Abstract:
A differential conversion circuit for converting an unbalanced signal to a balanced signal is provided. The differential conversion circuit is arranged with primarily passive components, and so avoids introducing noise or other disturbances to the signal. The differential conversion circuit has a terminal input for receiving an unbalanced signal that is related to a communication input signal. An inductor and a resonating capacitor connect to the terminal input, and a coupling capacitor connects to the inductor. One differential output line is provided by the resonating capacitor, while the other differential output line is provided by the coupling capacitor. The output from the capacitors is thereby a pair of lines that provide a balanced differential signal which is deliverable to a balanced load.

Description:
FIELD OF THE INVENTION 
     The field of the present invention is electronic circuits. More particularly, the present invention relates to an electronic configuration for use in a radio frequency device. 
     BACKGROUND OF THE INVENTION 
     Wireless devices are transforming how people work, relax, and communicate. These devices can enable convenient access to informational, educational, and entertainment data, and provide a convenient portal for worldwide communication. Some of the most popular wireless devices are portable, which benefit not only from a small footprint, but also require a consistent and robust communication link to be useful. Without the benefit of such a dependable communication link, users are unable to reasonably rely on the availability of their wireless devices. 
     Generally, a wireless device has a radio transceiver that communicates with other mobile devices or to a more permanent base station. Accordingly, the wireless device has an antenna that is used to both transmit and receive radio frequency signals. In particular, the antenna and wireless device are typically configured to operate on a particular range of radio frequencies, with an information signal modulated on the radio wave. 
     It is a particularly difficult problem to configure a wireless device to reliably and robustly receive signals in a manner that enables the information signal to be consistently demodulated and used. Several factors affect the quality of reception and the usability of the information signal. For example, the modulation signal may be subjected to physical interferences, such as buildings, that substantially attenuate the modulation signal. Further, distance from the modulation signal source also substantially attenuate the modulation signal. 
     The wireless device typically has an antenna that electrically couples to processing circuitry using a single-ended connection. A single ended connection is also known as an unbalanced connection. Such a single-ended, unbalanced connection provides a ground connector and a signal connector, with the signal connector transmitting all the signal information. However, such a single ended, or unbalanced, signal tends to be highly susceptible to noise, such as power supply ripple or crosstalk from other circuitry. Inducing such noise in a wireless device results in a lower signal to noise ratio, and increases the risk of losing the information signal. It is therefore desirable to use balanced signals and connections in a wireless device. However, although balanced antennas outputting balanced signals are known, the balanced antennas are generally larger than unbalanced antennas and therefore add an undesirable bulk and weight to wireless devices. Accordingly, it has not been practical to use balanced antennas on wireless devices. 
     Practical limitations therefore suggest the use of the single-ended antennae on wireless devices. However, it is also known that wireless devices would benefit from the use of balanced signals, as balanced signals are less susceptible to noise, for example. Accordingly, known conventional wireless devices have processing circuitry for converting the unbalanced signal to a balanced signal. In such a manner, the singled-ended antennae signal is converted to a balanced signal to enhance the efficiency and reduce susceptibility to noise. 
     In known wireless devices, single-ended signals are typically converted to a balanced signal using a device commonly referred to as a “balun” transformer. A balun, which is an abbreviation for “balanced-unbalanced”, may be arranged, for example, as a balancing transformer. The balun receives the single-ended signal and outputs a balanced signal, such as a differential signal. In the differential signal, the output signal is a function of the difference between two conductors, and is therefore less susceptible to noise or other disturbances. 
     Although the use of a balun in a wireless device advantageously provides balanced signals for processing, the balun is often bulky since it is not easily implemented inside on integrated circuit. The nature of the known balun device causes the balun to undesirably add size and cost to the wireless device. The balun may also cause a signal loss, which may translate into a degraded noise figure and lower performance characteristics for the wireless device. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a circuit for converting an unbalanced signal to a balanced signal without the disadvantages associated with using a balun transformer. 
     Briefly, the present invention provides a differential conversion circuit for converting an unbalanced signal to a balanced signal. The differential conversion circuit is constructed with primarily passive components which may be arranged on an integrated circuit to minimize size and cost. The differential conversion circuit has a terminal input for receiving an unbalanced signal that is related to a communication input signal. An inductor and a resonating capacitor connect to the terminal input, and a coupling capacitor connects to the inductor. One differential output line is provided by the resonating capacitor, while the other differential output line is provided by the coupling capacitor. The output from the capacitors is thereby a pair of lines that provide a balanced differential signal which is deliverable to a balanced load. 
     Advantageously, the differential conversion circuit uses primarily passive components, and so does not introduce noise or other disturbances due to the use of active components. Additionally, the differential conversion circuit is constructed from only a few components, and so adds little cost to manufacturing a wireless device. Further, the differential conversion circuit is compact and easily integrated into new or existing radio frequency (rf) circuit designs. 
