Patent Publication Number: US-2004053593-A1

Title: Signal mixers

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to circuits for mixing radio signals and in particular to double balanced diode mixers for generating an IF output signal from an RF input signal. It will be appreciated however that the invention is not limited to use with only double balanced diode mixers. The invention may also be implemented for single balanced mixers, doubly doubly balanced mixers and harmonic mixers, for example.  
       BACKGROUND TO THE INVENTION  
       [0002] Double balanced diode mixers (DBDM) are widely used in telecommunications equipment for converting an input message signal with a particular carrier frequency to an output message signal with a different carrier frequency. DBDMs can also be used as modulators and demodulators.  
       [0003] A typical DBDM circuit comprises an input transformer or balun for receiving an input signal, for example a radio frequency (RF) message signal. The transformer has a differential output for providing two opposite phase output signals, which are fed into two nodes of a diode ring. Two remaining nodes of the diode ring are connected to a differential input of a second or output transformer, which includes a centre tap for a local oscillator (LO) signal on a primary winding, and a single ended output on a secondary winding which contains a mixed output signal.  
       [0004] The diode ring mixes the LO signal with the RF message signal to produce a signal which includes various components including two intermediate frequency (IF) message signals. The output IF message signals have carrier frequencies of ±F IF =±F LO ±F RF  respectively. A DBDM arrangement will typically have an input filter circuit on the front-end for conditioning the RF input message signal, and also an output filter circuit or diplexer on the back-end for removing undesired components of the mixed signal and to terminate the RF signal in a load. The diplexer can be tuned to one of the IF carrier frequencies to pass the desired IF signal to the output.  
       [0005] The diode ring mixer, input filter and diplexer of a DBDM mixer can be constructed in a compact manner using surface mount and/or integrated circuit technology. However the input and output transformers are by nature bulky devices which increase the overall size of a DBDM mixer.  
       SUMMARY OF THE INVENTION  
       [0006] It is an object of the present invention to remove the need for an input transformer of a DBDM by implementing an equivalent circuit which approximates the functionality of the input transformer, or at least to provide an alternative to existing DBDM systems. In general terms the invention provides a circuit, which combines the functionality of the input filter with that of the input transformer. In one embodiment of the invention the circuit provides a LCR network which filters an input RF message signal as required, and further includes a portion which approximates the functionality of a transformer to provide RF differential output signals and to add an externally generated LO component to each differential output signal. In an alternative embodiment the circuit could be implemented using a microstrip arrangement.  
       [0007] In one aspect the invention may be said to consist in a method of generating output signals for transfer to a diode ring in a mixing circuit including: filtering a first input signal, phase splitting the first input signal to produce a differential signal, and combining each differential signal with a second input signal, to produce output signals for transfer to the diode ring.  
       [0008] In another aspect the invention may be said to consist in an input stage for a mixing circuit, including a LCR network having a filter for providing signal conditioning of a first input signal, a phase splitter for producing a differential output signal from the input signal, and additive functionality for combining each differential output signal with a second input signal.  
       [0009] In another aspect the invention may be said to consist in a LCR network for generating a differential signal for input to a diode ring including: a filter portion for conditioning a first input signal, and a portion implementing an approximation model of a transformer for producing a differential output signal and injecting a second signal into each differential output.  
       [0010] In another aspect the invention may be said to consist in a circuit for mixing electromagnetic signals including: a filter for signal conditioning of an input signal, a sub circuit for phase splitting the filtered input signal into differential output signals and adding a second input signal to each differential output signal, and a diode ring for switching each differential signal and added second signal, wherein the sub circuit approximates the functionality of a differential output transformer with a centre tap.  
