Patent Publication Number: US-2009221258-A1

Title: Balanced Series Mixer for High-Frequency Signals

Description:
FIELD OF THE INVENTION 
     The present invention relates to a balanced series mixer, having a balun transformer that has a HF-source port and an antenna port on one side and two diode ports on the other side, two diodes which in each case are connected between one of the. diode ports and a HF-ground, and a useful-signal tap for a mixed product that is generated by the diodes and is made up of a HF-signal supplied via the antenna port and a portion of a HF-signal supplied via the HF-source port. 
     BACKGROUND INFORMATION 
     A series mixer is described in German Patent No. DE 196 10 850. 
     For example, such series mixers are used in radar sensors for proximity warning systems and control systems in motor vehicles. In the case of radar systems having a monostatic antenna design, one and the same antenna is used for transmitting the radar signal and for receiving the radar echo. The series mixer is then used to relay the signal, supplied via the HF-source port, to the antenna, and at the same time, to mix the signal received by the antenna with a portion of the signal supplied via the HF-source port. The mixed product is then an intermediate-frequency signal whose frequency indicates the difference in frequency between the transmitted and the received signal. This intermediate-frequency signal supplies information about the Doppler shift occurring in response to the reflection of the transmitted signal at the radar target, and thus about the relative velocity of the radar target and, provided the frequency of the transmitted signal is modulated in ramp-shaped fashion as in the case of a FMCW (frequency modulated continuous wave) radar, about the propagation time of the radar signal and therefore about the distance of the radar target, as well. 
     For this intended purpose, German Patent No. DE 102 35 338 describes a non-balanced mixer having only a single non-linear diode for mixing the high-frequency signals. In this mixer, a direct voltage (bias) is fed via the useful-signal tap in order to bias the diode, so that the working point of the diode can be optimized. This is advantageous if the high-frequency signals have a relatively low power which is not sufficient for a “self-bias” of the diode. 
     On the other hand, however, such a non-balanced mixer has the disadvantage that it also de-modulates the amplitude noise contained in the high-frequency signals, thereby impairing the quality of the useful signal. To nevertheless obtain a sufficiently low-noise useful signal, an extremely low-noise HF-source must therefore be used, e.g., a waveguide-based Gunn oscillator. However, such oscillators are relatively costly, and in addition, require complex balancing processes which further increase the production costs. 
     Instead of Gunn oscillators, it would therefore be advantageous from the standpoint of cost to use familiar MMIC (monolithic microwave integrated circuit) oscillators. However, they exhibit a higher amplitude noise. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to provide a balanced series mixer which, on the one hand, may be used in conjunction with HF-sources of low power, and on the other hand, permits an effective suppression of amplitude noise. 
     This objective is achieved by a balanced series mixer of the type indicated at the outset, in which the diodes are each biased via a bias feed line connected on the side of the HF-ground, the diodes are separated, in terms of direct current, from the useful-signal tap and from the lines connected to the HF-source port and the antenna port, and the two bias feed lines have low-pass filters configured symmetrically relative to each other. 
     The bias feed lines make it possible to bias the two diodes in such a way that, already in open-circuit operation, they are functioning close to their optimal working point. The optimal working point of the diodes is then reached by the relatively low power supplied from the HF-source. The low-pass filters prevent a short-circuit or an attenuation of the useful signal via the bias feed lines. Since the useful-signal tap as well as the HF-source and the antenna are decoupled from the diodes in terms of direct current, it is ensured that the bias direct current fed across one of the two diodes completely flows off again across the other diode, so that both diodes are traversed by an identical current. In conjunction with the symmetrical configuration of the low-pass filters in the two bias feed lines, it is thus ensured that the symmetry of the balanced mixer is not disturbed, and this is the decisive condition for effectively suppressing the amplitude noise. 
     For example, if the mixer of the present invention is used in a radar sensor, it is thus possible to employ an inexpensive but low-power MMIC oscillator as HF-source, and nevertheless to obtain a high-quality, in particular, low-noise useful signal. 
     In this application case, the antenna port is connected to the antenna of the radar sensor, thus clarifying the term “antenna port.” However, the field of application of the present invention is not intended to thereby be limited to the cases in which an antenna is actually connected to this port. 
     The decoupling of the useful-signal tap in terms of direct current is preferably realized via a series capacitor. 
     Preferably quarter-wavelength line couplers are used for the separation, in terms of direct current, of the mixer from the HF-lines connected to the HF-source port and the antenna port. 
     The symmetrically configured low-pass filters in the two bias feed lines are preferably R-C networks whose symmetrically set resistances then at the same time allow the setting of the bias. 
     The balun transformer (balanced to unbalanced transformer) is preferably implemented using stripline technology, and may be formed by a rat-race coupler or quadrature coupler. The diodes are preferably connected to the balun transformer via matching networks which allow an exact tuning or a controlled detuning of the diodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram of the series mixer according to the present invention. 
