Patent Publication Number: US-2007120616-A1

Title: Multi-band frequency generation method and apparatus

Description:
TECHNICAL FIELD  
      This invention relates in general to communication systems and more particularly to the multi-band frequency generation is such systems.  
     BACKGROUND  
      The demand for multi-band communication systems has increased tremendously, particularly in the public safety arena where the interoperability of different communication systems is highly desirable. Fire departments, medical rescue and law enforcement are just a few examples of agencies that can benefit from the ability to communicate across different systems. The ability of a communication device, such as a radio, cell phone, digital assistant or the like, to operate amongst different frequency bands facilitates seamless mobility for the user.  
      The design of a multi-band communication device is complex as the different operating specifications for each band need to be addressed. In order to provide multi-band operation, circuit designers have typically increased the number of voltage controlled oscillators and provided multiple loop filters. Variations in frequency, tuning sensitivity, acquisition lock time and loop filter bandwidth are examples of operating parameters that need to be addressed in these circuits. Reduction of parts count and utilization of circuit board space are also of paramount importance in a portable multi-band radio design as these affect the cost, size and weight of each device.  
      Accordingly, a multi-band approach facilitating the issues discussed above is desirable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:  
       FIG. 1  is a block diagram for a multi-band frequency generation circuit formed in accordance with the present invention; and  
       FIG. 2  is a method for generating an output frequency in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.  
      The present invention may be embodied in several forms and manners. The description provided below and the drawings show exemplary embodiments of the invention. Those of skill in the art will appreciate that the invention may be embodied in other forms and manners not shown below. The invention shall have the full scope of the claims and shall not be limited by the embodiments shown below. It is further understood that the use of relational terms, if any, such as first, second, top and bottom, front and rear and the like are used solely for distinguishing one entity or action from another, without necessarily requiring or implying any such actual relationship or order between such entities or actions.  
      In accordance with the present invention, there is provided herein a frequency generation unit (FGU) having a minimum number of voltage controlled oscillators to satisfy all multi-band requirements in a single communication device. The selection of an appropriate intermediate frequency (IF) and VCO output frequency divider (N) allows for a minimum number of voltage controlled oscillators to selectively cover the desired bands. A programmable charge pump and a programmable reference divider are used to maintain a constant loop bandwidth within the frequency generation unit. The multi-band FGU of the present invention eliminates the need for multiple synthesizer loop filters and eliminates issues with loop filter bandwidth variations and loop filter switching components associated with past circuits.  
       FIG. 1  shows a block diagram of a frequency generation unit (FGU)  100  formed as part of a communication device in accordance with the present invention. FGU  100  comprises a synthesizer  102  having a programmable reference divider  104  and programmable charge pump  106 , a fixed loop filter  108 , and a plurality of selectable VCOs  110 . A selected VCO frequency  112  is fed back to synthesizer  102  along feedback path  114  to programmable feedback frequency divider  176  and phase detector  174  thereby closing the synthesizer loop. The selected VCO frequency  112 , with appropriate amplification, can be used for receiver injection via receive (RX) path  116  or for transmit injection via transmit (TX) path  118 .  
      In accordance with the present invention, the VCO frequency bands are chosen and set in conjunction with an appropriate intermediate frequency (IF) and divide-by-N (÷N) VCO output divider  152  such that a minimum number of VCOs can be used to satisfy all desired bands. As an example, for a selected IF of 109.65 MHz and a ÷N of two, all VHF, UHF, 700 MHz and 800 MHz bands can be covered with four VCO modules. For example, from the plurality of VCOs  110 , VCO 1  can be set for the 700/800 MHz RX bands, VCO 2  can be set for the 300/500 MHz and 400/600 MHz UHF TX bands; VCO 3  can be set for 200/400 MHz VHF TX band and the 300/599 MHz UHF RX band; and VCO 4  can be set for the 200/300 MHz VHF RX band and the 700/800 MHz TX band. Other band combinations can be used based on the selected IF and desired bands of operation. However, this may entail using additional VCO modules. By selecting an appropriate intermediate frequency (IF) and divide-by-N, a minimum number of VCOs can be used to satisfy the desired bands.  
