Integral mixer and oscillator device

Abstract

A device for providing the functionality of an oscillator and mixer is disclosed herein. The device uses a differential pair to provide first and second filter networks with a time varying signal. The first filer network generates an oscillating signal through the use of a generated negative resistance and provides the oscillating signal while filtering out unwanted signals. The second filter network receives the time varying input signal and the oscillating signal and provides a mixed output while preventing the transmission of oscillations at the oscillating signal frequency. A double balanced embodiment is also disclosed.

Description

BACKGROUND

[0002] 1. Technical Field

[0003] This invention relates generally to the fields of Radio Frequency (RF) mixers and oscillators. More particularly, the invention relates to the integration of mixers and oscillators.

[0004] 2. Description of the Related Art

[0005] Oscillators are used to provide a time varying signal in electronic circuits and are well known to those of skill in the art. FIG. 1 illustrates a typical oscillator known in the art. This oscillator is a negative resistance cell 10. A simple differential pair of transistors Q1 and Q2 is shown. The emitters of both devices, e1 and e2, are connected, either by a short circuit or other connection, thereby defining a common node n1. Current source I1 is connected to the common node n1. In the most basic case the current source I1 is a direct current (DC) source, as shown.

[0006] CN1 and CN2 are symmetric coupling networks, which may consist of short circuits, capacitors, transistors, or other circuit elements alone or in combination. For instance, if CN1 is an RC network, then CN2 is preferably an identical RC network. Connecting coupling networks CN1 between b1 and c2, and CN2 between b2 and c1, as shown, generates a differential negative resistance between nodes a and a′, or between any other two symmetric differential points in circuit 10 such as between bases b1 and b2, or even between corresponding differential points inside networks CN1 and CN2. For instance, if CN1 has a first resistor, then by symmetry CN2 preferably has a corresponding second resistor, and a differential negative resistance can be found between one node of the first resistor in CN1 and another corresponding symmetric nod of the second resistor in CN2. If a connection is made between a and a′, such that the connection provides an impedance with a real component greater than or equal to the real value of the negative resistance, circuit 10 will oscillate. The frequency of the oscillation will be determined by the impedance connected between a and a′. The oscillating output signal is produced over the connection between a and a′. In frequency conversion applications, the desired output signal is typically used as a signal source for another circuit.

[0007] FIG. 2 illustrates a single balanced mixer 20, which is known in the art. A differential pair of transistors Q3 and Q4 is shown. The emitters e3 and e4 are connected in the same manner as those in FIG. 1, thereby defining common node n7. A current source I2 is connected to the common node n7. In this instance, the current source I2 has both a DC component (Idc) and a frequency component (IRF) at frequency fRF.

[0008] Unlike the negative resistance cell 10, there is no cross coupling. Instead a frequency source, S1, providing an LO (Local Oscillator) signal at frequency fLO, is connected between bases b3 and b4. This LO signal varies the amount of current flowing through each transistor Q3 and Q4 such that the differential component of the current, provided by collectors c3 and c4 when a load is connected between nodes d and d′, has frequency components at DC, fLO, and fLO±fRF. In frequency conversion applications, the desired output signal is typically one of the frequency translated components of IRF, at either fLO−fRF (down-converter) or fLO+fRF (up-converter).

[0009] Although known oscillators and mixers can be combined, as they are in frequency conversion applications, the result is often far from ideal. In most applications, the mixer and oscillator are separate circuits. This results in an increase in both component area and power consumption, which results in a device that is costly to manufacture and operate. Furthermore, the physical distance between oscillator and mixer circuits often results in leakage of the local oscillator signal to other circuits, which is disadvantageous, as one of skill in the art will appreciate.

[0010] It is, therefore, desirable to provide a device that provides the functionality of both an oscillator and a mixer, which decreases the required power and area required to implement. Additionally, it would be desirable to provide a device that reduces the leakage of the oscillator signal to other circuits.

SUMMARY

[0011] It is an object of the present invention to obviate or mitigate at least one disadvantage of previous mixer-oscillator arrangements. It is a further object of the present invention to provide a device with integral oscillation and mixing functionality.

