Abstract:
A method and apparatus are disclosed for tuning a voltage controlled oscillator (VCO) having two point modulation used in a phase lock loop modulation system. A loop correction voltage applied to a first modulation input of the VCO when a first modulation signal, e.g., +1, is applied to a second modulation input of the VCO is compared to a loop correction voltage applied to the first modulation input when a second modulation signal, e.g., −1, is applied to the second modulation input of the VCO. The comparison produces a correction signal used to adjust the signal level of at least one of the signals, e.g., the second modulation input signal, applied to the two modulation inputs of the VCO.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for tuning a voltage controlled oscillator (VCO) within a two-point modulated phase lock loop (PLL) system. 
     BACKGROUND OF THE INVENTION 
     Phase lock loop (PLL) modulated transmission systems are known in which a carrier frequency, set by a VCO as the center frequency of the loop, can be modulated by applying a signal to a single point in the loop. 
     Single point modulation circuits modulate the VCO by connecting the modulating signal to one side of the loop filter or the other. With the pre-loop filter connection circuit, the modulating signal is low-pass filtered by the loop as it is modulated onto the carrier. Thus components of the modulating signal that are higher than the loop bandwidth will be suppressed. Unless there is no significant energy in the modulating signal above the loop bandwidth, the signal will suffer some loss (distortion) during the modulation. 
     With the post-loop filter correction circuit, the modulating signal is high-pass filtered by the loop as it is modulated onto the carrier. Thus components of the modulating signal that are lower than the loop bandwidth will be suppressed. The PLL output signal will suffer some loss (distortion) during the modulation unless there is no significant energy in the modulating signal below the loop bandwidth. 
     Two point modulation schemes address signal loss or distortion due to loop filter effects. In a two point modulation system, the modulating signal may be applied to the loop at two places—both before and after the loop filter. Assuming the gains in these two modulation paths are matched, the low-pass response seen by the pre-loop filter connection and the high-pass response seen by the post loop filter connection cancel each other out with the result being that the loop has a flat frequency response (same gain over all frequencies) for the modulating signal, avoiding distortion in the modulation signal as it is modulated onto the carrier. However, when the gains are not matched, the low-pass and high-pass responses do not cancel each other out and some distortion occurs. To avoid loss or distortion of the modulating signal as it is modulated by the loop onto the carrier, the gains of the two modulation paths must be matched. Gain matching has been accomplished in a variety of ways. 
     One technique to match gains first requires testing of each device incorporating a PLL then the special manufacturing or physical modification of each PLL&#39;s loop filter resulting in a unique loop filter for every IC and PLL combination. The special manufacturing of loop filters is time consuming and very expensive. 
     In another technique, tuning has been accomplished by writing a value into non-volatile memory that sets the gain in the Vkmod path so that it matches the gain in the loop correction signal Vkvco path. The non-volatile memory technique was not satisfactory as it consumed large amounts of chip area on device ICs and was not capable of tuning multiple carrier frequencies or channels after the initial manufacturing of the device, and thus added great cost to device manufacturing. The non-volatile memory technique also added additional testing time in the factory for each and every device manufactured, not just a particular model, adding to manufacturing costs. 
     The loop filter modification technique and the non-volitile memory technique also were highly unsatisfactory in that the tuning was accomplished once at the factory for a single channel. This single channel tuning matches Vkvco and Vkmod gains for only a single channel since the gains vary for each channel. Thus, gain mismatch worsens when any channel besides the tuned channel is used resulting in varying degrees of distortion that limited the performance and capabilities of a given device. 
     Another tuning method added a second internal VCO and PLL which is used as a reference during power-up of the associated transmitter for each burst of transmitted data. The difference between Vkvco and Vkmod path gains is detected to determine a value that sets gain in the Vkmod path so that it matches gain in the Vkvco path. The duplication of the PLL also consumed large areas of device ICs, was not useful in portable devices due to undesirable power consumption requirements and was much more complex and costly to manufacture. 