      Digital programming interface  172  under microprocessor control provides programming capability to the programmable reference divider  104 , programmable charge pump  106 , programmable feedback frequency divider  176  and a switching logic extender interface  130 . Interface  130  generates digital I/Os  128  for selecting the desired VCO and also generates other digital I/Os  132 ,  142 ,  160 ,  166  for controlling a variety of switching circuitry  134 ,  140 ,  156 ,  158  used throughout the FGU  100 .  
      In operation, the programmable reference divider  104  receives and divides a reference frequency  120  to provide a divided reference frequency  122  to the phase detector  174  which provides sink/source controls to the programmable charge pump  106 . The programmable charge pump  106  generates a charge pump output  124  which can sink or source current as appropriate. The charge pump output  124  is filtered through a loop filter  108  formed of fixed loop elements (capacitors, resistors, etc.) to produce a control voltage  126 . Fixed loop filter  108  maintains a predetermined bandwidth by changing the charge pump current and the reference frequency via programming. The control voltage  126  is used to steer a selected VCO from the plurality of VCOs  110 . Control I/O  128  is used to select the desired VCO and desired VCO band of operation within that VCO. For example, digital I/ 0   128  may consist of a series of logic levels to enable the desired VCO and enable the desired band of operation within the enabled VCO. Each VCO may provide one or more bands of operation as discussed above.  
      Digital input  132  also controls RF switch  134  for routing the output of the selected VCO back to synthesizer  102  along feedback path  114  with amplification of selected VCO frequency  112  occurring at amplifier  136 . The selected VCO output frequency  112  is also amplified at amplifier  138  prior to being injected into either the receive path  116  or the transmit path  118 .  
      Another RF switch  140  under logic control  142  takes amplified signal  144  and routes it to one of three routing paths  146 ,  148 ,  150 . Routing path  146  divides the signal through the divide-by-N frequency divider  152 , preferably integrated within synthesizer  102 , to produce divided output signal  154 . Divided output signal  154  is forwarded to two routing switches, RX routing switch  156  in the RX path  116  and TX routing switch  158  in the TX path  118 . RX routing switch  156  utilizes logic control  160  to route either the undivided output signal from routing path  148  or the divided output signal  154  to amplifier  162  to be used as a RX injection signal  164 . TX routing switch  158  utilizes logic control  166  to route either the undivided output signal from path  150  or the divided output signal  154  to amplifier  168  to be used as a TX injection signal  170 . The divide-by-N  152  and selection of intermediate frequency (IF) minimizes the number of VCOs required for the FGU  100 .  
       FIG. 2  is a flowchart summarizing the technique of providing multi-band frequency generation in accordance with the present invention. Technique  200  begins at step  202  by selecting an intermediate frequency (IF)  202  and divide-by-N value to provide a plurality of selectable VCOs covering multiple bandwidths at step  204 . A reference frequency is provided at step  206  which is divided with a programmable reference divider at step  208 . The divided reference frequency is applied to a phase detector at step  210  to generate sink and source controls at step  212  which are applied to a programmable charge pump at step  214 . The programmable charge pump generates a charge pump output current (sink/source) at step  216 . The charge pump output current is filtered at step  218  via a loop filter formed of fixed elements thereby producing a control voltage. The control voltage is applied to the plurality of selectable VCOs at step  220 . By applying the control voltage to the plurality of selectable VCOs at step  220  and selecting a frequency band (via control logic) from the plurality of selectable VCOs at step  222 , a VCO output signal is generated at step  224 .  
      Depending on the VCO frequency band selected at step  222 , the VCO output signal generated at step  224  is selectively routed at step  226  through either a RX path at step  228 , or a TX path at step  230  or is divided by the divide-by-N value at step  232 . If routed through the divide-by-N path, then the divide-by-N frequency is then routed to the RX path at step  234  or the TX path at step  236 . Signal amplification takes place along the various routing paths as appropriate.  
      By selecting the appropriate intermediate frequency (IF) in conjunction with selecting a frequency divider N, a minimum number of VCOs are used to satisfy any communication band frequency spectrum bands in a single communication device. Utilizing a programmable charge pump and a programmable reference divider compensates for variations in VCO operating bandwidth and tuning sensitivity. The frequency generation unit formed in accordance with the present invention thus allows for a minimum number of VCOs to achieve multi-band operation. This approach reduces parts count, cost and minimizes printed circuit board real estate thus providing significant advantages to a portable multi-band communication device.  
      While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.