Abstract:
A predistortion generator for coupling in-line with a non-linear device produces an output predistorion signal of useful amplitude, but with low composite triple beat and cross modulation distortions. The predistortion circuit comprises a distortion circuit which utilizes the non-linear current flowing through at least one diode, to provide a desired amount of signal attenuation bandwidth. The distortion generator circuitry is always matched to the non-linear device, thereby ensuring a frequency response that is predictable and predefined.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to communication systems employing amplification devices. More particularly, the invention pertains to a second order predistortion circuit for coupling between an amplifier and a laser transmitter to minimize the second order distortion output by the laser transmitter.  
           [0002]    Amplifiers are widely used in many types of communication applications. For certain communication systems, such as optical communication systems, the amplifier is coupled with a laser transmitter, which generates the optical communication signal. As the need for more precise and reliable communication systems increases, it has become imperative to minimize distortions and achieve a linear frequency response.  
           [0003]    Directly modulating the analog intensity of a distributed feedback (DFB) laser is widely used to transmit analog signals, (such as sound or video signals and data), on optical fibers over a long distance. Such amplitude modulation signal typically suffers from nonlinearity of the optical source. DFB lasers are limited primarily by second order distortion.  
           [0004]    Laser nonlinearities limit the optical modulation depth M that can be used in the laser. Since the carrier-to-noise ratio of the signal is proportional to the square of the optical modulation depth M, by reducing second order distortion products, the optical modulation depth M can be increased, thus greatly improving system dynamic range.  
           [0005]    Referring to FIG. 1, a common prior art method of using a standard RF push-pull amplifier to drive a laser transmitter is shown. A signal is input into the RF amplifier  10  which is connected to a laser transmitter via a balun  12 . A balun is a type of transmission line transformer (BALanced-UNbalanced) which allows for the transition between a unbalanced circuit and a balanced circuit and permits impedance matching. In FIG. 1, one leg of the balun  12  is connected to ground, while the other leg of the balun is output to the laser transmitter or other predistortion generating circuits. When an RF amplifier is used to drive a laser transmitter as shown in FIG. 1 over a broad frequency range of input signals, the laser output may be distorted in a non-linear fashion over the frequency range. This non-linear distortion, if not corrected, will degrade the signal performance transmitted by the laser as the output becomes less predictable.  
           [0006]    Prior art solutions require the use of numerous complex distortion circuits to correct for second and third order distortion over a broad frequency range. Each distortion circuit corrects a limited portion of the broad frequency range to be transmitted by the laser. For example, U.S. Pat. No. 5,523,716 (Grebliunas) discloses an in-line third order predistortion circuit for satellite applications. Because of the different frequency ranges, bandwidths and power ranges, this design is not appropriate for CATV applications. The power in a satellite applications is much greater than for CATV applications. Accordingly, the diodes used in a satellite application need not be biased. In contrast, for CATV applications, the diodes must be forward biased.  
           [0007]    U.S. Pat. No. 5,119,392 (Childs) discloses an inline second order predistortion circuit for use with a laser diode. A field effect transistor (FET) biased for square law operation generates the predistortions. Due to field and doping-dependent variations in carrier mobility, the exact exponent N that can be achieved with a FET varies from between 1.0 and 2.7. The deviation of the exponent N from an ideal number, (i.e., 2.0), causes third order distortion. The difficulties in achieving an ideal exponent N equal to 2, and a good RF frequency response across the entire frequency band by using single stage FET amplifiers limits the performance of this predistortion circuit.  
           [0008]    Likewise, U.S. Pat. No. 5,600,472 (Uesaka) and U.S. Pat. No. 5,798,854 (Blauvelt et al.) also generally disclose forward bias diodes used for inline second order or third order predistortion circuits.  
           [0009]    Most prior art distortion circuits also require complementary temperature correction circuits for proper operation over a wide range of temperatures. However, each predistortion circuit that is introduced creates additional distortions and losses that degrade the overall performance of the laser transmitter.  
