Patent Publication Number: US-11038461-B2

Title: Optocoupler emulating input stage for digital isolators

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This continuation application claims priority to U.S. patent application Ser. No. 16/223,545, filed Dec. 18, 2018, which application claims priority to Indian Provisional Application No. 201841038738, filed Oct. 12, 2018, both of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     An optocoupler transmits signals between isolated circuits. A transmitting end of an exemplary optocoupler emits light based on a signal to be transmitted. A receiving end of the exemplary optocoupler comprises a switch responsive to the light. The switch of the receiving end is activated to generate a signal based on the light. The generated signal reflects the signal activating a LED of the transmitting end emitting the light. 
     Utilizing the LED, the exemplary optocoupler transmits a signal while preventing voltage or current of the transmitting end from affecting the receiving end. The LED employed by the exemplary optocoupler has a current-voltage (IV) characteristic of a diode. Optocoupler emulators, designed to replace an optocoupler, generally fails to completely mimic the operational characteristics of an optocoupler employing a LED with a diode IV characteristic. 
     SUMMARY 
     An aspect of the present invention provides a diode emulating oscillator comprising a set of bipolar junction transistors coupled in series, an inductor capacitor (LC) oscillator coupled to the set of bipolar junction transistors, and a current mirror transistor coupled to the set of bipolar junction transistors and the LC oscillator, wherein the LC oscillator is configured to generate a modulated signal based on a current flowing through the current mirror transistor when a current flows through the set of bipolar junction transistors. 
     An aspect of the present invention also provides a digital isolator comprising a diode emulating oscillator configured to generate a modulated signal, an isolation barrier configured to transmit the modulated signal, and a receiver configured to receive and demodulate the modulated signal, wherein the diode emulating oscillator comprises a set of bipolar junction transistors and an inductor capacitor (LC) oscillator coupled to the set of bipolar junction transistors in parallel, and wherein the LC oscillator is configured to generate the modulated signal when a current flows through the set of bipolar junction transistors. 
     An aspect of the present invention further provides a digital isolator comprising a diode emulating oscillator configured to generate a spread spectrum modulated signal, an isolation barrier configured to transmit the spread spectrum modulated signal, and a receiver configured to receive and demodulate the spread spectrum modulated signal, wherein the diode emulating oscillator comprises a set of bipolar junction transistors, an inductor capacitor (LC) oscillator coupled to the set of bipolar junction transistors in parallel, and a spread spectrum module coupled to the LC oscillator, and wherein the LC oscillator is configured to generate a modulated signal when a current flows through the set of bipolar junction transistors and the spread spectrum module is configured to adjust a frequency of the modulated signal to generate the spread spectrum modulated signal. 
     An aspect of the present invention further provides a digital isolator comprising a diode emulating oscillator configured to generate a modulated signal, an isolation barrier configured to transmit the modulated signal via a channel, and a receiver configured to receive and demodulate the modulated signal, wherein the diode emulating oscillator comprises an inductor capacitor (LC) oscillator circuit configured to generate an on-off key based modulated signal after an application of current or voltage input above a threshold of the LC oscillator circuit, wherein the on-off key based modulated signal reflects the application of current or voltage input above the threshold and an amplitude of the input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of various examples, reference will now be made to the accompanying drawings in which: 
         FIG. 1  illustrates an exemplary digital isolator transmitting a signal with a LED; 
         FIG. 2  illustrates a digital isolator with an optocoupler emulated input according to a prior art; 
         FIG. 3  illustrates an exemplary circuit of a diode emulating isolator according to an aspect of the present invention; 
         FIG. 4  illustrates a digital isolator according to an aspect of the present invention; 
         FIG. 5  illustrates the IV sweep of the diode emulating oscillator according to an aspect of the present invention; 
         FIG. 6  illustrates an architecture of a diode emulating oscillator according to another aspect of the present invention; 
         FIG. 7  illustrates a digital isolator according to an aspect of the present invention; 
         FIG. 8  illustrates the change of frequency of the spread spectrum modulated signal transmitted from diode emulating isolator to receiver over time, and the corresponding amplitude of the spread spectrum modulated signal also over time; and 
         FIGS. 9-13  illustrates exemplary circuits of diode emulating isolator according to various aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to certain examples of the present invention. These examples are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other examples may be employed and that various structural, logical, and electrical changes may be made. Moreover, while specific examples are described in connection with a digital isolator, it should be understood that features described herein are generally applicable to other types of electronic parts or circuits. 