     Accordingly, it would be advantageous to have a method and apparatus for tuning the gain of the Vkvco and Vkmod signal paths with lower power consumption, fewer components and a simplified and time efficient tuning arrangement. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention presents a method and apparatus for tuning a two-point modulated phase-locked loop (PLL) such as may be used for modulated signal transmission in a communication system. The present invention provides tuning of a PLL for single or multiple carrier frequency channels with gain correction of an applied modulation signal. The Vkvco loop correction signal is sampled and held after applying a +1 modulation signal to the modulation input and then sampled with a −1 modulation signal applied to the modulation input. The sampled Vkvco values are compared to produce a gain correction signal which is scaled onto a modulation signal thereby producing a gain corrected modulation input signal Vkmod to the VCO. The gain through the two modulation paths to the VCO is matched using the correction signal to minimize distortion of the VCO output. 
     For a multiple channel transmission system, this method can be applied independently for each channel. Vkvco signals can be sampled for each channel to produce an associated gain correction signal for each channel. 
     These and other features and advantages of the invention will be better understood by reference to the detailed description below which is provided in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an exemplary PLL modulation system which may employ the invention; 
     FIG. 2 is a block diagram of a gain correction circuit which may be used with the FIG. 1 circuit; 
     FIG. 3 is a process sequence for balancing the Vkvco and Vkmod signal gains using the FIG.  1  and FIG. 2 circuits; 
     FIG. 4 discloses an alternative processing sequence which may be used for gain balancing; 
     FIG. 5 is a block diagram of an alternative structure which may be used for gain balancing; 
     FIG. 6 is a process sequence for balancing gain using the FIG.  1  and FIG. 5 circuits; and 
     FIG. 7 is a block diagram of another alternative structure which may be used for gain balancing. 
     FIG. 8 is a block diagram of another alternative structure which may be used for gain balancing. 
     FIG. 9 is a block diagram of another alternative structure which may be used for gain balancing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, FIG. 1 illustrates a PLL system for use in binary frequency shift keying (BFSK) scheme modulating a carrier signal to produce a modulated output signal Fout. The carrier center frequency is set by the VCO and frequency modulation of this carrier occurs in response to a signal applied to a modulation input terminal  15 . The PLL includes a phase detector  25 , loop filter  27 , a VCO  31  for receiving the loop correction modulating signal Vkvco  29  and a gain corrected modulation signal input Vkmod  33  and providing the output signal Fout, and a frequency divider  21  for dividing by the Fout signal by a scalable factor N which is then applied to phase detector  25  which also receives a reference signal  23 . A scaling circuit  37  is provided which receives a gain correction input signal  35  which is used to scale the gain of the modulation signal  15  input into the scaling circuit  37  thereby producing a gain corrected modulation input signal Vkmod  33  for input to the VCO  31 . 
     A carrier channel input  13  is provided which is used to program divider  21  to divide the Fout signal by an appropriate factor N for a given channel carrier frequency to produce a loop frequency which is input to phase detector  25  for comparison with reference frequency signal  23 . The carrier channel input  13  is provided by a control circuit  18 . The carrier channel input  13  is a number of a digital value which is used to determine the base carrier frequency in the Fout signal. For example, if the center frequency of Fout is to be 100 Mhz and the reference signal  23  is 10 Mhz, then the carrier channel input value would be the digital value corresponding to 11. The carrier channel input is also adjusted by the value of the applied modulation input  15  with an adder  17 . Alternatively the modulation input to the adder  17  can also be accomplished using a scaler which serves as a frequency deviation input for +1 and −1 deviation which would scale a +1 to a number representing a desired positive deviation in frequency and −1 to a number representing a desired negative deviation in frequency. 
     A fractionalization circuit  19  produces an average value of the carrier channel input as adjusted by the modulation input  15 . The N in the frequency divider  21  is obtained from the fractionalization circuit  19  output to create the desired average loop frequency value that is input into the phase detector  25 . Note that in this example, N does not equal carrier+modulation. Channel selection circuitry can incorporate a circuit which derives a sequence of divisor factors such that the average of said sequence of divisor factors is equal to the sum of the received signals. 