           [0010]    Accordingly, it is advantageous to minimize the number of pre- or post-distortion circuits which are utilized to correct for the distortion of the RF amplifier and the laser transmitter.  
         SUMMARY  
         [0011]    The present invention is a second order predistortion circuit for coupling between an amplifier and a laser transmitter. The circuit includes a non-linear attenuating circuit which is coupled to a transmission line transformer. The distortion amplifier circuitry is always matched to the laser transmitter, thereby ensuring a frequency response that is predictable and predefined.  
           [0012]    Objects and advantages of the of the present invention will become apparent to those skilled in the art after reading a detailed description of the preferred embodiment.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic diagram of a prior art circuit comprising an RF amplifier coupled with a laser transmitter.  
         [0014]    [0014]FIG. 2 is a schematic diagram of one embodiment of the distortion amplifier of the present invention having a distortion circuit with a single diode coupled to the ground path of an RF amplifier.  
         [0015]    [0015]FIG. 3 is a schematic diagram of a first alternative embodiment of the distortion amplifier of the present invention.  
         [0016]    [0016]FIGS. 4 and 5 are schematic diagrams of second and third alternative embodiments of the distortion amplifier of the present invention having distortion circuits with two series-coupled diodes.  
         [0017]    [0017]FIGS. 6 and 7 are schematic diagrams of fourth and fifth alternative embodiments of the distortion amplifier of the present invention having distortion circuits with two parallel-coupled diodes.  
         [0018]    [0018]FIG. 8 is the preferred method in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The preferred embodiment of the present invention will be described with reference to the drawing figures where like numerals represent like elements throughout.  
         [0020]    Referring to FIG. 2, the preferred embodiment of a predistortion circuit  60  made in accordance with the present invention is shown. The predistortion circuit  60  is coupled between an RF amplifier  10  and a laser transmission or other non-linear device (NLC) (not shown) and includes a transistor line transformer such as a balun  12  and a distortion circuit  23 .  
         [0021]    The balun  12 , is coupled between the output of the RF amplifier  10  and a laser transmitter (not shown). The balun  12  provides impedance matching and allows for the transition between the balanced RF amplifier  10  and the unbalanced laser transmitter (not shown). The RF amplifier  10  is preferably a push-pull type amplifier.  
         [0022]    The distortion circuit  23  is attached to one leg of the balun  12 , between the balun  12  and ground. The distortion circuit  23  comprises a resistor R 1 , a capacitor C 1  and diode D 1 . A DC bias current is also applied at a DC bias input  25  between the resistor R 1  and the capacitor C 1 .  
         [0023]    In any multichannel RF communication environment, the ratio of the peak RF current to the average RF current, (i.e., the peak to average ratio), is dependent upon the number of channels present. Accordingly, the peak to average ratio will change depending upon the number of channels in the system. In a CATV environment, due to the large number of channels, (“typically” 110 channels), the peak to average current ratio is 3 to 1. Of course, those of skill in the art should realize that a CATV environment can have from as few as 30 channels to as many as 132 or more channels. For ease of explanation, the foregoing description will assume a CATV environment of approximately 110 channels.  
         [0024]    The values of the components R 1 , C 1  and D 1  must be specifically selected so that: 1) approximately one-third to one-half of the RF output current from the RF amplifier  10  flows through the diode D 1  and 2) the DC bias current to the diode D 1  (which forward-biases the diode D 1 ) is approximately between two to three times the value of the RF current through the diode D 1 . These current ratios are critical to efficient operation of the distortion circuit  23 . If the magnitude of the DC bias current is not approximately three times the magnitude of the RF current, clipping of the signal output to the laser transmitter may result. When the RF current flows in the direction from point  3  to point  1 , the balun  12  output current flows in the direction from point  2  to point  4 .  
         [0025]    In general, the distortion characteristics of the diode D 1  are matched to the distortion characteristics of the particular laser transmitter. The distortion circuit  23  creates second order distortions to compensate for the distortions generated by the laser transmitter. The particular internal junction resistance of the diode D 1  (i.e., its IV curve), are exploited for matching with the IV curve of the laser transmitter to provide predistortion to the laser transmitter. The amount of RF current through the diode D 1  determines the level of distortion output by the distortion circuit  23 .  
