Abstract:
A mixer includes a first field effect transistor (FET) having a gate that receives a first signal of a balanced local oscillator (LO) signal, a first source/drain coupled to a ground voltage, and a second source/drain; and a second FET having a gate that receives a second signal of the balanced LO signal, a first source/drain that floats, and a second source/drain connected to the second source/drain of the first FET to form a mixing node, the second signal being out of phase with the first signal. A diplexer is connected between the mixing node and each of a radio frequency (RF) port and an intermediate frequency (IF) port. A first LO leakage caused by the first FET is substantially canceled by a second LO leakage caused by the second FET at the mixing node.

Description:
BACKGROUND 
   Frequency mixers may be included in many types of electronic systems. For example, frequency mixers in radio systems down convert a received radio frequency (RF) signal by combing the RF signal with a local oscillator (LO) signal. The combination of the RF signal and the LO signal yields an intermediate frequency (IF) signal, which has a frequency corresponding to a difference between the RF and LO signals. 
     FIG. 1  is a block diagram of a known mixer  100 , which includes LO-port  110  for inputting a LO signal, RF-port  120  for inputting a received RF signal and IF-port  130  for outputting an IF signal. The mixer  100  also includes a single grounded-source field effect transistor (FET)  112 , which may be a gallium arsenide field-effect transistor (GaAsFET), for example. FET  112  includes a gate connected to LO-port  110  for receiving the LO signal and a source connected to ground. 
   The mixer  100  also incorporates a diplexer  115 , which performs frequency separation to enable RF and IF signals to be received and sent on different frequencies. The diplexer includes capacitor C 1  connected to RF-port  120  and inductor L 1  connected to IF-port  130 . The mixer  100  is generally capable of low conversion loss and a low noise figure over its frequency range, and requires little LO drive power. Isolation of LO-port  110  is provided by the mixing FET  112 . However, FET  112  leaks LO energy at frequencies where parasitic capacitance between the gate and drain (Cgd) of FET  112  is significant. The LO-port isolation may therefore not be sufficient for system requirements. 
   Efforts to improve LO-port isolation (and to reduce LO energy leakage) have included the addition of baluns, which isolate a single, unbalanced input line and provide a corresponding balanced output, consisting of two output lines carrying out of phase signals. For example,  FIG. 2  is a block diagram of conventional mixer  200 , which includes balun  216  on LO-port  110  and balun  218  on IF-port  130 .  FIG. 3  is a block diagram of another conventional mixer  300 , which includes balun  316  on RF-port  120  and balun  318  on IF-port  130 . For each balun, an unbalanced signal is carried on two signal lines, one of which is tied to ground, and the balanced signal may be carried on three signal lines, one of which is tied to ground via a center tap (not shown) and the remaining two of which carry electrical signals having equal amplitudes, but opposite phases. 
   Although LO-port isolation may be improved by the configurations of mixers  200  and  300 , the baluns  216 ,  218  or the baluns  316 ,  318  respectively increase cost and size of the circuit, and cause additional loss. Further, when balun  316  is used on RF-port  120 , the noise figure degrades in a down converter application. Therefore, balun  316  usually must be realized as a passive structure, in an attempt to minimize impact on the noise figure. Also, when balun  316  is used on RF-port  120  or balun  218  and/or  318  is used on IF-port  130 , conversion loss is elevated. For example, with respect to IF-port  130 , the low frequencies usually require that baluns  218 ,  318  be realized as coil-core transformers, which are typically relatively large and expensive. 
   SUMMARY 
   In a representative embodiment, a mixer includes a first transistor and a second transistor. The first transistor includes a gate operative to receive a first signal of a balanced local oscillator (LO) signal, a first source/drain coupled to ground, and a second source/drain. The second transistor includes a gate operative to receive a second signal of the balanced LO signal, a first source/drain that is floating, and a second source/drain connected to the second source/drain of the first transistor. Leakage of LO energy from the second transistor substantially cancels leakage of LO energy from the first transistor. 
   In another representative embodiment, a mixer includes a balun connected to a local oscillator (LO) port of the mixer. The mixer also includes a first transistor comprising a first gate coupled to a first output of the balun and a first source coupled to a ground voltage; and a second transistor comprising a second gate coupled to a second output of the balun and a floating second source operative to increase isolation of the LO-port to a diplexer of the mixer. 
   In another representative embodiment, a mixer includes a first field effect transistor (FET) having a first gate that receives a first signal of a balanced local, oscillator (LO) signal, a first source coupled to a ground voltage, and a first drain. The mixer further includes a second FET having a second gate that receives a second signal of the balanced LO signal, a second source that floats, and a second drain connected to the first drain to form a mixing node. The second signal has a phase opposite to a phase of the first signal. A diplexer is connected between the mixing node and each of an RF-port and an IF-port. A first LO leakage caused by the first FET is substantially canceled by a second LO leakage caused by the second FET at the mixing node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The example embodiments are best understood from the following detailed description when read with the accompanying figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
       FIG. 1  is a block diagram illustrating a conventional mixer. 
