Patent Document

TECHNICAL FIELD 
     The present invention relates to a mixer. 
     BACKGROUND 
     Mixers have been used in radio circuits for a long time. A mixer may be used to produce an intermediate frequency (IF) signal dependent on a received radio frequency (RF) signal and a local oscillator (LO) signal. The IF signal is delivered to a detector circuit, which generates an audio frequency (AF) signal in response to the IF signal. Hence, the mixer in co-operation with a suitable detector circuit can operate to extract a message from a radio signal. 
     U.S. Pat. No. 5,589,791 to Gilbert describes a classic active mixer commonly known as the “Gilbert mixer”. The Gilbert mixer comprises a mixer core having four transistors whose bases are connected to an LO port for receiving a LO signal and whose collectors are connected to an IF output port. The mixer also has an RF input section having two transistors whose bases are connected to an RF port for receiving an RF signal and whose collectors are connected to the emitters of the transistors in the mixer core. 
     The operation of the mixer is as follows: In the absence of any voltage difference between the bases of the two transistors in the RF input section, the collector currents of these two transistors are essentially equal. Thus, a voltage applied to the LO port results in no change of output current. Conversely, if an RF signal is applied to the RF port, but no voltage difference is applied to the LO port, the output currents will again be balanced. Thus, it is only when a signal is applied to both the LO port and the RF port that an intermediate frequency signal appears at the IF port. A known problem with the classic active mixer is the switching noise generated by the mixer core transistors as they switch between their “on” and “off” states. U.S. Pat. No. 5,589,791 to Gilbert describes this problem, and illustrates noise bursts that are created during transition periods when the LO signal changes between high and low states. U.S. Pat. No. 5,589,791 to Gilbert also discloses a mixer having an RF input port, an LO input port and an active input driver connected to the LO input port. The input driver is a complex circuit including an input for receiving a single-sided LO signal, and no less than 21 transistors forming class AB emitter followers and an associated bias stage. The active input driver aims to cause a forced supply and withdrawal of charge from each of the LO input terminals of the mixer core for providing quicker transitions between the on and off states in the mixer core. 
     SUMMARY 
     An aspect of the invention relates to the problem of providing a mixer with improved performance at low cost. 
     This problem is addressed by a mixer, comprising: 
     a first terminal and a second terminal forming a first input port for receiving a first signal having a first frequency; 
     an second input port for receiving a second signal having a second frequency; 
     a mixer output port for a resulting signal; and 
     a first group of valves having their control inputs coupled to the first terminal for receiving the first signal; 
     a second group of valves having their control inputs coupled to the second terminal for receiving the first signal; and 
     a third group of two valves having their control inputs coupled for receiving the second signal; 
     said valves co-operating such that in operation the mixer produces the resulting signal responsive to the first and second signals. Moreover, the mixer comprises at least one passive low pass filter having an inductor; said at least one low pass filter being connected to the control input of a valve. 
     This filter advantageously operates to decrease the rise time of the signal controlling the valve. A quicker rise time of that signal causes a quicker transition of the valve from non-conducting to conducting state. Since the noise produced by a mixer emanates to a large extent from noise produced by a valve during transition from a conducting state to a non-conducting state, the total amount of noise is thereby reduced, when the mixer is provided with such filters. Since the filter has only passive components, the reliability of the mixer is improved and the noise contribution is minimal. An additional advantage attained with passive components is a low component cost. Hence, the mixer, when used in a receiver renders a reliable high fidelity radio receiver at a low cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For simple understanding of the present invention, it will be described by means of examples and with reference to the accompanying drawings, of which 
     FIG. 1 is a block diagram of an embodiment of a superheterodyne receiver. 
     FIG. 2 is a circuit diagram of an embodiment of the mixer shown in FIG.  1 . 
     FIG. 3A is a voltage/time diagram illustrating the temporal progression of the amplitude of a LO signal. 
     FIG. 3B is a more detailed voltage/time diagram illustrating the temporal progression of the amplitude of a positive edge of the LO signal shown in FIG.  3 A. 
     FIG. 3C is a voltage/time diagram illustrating the temporal progression of the amplitude of the signal at a valve gate in response to the positive edge signal illustrated in FIG.  3 B. 
     FIG. 4A is a circuit diagram of another embodiment of the mixer shown in FIG. 1, connected to a Local Oscillator. 
     FIG. 4B illustrates another version of the circuitry of FIG.  4 A. 
     FIG. 5 is a circuit diagram of yet another embodiment of the mixer shown in FIG.  1 . 
     FIG. 6 is a circuit diagram of yet another embodiment of the mixer shown in FIG.  1 . 
     FIG. 7 is a combined block diagram/circuit diagram of an embodiment of the mixer shown in FIG. 1, wherein the filters are represented as blocks. 
     FIG. 8 illustrates an embodiment of a filter having two inductors and two capacitors. 
    
    
     DETAILED DESCRIPTION 
     In the following description similar features in different embodiments will be indicated by the same reference numerals. 