     These and other features and advantages of the present invention will be appreciated from review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general schematic layout for a differential conversion circuit in accordance with the present invention; 
     FIG. 2 is a schematic diagram of an embodiment of a differential conversion circuit in accordance with the present invention; 
     FIG. 3 is a schematic diagram of another embodiment of a differential conversion circuit in accordance with the present invention; 
     FIG. 4 is a detailed schematic diagram of an embodiment of a differential conversion circuit in accordance with the present invention; 
     FIG. 5 is a current gain chart showing a simulation result using the circuit of FIG. 4; and 
     FIG. 6 contains current charts showing a simulation result using the circuit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a differential conversion circuit  10  is shown in FIG.  1 . The differential conversion circuit  10  is arranged for use in a wireless device, and in particular, a wireless device operating in a radio frequency band. However, it will be appreciated that the differential conversion circuit  10  may be modified for use in other devices and at other frequencies. For example, the differential conversion circuit  10  may be used in cable television receivers, satellite transceivers, cordless phones, or other devices where a conversion from an unbalanced signal to a balanced signal is desirable. 
     The differential conversion circuit  10  receives an input RF current  20  at terminal point  24 . The input RF current  20  may be provided by, for example, by transistor  16 . In the described example, transistor  16  receives an RF signal  12  at its base, and generates the input RF current  20  from its collector  13 . The RF signal  12 , and the RF current  20  are both single ended, i.e., unbalanced, signals. It will be appreciated that the input RF current  20  may be generated in alternative ways. 
     From the terminal point  24 , two electrical paths  44 ,  46  are provided. Each of the electrical paths  44 ,  46  provides one line of a differential output  48 . The first electrical path  44  extends from the terminal point  24 , through resonating capacitor  25  and load  34 , and results in one line  41  of the differential output  48 . The second electrical path  46  extends from the terminal point  24 , through inductor  23 , coupling capacitor  27 , and load  32 , and results in the other line  42  of the differential output  48 . 
     A collector voltage source  17  is provided to an isolation load  21 , which is connected to both the coupling capacitor  27  and the inductor  23 . The isolation load  21  is selected to provide a high impedance at the frequency of the RF signal, but to pass a DC current. Accordingly, the collector voltage source  17  is enabled to provide a collector voltage for transistor  16 . 
     The inductor  23  and the resonating capacitor  25  are selected to resonate at the frequency of the input RF signal  12 . More particularly, the inductor  23  and the resonating capacitor  25  are sized according to the well known equation: 
     
       
         RF=1/(2πSqrt(L×C)) 
       
     
     where, 
     RF is the frequency of the input signal  12 ; 
     L is the inductance value of the inductor  23 ; and 
     C is the capacitance value of the capacitor  25 . 
     The value of the capacitor  27  is selected such that capacitor  27  acts as a coupling capacitor at the frequency of the RF input signal  12 . 
     Loads  34  and  32  are balanced low impedance loads. Load  34  receives differential current phase  35 , and load  32  receives differential current phase  37 . The gain of the differential conversion circuit  10  is thereby determined by comparing the differential current phases  35  and  37  with the input RF current  20 . More particularly the gain is calculated with the formula:            Current                 Gain     =              (       I   37     -     I   35       )     /     (     I   20     )            =       WL   R     =     1   WCR           ,                          
     where, 
     R is resistance of loads  34 ,  32 ; 
     W=2 f o , where f o  is operating frequency; 
     L=value of inductor  23 ; 
     C=value of capacitor  25 ; 
     I 37  is the current  37  through load  32 ; 
     I 35  is the current  35  through load  34 ; and 
     I 20  is the RF input current  20 . 
     In operation, the differential conversion circuit  10  has been found to provide a substantial current gain, as defined above, without introducing noise due to the use of active components. Since the differential conversion circuit  10  is also implemented with only a few passive components, the differential conversion circuit  10  is easily and economically added to electronic designs. 
     Referring now to FIG. 2, another differential conversion circuit  60  is shown. The differential conversion circuit  60  receives an RF input current  65  from cascode transistors  67 . More particularly, an RF input signal  61  is provided to transistor  63 , which cooperates with transistor  64  to provide the RF input current  65 . 
     The RF input current  65  is received at terminal point  69  of the differential conversion circuit  60 . From the terminal point  24 , two electrical paths  66 ,  68  are provided. Each of the electrical paths  66 ,  68  provides one phase of a differential current output  99 . The first electrical path  66  extends from the terminal point  69 , through resonating capacitor  71  and load  86 , and provides a current  81 . The second electrical path  68  extends from the terminal point  69 , through inductor  70 , coupling capacitor  72  and load  87 , and provides a current  82 . Accordingly, the differential conversion circuit  60  is used to convert the single ended RF input signal  61  to the differential current output  99 . 
