About this project:

After messing with several different photodiode detector circuits, I decided to see if I could pass video through an optical-only ("lightbeam") video link.

The answer was:  Yes - but this is not the way to do it over longer distances.  (I'll get to this later.)

Comment:  Philips is apparently phasing out the Luxeon I, III, and V lines in favor of the lower-power Luxeon Rebel devices.  Since I have not used those other devices, the techniques described here may not directly apply.  For the time being, however, the Luxeon III devices are still available from various sources.

Video-Speed optical detector:

Figure 1:
Top:  Schematic of video-speed optical receiver.
Bottom:  As-built prototype of this receiver.
Click on either image for a larger version.

One of the most vexing things about using solid-state optical detectors (such as a PIN photodiode) is that you can either make such a detector very sensitive, or you can make it fast - but not both at the same time.  The main reason is capacitance:  It is the very nature of capacitance to resist a change in voltage - and high-speed operation clearly implies very fast changes in voltages over time.

For photodiodes, there are several approaches that one can take to improve the speed:

• Reduce the capacitance by using a smaller photodiode.  The smaller the photodiode, the less area there is, so it has lower capacitance - but this also means that there is a smaller area on which the detected light is to be focused and this can be a complication when using simple optics as it may be awkward to accurately focus the accumulated light on a piece of silicon that is smaller than 1 square millimeter.
• Reduce the capacitance by using reverse bias.  Like any diode, a silicon photodiode's intrinsic capacitance goes down as a reverse bias is applied - for precisely the same reason as it does for Varactor diodes:  The width depletion region of the diode (that area in the silicon where there is nothing to allow electron flow) increases with higher reverse bias.  While the capacitance can be reduced severalfold (compared to the zero-bias voltage) by doing this, one can only apply so much voltage before reaching the reverse voltage limits of the diode itself.
• Load down the photodiode to improve the speed speeding up the R/C constant.  Damping down the response with lower resistance increases the frequency response, but it does so at the expense of reducing sensitivity because the available output of the photodiode is reduced.  This doesn't take into account noise contribution from real-world components.

The works of P.C.D. Hobbs go into some detail about extracting higher frequency response from a relatively high-capacitance photodiode.  In his paper "Photodiode Front Ends - The Real Story" he details a circuit that takes several approaches to do this:

• The use of a transimpedance amplifier for current-to-voltage conversion.
  
• Application of reverse bias across the photodiode to reduce its junction capacitance.
• The use of a Cascode input to minimize Miller capacitance - essentialy, capacitive loading effects by the diode's amplifier.
• The use of a bootstrap circuit to minimize the voltage swing across the photodiode to reduce the "damping" effects of the photodiode's capacitance.

An adaptation of the circuit is shown at the top of Figure 1 and the circuit works approximately as follows:

• Q1 is wired as a bootstrap circuit.  Small changes in voltage are countered by Q1's current amplification capability and reduce the voltage swing across D1, the photodiode:  If the voltage isn't allowed to change as much, the capacitance will have less of an effect on frequency response.  This circuit offers roughly an 8x improvement in frequency response.
  
• Being that the base voltage will necessarily follow the emitter voltage (which changes as a result of the current through Q1) Q2, wired as a common-base amplifier, will respond to this attempted voltage change.
• U1 is wired as a transimpedance amplifier, detecting the changes in Q2's collector current.
• The loading on the photodiode (D1) is based on Q1 and Q2's bias current.  Note that Q1, the bootstrap circuit, operates at a much higher current than Q2, the common-base circuit, but Q2 provides most of the loading on the photodiode, ultimately determining the frequency response of the circuit.
• Reverse bias on D1, the photodiode, is derived from the voltage drop across the 10k resistor in parallel with C1 plus the base-collector voltage drop of Q1.
  

The upshot of all of this is that the output voltage of U1 will be generally proportional to the amount of light falling on D1, the photodiode.  U2 and its associated circuity simply provides an "artificial ground" to avoid the requirement of a bipolar (dual-voltage) power supply.

Important note concerning the circuit in Figure 1:

The circuit shown in Figure 1 was designed only to be fast and no attempt was made to optimize sensitivity:  I have little doubt that it can be improved in terms of both bandwidth and sensitivity.

Figure 2:
Top:  Schematic of video-speed modulator
Bottom:  As-built (and operating) video modulator.
Click on either image for a larger version.

Note also that with any photodetector that isn't inherently self-amplifying (like a photomultiplier or an avalanche photodiode) that the ultimate sensitivity is limited, in large part, by the noise contribution of the amplifier connected to the photodetector.  In this circuit, there are a number of noise-contributing components - namely the transistors Q1 and Q2, but note that the resistors (especially the 4.7k resistor in Q1's emitter circuit) can also contribute significant noise - so it is recommended that one use metal film resistors in the Q1/Q2/U1 circuits instead of the more-typical carbon-film resistors.  The resistors associated with U2 are not noise contributors in this circuit.