     For Binary Frequency Shift Keying (BFSK) modulation, the modulation input  15  can take on values of +1 or −1. When the carrier input  13  is set at a channel c 1  and the modulation input  15  is set at 0, the loop will settle to Fout=Fc 1  and the loop correction signal Vkvco  29  will settle to a voltage value of Vc 1 . When the modulation input is  15  changed to +1, the loop will settle out to Fout=Fc 1 +Δ, where Δ is the frequency deviation due to +1 modulation. If the gains in the Vkvco  29  and Vkmod  33  path are matched, the loop correction signal Vkvco  29  will again settle to Vc 1 . If the Vkvco and Vkmod signal path gains are mismatched, the loop correction signal Vkvco  29  will settle to a voltage value of Vc 1 +ε, where ε represents the voltage offset due to gain mismatch. When the modulation input  15  is changed to −1, the loop will settle out to Fout=Fc 1 −Δ and again the correction signal Vkvco  29  will settle to Vc 1  if the Vkvco and Vkmod signal path gains are matched. If the Vkvco and Vkmod signal path gains are mismatched the voltage Vkvco  29  will settle to Vc 1 −γ. If the varacter characteristic within VCO  31  is linear, ε=γ. The present invention seeks to make ε and γ approximate a value of zero by tuning the correction input  35  so that the gain from the modulation input  15  to Fout  39  through Vkmod  33  is matched to the gain from the modulation input  15  to Fout  39  through Vkvco  29 . It should be noted that the +1 indicates a positive deviation in frequency and the −1 indicates a negative deviation in frequency. Alternatively, a scaler can be placed prior to adder  17  that would scale a +1 to a number representing a desired positive deviation in frequency and −1 to a number representing a desired negative deviation in frequency. The coefficient for the scaling could be supplied by the control circuit  18  or by another control component. The control circuit  18  can be the same component supplying carrier input  13  and modulation input  15  or alternatively separate components such as in FIG.  5  and control circuit  18 ′. 
     Turning to FIG. 2, a comparator circuit is provided for comparing the loop correction signal Vkvco  29  when the modulation input is +1 to the correction signal Vkvco  29  when the modulation input is −1 during a correction period which precedes actual use of the FIG. 1 PLL for signal modulation. Using the compare results, a determine gain correction circuit  47  can then determine a gain correction signal input  35  which is used to scale a modulation input  15  to produce a gain corrected modulation signal input Vkmod  33  during subsequent operation of the PLL so that the Vkvco and Vkmod signal gains match) thereby eliminating the ε and γ error or gain mismatch. 
     The FIG. 2 circuit has unity gain amplifiers  43  receiving a correction signal Vkvco  29 , taken from the FIG. 1 circuit, then inputting the amplified Vkvco signal to a sample and hold circuit  45  and to the determine gain correction circuit  47 . The sample and hold circuit  45  is used to sample and hold a Vkvco signal value when one modulation value, e.g., +1 is generated. This value is held for comparison in the determine gain correction circuit  47  with a value of Vkvco produced when the other modulation value, e.g., −1, is used. The difference represents the differences in gain of the Vkmod and Vkvco signals and is used to develop a proportional gain correction signal  35  to reduce the difference to zero. The correction factor is applied as a gain correction signal input  35  to the scaling circuit  37  in FIG. 1 to scale the gain of a modulation signal  15  to produce a gain corrected modulation signal input Vkmod  33  which is input to the FIG. 1 VCO  31 . 
     The FIG. 2 circuit may determine the gain correction input signal  35  from one comparison of the Vkvco signal values for +1 and −1 modulation inputs or successive Vkvco signal comparisons can be employed to produce the gain correction input signal  35 . 
     FIG. 3 discloses one processing sequence that may be used with the FIG.  1  and FIG. 2 circuits in which a single comparative measurement of the Vkvco signal  29  values is used to develop the gain correction input signal  35 . Control circuit  18  (FIG.  1 ), which can be a logic circuit, microprocessor or any other type of control circuit, implements the process of FIG.  3 . It starts the PLL tuning cycle at processing segment  71  and sets the carrier input  13  to a desired channel at processing segment  73 . The control circuit  18  will then set a modulation input  15  of +1 at processing segment  75 , and will then allow the PLL to settle. The control circuit  18  will then operate sample and hold circuit  45 , causing it to store the Vkvco correction signal  29  at processing segment  77 . Next, the control circuit  18  will set the modulation input  15  to −1 and again allow the PLL to settle at processing segment  79 . The control circuit  18  at processing segment  81  operates the determine gain correction circuit  41  causing it to compare the Vkvco signals  29  corresponding to the −1 and +1 modulation inputs, the latter being stored in the sample and hold circuit  45 , and the former being taken directly from the PLL loop, and develops a gain correction input signal  35  from the difference between the two compared Vkvco values which is required to match the Vkmod gain to the Vkvco gain that is, to produce a difference in the compared Vkvco signals of zero. The gain correction input signal  35  is developed at processing segment  83  and is applied to scaling circuit  37  to appropriately scale the modulation input  15  in the Vkmod path at processing segment  85  for subsequent operation of the PLL. 
