Abstract:
A two transistor mixer for mixing radio frequency signals in a communications device. A mixer core section comprised of a pair of matched semiconductor devices having their drain ports connected to a first common node through matched load resistors and their source ports connected to a second common node end. A local oscillator (LO) signal input connected to the first and second nodes, wherein the connection to the second node is through a current source. A radio frequency (RF) signal input connected to the gate ports of said pair of semiconductor devices and an intermediate frequency (IF) output obtained from the drain ports of said pair of semiconductor devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio frequency mixers and, more particularly, to balanced mixers. 
     2. Prior Art 
     In the context of a communication device, such as a mobile phone, there is a need to combine the RF input signal with a local oscillator signal to generate an intermediate frequency signal for filtering. This function is performed in a mixer and it is this circuit component to which this application is directed. 
     A singly balanced mixer is shown in FIG. 3, and includes a differential pair of transistors connected to a collector of an RF-amplifier transistor. The local oscillator signal is connected to the bases of the differential switching pair and the RF input is connected to the base of the common emitter amplifier transistor. The RF-amplifier converts input RF-voltage to current at its collector. LO switching transistors alternate the routing of this current to either of the loads. 
     FIG. 4 shows a double balanced mixer, commonly known as a Gilbert cell mixer. A differential RF transistor pair converts differential RF input voltage to differential current. Two sets of LO switching transistor pairs are used to alternate the routing of the current at the output. The mixer operates as a sign-switcher. The double balanced mixer was designed, among other things, to eliminate the need, in the case of the singly balanced mixer, to cancel out. the local oscillator component from the IF signal. The proliferation of components over the singly balanced mixer, however, creates a sensitive matching challenge. For optimum performance, double balanced mixers require that the component pairs, be matched as close as possible in order to maximize port to port isolation and to reduce undesirable distortion. Conventional integrated double balanced mixer circuits may need compensation for any imbalance by either electrical means or even laser tuning. 
     It is a purpose of this invention to design a mixer having many of the attributes of the double balanced mixer, while minimizing matching problems. This is accomplished with a reduction in the number of components. 
     SUMMARY OF THE INVENTION 
     A mixer is provided for mixing radio frequency signals in a communications device. A single pair of matched transistors comprise the core of the mixer. Each of the transistors have their drain port connected to the in phase local oscillator signal(LO) through matching resistance loads and their source port connected to a reverse phase local oscillator signal(−LO) through a resistor acting as a current source. No DC bias is applied between such ports. The gate ports of each of the transistors in the pair receive the differential radio frequency signal. The intermediate frequency output is obtained from the drain ports of the matched transistors. In the positive half cycle of the local oscillator signal the mixer will operate as a differential pair having inverting power gain, while in the negative half cycle of the local oscillator signal, the mixer circuit will operate as two source followers, i.e. two non-inverting amplifiers having current gain. In this manner a mixer is provided with port to port isolation, low noise, and high linearity. This is accomplished with only two sets of matched components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a mobile station and a cellular communication system to which the mobile station is bidirectionally coupled through a wireless RF link; 
     FIG. 2 is a block diagram of the mobile station shown in FIG. 1 that is constructed and operated in accordance with this invention; 
     FIG. 3 is a circuit diagram of a typical singly balanced mixer; 
     FIG. 4 is a circuit diagram of a typical double balanced mixer; and 
     FIG. 5 is a circuit diagram of a two transistor balanced mixer illustrating one embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wireless user terminal or mobile station  10  is shown in FIGS. 1 and 2, in which the subject invention may be used. The mobile station  10  includes an antenna  12  for transmitting signals to and for receiving signals  10  from a base site or base station  30 . Base station  10  generally would include a base station sub-system (BSS) as well as a base transceiver station (BTS). For simplicity, these two components are collectively referred to simply as the base station  30 . The base station  30  is a part of a cellular network  32  that includes a mobile switching center (MSC)  34  or similar apparatus. The MSC  34  provides a connection to landline trunks when the mobile station  10  is involved in a call. 
     The mobile station includes a modulator (MOD)  14 A, a transmitter  14 , a receiver  16 , a demodulator (DEMOD)  16 A, and a controller  18  that provides signals to and receives signals from the transmitter  14  and receiver  16 , respectively. These signals include signaling information in accordance with the air interface standard of the applicable cellular system, and also user speech and/or user generated data. The air interface standard may be based on TDMA as used in the GSM system, although the use of this invention is not intended to be limited to a particular type of system. 