     In this description, the term “couple” or “couples” means either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, X may be a function of Y and any number of other factors. 
       FIG. 1  illustrates an exemplary optocoupler  100  transmitting a signal with a LED  111 . When voltage V F  is applied to the transmitting end  110 , current I F  flows through LED  111  and LED  111  emits a light. The light activates a bipolar junction transistor  121  of the receiving end  120  of the optocoupler  100 . When the bipolar junction transistor  121  is activated, current I C  flows through the bipolar junction transistor  121  and the receiving end  120  generates a signal S 1  reflective of the LED light. 
     When voltage V F  or current I F  represents data (e.g., when V F  or I F  is applied to the transmitting end  111  when a data to be transmitted is “1”), the LED  111  emits light that reflects the data (e.g., the LED  111  emits light when a data to be transmitted is “1”). Signal S 1  generated from the receiving end  120 , in turn, reflects the data (e.g., S 1  is output when a data transmitted by the transmitting end  110  is “1”). Accordingly, data has been transmitted from the transmitting end  110  to the receiving end  120  while the transmitting end  110  and the receiving  120  are electrically isolated. 
     Systems requiring power or noise isolation between two domains use an optocoupler, an example of which is illustrated in  FIG. 1 , to transmit data between the two domains. Such systems may include a sensor node and a motor interfaced by a mobile control unit, wherein the motor side of the domain uses a high voltage that needs to be isolated from the sensor node. Such system may also be a system that has very large voltage transients between two or more grounds. 
       FIG. 2  illustrates a digital isolator with an optocoupler emulated input according to a prior art. The digital isolator  200  according to a prior art comprises a voltage clamp  210 , to which voltage V F  or current I F  is applied based on a preferred configuration, a discharge circuit  220  to discharge excess voltage or current, and a spread spectrum oscillator  230  generating a spread spectrum modulated signal based on voltage V F  or current I F . 
     The spread spectrum oscillator  230  comprises a bias generation module  231 , an oscillator  232 , and a spread spectrum module  233 . The bias generation module  231  applies V Bias  to oscillator  232  after voltage V F  or current I F  over a predetermined threshold is applied to the digital isolator  200 . When V Bias  is applied, the oscillator  232  generates a modulated signal. Spread spectrum module  233  adjusts the frequency of the modulated signal of the oscillator  232  so that the frequency of the modulated signal is spread across spectrum and prevents high electromagnetic radiation. The spread spectrum modulated signal S 2  is output via V Out  of the spread spectrum oscillator  230 . 
     A goal of digital isolator  200  is to mimic the operational characteristics of a digital isolator with a LED with an IV characteristic of a diode. Another goal of digital isolator  200  is to be compatible with the systems and parts of an optocoupler. 
     The prior art digital isolator of  FIG. 2 , however, is constructed without diodes and fails to completely mimic the operational characteristics of an optocoupler or a digital isolator with a LED. Also, a digital isolator constructed using CMOS transistors, such as the prior art digital isolator of  FIG. 2 , is much more expensive than an optocoupler, making it less competitive. 
       FIG. 3  illustrates an exemplary circuit of a diode emulating isolator according to an aspect of the present invention. The diode emulating isolator  300  of  FIG. 3  comprises a set of bipolar junction transistors  301 ,  302  coupled in series, an inductor capacitor (LC) oscillator  310  coupled to the set of bipolar junction transistors  301 ,  302  in parallel, and a current mirror transistor  303  coupled to the set of bipolar junction transistors  301 ,  302  and the LC oscillator  310 . The diode emulating isolator  300  of  FIG. 3  may further comprise a diode  341  to prevent a reverse current flow when current is applied to the diode emulating oscillator  300 . According to an aspect of the present invention, diode  341  may be replaced with CMOS or any other reverse current flow preventing circuitry. 
     When a current or voltage above a current or voltage threshold of bipolar junction transistors  301 ,  302  is applied to diode emulating isolator  300 , bipolar junction transistors  301 ,  302  are turned on. A base of current mirror transistor  303  is coupled to a base of one transistor  302  of the set of bipolar junction transistors  301 ,  302 . When current flows through the one transistor  302  of the set of bipolar junction transistors  301 ,  302 , current mirror transistor  303  is turned on. 
