versus the state-of-the-art VCO at 5 GHz. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.