         [0026]    The predistortion circuit  23  basically includes two signal paths; the first signal path  31  comprising the resistor R 1  and the capacitor C 1 ; and the second signal path  35  comprising the diode D 1 . Referring to the first signal path  31 , this path  31  basically affects the operating point of the diode D 1 . The DC bias input  25  provides a DC bias current to forward bias the diode D 1 . The capacitor C 1  acts as a DC block, permitting the DC bias current from the DC bias input  25  to flow up through the first signal path  31  and over to the second signal path  35  to the diode D 1 . However, since the capacitor C 1  is invisible to the RF current, the RF current will be divided between the first signal path  31  and second signal path  35 . From an RF point of view, the resistor R 1  is in parallel with the diode D 1 . But from a DC point of view, the resistor R 1  and the diode D 1  are in series. This forces the DC bias current from the DC bias input  25  through the diode D 1  to forward bias the diode D 1 .  
         [0027]    The unique location of the DC bias input  25  provides a significant advantage over other prior art correction circuits. Since the DC bias input  25  is at ground potential, it is essentially self-isolated and does not require any additional measures for decoupling, thereby greatly simplifying the overall circuit design. The DC bias input  25  plays an important role in overall distortion level and frequency response control. Basically, when the bias current is low, low frequencies get more second order correction; whereas a higher bias current provides more correction at higher frequencies. In this manner, the bias current may be controlled for different laser distortion characteristics. Thus, an RF filter that is typically required in the prior art, is not required for the present invention. This eliminates the unwanted negative effects of an RF filter, which can change both the phase and the frequency of the distortion.  
         [0028]    It should also be noted that there is a negligible temperature dependency of the circuit. Since the DC bias current is relatively large, (i.e., 5-15 mA), the diode internal junction resistance is very small. This should be compared with prior art circuits having bias current in the microamp range. Therefore, the forward DC diode bias current across D 1  is primarily determined by the DC bias current input  25 . As a result, the current variation due to the temperature variation is minimal and a separate temperature correction circuit is not necessary. Experimental data shows that when temperature changes from 0-65° C., the correction results change less than 1-2 dB. This is a significant advantage over existing prior art circuits.  
         [0029]    Since the diode internal junction resistance is very small, the distortion circuit  23  adds an equivalent additional resistance of approximately 2 ohms. The influence of the distortion circuit  23  upon the operation of the RF amplifier  10  is therefore small.  
         [0030]    Since a laser is a square law device, it tends to minimize third order distortion but has a very limited immunity to second order distortion. The predistortion circuit  60  of the present invention can create a correction signal that may be tailored to a particular laser. This predistortion circuit  60  corrects second order distortion while having little effect on third order distortion. The tailoring of the predistortion circuit  60  is accomplished by the selection of the diode D 1 . Diode internal series resistance and diode internal junction resistance, which are determined by the diode current, determine the diode total resistance which, in turn, determines the amount of current flowing through the diode D 1 .  
         [0031]    There are several technical observations that can be made about the distortion circuit  23  made in accordance with the teachings of the present invention. As the RF current flowing through the diode D 1  is increased, the equivalent diode resistance is reduced and the total resistance between point  1  and ground is reduced. This causes the output internal resistance of the predistortion circuit  60  at point  2  to be reduced. As the RF current flowing through the diode D 1  is decreased, the output internal resistance of the predistortion circuit  60  at point  2  is increased. This change in resistance creates the second order predistortion needed for the laser transmitter.  
         [0032]    For an example, for a Fujitsu laser having a power range from 3-10 mW, the DC current flowing through the diode D 1  is 5-16 mA, which is much higher than prior art inline predistortion circuits. For example, the circuit disclosed in U.S. Pat. No. 5,600,472 includes a diode current of between 0 and 400 μA; and the circuit disclosed in U.S. Pat. No. 5,798,854, includes a diode current of 20 μA. Because of the large diode DC current, the amount of diode internal series resistance is also important for present invention, which is not important for the prior art distortion circuit.  