     The collector of the current mirror transistor  303  is coupled to LC oscillator  310 . When current flows through the set of bipolar junction transistors  301 ,  302 , and current mirror transistor  303 , LC oscillator  310  is excited to generate a high frequency modulated signal. In the example of  FIG. 3 , the LC oscillator  310  generates a set of differential signals that is output via port OUT-N and port OUT-P. 
     LC oscillator  310  comprises a set of inductors  311 ,  312 , which are coupled in series, and a set of capacitors  321 ,  322 , which are coupled in series. The set of capacitors  321 ,  322  is coupled to the set of inductors  311 ,  312  in parallel. The LC oscillator  310  further comprises a CMOS transistor  331 ,  332  amplifying the signals of LC oscillator when the LC oscillator  310  is excited upon a current application to the diode emulating oscillator  300 , wherein the current applied is above the current threshold of the bipolar junction transistors  301 ,  302 . According to an aspect of the present invention, LC oscillator  310  may comprise an oscillator of a different architecture, and is not limited to the exemplary circuitry illustrated in  FIG. 3 . 
     When LC oscillator  310  is excited by the above threshold current or voltage application to the diode emulating oscillator  300 , a set of high frequency differential signals is generated and output across the set of capacitors  321 ,  322 . As noted below regarding  FIG. 4 , the set of differential signals is transmitted across the isolator barrier to communicate to the receiver of a digital isolator presence of data. 
     When a current applied to the diode emulating oscillator  300  is below a current threshold of LC oscillator  310 , an output of a set of differential signals via port OUT-N and port OUT-P is not guaranteed because LC oscillator  310  requires a minimum current to start oscillating. This is similar to the operational characteristics of an optocoupler where a LED may or may not be fully lit where a current below a diode current threshold is applied to the optocoupler. 
     When no current is applied to the diode emulating oscillator  300 , LC oscillator  310  is off and communication channels across port OUT-N and port OUT-P are silent. 
       FIG. 4  illustrates a digital isolator according to an aspect of the present invention. The digital isolator of  FIG. 4  comprises a diode emulating oscillator  300 , an isolator barrier  410 , and a receiver  420 . The isolator barrier  410  comprises capacitors to transmit the set of differential signals output from port OUT-N and port OUT-P of diode emulating oscillator  300  to receiver  420 . According to an aspect of the present invention, isolation barrier  410  may also comprise inductors or other forms of capacitive or inductive circuitry to transmit the signal output. 
     Receiver  420  comprises a pre-amplifying module  421  amplifying the set of differential signals generated and transmitted from diode emulating oscillator  300 . Receiver  420  further comprises an envelop detector  422  to detect the modulated signal transmitted by the diode emulating oscillator  300  after the modulated signal is amplified by the pre-amplifying module  421 . 
     The examples illustrated in  FIG. 3  and  FIG. 4  provide a new, simple architecture for emulating an optocoupler. The diode emulating oscillator  300  of  FIGS. 3 and 4  modulates signals based on on-off keying. LC oscillator  310  is turned on when a data to be transmitted is present and an input current or voltage higher than the threshold value of the bipolar junction transistors  301 ,  302  is applied to the diode emulating oscillator  300 . This in turn excites the LC oscillator  310  to generate a set of differential signals output via port OUT-N and port OUT-P. There is no need for a separate power on circuit to turn on LC oscillator  310 . Conversely, when data to be transmitted is “0,” no current is applied to the bipolar junction transistors  301 ,  302 . LC oscillator  310  is turned off, and no signals are output via port OUT-N and port OUT-P. 
     Furthermore, a voltage of two diode drops is maintained across the set of bipolar junction transistors  301 ,  302 . As such, unlike the prior art of  FIG. 2 , the examples of  FIG. 3  and  FIG. 4  do not require an active internal voltage, current regulation. 
     In addition, differential signal drivers are not necessary for the diode emulating oscillator  300  to drive the capacitors of the isolation barrier  410  because the LC oscillator  310  inherently drives the capacitors of the isolator barrier  410  differentially. Low cost version of a digital isolator can be also realized because the CMOS transistors  331 ,  332  of LC oscillator  310  may be replaced with bipolar junction transistors. The architecture of  FIG. 3  and  FIG. 4  further minimizes the number of base masks and isolation of voltage or noise can be realized by having the isolator barrier ( 410 ), e.g., capacitors, implemented only on the receiver  420 . This further reduces the cost of the diode emulating oscillator  310  die. 
