A MIXER

Abstract: A mixer, comprising: a first terminal (70) and a second terminal (80) forming a first input port for receiving a first signal having a first frequency (f>LO<); an second input port (40, 50) for receiving a second signal having a second frequency (f>RF<); a mixer output port (100, 110) for a resulting signal; and a first group of valves (Q3, Q4) having their control inputs (233,243) coupled to the first terminal for receiving the first signal; a second group of valves (Q5, Q6) having their control inputs (253,263) coupled to the second terminal for receiving the first signal; and a third group of two valves (Q1, Q2) having their control inputs coupled for receiving the second signal; said valves (Q1, Q2, Q3, Q4, Q5, Q6) co-operating such that in operation the mixer produces the resulting signal responsive to the first and second signals. 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 (Q5, Q3, Q6, Q4).

Description: A Mixer Technical Field of the Invention The present invention relates to a mixer. Description of Related Art 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. US 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. US 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. US 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 Figure 1 is a block diagram of an embodiment of a superheterodyne receiver. Figure 2 is a circuit diagram of an embodiment of the mixer shown in Figure 1. Figure 3 A is a voltage/time diagram illustrating the temporal progression of the amplitude of a LO signal. Figure 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 Figure 3 A. Figure 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 Figure 3B. Figure 4 A is a circuit diagram of another embodiment of the mixer shown in Figure 1, connected to a Local Oscillator. Figure 4B illustrates another version of the circuitry of Figure 4A. Figure 5 is a circuit diagram of yet another embodiment of the mixer shown in Figure 1. Figure 6 is a circuit diagram of yet another embodiment of the mixer shown in Figure 1. Figure 7 is a combined block diagram/circuit diagram of an embodiment of the mixer shown in Figure 1, wherein the filters are represented as blocks. Figure 8 illustrates an embodiment of a filter having two inductors and two capacitors. Detailed Description of Embodiments In the following description similar features in different embodiments will be indicated by the same reference numerals. Figure 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 tuning oscillator signal from a local oscillator 90, and outputs 100, 110 for delivery of an intermediate frequency signal (IF). The outputs 100, 110 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. Figure 2 is a circuit diagram of an embodiment of the mixer 60 shown in Figure 1. The mixer of Figure 2 A has a first transistor Ql and a second transistor Q2, the gates of which are coupled to the inputs 40 and 50, respectively, for receiving the RF signal. The sources of transistors Ql and Q2 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 Q3 via an inductor 230, and to the gate 243 of a second switch transistor Q4 via an inductor 240. A capacitor 235 is connected between signal ground and the junction of inductor 230 and the gate 233 of transistor Q3, such that the LC circuit forms a first low pass filter F3. Another capacitor 245 is likewise connected between signal ground and the junction of inductor 240 and the gate 243 of transistor Q4, as illustrated in Figure 2A, so as to form a second low pass filter F4. The inductors 230 and 240 have an inductance and a series resistance. Because of the resistance the filters F3 and F4 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 Q5 via an inductor 250, and to the gate 263 of a fourth switch transistor Q6 via an inductor 260. Capacitors 255 and 265, respectively, are connected between the gates of transistors Q5 and Q6 respectively, and signal ground, as illustrated in Figure 2A. Inductor 250 in combination with capacitor 255 forms a third low pass filter F5, and inductor 260 in combination with capacitor 265 forms a fourth low pass filter F6. 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 Q3 and Q4 are connected to the first IF signal output 100, whereas the drain terminals of transistors Q5 and Q6 are connected to the second IF signal output 110. Figure 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 Figure 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). Figure 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 Figure 3 A. Figure 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 Figure 3B. With reference to Figure 2, the transistor Q3 is turned off when the amplitude at the gate is below the level vl shown in Figures 3B and 3C. When the amplitude at the gate is above the level v2 shown in Figures 3B/3C, the transistor Q3 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 Figures 3B and 3C it can be clearly seen that the time period t4-t3 in Figure 3C is shorter than the time period t2-tl in Figure 3B. As a matter of fact the rise time of the LO signal is shorter after having passed the LO signal through the filter F3 (See Figure 2). It can be seen, by comparing Figures 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 F3 is charged with reactive energy. Hence, the filter F3 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 Q3 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 F3, F4, F5 and F6. According to preferred embodiments the low pass filters F3, F4, F5 and F6 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 F3 are selected such that the time constant of the filter F3 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 Figure 3A above. According to embodiments of the invention the time constant of filter F3 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 F3 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 LO 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 Figure 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 0,03pF. 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 Figure 2 embodiment, but also to the other embodiments of the invention described in this text. Figure 4A includes a circuit diagram of another embodiment of the mixer 60 shown in Figure 1. Figure 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 4A. 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 Q3 and Q4. A second inductor 310 is coupled between input terminal 80 and the gates of transistors Q5 and Q6, and a capacitor 320 is coupled between the gates of transistors Q3/Q4 and Q5/Q6, as shown in Fig.4. The inductors 300, 310 are inductive and resistive for the same reason as mentioned for inductor 230 above. Figure 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. Figure 5 is a circuit diagram of yet another embodiment of the mixer 60 shown in Figure 1. According to the Fig. 5 embodiment a first inductor 300 is coupled between input terminal 70 and the gates of transistors Q3 and Q4 and a capacitor 330 is coupled between the gates of transistors Q3/Q4 and ground, as shown in Fig.5. A second inductor 310 is coupled between input terminal 80 and the gates of transistors Q5 and Q6, and a capacitor 340 is coupled between the gates of transistors Q3/Q4 and ground. Figure 6 is a circuit diagram of yet another embodiment of the mixer 60 shown in Figure 1. The Fig. 6 embodiment corresponds to the Figure 2 embodiment, but whereas the capacitors 235, 245, 255, 265 in Figure 2 are connected to ground, each of the corresponding capacitors 435, 445, 455, 465 in the Figure 6 embodiment are connected to the drain terminal of the respective transistor. Figure 7 is a combined block diagram/circuit diagram of an embodiment of the mixer 60 shown in Figure 1, wherein the filters are represented as blocks. The filters F3, F4, F5 and F6 can consist of a simple passive filter stage having a single inductor and a single capacitor as described with reference to Figure 2 above. In an alternative embodiment each filter has plural LC pairs. Figure 8 illustrates an embodiment of the filter F3 having two inductors and two capacitors. Also when the filter has plural LC pairs, as shown in Figure 8, the above described relation between the filter time constant and the rise time of the LO signal applies.