[0012] In a first aspect, the present invention provides a device, which has a plurality of nodes, for generating both an oscillating output signal and a mixed output signal based on a time varying input signal. The mixed output signal represents a translation of the input signal by the oscillating output signal. The device has a differential pair of transistors for receiving the time varying input signals and for generating a differential pair output signal having a time varying input component and other components, and a first filter network that is connected to the differential pair for receiving the varying input signal component from the differential pair. The first filter network selectively generates a negative resistance at the desired oscillating frequency across two nodes connected to the first filter network, and selectively provides the oscillating output signal by providing an impedance across the two nodes. Once the oscillating signal is established, the outputs of the differential pair contain frequency components of both the oscillating signal and signal components caused by the mixing of the input signal with the oscillating signal in the differential pair. Connected to the first filter network and the differential pair is a second filter network. The second filter network receives the oscillating output signal, and receives the other signal components, generates, and selectively passes the mixed output signal.

[0013] In a further embodiment, the time varying input signal has both time varying and constant components. In other embodiments, the negative resistance is generated across nodes connected to the collectors of the differential pair, and the magnitude of the resistance is proportional to the magnitude and frequency of the time varying input signal. Alternate embodiments include means for generating the mixed output signal by translating the oscillating output signal by the frequency of the time varying signal, this translation can be both positive and negative.

[0014] In a further embodiment of the present invention, the device has a second differential pair of transistors for receiving a second time varying input. The second differential pair provides this input to the first and second filter networks. The first filter network generates a negative resistance across two pairs of nodes in parallel with each other and the second filter network cancels output signals that have the frequency of the oscillating output signal. In alternate embodiments the time varying signals are out of phase with each other, and the overall negative resistance is substantially constant. In various embodiments, the first and second networks can be either high or low pass filters, either of which can be tuneable. In yet another embodiment, there is an input circuit that generates the out of phase time varying input signals using a differential pair of transistors connected to a constant signal, and a time varying signal and a biasing signal.

[0015] In a further aspect, the present invention provides a device for receiving a time varying input signal and for generating both an oscillating output signal, and a mixed output signal, the mixed output signal representing a translation of the oscillating output signal, the device having a plurality of nodes. The device has first and second differential pairs of transistors, each pair receiving a time varying input signal. A first filter network is operatively connected to the differential pairs and receives from them the two time varying input signals. These input signals are used to generate a negative resistance across two pairs of nodes connected in parallel to the first filter network. The first filter network selectively provides the oscillating output signal by providing an impedance across the parallel negative resistance. A second filter network is operatively connected to the two pairs of nodes, for receiving the oscillating output signal, and is also operatively connected to the differential pairs for receiving the time varying signals. The second filter network selectively passes the mixed output signal. In alternate embodiments of the present invention the time varying input signals are out of phase, the first filter network is a tuneable high pass filter, and the second filter network is a low pass filter.

[0016] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the present invention will now be described, by way of example only, with reference to the attached figures, wherein:

[0018] FIG. 1 is a schematic diagram showing an oscillator of the prior art;

[0019] FIG. 2 is a schematic diagram showing a mixer of the prior art;

[0020] FIG. 3 is a schematic diagram showing an exemplary single balanced mixillator, according to an embodiment of the claimed invention;

[0021] FIG. 4 is a schematic diagram showing an exemplary dual balanced mixillator according to a further embodiment of the claimed invention;

[0022] FIG. 5 is a schematic diagram showing an exemplary HPF (High Pass Filter) suitable for use as a filter network in a frequency down conversion application of the mixillator of FIG. 4;

[0023] FIG. 6 is a schematic diagram showing an exemplary LPF (Low Pass Filter) suitable for use as a filter network in a frequency down conversion application of the mixillator of FIG. 4; and

[0024] FIG. 7 is a schematic diagram showing an exemplary input stage suitable for use as the two current sources for the dual balanced mixillator of FIG. 4.

[0025] Same reference numerals are used in different figures to denote similar elements.

DETAILED DESCRIPTION OF THE DRAWINGS

[0026] Generally, the present invention provides a system for generating a mixed output signal and an oscillating output signal based on a time varying input signal. The mixed output signal is the oscillating signal translated by a time varying factor of the input signal. A dual balanced embodiment of the device that makes use of a differential pair to provide out of phase input signals is also disclosed. The device provides the functionality of both an oscillator and a mixer, which decreases the required power and area required to implement. Additionally it reduces the leakage of the oscillator signal to other circuits.