     According to an example of the present invention, the current gain value of current mirror transistor  303  in relation to the current gain value of transistor  302  can be adjusted. For instance, when the current gain value of current mirror transistor  303  is ten (10) times the current gain value of transistor  302 , a current applied to LC oscillator  310  is ten times the current applied to the set of bipolar junction transistors  301 ,  302 . By adjusting the current gain value of current mirror transistor  303  in relation to the current gain value of transistor  302 , diode emulating oscillator  300  can adjust the amount of current channeled to LC oscillator  310  to generate a modulated signal to be transmitted. 
     In the examples of  FIGS. 3 and 4 , the amplitude of the modulated signal output from the LC oscillator  310  is dependent on the value of input to the diode emulating oscillator  300 . According to an aspect of the present invention, the change in the amplitude of the modulated signal output from the LC oscillator  310  is detectable from the receiver  420 . For instance, the envelop detector  422  may detect the change in the amplitude of the modulated signal transmitted across the isolation barrier  410 , which reflects the change in the size of current or voltage applied to the diode emulating oscillator  300 . Accordingly, analog signal can be transmitted more effectively while maintaining analog signal isolation. 
       FIG. 5  illustrates the IV sweep of the diode emulating oscillator  300  according to an aspect of the present invention. The input V indicates the voltage applied to the anode and cathode of the diode emulating oscillator  300 . The current line of the prior art of  FIG. 2 , S 11 , illustrates a current jump around input V of 1.6-1.8V. The current line of the diode emulating oscillator  300 , S 12 , illustrates no current jump around input V of 1.6-1.8V, more accurately mimicking the IV characteristics of a diode. 
       FIG. 6  illustrates an architecture of a diode emulating oscillator according to another aspect of the present invention. Diode emulating oscillator  600  of  FIG. 6  comprises a set of bipolar junction transistors  601 ,  602 , a current mirror transistor  603 , a LC oscillator  610 , and a diode  641 . The operations and functions of bipolar junction transistors  601 ,  602 , current mirror transistor  603 , LC oscillator  610 , and diode  641  is comparable to the operations and functions of bipolar junction transistors  301 ,  302 , current mirror transistor  303 , LC oscillator  310 , and diode  341  of  FIG. 3 . According to an aspect of the present invention, LC oscillator  610  may comprise an oscillator of a different architecture, and is not limited to the exemplary circuitry illustrated in  FIG. 6 . 
     Diode emulating oscillator  600  of  FIG. 6  example further comprises a spread spectrum module  650 . Spread spectrum module  650  comprises a set of switchable capacitors  651 ,  652 , and a spread spectrum module controlling unit  655  configured to turn on or turn off the set of switchable capacitors  651 ,  652 . Although  FIG. 6  illustrates two switchable capacitors  651 ,  652 , more or less capacitors may be employed depending on a need to adjust the modulating frequency. 
     The set of switchable capacitors  651 ,  652  is coupled to the set of capacitors  621 ,  622  of LC oscillator  610 . By turning on and off the set of switchable capacitors  651 ,  652 , spread spectrum module  650  adjusts the frequency of the modulated signal generated by LC oscillator  610 . The frequency of the modulated signal output of LC oscillator  610  is based on the resonating frequency of the inductors and capacitors of a LC oscillator. By turning on and off the switchable capacitors  651 ,  652 , the effective capacitance in the LC tank changes, hence changing the resonating frequency of the LC oscillator. The modulated signal frequency of the LC oscillator  610  is hence spread across a wider spectrum. 
     In  FIG. 6 , a voltage over the set of bipolar junction transistors  601 ,  602  operates as a power source to spread spectrum module  650 . No separate power source is required to turn on or off spread spectrum module  650 . In addition, the voltage over the set of bipolar junction transistors  601 ,  602  is maintained to two diode voltage drops. Accordingly, no current or voltage regulation is required. 
       FIG. 7  illustrates a digital isolator according to an aspect of the present invention. The digital isolator of  FIG. 7  comprises the diode emulating oscillator  600  of  FIG. 6 , an isolator barrier  710 , and a receiver  720 . The operations and functions of isolation barrier  710  and receiver  720  are comparable to the operations and functions of isolation barrier  410  and receiver  420  of  FIG. 4 . 