         [0033]    An alternative embodiment of a predistortion circuit  62  made in accordance with the teachings of the present invention is shown in FIG. 3. This embodiment differs from the embodiment shown in FIG. 2 only in the polarity of the diode D 1 . The embodiments shown in FIGS. 2 and 3 generate different second order distortion. However, the goal of a simplified second order distortion generation can be achieved utilizing either of these configurations.  
         [0034]    As a laser transmitter optical output power becomes larger, it typically requires more RF drive power and more DC drive current, resulting in Schottky diodes which require more DC bias drive current. Referring to FIGS. 4 and 5, two additional alternative embodiments of predistortion circuits  70 ,  80  made in accordance with the present invention are shown. These embodiments are utilized for applications where higher output powers are required than can be achieved utilizing a single diode.  
         [0035]    As shown in FIG. 4, the predistortion circuit  70  includes a balun  12  and a distortion circuit  27 . The predistortion circuit  27  is attached one leg of the balun  12 , between the balun  12  and ground. The distortion circuit  27  includes two resistors R 1 , R 2 , a capacitor C 1  and two diodes D 1 , D 2 . A DC bias current is applied at a DC bias input  25  between the resistor R 2  and the capacitor C 1 . The distortion circuit  27  includes a first signal path  38  and a second signal path  39 . The first signal path comprises resistors R 1 , R 2 , DC bias input  25  and capacitor C 1  in series. The second signal path  39  comprises diodes D 1  and D 2  in series. The two diode/resistor pairs (i.e., D 1 /R 1  and D 2 /R 2 ), are coupled in series in order to achieve greater output powers from the distortion circuit  27 .  
         [0036]    The predistortion circuit  80  shown in FIG. 5 includes a balun  12  and a distortion circuit  33 . The distortion circuit  33  is attached to one leg of balun  12 , between balun and ground. The distortion circuit  33  comprises two resistors R 1 , R 2 , a capacitor C 1  and two diodes D 1 , D 2 . A DC bias current is applied at a DC bias input  25  between the resistor R 2  and the capacitor C 1 . The distortion circuit  33  includes a first signal path  40  and a second signal path  41 . The first signal path  40  comprises resistors R 1  and R 2 , DC bias current input  25  and capacitor C 1  in series. The second signal path  41  comprises diodes in series. This predistortion circuit  33  operates in the same manner as the distortion circuit  27  shown in FIG. 4 except that a different response characteristic is achieved due to the reverse polarity of the diodes D 1 , D 2 .  
         [0037]    Referring to FIG. 6, an alternative embodiment of the predistortion circuit  90  of the present invention is shown. The predistortion circuit  90  is coupled to a laser transmitter (not shown). The distortion amplifier  90  includes a balun  12  and a distortion circuit  20 . The distortion circuit  20  is attached to one leg of the balun  12 , between the balun  12  and ground. The distortion circuit  20  comprises an inductor L 1 , a resistor R 1 , a capacitor C 1  and two diodes, D 1 , D 2 . A DC bias current is also applied at a DC bias input  25  between the resistor R 1  and the capacitor C 1 .  
         [0038]    The distortion circuit  20  of this embodiment includes two signal paths  28 ,  29 , and operates upon the same physical principals as were discussed with reference to FIGS. 2, 3,  4  and  5 . However, this embodiment includes an inductor L 1  in the first signal path  28  and includes parallel-coupled diodes D 1  and D 2  in the second signal path. The inductor L 1  improves the frequency response of the distortion circuit  20  over the entire frequency bandwidth and optimizes the frequency response of the distortion circuit  20 .  
         [0039]    The parallel-coupled diodes D 1 , D 2  present a unique solution for RF current distribution between the two signal paths. By coupling diodes D 1  and D 2  in parallel, it is possible to obtain an equivalent diode internal series resistance. Since, in most cases, diode internal series resistance is determined during the manufacturing process of the diodes and cannot be changed by a user, selecting a unique combination of two different types of diodes D 1  and D 2 , it is possible to obtain the equivalent diode internal series resistance that is required for matching with the laser transmitter. For the embodiment shown in FIG. 6, the two diodes D 1 , D 2  have been combined to form a new equivalent diode, which has the specific equivalent diode internal series resistance that is desired.  