[0027] FIG. 3 illustrates an embodiment of the present invention. The device 30 comprises a circuit with both oscillator and mixer functionality. Device 30, which is preferably referred to as a “mixillator”, includes a simple differential pair of transistors Q5 and Q6, as well as two filter networks FN1 and FN2, for generating an oscillating output signal, LO, and mixing LO with an external input signal to produce an output mixed signal

[0028] As with the separate oscillator 10 and mixer 20 of FIGS. 1 and 2, circuit 30 has a differential pair of transistors, Q5 and Q6, connected at the emitters e5 and e6 thereby defining common node n11. A current source I3, which has both DC (Idc) and time varying or RF (IRF) components is connected to the common node n11. Connected to the collectors c5 and c6 of Q5 and Q6, are two filter networks, FN1 and FN2. FN1 is further connected to bases b5 and b6 of Q5 and Q6. FN2 provides a differential output between nodes f and f′ which corresponds to the output of a conventional mixer driven by a conventional oscillator.

[0029] To understand the operation of device 30, consider the circuit of FIG. 3 in the absence of FN2, where IRF=O. Under these conditions, FN1 connects b5 and b6 to c5 and c6. Thus FN1 is analogous to a combination of CN1 and CN2 of FIG. 1, which connect b1 to c2 and c1 to b2 respectively. In this configuration, it is apparent that FN1 must provide a negative resistance across two nodes connected to FN1. These nodes can be internal to FN1, as were the nodes in the prior art example, or external nodes such as n12 and n13 or n14 and n15. In the presently illustrated embodiment, FN1 provides a negative resistance across nodes n14 and n15. A negative resistance must also be seen at n12 and n13, although it has a different value.

[0030] To further understand FN1, consider the circuit of FIG. 3 in the absence of FN2 and where IRF is non-zero. The single balanced mixer 20 of FIG. 2, provides current at the output d/d′ of the circuit 20, the current having components at DC, fLO, and fLO±fRF. Therefore, in the presence of an RF component, it is no longer sufficient for FN1 to simply generate a negative resistance at the LO frequency, as was done in the prior art. In the embodiment of FIG. 3, FN1 still generates a negative resistance at the desired oscillator frequency fLO, but FN1 preferably does not generate a negative resistance at the desired mixer output frequencies, fLO−fRF or fLO+fRF. FN1 provides negative resistance, and thus an oscillating output, between b5 and b6 at fLO, but does not provide an oscillating output at the mixer output frequencies of fLO−fRF or fLO+fRF. This frequency selectivity in FN1 provides oscillator functionality in the presence of an RF component in the source current. If the fLO signal is to be used by other components, such as a PLL (Phase Locked Loop), the circuit should not be heavily loaded, and should be buffered by use of signal followers or other known techniques to isolate the circuit from the PLL. If it is desired, a pure oscillator signal can be obtained across nodes n14 and n15. This signal can be provided to other circuits in addition to the use described below.

[0031] The role of FN2 in circuit 30 is now considered. FN2 is complimentary to FN1 to integrate the mixer functionality. FN2 selectively passes the desired mixer output components, fLO−fRF or fLO+fRF, to the differential output between f and f′. Additionally, FN2 blocks the oscillation of the input components, at the desired oscillator frequency fLO, from transmission by the differential output. In this manner FN2 allows device 30 to provide mixer functionality integral with oscillator functionality in a single circuit.

[0032] Advantageously, in device 30, the same DC current, Idc, provides a bias for both the mixer and oscillator functions, which provides power, cost, and component area savings. Furthermore, the proximity between oscillator function and mixer function achieved via integration of these functions reduces leakage of the local oscillator signal to other circuits.

[0033] The concepts taught by example in FIG. 3 are adapted for use in a double balanced circuit 40 in FIG. 4. In this embodiment, the device 40 consists of two differential pairs, the pair formed by Q7 and Q8, and the pair formed by Q9 and Q10. The emitters of Q7 and Q8, e7 and e8 respectively, are connected to common emitter n21, which is connected to current source I4. The emitters of Q9 and Q10, e9 and e10 respectively, are connected to common emitter n23, which is connected to current source 15. The current source I4 for the Q7 and Q8 pair contains components Idc+IRF, while the current source I5 for the Q9 and Q10 pair contains components Idc−IRF.