     It should be noted that the FIG. 3 sequence can be repeated each time a channel is changed so that an appropriate gain correction input signal  35  is set for a corresponding selected channel. 
     The FIG. 3 processing sequence is but one way to develop a gain correction input signal  35 . In another processing sequence shown in FIG. 4, a three-step approach is shown. Again, this sequence can be implemented by control circuit  18 . The processing sequence is started at processing segment  89 . At processing segment  91 , a number of check bits n will be set and a counter will be initialized to zero. In this example using a three step approach, n will equal 3. The carrier channel input  13  is set to a desired channel by control circuit  18  at processing segment  93  and the modulation input  15  is set to +1 at processing segment  95  then the PLL will be allowed to settle. The control circuit  18  will then operate sample and hold circuit  45  to store the Vkvco signal  29  at processing segment  97 . Next, control circuit  18  will set the modulation input  15  to −1 at processing segment  99  and then the PLL will be allowed to settle. The control circuit  18  will then operate the determine gain correction circuit  47  so that it compares the Vkvco signals for the +1 and −1 modulation inputs to develop a gain correction input signal  35  required to match the Vkmod gain to the Vkvco gain  103 . The gain correction input signal  35  is applied to scaling circuit  37  in the Vkmod signal path at processing segment  105 . The control circuit  18  will then determine if the counter, which currently stores a value of 0, equals n−1 at processing segment  107 . For this first iteration, where n=3 and the counter=0, processing segment  107  will yield a “no” response, in which case the counter value will be incremented by one in processing segment  109  and the processing sequence is repeated from segments  95  through  107 . A new gain correction input signal  35  is thus developed each time the sequence is repeated and each time the correction input value is refined. If the counter =n−1, as determined in processing segment  107 , then the successive measurement and correction loop processing sequence will be terminated. Thus, if the counter value does not equal n−1, then the counter value will be incremented by one in processing segment  109  and the processing scheme will repeat until the counter holds a value of 2 which equals n−1, as determined in step  107 . The final gain correction input signal  35  is applied to scaling circuit  37  where it is used to scale the gain of subsequent modulation inputs to the PLL in the Vkmod path thereby producing a gain corrected modulation input  33  for input into the VCO  31 . The FIG. 4 sequence permits an appropriate gain correction input signal  35  to be developed and modified in three stages, thereby minimizing possibility of a signal aberration which might occur using only one set of Vkvco samples. 
     Although the FIG.  1  and FIG. 2 circuits are shown as utilizing primarily analog circuits, the invention may also be implemented using analog or digital circuits or a combination of analog and digital circuits. 
     Turning to FIG. 5, another embodiment of the invention is shown which uses a digital circuit to compare the voltage Vkvco obtained when the modulation input is +1 to the voltage Vkvco obtained when the modulation input is −1. The circuit determines a gain correction input signal  35  to scale a modulation input  15  in the scaling circuit  37  to produce a gain corrected modulation signal input Vkmod  33  so that the Vkvco and Vkmod gains match. 
     The FIG. 5 circuit employs a pair of unity gain amplifiers  43 , a sample and hold circuit  45  for sampling and holding a Vkvco value when one modulation value, e.g., +1 is generated. This value is then compared in a digital gain correction circuit  203  with a value of Vkvco produced when the other modulation value, e.g., −1, is used. The differences represent the differences in gain of the Vkmod and Vkvco signals which is used to develop a proportional correction factor to reduce the difference to zero. The correction factor is applied as a gain correction input signal  35  to the scaling circuit  37  in the FIG. 1 circuit. 