         [0040]    The inductance L 1  is used to raise the second order correction ability of the circuit in the higher frequency ranges. The inductor L 1  helps the RF response of the distortion circuit  20  across the entire frequency bandwidth. Without the inductor L 1 , the RF response of the distortion circuit  20  would essentially be flat. The inductor L 1  tilts the frequency response of the distortion circuit  20  such that a greater amount of distortion is provided at higher frequencies, where it is most needed. This permits the distortion circuit  20  to better match the distortion of the laser transmitter across the entire frequency bandwidth.  
         [0041]    Referring to FIG. 7, an alternative embodiment of a predistortion circuit  100  made in accordance with the present invention is shown. This embodiment is similar to the embodiment shown in FIG. 6, except that the first signal path  31  does not include the inductor L 1 . Although the inductor L 1  improves the frequency response over the entire frequency bandwidth and optimizes the frequency response of the distortion circuit  20 , it is not necessary for operation. Accordingly, this embodiment of the distortion circuit  22  is slightly less complicated than that shown in FIG. 6.  
         [0042]    Table 1 shows the specifications for the components in the embodiments described hereinbefore. Those of skill in the art should recognize that the specific components will change depending upon the response desired. This depends upon the type of RF amplifier  10 , the balun  12 , the laser transmitter (not shown) and/or other circuits or NLDs (not shown) to which the predistortion circuit of the present invention is coupled.  
                           TABLE 1                                   COMPONENT   SPECIFICATIONS                           C 1     0.1 microfarad           D 1     Hewlett-Packard HSMS-2822           D 2     Alpha Industries SMS 7621 Schottky           L 1     1.5 nano Henries           R 1     3 ohms           Bias Control   15 milliamp           Amplifier   Analogic ACA 0860                      
 
         [0043]    As shown and described, the present invention: a) reduces the number of second order distortion generating circuits to a single second order distortion generating circuit operating over a very wide frequency bandwidth (55-860 MHz and greater); b) eliminates or minimizes the need for additional temperature compensation circuitry thereby making the distortion circuit temperature independent; c) has limited impact upon the signal to be transmitted with respect to third order distortion, thereby simplifying any third order distortion generating circuit that may be needed; and d) due to its simplicity, it occupies significantly smaller silicon surface area when implemented than the prior art distortion generating circuits.  
         [0044]    Utilizing the present invention, the CSO correction ability can be quite large. Usually it can correct about 10 dB across the 55-750 MHz frequency band. The largest CSO correction ability is −55 dBc. In this case, the worse case CSO at −55 dBc can be corrected to better than −65 dBc across the 55-860 MHz frequency band.  
         [0045]    By eliminating the prior art need for multiple second order distortion circuits and their associated multiple temperature compensation circuits, the present invention will permit significantly smaller and cheaper laser transmitters. Utilizing the present invention, for example, will permit a doubling in the number of laser transmitters and the same amount of module space.  
         [0046]    It should also be noted that any of the embodiments of the predistortion circuit in accordance with the present invention may be coupled together with an RF amplifier as a single unit to create an RF distortion amplifier. All of the teachings herein are equally applicable to such a configuration.  
         [0047]    Referring to FIG. 8, the preferred method  200  of generating a distorted RF signal in accordance with the present invention is shown. The method is initiated when a linear differential RF signal is input into a transmission line transformer (step  202 ). It should be understood that the transmission line transformer is coupled to an NLD in a first winding and a distortion circuit in a second winding. The distortion circuit is then forward-bias, as desired, (step  204 ) and the linear differential RF signal that is input is distorted by the distortion circuit to create a distorted RF signal (step  206 ). The distorted RF signal is then coupled from the second winding to the first winding (step  208 ) and then output to the NLD (step  210 ).