         FIG. 2  shows an enlarged representation of a balun transformer in the series mixer according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The balanced series mixer shown in  FIG. 1  has a balun transformer  10  which, in the example shown, is formed by a quadrature coupler realized using stripline technology. This balun transformer  10 , which is shown again enlarged in  FIG. 2 , has four ports interconnected in a rectangular configuration, namely, a HF-source port  12 , an antenna port  14  and two diode ports  16 ,  18 , of which one is situated opposite HF-source port  12 , and the other is opposite antenna port  14 . 
     The mixer shown in  FIG. 1  also has two non-linear, identical diodes  20 ,  22  which are connected with opposite polarity between a respective HF-ground  24 ,  26  and one of diode ports  16 ,  18  of balun transformer  10 . Matching networks  28 ,  30 , respectively, are inserted between the diodes and the diode ports. 
     HF-source port  12  of balun transformer  10  is connected via a quarter-wavelength line coupler  32  to a line  34 , that is connected via an input  36  to a HF-source, e.g., a MMIC oscillator  38 . 
     Antenna port  14  of balun transformer  10  is connected via a further quarter-wavelength line coupler  40  to a line  42 , that is connected via an input and output  44  to an antenna  46 . MMIC oscillator  38  and antenna  46  are indicated here only by dot-dash lines, since they are not components of the mixer. 
     Balun transformer  10 , taking the form of a quadrature coupler, has the characteristic that it is adapted to its HF-source port and antenna port  12 ,  14  when identical impedances are present at the two diode ports  16 ,  18 . This adaptation may be achieved here with the aid of diodes  20 ,  22 . 
     A HF-signal, generated by MMIC oscillator  38 , is fed via line  34  to HF-source port  12 , and is passed on via the two matching networks  28 ,  30  to diodes  20 ,  22 . Depending on the tuning of diodes  20 ,  22 , a greater or smaller portion of this signal is reflected and is fed via balun transformer  10  into line  42 , and ultimately into antenna  46 . 
     A HF-signal received by antenna  46  arrives via line  42  and balun transformer  10  at diodes  20 ,  22 , and is mixed with the non-reflected portion of the HF-signal supplied via line  34  to form a useful signal which may be tapped at a useful-signal tap  48 . The relation between the power converted by diodes, with respect to the power reflected and radiated via antenna  46  is set by controlled mismatch of diodes  20 ,  22 . 
     To optimize the working point of diodes  20 ,  22 , one bias feed line  50  and  52 , respectively, is provided for each diode. Bias feed lines  50  and  52  are exactly symmetrical relative to each other and in each case include a low-pass filter  54 . 
     In the case of bias feed line  50  for diode  20 , a direct voltage V 1  is applied to a bias terminal  56 . This bias terminal is connected to HF-ground  24  via a resistor  58  of low-pass filter  54  and a filter  60  used as a high-frequency block. Low-pass filter  54  is formed by an R-C network that, in addition to resistor  58 , has a capacitor  62  which is connected between the pole of resistor  58  connected to the high-frequency ground and a DC-ground  64 . 
     Bias feed line  52  for diode  22  has the same construction, only a direct voltage V 2  is applied to direct-voltage terminal  56 . Preferably, it holds that V 2 =V 1 . 
     Useful-signal tap  48  is connected to antenna port  14  of balun transformer  10  via a series capacitor  66  of a further R-C network  68  and a filter  70  used as a high-frequency block, in order to tap the useful signal demodulated by diodes  20 ,  22 . 
     With the aid of resistors  58 , which have identical resistance values, diodes  20 ,  22  are biased in such a way that, in conjunction with the HF-power supplied by MMIC oscillator  38 , they reach their optimal working point. At the same time, low-pass filters  54  prevent the useful signal, which is an intermediate-frequency signal, from leaking away via bias feed lines  50  and  52 . 
     In addition to series capacitor  66 , R-C network  68  also includes a resistor  72  connected to ground, and with its series capacitor  66 , prevents the direct current, supplied by bias feed lines  50  and  52 , from leaking away via useful-signal tap  48 . In the same way, quarter-wavelength line couplers  32 ,  40  prevent the direct current from leaking away via lines  34  and  42 . It is thus ensured that both diodes  20  and  22  are always traversed by the same current. 
     Filters  60  and  70  prevent the HF-signals from leaking away via bias feed lines  50  and  52  or useful-signal tap  48 . 
     The symmetrical configuration of low-pass filters  54  ensures a perfect symmetry between diodes  20  and  22 , so that the mixer operates as a balanced mixer and effectively suppresses an amplitude noise contained in the HF-signals, for instance. 
     Under suitable conditions, using the set-up described, it is possible to achieve a suppression of the amplitude noise by up to 50 dB. On the other hand, if resistor  58  and/or the capacitive coupling to DC-ground  64  is omitted in only one of the two low-pass filters  54 , the suppression is only approximately 10 dB. In comparison, the sum of direct voltages V 1  and V 2  has only a slight influence on the suppression of the amplitude noise, so that it is also possible, for example, to set one of these two voltages to zero, and thus to dispense with a bipolar voltage supply. 
     By suitable bias of diodes  20  and  22 , in general, it may be achieved that the suppression of the amplitude noise, as well as the AM/FM conversion are nearly constant in a relatively wide range of oscillator powers.