     The spread spectrum modulated signal output from diode emulating oscillator  600  is output via port OUT-N and port OUT-P and transmitted to receiver  720  via channel formed across the capacitors of isolation barrier  710 . The capacitors of isolation barrier  710  transmits the spread spectrum modulated signal to the receiver  720  while preventing voltage and noise from being transmitted. According to an example of the present invention, isolation barrier  710  may comprise an inductor, or any other types of capacitive or inductive circuitry. 
       FIG. 8  illustrates the change of frequency (S 21 ) of the spread spectrum modulated signal transmitted from diode emulating isolator  600  to receiver  720  over time, and the corresponding amplitude of the spread spectrum modulated signal (S 22 ) also over time. By adjusting the frequency of the modulated signal and spreading the frequency across spectrum, electromagnetic emissions may be controlled. 
       FIG. 9  illustrates an exemplary circuit of diode emulating isolator according to an aspect of the present invention. In particular, the example of  FIG. 9  comprises a set of bipolar junction transistors (BJTs)  901 ,  902  coupled in series, LC oscillator  910  coupled to the set of BJTs  901 ,  902  in parallel, a current mirror transistor  903  coupled to the set of BJTs  901 ,  902  and LC oscillator  910 . The diode emulating isolator  900  of  FIG. 9  may also comprise a diode  941  to prevent reverse current flow when current is applied to the diode emulating oscillator. According to an aspect of the present invention, diode  941  may be replaced with CMOS transistor or any other reverse current flow preventing circuitry. The set of BJTs  901 ,  902 , current mirror transistor  903 , and diode  941  of  FIG. 9  are comparable and operate similarly to the set of BJTs  301 ,  302 , current mirror transistor  393 , and diode  341  of  FIG. 3 , respectively. 
     After an input of current or voltage over a threshold of the set of BJTs  901 ,  902  and a threshold of LC oscillator  910  is applied to diode emulating isolator  900 , LC oscillator  910  is excited to output a pair of differential signals reflecting the current or voltage input via ports OUT-N and OUT-P. The pair of differential signals is on-off key based modulated signals and the amplitude of the modulated signals is positively correlated with the amplitude of the current or voltage input. For instance, when a higher voltage or current is applied to the diode emulating isolator  900 , the pair of differential signals output has a bigger amplitude. The amplitude of the differential signals may be detected by an envelope detector of a receiver (e.g.,  422 ,  722 ). 
     LC oscillator  910  of  FIG. 9  example comprises an inductor capacitor (LC) tank circuit  911 , and a set of transistors  931 ,  932 ,  933 ,  934  coupled to LC tank circuit  911 . In particular, LC tank circuit  911  comprises inductor  913 , and capacitor  912  coupled to inductor  913  in parallel. The set of transistors  931 ,  932 ,  933 ,  934  comprises a first pair of cross coupled NMOS transistors  931 ,  932  and a second pair of cross coupled PMOS transistors  933 ,  934 . 
     A drain of first transistor  931  of first pair of NMOS transistors  931 ,  932  is coupled to a first end of LC tank circuit  911 , and a drain of a second transistor  932  of first pair of NMOS transistors  931 ,  932  is coupled to a second end LC tank circuit  911 . Further, sources of first and second transistors  931 ,  932  of the first pair of NMOS transistors  931 ,  932  are coupled to a collector of current mirror transistor  903   
     A drain of first transistor  933  of second pair of PMOS transistors  933 ,  934  is coupled to the first end of LC tank circuit  911  and a drain of second transistor  934  of second pair of PMOS transistors  933 ,  934  is coupled to the second end of LC tank circuit  911 . Further, sources of first and second transistors  933 ,  934  of second pair of PMOS transistors  933 ,  934  are coupled to an end of the set of BJTs  901 ,  902  configured to receive input current or voltage. 
     The current or voltage input to diode emulating oscillator  900  is applied to the center tap of LC oscillator  910  coupled to the sources of transistors  933 ,  934  of second pair of PMOS transistors  933 ,  934 . 
