Abstract:
A method and apparatus for modulating a plurality of information signals onto respective unique intermediate frequency (IF) carrier signals, summing the respective IF carrier signals to provide a stacked IF carrier signal and upconverting the stacked IF carrier signal to a radio frequency (RF). Advantageously, the invention reduces the cost and complexity of modulation and frequency conversion processes in those systems requiring a multiplicity of audio, video or data carriers placed side by side or in a consecutive order within a defined frequency spectrum.

Description:
The invention relates to communication systems generally and, more specifically, the invention relates to a low noise block converter method and apparatus suitable for use in a data transmission system. 
     BACKGROUND OF THE DISCLOSURE 
     In information distribution systems utilizing radio frequency (RF) modulation techniques it is well known to modulate each of a plurality of information signals, such as audio, video or data signals onto respective intermediate frequency (IF) carrier signals having a common frequency. The respective IF carrier signals are then modulated onto respective RF carrier frequencies, which are then transmitted to a receiver. The receiver tunes and demodulates an RF carrier frequency including a desired IF modulated information signal. An IF demodulator then retrieves the desired information signal. 
     Currently in Community Access Television or CATV applications, it is common practice to use Frequency Division Multiplexing (FDM) as a means of inserting a multiplicity of audio, visual or data carriers onto a single cable or optical fiber for transport to the subscriber. That is, each of a plurality of, illustratively, 6 MHz bandwidth television signals are modulated onto adjoining radio frequency RF channel slots and transmitted to receivers within the CATV system. These channels are typically arranged side by side in a consecutive manner with little or no gaps from the first RF carrier (Lowest Frequency) to the last RF carrier (Highest frequency). 
     Specifically, a first step in the CATV FDM process comprises modulating each of a plurality of baseband information signals (e.g., each television signals) onto a standard intermediate frequency to produce a corresponding plurality of IF modulated signals. In the United States, the IF is typically 43.75 MHz for analog video and 44 MHz for digitally modulated video or digitally modulated data. A second step in the CATV FDM process comprises upconverting each of the plurality of IF modulated signals onto a corresponding plurality of respective RF carrier signals, which are then transmitted. Thus, the CATV FDM process utilizes, for each baseband information signal, a respective IF modulator and a respective RF upconverter. 
     Therefore, it is seen to be desirable to provide a reduced cost method and apparatus for providing a plurality of information streams to receivers within an information distribution system. Moreover, it is seen to be desirable to specifically reduce the amount of processing (and associated circuitry) required to prepare and transmit the plurality of information streams, such as within a CATV system. 
     SUMMARY OF THE INVENTION 
     The disclosure describes a method and apparatus for modulating a plurality of information signals onto respective unique intermediate frequency (IF) carrier signals, summing the respective IF carrier signals to provide a stacked IF carrier signal and upconverting the stacked IF carrier signal to a radio frequency (RF). Advantageously, the invention reduces the cost and complexity of modulation and frequency conversion processes in those systems requiring a multiplicity of audio, video or data carriers placed side by side or in a consecutive order within a defined frequency spectrum. 
     Specifically, an upconverter according to the invention comprises: a plurality of modulators for modulating respective information signals onto respective intermediate frequency (IF) carrier signals, the respective IF carrier signals being separated by a predetermined frequency; a summation module, for summing the plurality of IF carrier signals to produce a stacked IF carrier signal; a first mixer, for spectrally shifting to a first frequency range, the stacked IF carrier signal, the first frequency range comprising frequencies greater than frequencies of the IF carrier signals; and a second mixer, for spectrally shifting to one of a plurality of spectral portions within a second frequency range, information within the first frequency range, the second frequency range comprising frequencies less than the first frequency range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawing, in which: 
     FIG. 1 depicts a high level block diagram of a multiple carrier frequency up conversion system. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described within the context of an information distribution system, illustratively a video information distribution system. However, it will be apparent to one of ordinary skill in the art that the invention is also applicable other information distribution systems utilizing intermediate frequency (IF) modulation of a plurality of information streams prior to modulating the IF modulated information stream onto a radio frequency (RF) carrier frequency for subsequent distribution to one or more information consumers. 
     FIG. 1 depicts a high level block diagram of a multiple carrier frequency “up conversion” system. Specifically, the up converter  100  of FIG. 1 comprises a controller  105 , a plurality of (i.e., N) quadrature amplitude modulators (QAM)  110 - 1  through  110 -N (collectively modulators  110 ), a corresponding plurality of low-pass filters  120 - 1  through  120 -N (collectively low-pass filters  120 ), a frequency summation module  130 , a first mixer  140 , a first local oscillator  145  having associated with it a crystal  145 X, a band-pass filter  150 , a second mixer  160 , a second local oscillator  165  having associated with it a crystal  165 X, and a final low-pass filter  170 . 