       [0011] The invention may also be said to consist in any alternative combination of parts or features as described or shown in the accompanying drawings. Known equivalents of these parts or features not expressly set out are nevertheless deemed to be included. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] Preferred embodiments of the invention will now be described with reference to the accompanying figures, of which:  
     [0013]FIG. 1 is a receiver block diagram showing an input filter, DBDM and diplexer of a typical implementation of a DBDM for frequency conversion,  
     [0014]FIG. 2 shows in further detail the DBDM stage of the mixing circuit,  
     [0015]FIG. 3 shows a circuit diagram of the DBDM stage,  
     [0016]FIG. 4 shows a circuit diagram of the input filter stage,  
     [0017]FIG. 5 shows a block diagram of a modified input stage which implements the combined functionality of a front-end filter and differential output transformer,  
     [0018]FIG. 6 shows a circuit diagram of a preferred embodiment of the modified input stage, and  
     [0019]FIG. 7 shows a circuit diagram of an alternative equivalent circuit for replacing the differential output transformer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0020] Referring to the drawings it will be appreciated that the input circuit according to the invention can be implemented in various forms. The following examples are given by way of example only. It will also be appreciated that details relating to filter design and component selection will be understood by those skilled in the art and need not be described in detail here. It should also be appreciated that, although illustrated and described as a down conversion mixer, the DBDM is well known to those skilled in the art as a bi-directional mixer and can be equally used as an up-conversion mixer, modulator or demodulator. The input circuit can be used in conjunction with the output circuit from the mixing circuit which is disclosed in NZ application no. 508053.  
     [0021]FIG. 1 shows the general structure of an existing mixing circuit  10  which implements a DBDM  12 . The operation of such a circuit will be known to those skilled in the art although will be briefly explained for reasons of clarity in the description overall. The circuit includes a front-end filter  11  which has a single ended input for receiving, for example, a RF input message signal  14 . The filter  11  performs signal conditioning on the input signal  14  to remove undesirable noise and unwanted signals. The filtered signal  15  is then passed to the DBDM  12  where an externally generated LO signal  16  is mixed with the filtered signal  15  to produce a mixed signal  17  which contains various components as a result of the mixing operation. The mixed signal is passed to a diplexer  13  where undesired components, including the original RF message signal  15  and LO signal  16  are terminated in a load to provide a single ended IF output message signal  18  which can be used as required by the remaining portion of telecommunications equipment in which the mixer is being implemented.  
     [0022]FIG. 2 shows the DBDM  12  in further detail. The filtered signal  15  is passed into one input terminal of a transformer  20  of which the other input terminal is grounded. The transformer  20  generates a differential output  23 ,  24  on a secondary winding, one output comprising the filtered signal  15  and another comprising the same signal but phase shifted by 180°. The LO  16  signal is fed into an input centre tap of the transformer  20 . Each output  23 ,  24  is coupled to an opposing node of a diode ring  21 . Two other opposing nodes of the diode ring are connected to differential input terminals  25 ,  26  of an output transformer  22 . A centre tap  27  of the primary winding of the output transformer  22  is connected to earth. A single ended output terminal of the transformer  22  is coupled to the diplexer, while the other output terminal is grounded. It should be noted that somebody skilled in this area of technology would appreciate that the input and output transformers could be replaced by baluns.  
     [0023]FIG. 3 shows a circuit diagram of the DBDM  12  shown in FIG. 2. The input transformer  20  includes a single ended primary winding  30  with a centred tapped differential secondary winding  31 . Similarly the output transformer  22  includes a centred tapped differential input primary winding  32  and a single ended secondary winding  33 . The diode ring consists of four diodes arranged such that only two of the diodes conduct at any one time depending on the polarity of the LO signals at the nodes  34 ,  36 . The filtered input signal  15  is fed into one terminal of a primary winding  30  of the input transformer  20 . The differential output terminals  38   a ,  38   b  of the secondary winding  31  are coupled to opposite nodes  34 ,  36  of the diode ring while the two other opposing nodes  35 ,  37  are coupled to the differential inputs terminals  39   a ,  39   b  of the primary winding  32  of the output transformer  22 .  
     [0024] The LO signal  16  is fed into the centre tap  38   c  of the input transformer  20  secondary winding  31  and in turn fed into the diode ring  21  to input nodes  34 ,  36  via the differential output terminals  38   a ,  38   b . The single ended output signal  17  is generated on an output terminal of the output transformer&#39;s  22  secondary winding  33 , while the other output terminal is earthed. During operation of the mixer  12  the LO signal  16  is fed into the diode ring and alternately switches opposing diode pairs on and off which alternates the differential RF output signal  23 ,  24  between the differential input terminals  39   a ,  39   b  of the output transformer  22 . It will be appreciated by those skilled in the art that in an alternative embodiment the LO  16  could readily be injected into a centre tap  39   c  of the primary winding  32  of the output transformer  22  with the centre tap  38   c  of the primary winding  31  of the input transformer  20  being connected to ground. Either configuration of LO  16  and ground connections to the centre taps  38   c , 39   c  can be used as required. One configuration may be preferable to the other in certain implementations.  
