Abstract:
An electrical isolation device wherein a signal to be electrically isolated is mixed with a carrier signal by a multiplier to produce a modulated carrier signal which is passed through an isolation barrier and demodulated and filtered to extract thereby the initial input signal.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to signal analysis devices and, more particularly, to a method and apparatus for providing electrical isolation such as between a signal under test (SUT) and an electronic testing device. 
   BACKGROUND OF THE INVENTION 
   Signal acquisition devices or test and measurement devices such as digital storage oscilloscopes (DSOs) and the like receive one or more signals under test (SUT) via one or more respective input channels. It is important that the input channels be electrically isolated from each other and from the chassis ground of the test and measuring device. Such isolation is necessary for signals having a frequency from DC to the bandwidth of the measuring device. Such isolation allows multiple signals under test to be provided to, and analyzed by, the same measuring device without affecting the systems/signals under test. 
   One isolation method known to those skilled in the art for use in wide bandwidth systems is the so-called “two path scheme” in which an input signal is broken up into two signals; namely, a low frequency input signal and a high frequency input signal. Optocouplers are used for low frequency signals (e.g., signals under 1 MHz) and wide band transformers are used for the high frequency signals. Unfortunately, optocouplers are difficult and expensive to linearize, and wide band linear transformers are expensive. Additionally, it is difficult to insure that the resulting isolated low end high frequency signal components of the signals under test are sufficiently well matched such that they may be recombined to produce a “flat” signal suitable for further processing by the test and measurement device. 
   SUMMARY OF INVENTION 
   These and other deficiencies of the prior art are addressed by the present invention of a method and apparatus enabling the isolation of wide band signals in a manner avoiding the “two path scheme” and its inherent difficulties. One embodiment comprises an electrical isolation device (isolator) wherein a signal to be electrically isolated is mixed with a carrier signal by a multiplier to produce a modulated carrier signal which is passed through an isolation barrier and demodulated and filtered to extract thereby the initial input signal. 
   The subject invention is adapted, in one embodiment, to an apparatus comprising a modulator, for modulating an input signal onto a carrier signal to produce thereby a modulated carrier signal having associated with it a first ground reference; an isolation device, for processing the modulated signal to produce a corresponding modulated signal having associated with it a second ground reference; a demodulator, for demodulating the corresponding modulated signal and a corresponding carrier signal to produce thereby a demodulated signal; and a smoothing device, for retrieving a corresponding input signal from the demodulated signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high level block diagram of a signal isolation system according to an embodiment of the invention; 
       FIG. 2  depicts a flow diagram of a method according to an embodiment of the invention; and 
       FIGS. 3–6  graphically depicts several waveforms useful in understanding the present invention. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The subject invention will be primarily described within the context of test and measurement devices such as digital storage oscilloscopes, analog oscilloscopes and the like. However, it will be appreciated by those skilled in the art that the invention may be advantageously employed in any environment where electrical isolation of a relatively wide bandwidth signal is desired. 
     FIG. 1  depicts a high level block diagram of a signal isolation system according to an embodiment of the invention. Specifically, the system  100  of  FIG. 1  comprises a first buffering device  105 , a first active mixer  110 , a first isolating device  120 , a second isolating device  130 , a second active mixer  140 , a second buffering device  145 , an oscillator  150  and a filter  160 . It is noted that the first buffering device  105  and first mixer  110  are electrically isolated from the second active mixer  140 , second buffering device  145 , oscillator  150  and filter  160 . 
   The system  100  receives a differential input signal and responsively produces a differential output signal which comprises an isolated or a corresponding version of the differential input signal. It will be appreciated by those skilled in the art and informed by the teachings of the present invention that the system  100  of  FIG. 1  may also operate using non-differential input and output signals. However, noise immunity and other factors make the use of a differential input signal desirable, though not strictly necessary. 