     The digital gain correction circuit  203  includes an analog comparator  205  to compare analog values of the +1 modulation Vkvco signal from the sample and hold circuit  45  to the −1 modulation Vkvco signal. The result of the comparison is sent to an amplifier  207  and then to an analog-to-digital converter (ADC)  209 . The ADC output signal is then sent to control circuit  18 ′ which controls the production of the gain correction input signal  35 . The control circuit  18 ′ evaluates the Vkvco difference signal and develops a digital correction signal which is applied to a digital to analog circuit (DAC)  215  which converts the digital signal to an analog correction signal which is output as the gain correction input signal  35 . In this embodiment, control circuit  18 ′ develops a three bit digital value used for the gain correction value and which is applied to DAC  215 . It should be noted that the control circuit  18 ′ illustrated in FIG. 5 replaces the control circuit  18  shown in FIG.  1 . 
     A register  213  may also be used with control circuit  18 ′ to develop the gain correction input signal  35  using a successive approximation technique. FIG. 6 discloses a processing sequence which may be used with the FIG.  1  and FIG. 5 circuits using a successive approximation technique to develop the gain correction input signal  35 . In this arrangement, the control circuit  18 ′ controls the processing sequence shown in FIG.  6 . Referring to FIG. 6, control circuit  18 ′ (FIG. 5) will start the PLL tuning cycle at processing segment  299 . A carrier channel input is set to a desired value by control circuit  18 ′, for example  110  for a 100 Mhz carrier signal at processing segment  301 . Control circuit  18 ′ will also set the 3 bit successive approximation register  213  to an initial binary value of 100 at processing segment  301 . Control circuit  18 ′ at processing segment  303  will next set the modulation input  15  to a first value, e.g. +1, and the loop will be allowed to settle. The Vkvco signal will be sampled and held by a sample and hold circuit  45  at processing segment  305 . The control circuit  18 ′ will next change the modulation input to another value, e.g. −1 at processing segment  307 . Next, the first Vkvco signal obtained with a +1 modulation input, which is stored in the sample and hold circuit  45 , will be compared with the second Vkvco signal obtained with the −1 modulation input by the analog comparison circuit  205 , and a signal will be output to amplifier  207  at processing segment  309 . The comparison output signal will next be converted to a digital signal by a digital signal to analog converter  209  in processing segment  311 . 
     The control circuit  18 ′ examines the digital comparison signal at processing segment  313 . If the comparison signal indicates a zero difference at segment  313 , then the successive approximation register values are not modified and a gain correction input signal  35 , based on the value in the successive approximation register  213 , is generated by control circuit  18 ′ and the analog output thereof is applied as the gain correction input signal  35  to scaling circuit  37  in FIG. 1 (segment  329 ). If the comparison signal is not zero, then the control circuit  18 ′ determines if the register value should be higher or lower than a binary value of 100 (the current value in the register  213 ) in processing segment  313 . If the Vkvco comparison indicates the register value should be lower, then the control circuit  18 ′ decrements the MSB value by setting it to zero and the next lower MSB (here the middle bit position) is selected and set to 1 at processing segment  321 . If the comparison of the two Vkvco values indicate the need for higher gain in the Vkmod path, or in other words a higher correction register value, the MSB is left alone (the MSB value of 1 is retained), and the next lower MSB is set to 1 at processing segment  321 . Then the control circuit  18 ′ uses the register  213  value to produce a digital correction signal which is converted back to an analog signal by DAC  215  at processing segment  323  and applied as an analog gain correction input signal  35  to scaling circuit  37  at processing segment  325 . 
     At processing segment  326  control circuit  18 ′ checks to see if there have been three passes through the processing sequence. If so, the processing sequence ends. If not, the processing sequence from segments  303  to  326  is repeated again. 