[0034] FN3, a filter network analogous to FN1 of device 30, connects the collectors (c7, c8, c9, and c10) in pairs so as to generate a frequency selective negative resistance, analogous to the one described above in reference to FIG. 3. This frequency selective negative resistance is used in the oscillator function of device 40. Collectors c7, c8, c9 and c10 are connected to nodes n28, n26, n27 and n29 of FN3 respectively. Bases b7 and b10 are connected to the common connection point n25 whereas bases b8 and b9 are connected to the common connection point n24, both n25 and n24 are connected to FN3.

[0035] FN3 preferably connects nodes n28 and n29 with node n24 thereby connecting collectors c7 and c10 with bases b8 and b9 respectively, as well as preferably connecting nodes n26 and n27 with node n25 thereby connecting collectors c8 and c0 with bases b7 and b10 respectively. In so doing, FN3 provides two negative resistance cells in parallel. The first negative resistance cell is provided by connecting collectors c8 and c7 of transistors Q8 and Q7 to bases b7 and b8 respectively. This connection provides a first cell impedance between bases b7 and b8. The second negative resistance cell is provided by connecting collectors c9 and c10 of transistors Q9 and Q10 to bases b10 and b9 respectively. This second connection creates a second cell impedance between bases b9 and b10. The two negative resistance cells are connected in parallel because bases b7 and b8 of the first negative resistance cell are connected to bases b10 and b9 respectively of the second negative resistance cell.

[0036] A connection between any of n28 and n26, n29 and n27, n24 and n25, and various nodes internal to the FN3 network, that has an impedance with a real component larger than the effective negative resistance provided by the parallel arrangement of the negative resistance cells causes circuit 40 to oscillate. As before the frequency of the oscillation is determined by the impedance of the connection. By selecting the impedance of the connection appropriately, circuit 40 provides an oscillating signal with the desired frequency. FN3 is connected to the rest of circuit 40 via nodes n24, n25, n26, n27, n28 and n29. The value of the negative resistance created by FN3 is dependant upon the source current. In device 40, FN3 is driven by I4 and I5, whereas circuit 30 is driven by only I3. I4=Idc+IRF, while I5=Idc−IRF, so the net current provided, I4+I5, is (Idc+IRF)+(Ic−IRF) or 2Idc. Because the negative resistance is dependant upon the source current, the overall negative resistance provided by the parallel negative resistance cells remains substantially constant over variations in IRF. This is an advance over the novel device 30 as illustrated in FIG. 3. Knowing that the impedance of the parallel negative resistance cells is substantially constant, all that must be done to provide oscillator functionality to circuit 40 is to have FN3 provide an impedance that has a real component greater than the real component of the parallel negative resistance cells.

[0037] Advantageously, circuit 40 retains features of a traditional oscillator function as the oscillator output can be taken between bases b7 and b8, bases b9 and b10, or nodes n24 and n25.

[0038] FN4, a filter network analogous to FN2 of FIG. 3, is used to selectively pass the desired output frequencies to the mixer output between g and g′, while rejecting components at the desired oscillator frequency. FN4 is connected to circuit 40 via nodes n30, n31, n32, n33, n34, and n35 which are connected to collectors c7, c8, c9 and c10, and mixer outputs g and g′, respectively.

[0039] Advantageously, the circuit 40 retains features of a traditional double balanced mixer function, with the mixer output available between nodes g and g′. With a double balanced topology such as that shown in FIG. 4, FN4 can be designed such that the components at the LO frequency cancel, thereby rejecting the component at the LO frequency, to facilitate selectively passing the desired output components, as will be described in greater detail below in reference to FIG. 6.