     FIG. 1 is a block diagram of an embodiment of a superheterodyne receiver  10  having an antenna  20  coupled to a radio frequency circuit  30 . When a signal is received by the antenna, the radio frequency circuit  30  delivers a radio frequency signal (RF) to the inputs  40 ,  50  of a mixer  60 . 
     The mixer  60  also has inputs  70 ,  80  for receiving a turning oscillator signal from a local oscillator  90 , and outputs  110 ,  10  for delivery of an intermediate frequency signal (IF). The outputs  110 ,  10  are coupled to inputs  120 ,  130  of an intermediate frequency (IF) amplifier  140  having outputs for delivering an amplified IF signal to a detector circuit  150 . The detector circuit  150  generates an audio-frequency (AF) signal in response to the IF signal, and the AF signal is delivered to an audio-frequency amplifier  160  operating to amplify the AF signal, and to deliver it to a load  170 , such as a loudspeaker. 
     FIG. 2 is a circuit diagram of an embodiment of the mixer  60  shown in FIG.  1 . 
     The mixer of FIG. 2 has a first transistor Q 1  and a second transistor Q 2 , the gates of which are coupled to the inputs  40  and  50 , respectively, for receiving the RF signal. The sources of transistors Q 1  and Q 2  are coupled to signal ground  180 , preferably via a biasing current device  190 . 
     The first LO input  70  is coupled to the gate  233  of a first switch transistor Q 3  via an inductor  230 , and to the gate  243  of a second switch transistor Q 4  via an inductor  240 . A A capacitor  235  is connected between signal ground and the junction of inductor  230  and the gate  233  of transistor Q 3 , such that the LC circuit forms a first low pass filter F 3 . Another capacitor  245  is likewise connected between signal ground and the junction of inductor  240  the gate  243  of transistor Q 4 , as illustrated in FIG. 2,  50  as to form a second low pass filter F 4 . The inductors  230  and  240  have an inductance and a series resistance. Because of the resistance the filter F 3  and F 4  are dampened, thereby avoiding undesired oscillation in the circuitry. 
     The second LO input  80  is coupled to the gate  253  of a third switch transistor Q 5  via an inductor  250 , and to the gate  263  of a fourth switch transistor Q 6  via an inductor  260 . Capacitors  255  and  265 , respectively, are connected between the gates of transistors Q 5  and Q 6  respectively, and signal ground, as illustrated in FIG.  2 . Inductor  250  in combination with capacitor  255  forms a third low pass filter F 5 , and inductor  260  in combination with capacitor  265  forms a fourth low pass filter F 6 . The inductors  250  and  260  also have an inductance and a resistance so as to avoid undesired oscillation in the circuitry. 
     The drain terminals of transistors Q 3  and Q 4  are connected to the first IF signal output  100 , whereas the drain terminals of transistors Q 5  and Q 6  are connected to the second IF signal output  110 . 
     FIG. 3A is a voltage/time diagram illustrating the temporal progression of the amplitude of the LO signal, i.e. the signal provided by the local oscillator  90  in FIG.  1 . The rise time T r  of a positive edge of a signal is usually defined as the duration for the signal amplitude to progress from 20% of a top value to 80% of the top value (see FIG.  3 A). 
     FIG. 3B is a more detailed voltage/time diagram illustrating the temporal progression of the amplitude of a positive edge of the LO signal shown in FIG.  3 A. 
     FIG. 3C is a voltage/time diagram illustrating the temporal progression of the amplitude of the signal at gate  233  (See FIG. 2) in response to the positive edge signal illustrated in FIG.  3 B. With reference to FIG. 2, the transistor Q 3  is turned off when the amplitude at the gate is below the level v 1  shown in FIGS. 3B and 3C. When the amplitude at the gate is above the level v 2  shown in FIGS.  3 B/ 3 C, the transistor Q 3  is conducting so well that there is no voltage swing at its output, i.e. the transistor is saturated. 
     By comparing the signal portions shown in FIGS. 3B and 3C it can be clearly seen that the time period t 4 -t 3  in FIG. 3C is shorter than the time period t 2 -t 1  in FIG.  3 B. As a matter of fact the rise time of the LO signal is shorter after having passed the LO signal through the filter F 3  (See FIG.  2 ). 
     It can be seen, by comparing FIGS. 3B and 3C that when a positive edge of the LO signal, having a certain slope, is delivered to terminal  70  the amplitude at gate terminal  233  (See FIG. 2) is initially unaffected. During this phase, however, the filter F 3  is charged with reactive energy. Hence, the filter F 3  will initially cause a delay, and thereafter the filter will cause the signal level at gate terminal  233  to have a steeper slope than that of terminal  70 . A quicker rise time of the signal causes a quicker transition of transistor Q 3  from non-conducting state to conducting state. Since the noise contribution from a transistor is predominantly generated during transition between a conducting state and a non-conducting state, the amount of noise is thereby advantageously reduced, when the mixer is provided with filters such as F 3 , F 4 , F 5  and F 6 . 