       FIGS. 10-13  illustrate additional exemplary circuits of diode emulating isolator according to various aspects of the present invention. In particular, the examples of  FIGS. 10-13  illustrate diode emulating oscillator  1000  including various LC oscillator circuit according to an example of the present invention. A LC oscillator circuit of diode emulating oscillator  1000  is excited upon an application of current or voltage input above a threshold of the LC oscillator circuit. When the LC oscillator circuit of diode emulating oscillator  1000  operates, it generates a pair of differential signals output from port OUT-N and port OUT-P. The pair of differential signals comprises an on-off key based modulated signal and the amplitude of the pair of differential signals reflect the amplitude of the input current or voltage. An envelope detector of a receiver of the on-off key based modulated signal may detect the amplitude of the pair of differential signals, in addition to the presence of above threshold input or current input to diode emulating isolator  1000 . 
     An IV sweep result of diode emulating oscillator  1000  of  FIGS. 10-13  is analogous to the IV sweep line S 12  of  FIG. 5  and closely tracks the IV operational characteristics of a diode. 
     In  FIGS. 10( a ) and 10( b ) , diode emulating oscillator  1000  comprises LC oscillator circuit  1010  including LC tank circuit  1020  and a pair of cross coupled NPN BJTs  1031 ,  1032  coupled to LC tank circuit  1020 . LC tank circuit  1020  of  FIGS. 10( a ) and ( b )  comprises a pair of inductors  1021 ,  1022  coupled in series, and a pair of capacitors  1023 ,  1024  coupled in series. The pair of capacitors  1023 ,  1024  is coupled to the pair of inductors  1021 ,  1022  in parallel, a collector of first BJT  1031  of the pair of cross coupled NPN BJTs  1031 ,  1032  is coupled to an end of LC tank circuit  1020 , and a collector of second BJT  1032  of the pair of cross coupled NPN BJTs  1031 ,  1032  is coupled to another end of LC tank circuit  1020 . 
     In the example of  FIGS. 10( a ) and 10( b ) , when an input current or voltage above a threshold of LC oscillator circuit is applied to a center tap of the pair of inductors  1021 ,  1022 , an on-off key based modulated signal is output from the ends of the LC tank circuit  1020  as a pair of differential signals output via ports OUT-N and OUT-P. 
     Further, as illustrated in  FIG. 10( b ) , diode emulating oscillator  1000  may comprise voltage drop NPN BJT  1040 . A base and a collector of the voltage drop NPN BJT  1040  is coupled to an emitter of each the BJTs of the pair of cross coupled NPN BJTs  1031 ,  1032 . 
     In  FIGS. 11( a ) and 11( b ) , diode emulating oscillator  1000  comprises LC oscillator circuit  1110  including an LC tank circuit  1111 , and a set of transistors  1131 ,  1132 ,  1133 ,  1134  coupled to LC tank circuit  1111 . In particular, LC tank circuit  1111  comprises inductor  1113  and capacitor  1112  coupled to the inductor  1113  in parallel. The set of transistors  1131 ,  1132 ,  1133 ,  1134  comprises a first pair of cross coupled NPN BJTs  1131 ,  1132  and a second pair of cross coupled PNP BJTs  1133 ,  1134 . 
     A collector of first BJT  1131  of the first pair of NPN BJTs  1131 ,  1132  is coupled to a first end of the LC tank circuit  1111  and a collector of a second BJT  1132  of the first pair of NPN BJTs  1131 ,  1132  is coupled to a second end the LC tank circuit  1111 . A collector of first BJT  1133  of the second pair of PNP BJTs  1133 ,  1134  is coupled to the first end of LC tank circuit  1111 , a collector of second BJT  1134  of the second pair of PNP BJTs  1133 ,  1134  is coupled to the second end LC tank circuit  1111 , and an emitter of first BJT  1133  of the second pair of PNP BJTs  1133 ,  1134  is coupled to an emitter of second BJT  1134  of the second pair of PNP BJTs  1133 ,  1134 . 
     After input current or voltage above the threshold of local oscillator circuit  1110  is applied to a center tap coupled to the emitters of the BJTs  1133 ,  1134  of the second pair of PNP BJTs  1133 ,  1134 , a pair of differential signals, an on-off key based modulated signals, is output from the ends of LC tank circuit  1111  via ports OUT-N and OUT-P. 