[0040] FIG. 5 shows an exemplary HPF (High Pass Filter) 50a suitable for use as a filter network in a frequency down conversion application of the device 40 as illustrated in FIG. 4. Capacitors X1, and X2 are connected in series and are arranged to define node n28 connected to the first capacitor X1, node n24 common to capacitors X1 and X2 and node n29 connected to capacitor X2. Capacitors X3, and X4 are connected in an analogous fashion defining node n26 connected to capacitor X3, node n25 common to capacitors X3 and X4, and node n27 connected to capacitor X4. Variable capacitor X5 and inductor L1 are connected in parallel between nodes n25 and n24.

[0041] Capacitors X1, X2, X3 and X4, when connected as described herein and depicted in FIG. 5, provide cross coupling to generate a negative resistance necessary for oscillator functionality in device 40. The capacitive coupling creates a high pass network and ensures that the generated negative resistance is seen only at high frequencies, which is desirable for frequency down-conversion applications. The combination of X5 and L1 form an exemplary tank circuit that can be used to generate sufficient real impedance to support oscillation. Additionally, the variable nature of X5 provides means for tuning the oscillator frequency.

[0042] FIG. 6 shows an exemplary LPF (Low Pass Filter) 60a suitable for use as a filter network in a frequency down conversion application of the circuit 40 as illustrated in FIG. 4. Inductors L2 and L4 are connected in series and are arranged to define node n30 connected to inductor L2, node n34 common to inductors L2 and L4 and node n32 connected to inductor L4. Inductors L3, and L5 are connected in an analogous fashion defining node n31 connected to inductor L3, node n35 common to inductors L3 and L5, and node n33 connected to inductor L5.

[0043] At low frequencies LPF 60 operates to connect c7 to c9 and c8 to c10 as typically found in a double balanced mixer. These connections allow LPF 60 to cancel the RF and LO frequency components at the output, while selectively passing the RF−LO frequency component.

[0044] Referring to FIG. 7, an input stage circuit 70 is illustrated. Input stage circuit 70 is used, in one embodiment, to provide input currents for the double balanced device 40 of FIG. 4. A differential pair of transistors Q11 and Q12 is shown. An RF signal source S2 drives a radio frequency signal I7∝IRF between bases b11, and b12, with output at nodes h and h′.

[0045] To reduce the noise contributions of the input differential pair biasing for Q11 and Q12 is provided in this embodiment. The biasing is such that at low frequencies it presents low impedance, in order to minimize noise appearing at b11 and b12. This noise, if unimpeded, modulates the oscillator and appears as phase noise at the oscillating nodes (bases b7 and b8, bases b9 and b10, or other nodes internal to FN3).

[0046] Inductors L6 and L7 are connected in series between bases b11 and b12 to provide at low frequencies, typically from DC to 5 or 10 MHz, low impedance which reduces phase noise. At high frequencies, typically near RF, L6 and L7 provide high impedance to avoid shunting the desired RF signal away from the bases. A bias voltage Vbias is provided between inductors L6 and L7 to satisfy the biasing requirements of both Q11 and Q12.

[0047] If linearity requirements are such that degenerative impedance is suggested, inductors L8 and L9 connected in series between emitters e11 and e12 are used for the same reasons as L6 and L7 above. However, if degenerative impedance is not suggested, then inductors L8 and L9 can be replaced by short circuits. A DC source, I8=2*Idc, is connected between inductors L8 and L9.

[0048] The two output signals, I9=Idc+IRF and I10=Idc−IRF, are provided by collectors c11 and c12 of transistors Q11 and Q12, at nodes h and h′ respectively. Circuit 70 is therefore used in a presently preferred embodiment to provide source I4 and I5 for device 40 by connecting nodes h and h′ of input stage circuit 70 to nodes n21 and n23 of device 40, respectively.

[0049] Although not expressly shown in the drawings, any type of transistor can be used, including but not limited to Field Effect Transistors (FET's), GaAs devices, PHemt, and the like. Furthermore, although NPN transistors are used in the drawings, it would be obvious to a person skilled in the art to use complementary devices and invert the structure illustrated, or to use both non-complementary and complementary devices. Further still, current sources can be realized in different ways such as by using resistors and inductors and substitution of one current source for another will be obvious to a person skilled in the art. Also, although the drawings illustrate using a HPF for FN3 and a LPF for FN4 in a frequency down conversion application, it would be obvious to a person skilled in the art to invert their roles and use a LPF for FN3 and a HPF for FN4 in a frequency up conversion application. It Would be obvious as well to a person skilled in the art to use voltage signals instead of or in conjunction with the current signals illustrated. Other types of filter structures can be used, and the use of alternate filter structures will also be obvious to a person skilled in the art. Other types of filters can lead to other applications, which are also considered to be within the scope of the invention.