Claims: Claims 1. A mixer, comprising: a first terminal (70) and a second terminal (80) forming a first input port for receiving a first signal having a first frequency (f o); an second input port (40, 50) for receiving a second signal having a second frequency (f^); a mixer output port (100, 110) for a resulting signal; and a first group of valves (Q3, Q4) having their control inputs (233,243) coupled to the first terminal for receiving the first signal; a second group of valves (Q5, Q6) having their control inputs (253,263) coupled to the second terminal for receiving the first signal; and a third group of valves (Ql, Q2) having their control inputs coupled for receiving the second signal; said valves (Ql, Q2, Q3, Q4, Q5, Q6) co-operating such that in operation the mixer produces the resulting signal responsive to the first and second signals; characterized in that 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 (Q5, Q3, Q6, Q4). 2. The mixer according to claim 1, wherein the inductor is a passive component having a resistance and an inductance. 3. The mixer according to claim 1 or 2, wherein at least one inductor is connected between the first terminal and the control inputs (233,243) of the first group of valves (Q3, Q4). 4. The mixer according to claim 3, wherein at least one inductor is connected between the second terminal and the control inputs (253,263) of the second group of valves (Q5, Q6). 5. The mixer according to any of claims 1 to 4, wherein a capacitor (320) is connected between the control inputs (233,243) of the first group of valves (Q3, Q4) and the control inputs (253,263) of the second group of valves (Q5, Q6). 6. The mixer according to any of claims 1 to 4, wherein each one of the valves in the first and second group of valves (Q3, Q4 Q5, Q6) has its control input (233, 243, 253,263) connected to the second terminal via an inductor (230, 240, 250, 260). 7. The mixer according to claim 6, wherein the junction of the valve control input (233, 243, 253,263) and the inductor (230, 240, 250, 260) is connected to one terminal of a capacitor (235, 245, 255, 265). 8. An electronic circuit, comprising: a mixer according to any of claims 1 to 7; and an oscillator (90) having a first output terminal and a second output terminal forming an oscillator output port which, when the circuit is in operation, delivers an oscillating signal having a certain rise time; wherein the oscillator output port is connected to the first input port of the mixer; and the time constant of the low pass filter has a value in the range from 0,2 to 10 times said certain rise time. 9. A receiver comprising a mixer according to any of claims 1 to 7.

Inventors: ARNBORG TORKEL, NYSTROEM CHRISTIAN, ARNBORG, TORKEL, NYSTROEM, CHRISTIAN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON (PUBL)
Application year: 2001
Main classification: H04B1
IPC codes: H03D7