     Additionally, as illustrated in  FIG. 11( b ) , diode emulating oscillator  1000  may further comprises voltage drop NPN BJT  1140 . A base and a collector of voltage drop NPN BJT  1140  is coupled to an emitter of each transistor of the first pair of cross coupled NPN BJTs  1131 ,  1132 . 
     In  FIG. 12 , diode emulating oscillator  1000  comprises LC oscillator circuit  1210  including LC tank circuit  1220 , and a pair of cross coupled NMOS transistor  1231 ,  1232  coupled to LC tank circuit  1220 . In particular, LC tank circuit  1220  comprises a pair of inductors  1221 ,  1222  coupled in series, and a pair of capacitors  1223 ,  1224  coupled in series. 
     The pair of capacitors  1223 ,  1224  is coupled to the pair of inductors  1221 ,  1222  in parallel, a drain of first transistor  1231  of the pair of cross coupled NMOS transistors  1231 ,  1232  is coupled to an end of LC tank circuit  1220 , and a drain of second transistor  1232  of the pair of cross coupled NMOS transistors  1231 ,  1232  is coupled to another end of LC tank circuit  1220 . 
     Input current or voltage to diode emulating oscillator  1000  is applied to a center tap of the pair of inductors  1221 ,  1222 . When the input is above the threshold of LC oscillator circuit  1210 , an on-off key based modulated signal is output from the ends of the LC tank circuit ( 1120 ) as a pair of differential signals. 
     Diode emulating oscillator  1000  of  FIG. 12  further comprises voltage drop NPN BJT  1240 , wherein a base and a collector of voltage drop NPN BJT  1240  is coupled to a source of each of the transistors of the pair of cross coupled NMOS transistors  1231 ,  1232 . 
     In the example of  FIG. 13 , diode emulating oscillator  1000  comprises LC oscillator circuit  1310  including LC tank circuit  1311 , and a set of transistors  1331 ,  1332 ,  1333 ,  1334  coupled to LC tank circuit  1311 . LC tank circuit  1311  comprises inductor  1313  and capacitor  1312  coupled to inductor  1313  in parallel. The set of transistors  1331 ,  1332 ,  1333 ,  1334  comprises a first pair of cross coupled NMOS transistors  1331 ,  1332  and a second pair of cross coupled NMOS transistors  1333 ,  1334 . 
     A drain of first NMOS transistor  1331  of the first pair of NMOS transistors  1331 ,  1332  is coupled to a first end of LC tank circuit  1311  and a drain of second NMOS transistor  1332  of the first pair of NMOST transistors  1331 ,  1332  is coupled to a second end of LC tank circuit  1311 . A drain of first PMOS transistor  1333  of the second pair of PMOS transistors  1333 ,  1334  is coupled to the first end of LC tank circuit  1311 , a drain of second PMOS transistor  1334  of the second pair of PMOS transistors  1333 ,  1334  is coupled to the second end of LC tank circuit  1311 , and a source of first PMOS transistor  1333  of the second pair of PMOS transistors  1333 ,  1334  is coupled to a source of second PMOS transistor  1334  of the second pair of PMOS transistors  1333 ,  1334 . 
     When the input current or voltage of diode emulating oscillator  1000  is applied to a center tap coupled to the sources of transistors  1333 ,  1334  of the second pair of PMOS transistors  1333 ,  1334 , the modulated signal, on-off key based, is output from the ends of LC tank circuit as a pair of differential signals via ports OUT-N and OUT-P. 
     Diode emulating oscillator  1000  of  FIG. 13  further comprises voltage drop NPN BJT  1340 . A base and a collector of voltage drop NPN BJT  1340  is coupled to a source of each the NMOS transistors of the first pair of cross coupled NMOS transistors  1331 ,  1332 . 
     The diode emulating isolator  1000  of  FIGS. 10-13  may also comprise diode  1011 ,  1135 ,  1211 ,  1335 , respectively, to prevent reverse current flow when current is applied to the diode emulating oscillator. According to an aspect of the present invention, diode  1011 ,  1135 ,  1211 ,  1335  may be replaced with CMOS transistor or any other reverse current flow preventing circuitry. 
     The above description and drawings are only to be considered illustrative of an example of the present invention which achieves the features and advantages described herein. Modifications are possible in the described examples, and other examples are possible, within the scope of the claims. Accordingly, the examples of the present invention described herein are not considered as being limited by the foregoing description and drawings.