   The differential input signal is buffered by the first buffering device  105  and coupled to the active mixer  110 . The active mixer  110  operates to multiply or modulate the buffered differential input signal and a first carrier signal CARRIER to produce a modulated signal MOD. The modulated signal MOD is passed to an input portion of the second isolating device  130 . The resulting isolated signal produced at the output portion of the second isolating device  130  is coupled to the second active mixer  140 . 
   The second active mixer  140  receives a second carrier signal CARRIER′ that is substantially identical to (though isolated from) the first carrier signal CARRIER provided to the first active mixer  110 . The second active mixer  140  operates to demodulate the isolated and modulated differential input signal to produce therefrom a demodulated differential signal DEMOD. 
   The demodulated differential signal DEMOD is coupled to the filter  160  via the second buffering element  145 . The filter  160  operates to smooth the demodulated differential signal DEMOD to produce therefrom a differential output signal having signal characteristics substantially similar to those of the differential input signal. 
   The oscillator  150  produces the second carrier signal CARRIER′, illustratively a square wave, sine wave, triangular wave and the like having a frequency of tens to hundreds of MHz. The carrier signal CARRIER′ is coupled directly to the second active mixer  140  and indirectly (through the first isolating device  120 ) to the first active mixer  110 . Thus, in this embodiment, the output signal of the oscillator  150  is used to initially modulate the differential input signal and subsequently demodulate the resulting isolated and modulated differential input signal to produce thereby the demodulated differential signal DMOD. While the above-described frequencies are useful within the context of a test and measurement device requiring such bandwidth, much smaller frequencies (i.e., under 1 MHz) may be used depending upon the particular application. 
   In one embodiment of the invention, each of the buffers  105 ,  145  comprise Model AD8131 buffers, and model AD8343 active mixers, all manufactured by Analog Devices Incorporated of Norwood, Mass. The first  120  and second  130  isolators may comprise respective pulse transformers packaged together as the model A6801 transformer provided by Pulse Engineering Incorporated of San Diego, Calif. The first  120  and second  130  isolators may also comprise linear optocouplers, low power radio frequency (RF) transmitter/receiver pairs (i.e., antennas and associated circuitry), fiber optic devices and the like. It is noted that the frequencies requiring isolation are relatively high, therefore, optical isolation devices like linear optocouplers having operating ranges down to DC are not necessary, though such devices may be employed within the context of the present invention. The filter  160  may comprise a Bessel filter, a Gaussian filter, an Elliptical filter and the like. 
   In one embodiment, the oscillator  150  comprises a 242.42 MHz oscillator producing a square wave, the filter comprises a 200 MHz fifth-order low pass Bessel filter and the first and second isolators  120  and  130  comprise pulse transformers. In this embodiment, the first active mixer  110  operates to modulate the differential input signal by multiplying it by ±1 (i.e., convolution about the carrier signal). In this manner, the signal MOD produced by the first active mixer  110  comprises the differential input signal multiplied by a square wave via, for example, a switching operation. In this embodiment, the second active mixer  140  operates to synchronously demodulate the isolated and modulated signal provided by the second isolation device by convolution. 
     FIG. 2  depicts a flow diagram of a method according to an embodiment of the invention. The method  200  of  FIG. 2  is entered at step  210 , when a modulator (e.g., first active mixer  110 ) modulates an input signal onto a carrier signal. Referring to box  215 , the carrier signal may comprise a sine wave, square wave, saw tooth wave or other wave shape. 
   At step  220 , the modulated signal is coupled to a demodulator (e.g., second active mixer  140 ) using an isolation device (e.g., second isolation device  130 ). Referring to box  225 , the isolation device may comprise a transformer, optocoupler, or other isolation device. The isolation device is used to ensure that the ground reference associated with the input signal may float with respect to the ground reference associated with a subsequent output signal. 
   At step  230 , the isolated signal received from the isolation device is demodulated using a corresponding carrier signal. Referring to box  235 , the demodulator (i.e., the second active mixer  140 ) employed for this task may comprise a balance mixer, an unbalanced mixer, a demodulator or other suitable device depending upon the type of modulation scheme employed by the first active mixer. 