     Thus, the control circuit  18 ′ again sets the modulation to a first value, e.g. +1 (segment  303 ), and a Vkvco signal is stored by the sample and hold circuit  45  after the PLL settles (segment  305 ). The control circuit  18 ′ then sets the modulation input to a second value, e.g. −1 (segment  307 ), and a second Vkvco signal (−1) is acquired. Next, the first and second Vkvco (+1 and −1) signals are compared by the analog comparator  205  (segment  309 ) which generates a comparison signal which is digitized by ADC  209  (segment  311 ) and the result is checked by control circuit  18 ′ for a zero value (segment  313 ). If the value is zero, then the correction signal  35  is output and scaled onto the Vkmod path  33 . If the value is not zero, the digital comparison signal is checked by the control circuit  18 ′ to see if the digital value stored in the successive approximation register  213  should be higher or lower than its current value (segment  321 ). If the digital comparison signal indicates the register value should be lower, then the middle bit (in this 3 bit successive approximation register embodiment) is selected, set to zero and the least significant bit is set to one (segment  321 ). The digital correction input signal corresponding to the contents of register  213  is then generated by control circuit  18 ′ and converted back to analog form by DAC  215  for use by scaling circuit  37 . If the digital comparison value indicates that the value in the successive approximation register  213  should be higher than the current value, then the middle bit is left alone (i.e., a “1” value) and the least significant bit is set to 1. Once again, control circuit  18 ′ checks to see if there have been three passes through the FIG. 6 sequence. If so, the processing sequence ends and the correction input value is not further modified until the tuning cycle is again triggered by the control circuit  18 ′. If not, control circuit  18 ′ causes the process from segments  303  to  326  to repeat again. In the last pass through the sequence, the control circuit  18 ′ will compare the digital difference between the Vkvco (+1) and Vkvco (−1) signals and determine if the value stored in the successive approximation register should be higher or lower. If the value should be higher, then the least significant bit is left alone. If the value should be lower, then the least significant bit is set to zero. When process segment  326  is reached on the third pass through the FIG. 6 sequence, the process is terminated and control circuit  18 ′ uses the then stored value in register  213  as the digital correction value which is passed to DAC  215  which produces gain correction input signal  35 . 
     The digital value of one bit of the successive approximation register  213  will thus be determined at each iteration of the process sequence of FIG. 6 until the least significant bit is determined and the successive approximation register  213  contains a final value which is used as the final gain correction input signal  35  to scale the applied modulation signal in scaling circuit  37 . 
     FIG. 7 discloses yet another arrangement for generating the gain correction input signal  35  to scaling circuit  37 . This embodiment uses a unity gain amplifier  402  and an analog to digital converter (ADC)  403  which respectively amplify and convert an input analog Vkvco signal  29  to digital form. A carrier channel value  13  is set in the FIG. 1 system, and control circuit  18 ″ inputs a first modulation value, e.g. +1 into the FIG. 1 system which, after the PLL settles, produces a first correction voltage Vkvco (+1). The Vkvco (+1) signal is then converted to a 3 bit digital value by the ADC  403  which is stored in a 3 bit storage register  407 . Next, the control circuit  18 ″ inputs a second modulation value, e.g. −1, which, after the PLL settles, is converted to a digital value Vkvco (−1) by the ADC  403  which is stored in a 3 bit storage register  407 ′. The first (+1) and second (−1) Vkvco signals are then compared by digital comparator  411 , which produces a digital correction signal which is converted back to analog form by the digital to analog converter (DAC)  413 . The output of DAC  413  is the gain correction input signal  35  which is applied to scaling circuit  37  to scale a modulation input  15  in the Vkmod path to produce the applied gain corrected modulation input signal Vkmod  33  signal during subsequent operation of the PLL. 
     FIG. 8 discloses another two point modulation system for the PLL which may be used in the invention. In this arrangement, the modulation signal Vkmod  33  is added to the correction signal Vkvco  29  and the combined signal is used to modulate VCO  31 . 
     FIG. 9 discloses yet another two point modulation scheme for the PLL which may be used with the invention. The primary difference between this embodiment and the embodiment in FIG. 1 is the application of the modulation signal Vfm to the system as the signal Vfm 1  at an additive point  17  before the loop filter  27  through an integrator  401  and the application of the modulation signal Vfm as the signal Vfm 2  to an additive point  17 ′ downstream of the loop filter Vkvco  29  signal. The modulation signal Vfm 2  is scaled in scaling circuit  37  by the correction input. The scaling circuit  37  scales the modulation input signal Vfm 2  to produce the gain corrected modulation input Vkmod  33 . 
     As illustrated by the various disclosed embodiments, the present invention may employ analog, digital or a combination of analog and digital circuits to modulate and correct the modulated VCO output of a PLL. Also, control circuits  18 ,  18 ′,  18 ″ have been described which may also be implemented as analog, digital or a combination of analog and digital circuits. Also, various two point modulation schemes may be used with the invention to produce the desired gain matching. Thus, while exemplary embodiments of the invention have been described and illustrated, the present invention is not to be considered as limited by such description and illustration, but is only limited by the scope of the appended claims.