     According to preferred embodiments the low pass filters F 3 , F 4 , F 5  and F 6  has only passive components, thereby providing a high reliability and a minimal noise contribution. An additional advantage attained with passive components is a low component cost. Hence, the mixer  60 , when used in a receiver renders a reliable high fidelity radio signal receiver at a low cost. 
     According to an embodiment the inductance of the inductor  230  and the capacitance of the capacitor  235  in filter F 3  are selected such that the time constant of the filter F 3  has a value similar to the rise time of the LO signal. In this connection the rise time is defined as mentioned in connection with FIG. 3A above. According to embodiments of the invention the time constant of filter F 3  has a value in the range from 0,2 to 10 times the rise time of the LO signal. According to some embodiments of the invention the time constant of filter F 3  has a value in the range from 0,5 to 2 times the rise time of the LO signal. According to a preferred embodiment the time constant of the filter is selected in the range from 90% to 110% of the rise time of the LO signal. 
     Since, normally, the fall time of the LO signal is substantially the same as the rise time thereof, the relation between the fall time and the time constant of the filter will normally be the same. If, however, there is a distinct difference between the rise and fall time of the LO signal, then the above relations should apply to the time constant of the filter as compared to the mean value of the rise and fall time of the LO signal. 
     According to one embodiment of the invention the time constant of the filters is selected to 20 picoseconds for a circuit wherein the LID signal has a rise time of 20 picoseconds. The LO signal may, for example, have a period of 120 ps. 
     An example of a mixer with the circuit diagram of FIG. 2 includes the following component values: Each of the inductors  230 ,  240 ,  250 ,  260  has an inductance of 1 nH, and a series resistance of 50 Ohm. Each of the capacitors  235 ,  245 ,  255 ,  265  has a capacitance of 0,03 pF. The time constant of the filter is thus 34 ps, obtained as 2* pi *.sqrt(LC). 
     The LO signal has an input period time of 120 ps, a rise time of 20 ps, and a fall time of 20 ps. The puse width of the LO signal in this example is 40 ps. 
     The above discussion about the relation between the filter time constant and the LO signal is applicable, not only to the FIG. 2 embodiment, but also to the other embodiments of the invention described in this text. 
     FIG. 4A includes a circuit diagram of another embodiment of the mixer  60  shown in FIG.  1 . FIG. 4A also illustrates a block diagram of a Local Oscillator  90 . having a DC bias source  270  and AC signal sources  275 ,  280 . The signal source  280  is 180 degrees phase shifted in relation to signal source  275 , as indicated by the polarity references + and − in FIG.  4 A. According to the FIG. 4A embodiment a filter F is connected to terminals  70 ,  80 . The filter F includes a first inductor  300  which is coupled between input terminal  70  and the gates of transistors Q 3  and Q 4 . A second inductor  310  is coupled between input terminal  80  and the gates of transistors Q 5  and Q 6 , and a capacitor  320  is coupled between the gates of transistors Q 3 /Q 4  and Q 5 /Q 6 , as shown in FIG.  4 . The inductors  300 ,  310  are inductive and resistive for the same reason as mentioned for inductor  230  above. 
     FIG. 4B differs from FIG. 4A in that two capacitors  321  replaces the single capacitor  320 . Each capacitor  321  has twice the capacitance value of capacitor  320 , and the terminal between the capacitors is connected to ground  322 . In this manner the capacitors  321  co-operate to provide a capacitance of the same value as capacitor  320 , but with the additional advantage of being grounded so as to obtain a defined DC-level. 
     FIG. 5 is a circuit diagram of yet another embodiment of the mixer  60  shown in FIG.  1 . According to the FIG. 5 embodiment a first inductor  300  is coupled between input terminal  70  and the gates of transistors Q 3  and Q 4  and a capacitor  330  is coupled between the gates of transistors Q 3 /Q 4  and ground, as shown in FIG. 5. A second inductor  310  is coupled between input terminal  80  and the gates of transistors Q 5  and Q 6 , and a capacitor  340  is coupled between the gates of transistors Q 3 /Q 4  and ground. 
     FIG. 6 is a circuit diagram of yet another embodiment of the mixer  60  shown in FIG.  1 . The FIG. 6 embodiment corresponds to the FIG. 2 embodiment, but whereas the capacitors  235 ,  245 ,  255 ,  265  in FIG. 2 are connected to ground, each of the corresponding capacitors  435 ,  445 ,  455 ,  465  in the FIG. 6 embodiment are connected to the drain terminal of the respective transistor. 
     FIG. 7 is a combined block diagram/circuit diagram of an embodiment of the mixer  60  shown in FIG. 1, wherein the filters are represented as blocks. The filters F 3 , F 4 , F 5  and F 6  can consist of a simple passive filter stage having a single inductor and a single capacitor as described with reference to FIG. 2 above. In an alternative embodiment each filter has plural LC pairs, FIG. 8 illustrates an embodiment of the filter F 3  having two inductors and two capacitors. Also when the filter has plural LC pairs, as shown in FIG. 8, the above described relation between the filter time constant and the rise time of the LO signal applies.

Technology Category: 5