     The present invention could be used with any suitable type of radio telephone system or suitable electronic device. With general regard to GSM mobile stations and networks, reference can be had to “The GSM System for Mobile Communications”, by Michel Mouly and Marie-Bernadette Pautet, 1992, the disclosure of which is incorporated by reference in its entirety. 
     Controller  18  also includes the circuitry required for implementing the audio and logic functions of the mobile station. By example, the controller  18  may be comprised of a digital signal processor device, a microprocessor device, and various analog to digital converters, digital to analog converters, and other support circuits. The control and signal processing functions of the mobile station are allocated between these devices according to their respective capabilities. 
     A user interface may include a conventional earphone or speaker  17 , a conventional microphone  19 , a display  20 , and a user input device, typically a keypad  22 , all of which are coupled to the controller  18 . The keypad  22  includes the conventional numeric ( 0 - 9 ) and related keys (#,*)  22   a,  and other keys  22   b  used for operating the mobile station  10 . These other keys  22   b  may include, by example, a SEND key, various menu scrolling and soft keys, and a PWR key. The mobile station  10  also includes a battery  26  for powering the various circuits that are required to operate the mobile station. The mobile station  10  also includes various memories, shown collectively as the memory  24 , wherein are stored a plurality of constants and variables that are used by the controller  18  during the operation of the mobile station. Certain TDMA timing related parameters that are transmitted from the base station  30  to the mobile station  10  are typically stored in the memory  24  for use by the controller  18 . It should be understood that the mobile station  10  can be vehicle mounted, handheld, or a stationary device. It should be further appreciated that the mobile station  10  can be capable of operating with one or more air interface standards, modulation types, and access types, and may thus be dual (or higher) mode device. 
     The receiver  16  also includes circuitry required for implementing the well known process of multiplexing a signal with a periodic signal to obtain a new center frequency, i.e., mixing. Mixing generally occurs immediately before multiple filter stages and receives the radio frequency (RF) signal, combines it with a periodic signal, the local oscillator (LO) signal to obtain an intermediate frequency (IF) signal which is filtered. 
     An embodiment of the two transistor mixer circuit  20  incorporating features of the present invention is shown in FIG.  5 . As seen in FIG. 5 the circuit comprises a pair of active semiconductor devices  21  and  22 . Semiconductors  21 and  22  are shown as a pair of transistors such as JFETS. However any suitable pair of matched semiconductors could be used, e.g., BJT, or MOSFETs. 
     The gates  23  and  24  of transistors  21  and  22  respectively are the differential signal input paths for the RF signals. The LO signal is received at node  27  and is passed to drains  25  and  26  of transistors  21  and  22  respectively, through matched load resistors R L1  and R L2  Source terminals  28  and  29  of transistors  21  and  22  respectively are connected to receive the LO signal through the current source resistor R s . The differential output for the IF signal is taken from drains  25  and  26 . 
     The transistors  21  and  22  of mixer  20  will operate in a reversible bias mode depending upon the half cycle of the LO signal. The supply voltage for this transistor pair  21  and  22  is a large LO AC signal. For positive half cycles of the LO signal, the transistor terminals  25  and  26  connected to their respective loads R L1  and R L2  are at a higher potential than the terminals  28  and  29  connected together and to the current source resistor R s . The circuit operates as a differential pair having inverting power gain. In this part of the LO signal cycle, the terminals  25  and  16  connected to the loads R L1  and R L2  are identified as the drains, while the terminals  28  and  29  are identified as the sources. For negative LO half cycles the biasing of the circuit is reversed and the operation is different. Now the transistor terminal designations are interchanged i.e., the terminal previously identifiable as a drain is now the source. Therefore, during the negative half cycle of the LO signal, mixer circuit  20  operates as two source followers, that is, two non-inverting amplifiers having current gain. At the IF outputs  30  and  31  there is present an amplified RF signal having its phase inversion triggered by the LO signal. 
     Thus, the mixer circuit  20  performs the function of a double balanced mixer, in the sense that it operates as a sign-switcher and the LO-signal is balanced out from both the RF and IF ports. The RF-signal is not balanced out from the IF output unlike in a true double balanced mixer. In addition, in the two transistor mixer of this invention, there can be some difference in gain between differential pair and dual source-follower modes. The mixer function is accomplished with fewer matched parts, namely, one matched pair of transistors and one matched pair of resistors. Additionally, compared to most known double balanced mixers the present invention does not have dedicated LO switch devices which in turn makes the present invention less noisy than conventional mixers. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For example, all or some of the resistors, shown in FIG. 5, could be replaced by inductor or resonant circuits. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.