     The up conversion system  100  of FIG. 1 receives a plurality of (i.e., N) input signals S 1 - 1  through S 1 -N (collectively input signals S 1 ). Each of the input signals S 1  is received by a respective one of the modulators  110 . Each of the modulators  110  modulates the respective received input signal S 1  onto a respective intermediate frequency (IF) carrier frequency to produce respective output signals S 2 - 1  through S 2 -N (collectively IF signals S 2 ). 
     First modulator  110 - 1 , in response to a control signal MC- 1  produced by the controller  105 , modulates first input signal S 1 - 1  onto, illustratively, a 44 MHz carrier frequency to produce first intermediate frequency (IF) carrier signal S 2 - 1 . Similarly, second modulator  110 - 2 , in response to a control signal MC- 2  produced by the controller  105 , modulates second input signal S 1 - 2  onto, illustratively, a 38 MHz carrier frequency to produce the second IF carrier signal S 2 - 2 . Finally, assuming that N=3, third modulator  110 -N, in response to a control signal MC-N produced by the controller  105 , modulates third input signal S-N onto, illustratively, a 32 MHz carrier frequency to produce third IF carrier signal S 2 -N. 
     Each of the IF carrier signals S 2 - 1  through S 2 -N are coupled to a respective low pass filter  120 - 1  through  120 -N. The low-pass filters  120  have cutoff frequencies selected to filter out at least the respective second harmonics of the IF carrier signals S 2 - 1  through S 2 -N. Each of the low-pass filters  120  produces a corresponding low-pass filter output signal S 3 - 1  through S 3 -N (collectively low-pass filtered signals S 3 ) which is coupled to a respective input of frequency summation module  130 . 
     The frequency summation module  130  combines the low-pass filtered signals S 3  to produce a summed or “stacked” IF signal S 4  comprising a plurality of IF modulated signals. The stacked IF signal S 4  is coupled to a first input of first mixer  140 . 
     Referring to FIG. 1, a spectral diagram of the output of summation module  130  is depicted with reference to the output signal S 4 . Specifically, the spectral diagram shows the IF carrier signal frequencies of modulators  110 - 1  and  110 - 2  in the exemplary embodiment (i.e., 38 MHz and 44 MHz). It can be seen that the IF carrier signals are spectrally separated by 6 MHz. This 6 MHz separation was selected in response to standard television signal separation parameters. In the case of N being greater than 2, additional modulators  110  produce additional IF carrier frequencies of, e.g., 26 MHz, 32 MHz, 50 MHz, 56 MHz and the like. 
     A second input of first mixer  140  receives a 906 MHz oscillation signal S 5  from a first local oscillator  145 , illustratively a fixed frequency synthesizer cooperating with the first crystal  145 X. 
     The first mixer  140  mixes the stacked IF signal S 4  and the 906 MHz oscillation signal S 5  to produce an output signal S 6  comprising a 906 MHz carrier signal having upper and lower side bands including the signal information within the stacked IF signal S 4  and its mirror image. The output signal S 6  produced by the first mixer  140  is coupled to the band-pass filter  150 . 
     Band-pass filter  150  comprises, illustratively, a 947 MHz dielectric band-pass filter having a 24 MHz bandwidth. Thus, a band-pass filter  150  will pass those frequencies between approximately 935 MHz and 959 MHz. In the case of the system of FIG. 1 utilizing on a 44 MHz modulator (e.g.,  110 - 1 ) and a 38 MHz modulator (e.g.,  110 - 2 ), the 947 MHz center frequency 24 MHz bandwidth is appropriate. However, in the case of a third modulator (e.g.,  110 -N) utilizing a 32 MHz carrier frequency, the band-pass filter  150  is adapted to have a 36 MHz passband and a 944 MHz center frequency. More generally, the band-pass filter  150  is adapted to have approximately an N*12 MHz pass band centered upon the a median frequency of the modulation frequencies (assuming the modulation frequencies comprise contiguous grouping of carrier frequencies). 
     Referring to FIG. 1, a spectral diagram of the output of local oscillator  145  is depicted with reference to the output signal S 6 . Specifically, since the oscillation frequency (906 MHz) is greater than the frequency of the two IF carrier signals (i.e., 38 MHz and 44 MHz), the first mixer  140  output signal S 6  includes a summation frequency group (i.e., the upper sideband) and a difference frequency group (i.e., the lower sideband). Thus, as depicted in the spectral diagram, the operation of the band-pass filter  150  will remove all frequency components except those associated with the relevant portion of the upper sideband. 
