Abstract:
A linear voltage controlled current sink is used to power a linear LED with fast optical rise time. The signal is transmitted through a small aperture in a grounded shield plate. The receiver is a PIN photodiode with fast optical rise time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical isolators and in particular optical isolators capable of wideband transmission capabilities. In even more particularity it is directed to wideband isolators with linear analog response. 
     2. Description of the Prior Art 
     Methods of eliminating ground loops are well known. One such method is electro-optical isolators that use optical wavelengths of the electromagnetic spectrum to traverse a gap in the electrical circuitry. 
     The slow rise time of the receivers with linear analog response has limited the bandwidth that can be transmitted. Receivers with fast optical rise time are nonlinear and therefore used as digital (on-off) systems. In such systems, the largest bandwidths are on the order of 5 MHZ. A further limitation in optical isolators is the coupling capacitance between the transmitter and receiver circuits. 
     SUMMARY OF THE INVENTION 
     Light emitting diodes, LED, with fast optical rise times exist commercially. The same is true for PIN photodiodes. By using the two as transmitter and receiver a wide band optical frequency on the order of 10 MHZ can be obtained. Use of continuous wave (CW) laser diodes, instead of LED&#39;s, provide even larger bandwidths. 
     As part of the transmitting circuit a voltage controlled current sink with linear response is used to modulate the incoming signal. The entire transmitting circuit is shielded. 
     The photodiode output is sent through a transimpedence amplifier and then a postamplifier. The entire receiving circuit is shielded to provide a low coupling capacitance, between transmitter and receiver. 
     An air gap separates the two circuits and in the middle of it is another shield. This shield is grounded. As a result of this shield there is very little coupling between the two circuits. 
     In practice this permits high resolution video transmission. The term high resolution refers to television with significantly more lines per scan than commercial television. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a traditional cable system with ground loops. 
     FIG. 2 is a schematic of the present invention where an optical isolator is employed. 
     FIG. 3 is a block diagram of an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a traditional wired circuit. Camera 10 and monitor 12 are connected by coaxial cable 14. Since the grounds can have different potentials, ground loops arise. Such loops are random and unpredictable. The randomness and unpredictability lower the signal quality. Disconnecting the ground at either the camera 10 or the monitor 12 will prevent ground loops, but a floating end can be a severe shock hazard. 
     FIG. 2 shows how optical isolators solve the problem of ground loops with no shock hazard. The components in the circuit are the same as FIG. 1 except there is a break in shielded wire 14. At one end an optical transmitter 16 is attached and to the other side of the break an optical receiver 18 is attached. By having both sides grounded, there is no electrical floating, which in turn avoids shock hazards. The gap between the transmitter 16 and receiver 18 prevents the electrical current which is previously discussed as ground loops. 
     FIG. 3 is a diagram of the present invention. The transmitter 16 and receiver 18 are shown with their circuit components. Transmitter 16 is composed of a linear voltage controlled current sink 20 for modulating the input signal. The output of sink 20 is fed to a light emitting diode, LED, 22 or CW laser diode with a fast optical rise time. 
     The response time or rise time in a circuit is inversely proportional to the frequency of the circuit. A &#34;fast&#34; rise time is one that is very short (i.e. rapid response). Thus use of fast rise time components permits large frequency bands to be transmitted. Experimental optical isolators, as disclosed above, have been built with 10 MHZ or better bandwidth. This is a rise time of 35 nanoseconds. The term &#34;fast&#34; refers to a rise time on the order of up to 35 nanoseconds for LED&#39;s and to 1 nanosecond for laser diodes. The rise time is the time it takes the signal to go from 10% to 90% of its peak energy level. This is the definition of rise time in A New Dictionary of Physics by Gray and Isaacs. 
     To be an analog device, the isolator must use the magnitude of the signal present as part of its information. A linear response of circuit components provides a direct way of comparing signal strengths. 
     The transmitter 16 includes a shield 24 which is attached to the current sink 20 input ground. Shield 24 has a hole in it so the light from LED 22 can be emitted. 
     The receiver 18 consists of a PIN photodiode 26 with linear response and fast optical rise time. The photodiode 26 output is attached to transimpedance amplifier 28 which serves as a preamp. Postamp 30 further amplifies the signal until it reaches a level predetermined by the desired use of the output and sensitivity of available output devices. Shield 32 protects the receiver components and is attached to output ground. 
     In the air gap, between transmitter 16 and receiver 18, is chassis grounded shield 34. This shield 34 reduces coupling capacitance between the transmitter and receiver. Appropriately placed in the shield 34 is a hole to permit optical light from LED 22 to pass to photodiode 26. The hole is limited in size so longer wavelengths than those of the optical range will not be transmitted.