       FIG. 2  is a block diagram illustrating a conventional mixer. 
       FIG. 3  is a block diagram illustrating a conventional mixer. 
       FIG. 4  is a block diagram illustrating a mixer according to a representative embodiment. 
       FIG. 5  is a graph illustrating energy loss characteristics of a conventional mixer. 
       FIG. 6  is a graph illustrating energy loss characteristics of a mixer, according to a representative embodiment. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
     FIG. 4  is a block diagram illustrating a mixer, according to a representative embodiment. A mixer  400  includes a mixing transistor, FET  412 , and a companion transistor to the mixing transistor, FET  413 . Both FET  412  and FET  413  may be GaAs FET transistors, for example, although other types of FETs (and/or other types of transistors) within the purview of one of ordinary skill in the art may be incorporated into mixer  400 , without departing from the spirit and scope of the present teachings. For example, the mixing transistor and the companion transistor may be high electron mobility transistors (HEMTs), pseudomorphic HEMTs, heterostructure FETs (HFETs), etc. 
   Also, FET  412  and FET  413  may vary in size, indicated for example by total gate width, although the particular size of each transistor may be adjusted to provide unique benefits for any particular situation or to meet various design requirements. For example, in an illustrative embodiment, FET  412  may be 60 μm in total gate width and FET  413  may be 24 μm in total gate width. 
   Gates of FET  412  and FET  413  are respectively connected to receive balanced signals based on an LO signal input through LO-port  410 . The balanced signals received by FETs  412 ,  413  may have the same amplitudes, but different phases. For example, the signal input to the gate of FET  413  may be 180 degrees out of phase with the signal input to the gate of FET  412 . The LO signal from LO-port  410  may be naturally balanced, in which case the already balanced LO signal(s) may be directly input to the gates of FETs  412 ,  413 , respectively. However, the LO signal from LO-port  410  is usually an unbalanced signal, in which case the LO signal must be subjected to a balance function to generate the balanced signals input to the gates of FETs  412 ,  413 . 
   In various embodiments, the balanced signals may be provided by any component capable of generating or producing them. In the depicted representative embodiment, the gates of FETs  412 ,  413  are connected to first and second output lines of balun  416 , to receive the balanced LO drive signals. A third line or center tap (not shown) of balun  416  may be connected to ground. A first input line of balun  416  is connected to LO-port  410  for receiving the unbalanced LO signal, and the second input line of the balun  416  is connected to ground. 
   Alternatively, balun  416  may provide the balanced signals with no grounded center tap, in which case balun  416  simply acts as a differential circuit, having a virtual point of balance. In other embodiments, any similar component that provides an appropriate balance function may be substituted for balun  416 . Also, as stated above, if the LO-port  410  generates a naturally balanced signal, there is no need for balun  416  or other component to perform the balance function. 
   Balun  416  may be realized as a transformer, for example, and is represented as a transformer in  FIG. 4 , for convenience of explanation. Because LO-port  410  does not substantially impact conversion loss or noise figure, balun  416  may be a small, relatively inexpensive balun. The size of balun  416  depends, for example, on the frequency of operation of the mixer  400 . Also, balun  416  may be implemented as a stand-alone component, inserted between the LO-port  410  and the gates of FETs  412 ,  413 , thus being easily and inexpensively incorporated into the functionality of mixer  400 . The circuit of mixer  400  benefits from the gain of the active balun  416 , which effectively helps reduce the LO drive requirement of the mixer  400 . 
     FIG. 4  depicts a drain of FET  412  connected to a drain of FET  413 , forming mixing node  414  in which the LO signal received through LO-port  410  is mixed with an RF signal received through RF-port  420 . A source of FET  412  is shown connected to ground. However, a source of FET  413  is not grounded, allowing it to float, thus creating a single-ended mixing structure. Accordingly, FET  413  does not substantively contribute to the actual mixing operation. However, FET  413  does introduce LO leakage into the mixing node  414 . 
   It is understood that the terminals of FETs  412 ,  413  are designated sources and drains in  FIG. 4  for convenience of explanation. As such, in various embodiments, the sources and drains may be reversed, without departing from the spirit and scope of the present teachings. For example, FET  413  may have a floating drain, as opposed to a floating source, and the mixing node  414  may be formed by connected sources of FET  412  and  413 . 
   Since the gate of FET  413  is driven by a signal that is out of phase with the signal driving the gate of FET  412 , the leakage from FET  413  tends to cancel the leakage from FET  412  at the mixing node  414 . The scope of leakage cancellation depends, in part, on the relative sizes of FET  412  and FET  413 , which may be selected such that the amplitude of leakage can be matched between FET  412  and FET  413  to reduce or eliminate the LO-port leakage of the mixer  400 . For example, as stated above, FET  412  may be substantially larger than FET  413 , e.g., FET  412  may be 60 um and FET  413  may be 24 um in an illustrative embodiment. 