     The output of the band-pass filter  150  comprises a band-pass filtered signal S 7  that is coupled to a first input of the second mixer  160 . A second input of the second mixer  160  receives an oscillation signal S 8  from the second local oscillator  165 , illustratively a variable frequency synthesizer responsive to a control signal OC produced by the controller  105 , provides an output oscillation ranging in frequency from 1347 MHz to 1807 MHz in 1 MHz steps. That is, the oscillation signal S 8  is modified by the second local oscillator  165  in steps of 1 MHz between 1347 MHz to 1807 MHz. 
     The second mixer  160  mixes the band-pass filter signal S 7  and the adjustable oscillator signal S 8  to produce a output signal S 9  comprising the stack of converted IF signals and their respective image frequencies. 
     It is important to note that the second local oscillator  165  provides a relatively course frequency adjustment suitable for locating the stacked IF signal information into one of a plurality of spectral regions. Since these spectral regions are adjusted in 1 MHz steps, the impact of phase noise upon the frequency accuracy of the second local oscillator  165  is relatively low. 
     The output signal S 9  produced by the second mixer  160  is coupled to a low-pass filter  170 , illustratively a 1 GHz low-pass filter. The low-pass  170  attenuates those frequencies above 1 GHz to produce an output RF signal S 10 , which is coupled to transmission circuitry (not shown). The cutoff frequency is selected based upon the maximum RF frequency utilized by the system (i.e., approximately 900 MHz). 
     Referring to FIG. 1, a spectral diagram of the lower sideband produced by the second mixer  160  and a frequency response of the low-pass filter  170  is depicted with reference to the output signal S 10 . Specifically, since the second oscillation frequency ranges from 1347 MHz to 1807 MHz, the low-pass filter  170  operates to attenuate the upper sideband of the second mixed signal S 9 . 
     It should be noted that the low pass filters  120  are depicted in FIG. 1 as being between the respective modulators  110  and the summation module  130 . In this manner, relatively straightforward low pass filters may be employed to perform the necessary second harmonic reduction and/or removal function. However, it should be noted that a single filter may be coupled between the summation module  130  and the first mixer  140  to achieve the same purpose. This single filter may comprise a low pass filter if the respective second harmonics do not intrude into spectral regions including frequencies of interest. This single filter may also comprise a comb filter. 
     In the embodiment of FIG. 1 the modulators  110  are implemented using model BCM 3033 QAM modulators by Broadcom, Inc. of Irvine, Calif. Since it is typical for the frequency setability or step size of most commercial modulators to be in steps of 125 KHz with a 12.5 KHz and 25 KHz offset capability, at least one of the local oscillator (LO) synthesizers in the upconverter must have a phase detector comparison frequency of 12.5 KHz to satisfy the 12.5 KHz offset requirement. Since a 12.5 KHz comparison frequency with VCO frequencies in the hundreds of MHz promotes phase noise in the VCO, it is desirable to use the highest possible comparison frequency in the LO synthesizers and still maintain setability. The BCM 3033 is utilized, at least in part, because of its frequency setting capability. 
     The invention provides for reductions in the phase noise of the local oscillators by increasing the comparison frequency of the synthesizers to 1 MHz, while also increasing the frequency setability or frequency step size of the upconverter to less than 100 Hz at the highest frequency of, illustratively, 860 MHz. The frequency setting resolution of this embodiment is achieved by utilizing two frequency setting elements. The first element is the modulator  110 , which is used for fine frequency adjustment of less than 100 Hz. The second element is the frequency synthesizer in the second local oscillator  165 , which provides the coarse frequency adjustment of 1 MHz steps. 
     Because the modulator  110  (such as the BCM 3033 QAM Modulator) uses Direct Digital Synthesis (DDS) to generate its IF output, excellent spectral purity and extremely low phase noise can be achieved. Additionally, since the phase comparison frequency of both the first  145  and second  165  local oscillators is kept as high as 1 MHz (rather than the 12.5 KHz used in conventional upconverter technology), additional phase noise reduction over existing agile upconverter technology is realized. 
     Advantageously, the above-described invention provides for the processing of a plurality of modulated IF carriers by a single upconverter. Additionally, a type of noise typically associated with the agile upconversion process itself is reduced. This noise, if excessive, causes errors in digital signals when said signals are transported over coaxial cable or fiber optical cable systems. 
     Although various embodiments which incorporate the teachings at the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.