   At step  240 , the demodulated signal is filtered or smoothed to retrieve the input signal. Referring to box  245 , the filtering or smoothing function employed may comprise a Bessel filter function, a Gaussian filter function, an Elliptical filter function or some other appropriate function. 
     FIG. 3  graphically depicts several waveforms useful in understanding the present invention. Specifically,  FIG. 3A  depicts a waveform of the modulated signal MOD produced by the first active mixer  1110  (operating as a balanced mixer) in the case where a sine wave carrier signal is employed and the input signal frequency is lower than the carrier signal frequency. illustrating the output signal MOD produced by the first active mixer  110 .  FIG. 3B  depicts a waveform illustrating the demodulated signal DEMOD produced by the second active mixer  140  (operating as a balanced mixer).  FIG. 3C  depicts the differential output signal produced by the filter  160 . Each of the  FIGS. 3A ,  3 B and  3 C show for reference purposes an input signal and the actual signal at the point described. 
     FIG. 4  graphically depicts several waveforms useful in understanding the present invention. Specifically,  FIG. 4A  depicts a waveform of the modulated signal MOD in the case where a square wave carrier signal is employed and the rise time of the input signal is faster than half the carrier signal period.  FIG. 4B  depicts a waveform illustrating the demodulated signal DEMOD produced by the second active mixer  140 . It is noted that the demodulated signal comprises substantially the original input signal with several high speed glitches  410 ,  420  imparted to the input signal during the modulation process. It is noted that the glitches  410 ,  420  are very high in frequency in comparison to the input and carrier signals due to the use of a square wave carrier signal. As such, the realization of the filter  160  is simplified.  FIG. 4C  depicts the output signal provided by the filter  160 . Each of the  FIGS. 4A ,  4 B and  4 C show for reference purposes an input signal and the actual signal at the point described. 
   Referring to  FIG. 4A , it is noted that the input signal has an initial rising edge that occurs within the middle portion of the square wave carrier signal rather then at a transition portion of the square wave signal. It is important to note that the square wave modulation enables the capturing of substantially all of the phase/timing information. By contrast, using a sine wave carrier signal, the phase/timing information would not be captured as accurately due to the comparatively inhibited rise time of the sine wave edge. The graphical depictions herein illustrate the advantage of a square wave carrier signal in comparison to a sine wave carrier signal. In addition to the simplified realization of filter  160 , the modulation/demodulation may be performed using a switching technique rather than the balanced multiplication technique (used with a sine wave carrier). 
     FIG. 5  graphically depicts several waveforms useful in understanding the present invention. Specifically,  FIG. 5A  depicts a waveform of the modulated signal MOD in the case where a square wave carrier signal is employed and the rise time of the input signal is slower than half the carrier signal period.  FIG. 5B  depicts a waveform illustrating the demodulated signal DEMOD produced by the second active mixer  140 . It is noted that the demodulated signal comprises substantially the original input signal with several high speed glitches  510  imparted to the input signal during the modulation process. It is noted that the glitches  510  are very high in frequency in comparison to the input and carrier signals due to the use of a square wave carrier signal, thereby simplifying the realization of filter  160 .  FIG. 5C  depicts the output signal provided by the filter  160 . Each of the  FIGS. 5A ,  5 B and  5 C show for reference purposes an input signal and the actual signal at the point described. 
     FIG. 6  graphically depicts several waveforms useful in understanding the present invention. Specifically,  FIG. 6A  depicts a waveform of the modulated signal MOD in the case where a square wave carrier signal is employed and the input signal is of a higher frequency that the carrier signal.  FIG. 6B  depicts a waveform illustrating the demodulated signal DEMOD produced by the second active mixer  140 . It is noted that the demodulated signal comprises substantially the original input signal with several high speed glitches  610  imparted to the input signal during the modulation process.  FIG. 6C  depicts the output signal provided by the filter  160 . Each of the  FIGS. 6A ,  6 B and  6 C show for reference purposes an input signal and the actual signal at the point described. 
   While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.