   The mixing node  114  is connected to diplexer  415 , which connects the mixing node  114  with RF-port  420  and IF-port  430 , for outputting the intermediate frequency based on mixing the LO signal and the RF signal. For example, the diplexer  415  includes capacitor  422  connected to RF-port  120  and inductor  432  connected to IF-port  130 . The diplexer  415  performs frequency separation to enable RF and IF signals to be received and sent on different frequencies. Various realizations of the diplexer  415  may be included in mixer  400  without affecting the spirit and scope of the description. For example, the values of the capacitor  422  and the inductor  432  may be determined in a known manner in accordance with the specific RF and IF frequencies involved in the mixing operation. Also, the inductor  432  may have a specified quality factor (INDQ), for example, given at a specified frequency. 
   The diplexer  415  may also be realized by a network of arbitrary complexity for the purpose of enhancing the selectivity of the diplexer and enabling the passage of RF and IF signals of arbitrarily close frequency proximity. The diplexer  415  may also be realized as a directional coupler or circulator. For example, the RF and IF signals propagate in opposite directions to each other in that the RF input signal travels toward the mixing node  414  whereas the IF output signal travels away from the mixing node  414 . As such, the RF and IF signals are amenable to separation by apparatuses that separate forward and reverse traveling waves, such as directional couplers and circulators. 
   Accordingly, mixer  400  receives an LO signal through LO-port  410 , which is to be mixed with an RF signal received through RF-port  420  to output a desired IF signal through IF-port  430 . A balance function is performed on the received LO signal (assuming the LO signal is initially unbalanced), such that balanced, out of phase LO signals are input to the gates of FET  412  and FET  413 , respectively. Corresponding LO energy leaks into the mixing node  114  from both FET  412  and FET  413 . However, because the balanced input signals are out of phase, the leakage from FET  413  substantially cancels the leakage from FET  412 . This cancellation, in turn, reduces LO energy leakage from RF-port  420  and improves isolation of LO-port  410  from RF-port  420 . 
   For comparison purposes,  FIG. 5  provides a graph illustrating energy lost in a conventional mixer from an RF-port due to LO leakage in the mixing node. For example,  FIG. 5  may indicate energy lost from the conventional mixer  100  depicted  FIG. 1 . The vertical axis of  FIG. 5  shows LO to RF isolation, which indicates isolation of the LO-port (e.g., LO-port  110 ) from the RF-port (RF-port  120 ), in decibels (dB). The horizontal axis shows frequency of the LO signal input through the LO-port in Gigahertz (GHz). 
   The curve of  FIG. 5  shows a substantial loss of LO energy (through the RF-port) as the frequency of the LO signal increases. For example when the frequency of the LO signal is 13 GHz, the energy output from the RF-port is 20 dB below the LO energy incident at the LO-port, and the isolation is said to be 20 dB. This indicates significant leakage in the mixing node. 
   In comparison,  FIG. 6  is a graph illustrating energy lost in a mixer, according to a representative embodiment, from an RF-port due to LO leakage in the mixing node. For example,  FIG. 6  may indicate energy lost from the mixer  400  depicted in  FIG. 4 . The vertical axis of  FIG. 6  shows isolation of the LO-port (e.g., LO-port  410 ) from the RF-port (RF-port  420 ) in dB and the horizontal axis shows the frequency of the LO signal input through the LO-port in GHz. 
   The curve of  FIG. 6  shows less loss of LO energy through the RF-port as the frequency of the LO signal increases, as compared to the curve illustrated in  FIG. 5 . For example when the frequency of the LO signal is 13 GHz, the energy output from the RF-port is about 30 dB below the LO energy incident at the LO-port, and the isolation is said to be 30 dB. There is therefore a 10 dB improvement in isolation at 13 GHz, relative to the isolation shown in the graph of  FIG. 5 . This indicates substantially less leakage in the mixing node, which is due to the leakage cancellation (e.g., by FETs  412  and  413 ) with respect to the balanced input signals. Accordingly, the curve of  FIG. 6  likewise shows that the isolation of LO-port  410  from RF-port  420  has increased. 
   In alternative embodiments, the mixer (e.g., mixer  400 ) may function as an up-converter. In such embodiments, the input signal is the IF signal and the signal that emerges from the RF port is the output signal. The output signal thus may be a sum frequency or a difference frequency, e.g., Frequency (LO)+Frequency (IF) or Frequency (LO)−Frequency (IF). 
   Further, the various embodiments likewise provide the same degree of improvement with respect to leakage from the LO-port (e.g., LO-port  410 ) to the IF-port (IF-port  430 ). Typically, though, IF-port leakage is less of a concern because in down converter applications, the IF-port is treated with a low pass filter (e.g., as part of the diplexer), which substantially rejects LO leakage. 
   In view of this disclosure it is noted that variant mixers can be implemented in keeping with the present teachings. Further, the various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.