     [0025]FIG. 4 is a circuit diagram of a front-end input filter  11  for the mixing circuit  12 . The circuit includes a terminal  40  for the RF input message signal  14  and an output terminal  41  for the filtered signal  15 . The circuit further includes a tuning terminal  42  and varicap tuned diodes  46 ,  47  to facilitate adjustment of the circuit for operation at desired frequencies. In this manner the two LC arrangements  43 ,  44  can be customised in conjunction with the resistor network  45  to effect a low impedance path to ground for undesired frequency components of the input signal  14 . In particular the arrangement can be adjusted to remove unwanted noise, distortion and other components of specific frequencies from the RF input signal  14 .  
     [0026]FIG. 5 shows a block diagram of a preferred embodiment of the invention including a modified front-end filter  50  which is implemented to approximate the combined functionality of the front-end filter  11  and input transformer  20  as encircled  28  in FIG. 2. Effectively the circuit is an LCR model, or equivalent circuit, of the front-end filter  11  and transformer  20 . Alternatively the equivalent circuit could be implemented using microstrip technology. The modified filter SO generates a differential output  23 ,  24  which can be fed into a diode ring to enable the modified filter  50  to be directly substituted for the front-end filter  11  and input transformer  20  which are typically used in a mixing circuit  10 .  
     [0027]FIG. 6 is a circuit diagram of a preferred embodiment of the modified filter  50 . A single ended input terminal  60  is provided in an LCR network which contains the adjustment terminal  42 , resistor network  45  and two LC arrangements  43 ,  44  of the original front-end filter  11 . Unwanted signal components of an input signal present on the input terminal  60  are removed by the LC networks  43 ,  44  in conjunction with capacitors  63 ,  65  in the manner as explained with reference to FIG. 4. The filtered signal is then passed to a first differential output terminal  61 . Due to the fact that LC network  44  is resonant with capacitors  63  and LC network  43  is resonant with capacitors  65  the signal passed to the output  62  is phase shifted by 180° relative to the signal on output terminal  61 . An LO signal is injected into each differential output signal by way of a second input terminal  64  and two series capacitors  65 . The relative phase of the LO components in each output signal is 0°. Each output signal therefore includes a RF message signal added to a LO signal with the RF message signal in each output having a relative phase shift of 180°. As noted above it will be appreciated that in an alternative embodiment the LO input terminal  64  could be the ground terminal, and the LO signal could be injected to the centre tap  39   c  of the transformer  22 .  
     [0028]FIG. 7 shows an alternative embodiment of the invention in which the functionality of the front-end filter  11  and input transformer  20  is not combined in a single circuit. Rather the original front-end filter  11  is retained and the transformer  20  is replaced by a LCR equivalent circuit  70  of the transformer  20 . The filtered signal  15  from the front end filter  11  is fed into an input terminal  71  of the equivalent circuit  70 . The filtered input signal  15  undergoes further filtering using a LC network and the resulting signal is fed to one differential output terminal  72 . The resulting signal is also fed through a series capacitor network  73  and LC circuit  76  which phase shifts the signal by 180° for subsequent transmission to a second differential output terminal  74 . An LO signal is injected via terminal  75  into each differential output signal by way of a second input terminal  75  and the two series capacitors  73  which keep a relative phase shift of 0° between the LO components in each output signal. The differential output terminals  72 ,  74  can be coupled to opposing nodes of a diode ring to feed the combined LO and RF signals to the ring for mixing. Bach output signal therefore includes a RF message signal added to a LO signal, with the RF message signal in each output having a relative phase shift of 180°. The equivalent circuit further includes an input terminal  42  and associated circuitry for adjusting the frequency response of the circuit as required. Again it will be appreciated that in an alternative embodiment the LO terminal  75  could be the ground terminal instead with the LO fed to the centre tap  39   c  of the transformer  22 .