[0050] The above-described embodiments of the present invention are intended to be examples only. Those of skill in the art may effect alterations, modifications and variations to the particular embodiments without departing from the scope of the invention, which is defined solely by the claims appended hereto.

Claims

1. A device for receiving a time varying input signal and for generating both an oscillating output signal and a mixed output signal, the mixed output signal representing a translation of the oscillating output signal, the device comprising: (a) a differential pair of transistors for receiving the time varying input signal and for generating a differential pair output signal having a time varying input signal component and other signal components; (b) a first filter network, operatively connected to the differential pair for receiving the varying input signal component from the differential pair, for generating a negative resistance across two nodes connected to the first filter network, and for selectively providing the oscillating output signal by providing an impedance across the two nodes; and (c) a second filter network, operatively connected to the two nodes, for receiving the oscillating output signal, and operatively connected to the differential pair for receiving the other signal components, for generating and selectively passing the mixed output signal.

2. The device of claim 1, wherein the input signal has time varying and constant components.

3. The device of claim 1, wherein the magnitude of the negative resistance is proportional to the magnitude and frequency of the time varying input signal.

4. The device of claim 1, wherein the first filter network generates the negative resistance across nodes connected to the collectors of the differential pair of transistors.

5. The device of claim 1, wherein the second filter network includes means for generating the mixed output signal by translation of the oscillating output signal by the frequency of the time varying signal.

6. The device of claim 1, wherein the mixed output signal has a frequency equal to the frequency of the oscillating output signal plus the frequency of the time varying input signal.

7. The device of claim 1, wherein the mixed output signal has a frequency equal to the frequency of the oscillating output signal minus the frequency of the time varying input signal.

8. The device of claim 1, further comprising: (d) a second differential pair of transistors for receiving a second time varying input signal, the second differential pair being operatively connected to the first and second filter networks for providing the second time varying input signal; wherein the first filter network creates a negative resistance across two pairs of nodes in parallel with each other, and wherein the second filter network cancels output signals at the frequency of the oscillating output signal.

9. The device of claim 8, wherein the first and second time varying signals have time varying and constant components.

10. The device of claim 9, wherein the first and second time varying signals are out of phase with each other.

11. The device of claim 8, wherein the overall negative resistance of the parallel negative resistances is substantially constant.

12. The device of claim 8, wherein the first filter network is a high pass filter.

13. The device of claim 12, wherein the high pass filter is tuneable.

14. The device of claim 8, wherein the second filter network is a low pass filter.

15. The device of claim 8, wherein the first filter network is a low pass filter.

16. The device of claim 15, wherein the low pass filter is tuneable.

17. The device of claim 8, wherein the second filter network is a high pass filter.

18. The device of claim 10, further comprising a input staging circuit operatively attached to the differential pairs for providing the time varying input signals, the input staging circuit having a differential pair of transistors operatively connected to a constant signal, a time varying signal and a biasing signal for generating the two out of phase time varying input signals.

19. A device for receiving a time varying input signal and for generating both an oscillating output signal, and a mixed output signal, the mixed output signal representing a translation of the oscillating output signal, the device comprising: (a) first and second differential pairs of transistors for each receiving the time varying input signal and for each generating a differential pair output signal having a time varying input signal component and other signal components; (b) a first filter network, operatively connected to the differential pairs for receiving the varying input signals from the differential pairs, for generating a negative resistance across two pairs of nodes connected in parallel to the first filter network, and for selectively providing the oscillating output signal by providing an impedance across the parallel negative resistances; and (c) a second filter network, operatively connected to the two pairs of nodes, for receiving the oscillating output signal, and operatively connected to the differential pairs for receiving the other signal components, for generating and selectively passing the mixed output signal.

20. The device of claim 19, wherein the time varying input signals are out of phase.

21. The device of claim 18, wherein the first filter network is a tuneable high pass filter, and the second filter network is a low pass filter.