     A collector voltage source  80  is provided to an isolation load  75 , which is connected between the coupling capacitor  72  and the inductor  70 . The isolation load is a parallel LC circuit having inductor  76  and capacitor  77 . Inductor  76  and capacitor  77  are selected to resonate at the frequency of the RF signal input  61 . 
     The inductor  70  and the resonating capacitor  71  are also selected to resonate at the frequency of the input RF signal  61 , while the value of the capacitor  72  is selected such that capacitor  72  acts as a coupling capacitor at the frequency of the RF input signal  61 . 
     Loads  86  and  87  are balanced low impedance loads, due to their configuration as a differential common base amplifier. In FIG. 2, loads  86  and  87  are arranged to receive differential current phases  81  and  82 , respectively. Load  86  has a dc current  97  for biasing transistor  95 , which receives differential current phase  81  at its emitter. Power supply voltage  91  couples to the collector of transistor  95 , which provides one pole for the voltage output  88 . In a similar manner, load  87  has a dc current  98  for biasing transistor  96 , which receives differential current phase  82  at its emitter. Power supply voltage  92  couples to the collector of transistor  96 , which provides the opposing pole for the voltage output  88 . Accordingly, a balanced differential signal is provided at output  88  in response to the unbalanced, i.e., single-ended, input signal  61 . 
     FIG. 3 shows another circuit having a differential conversion circuit  100 . As the differential conversion circuit  100  is similar to differential conversion circuits  10  and  60  described above, differential conversion circuit  100  will only be generally described. 
     An RF signal  101  is received by transistor  103 , which provides an RF input current  102  for the differential conversion circuit  100 . Using inductor  105 , resonating capacitor  107 , coupling capacitor  106 , and isolation load  107 , the differential conversion circuit  100  converts the single-ended RF input current  102  into differential current output  110 . In FIG. 3, the differential current output  110  is received into a Gilbert mixer quad  112 , which provides a balanced load for each current phase of differential current output  110 . 
     Referring now to FIG. 4, a specific circuit incorporating a differential conversion circuit  150  is shown. As the differential conversion circuit  150  is similar to differential conversion circuits  10 ,  60 , and  100 , described above, differential conversion circuit  150  will only be generally described. Differential conversion circuit  150  receives an RF input current  152  from an RF signal source  151 . The RF signal source  151  provides an RF input signal of about 1.9 GHz. Although FIG. 4 uses a specific signal source  151  for providing the RF signal current  152 , it will be understood that other circuits can be substituted that provide other frequencies and currents. 
     The differential conversion circuit  150  converts the RF input current  152  into a balanced differential current output  171 . The balanced differential current output  171  comprises differential current phase  161 , which is received into load  167 , and differential current phase  162 , which is received into load  169 . 
     In FIG. 4, inductor  155  and resonating capacitor  160  are selected to resonate at the frequency of the RF input signal, which is about 1.9 GHz. Accordingly, the inductor  155  is selected to be 5 nH, and the resonating capacitor  160  is selected to be 1.4 pF. The coupling capacitor  157  is selected to be substantially larger than the resonating capacitor  160 , and so is selected to be at 39 pF. In a similar manner the inductor and capacitor of the isolation load  165  are selected to resonate near 1.9 GHz. Although FIG. 4 discloses particular values for the capacitors and inductor, it will be appreciated that the selected values may be adjusted according to the manner described herein. It will also be appreciated that different loads  167  and  169  may be substituted, as well as different technologies used to implement the circuit. Preferably the differential conversion circuit is implemented as an integral section of an integrated circuit package, but it will be appreciated that the differential conversion circuit may be implement using surface mount or other discrete components. 
     The circuit shown in FIG. 4 was used in an electric circuit simulation program with the results of the simulation shown in FIGS. 5 and 6. FIG. 5 shows a current gain chart  200  with the x-axis  201  representing input frequency in gigaHertz, and the y-axis  202  representing a numerical gain factor. As can be seen from result line  204 , the current gain peak  206  occurs near the input frequency of about 1.9 GHz. 
     FIG. 6 has an input current chart  210  and an output current chart  220 . Each chart  210  and  220  has time represented on the x-axis  211 ,  221 , and current in miliamps represented on the y-axis  217 ,  227 . As can be seen from the charts, the input current  214  cycles between about 1 mA, while each of the differential output currents  224 ,  225  cycle between about 2 mA. 
     Advantageously, the differential conversion circuit provides an efficient conversion from an unbalanced signal to a balanced signal without introducing substantial noise associated with active components. Accordingly, the differential conversion circuit may be incorporated into a wide range of telecommunication and other devices where a low-noise conversion is desirable. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments which are presented in this description for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow. It is noted that equivalents for the particular embodiments discussed in this description may practice the invention as well.