Finally, this circuit does not have sufficient gain to provide the 1 volt peak-to-peak video signal necessary to drive a typical video monitor under most conditions and additional amplification is usually required.  While this could have been done using a number of different circuits (another LM7171 or two, a UA733-based amplifier, etc.) I used what was available:  The "Channel 1 Vertical Output" from my old Tektronix 465B oscilloscope.  I simply connected the video monitor to the appropriate jack on the back of my oscilloscope and used the 'scope itself as a variable-gain video amplifier.

Comment:  Again, if extreme sensitivity is required, the addition of the extra circuitry shown in the Hobbs circuit (e.g. bootstrapping, additional resistors) will introduce noise into the system, fundamentally limiting the degree to which the absolute maximum sensitivity can be obtained.  For a more thorough discussion of a circuit that has far better sensitivity - at the expense of bandwidth - see the page Optical Receivers for low-bandwidth through-the-air communications on this web site.

Video-Speed optical modulator:

Initial tests were done using an LED strongly driven by a function generator and with this setup I could verify that the circuit in Figure 1 did, in fact, have MHz-range frequency response, albeit with some rolloff.  With these encouraging results, I quickly (in 10 minutes or so) threw together about the simplest modulator that I could, the result being shown in Figure 2.

While this modulator works, it does not work very well as there are several things wrong with it:

• It does not have very good gain:  It would be nice to have a bit stronger drive signal than that of the 75 ohm video output from the source.  Rbias (the 150 ohm potentiometer) also robs much of the drive signal.
  
• It does not properly terminate the video source:  Although this wasn't important in this quick test, this would have been more important had a longer video cable been used.
• This circuit has limited modulation depth and rather poor linearity.  This circuit will operate with reasonable modulation depth over only a narrow range of settings of Rbias.  At the "optimum" setting (where reasonable modulation depth, linearity, and frequency response is obtained) the LED is driven well below its rated capabilities.
  
• The circuit is not thermally stable.  As the transistor heats up, its beta will increase, requiring readjustment of Rbias for best results.
• The frequency response is terrible.  As evidenced by monitoring the voltage across the 0.5 ohm (carbon-type - not wirewound) resistor in the emitter, it was clear that this circuit's frequency response rolled off significantly by the time one got to the chroma frequency (3.58 MHz for NTSC) with the amount of rolloff being about 6dB.

As I mentioned, I threw this circuit together in just a few minutes just to provide a usable source of video-modulated light.  There are any number of ways in which this circuit's performance could have been increased, but I won't mention them here.  Note that the driving method described is not capable of fully-driving the LED at the highest video component frequencies, hence some of the noted rolloff:  A better system would involve a "stiffer" driver as well as appropriate pre-emphasis designed into the transmit and receive system to maximize bandwidth at the available modulation depth.

Performance of the entire system:

Despite its many faults, I did succeed in sending video across my basement (a distance of about 20 feet or 6 meters) using an LED and photodiode receiver with plenty of margin to spare.  For transmitting, I placed the LED at the focus of a 250x310mm Fresnel lens to concentrate the beam (but losing at least half of the optical flux by not using a secondary lens) with the LED simply sitting atop of CD-ROM Jewel case as shown in Figure 2 (the Fresnel lens is visible in the bottom picture of Figure 3.)

Figure 3:  The video gear in action.
Top:  The video receiver is shown being bathed in red light from the transmitter with the actual video being received being visible on the monitor in the background.
Bottom:  The video transmitter.  The video source is from a video camera in the foreground while the Fresnel lens (being illuminated by the LED) is in the background, being held up by some lead-acid batteries.
Click on either image for a larger version.

The receive circuit (see the bottom picture in Figure 1 and the top picture in Figure 3 was attached to some scrap cardboard using double-sided tape and a simple lens holder was fashioned (also from cardboard) to hold a large Plano-Convex (PCX) lens such that the photodiode in the receiver was approximately at its focus.  Despite about 10dB of chroma-frequency rolloff, the pictures received were fairly good (on par with a home videotape recorder.)  In the top picture of Figure 3 one may see, in the background, the video monitor with the actual transmitted signal:  The white horizontal line on the monitor is from the pre-flash of the digital camera that I used.

As previously implied, it takes a bit of careful adjustment to get this system to work:  The setting of Rbias on the transmitter is critical for best bandwidth and linearity, as is the gain setting of the video amplifier (my oscilloscope in this case.)  I also noted that at this distance it was very easy to saturate the photodiode, so the transmitter was purposely mis-aimed to reduce the signal enough to allow proper operation of the receiver.

Quick back-of-the envelope calculations indicate that this system, using for the receiver the same-sized Fresnel as used on the transmitter, would be capable of spanning a distance of at least 1 mile (1.6km) with usable results:  Probably much more with more circuit optimizing - assuming that going through that much air wasn't going to cause its own problems...

Why this method is not good for longer-distance through-the-air video transmitting:

Having said all of this, I would NOT recommend these circuits for video communications at a distance of more than several hundred feet under any circumstances.  If you ignore the fact that they are finicky (requiring careful adjustments of both the transmitter and receiver to work) and that their frequency response is rather poor (something that could have been remedied with a little more work) and that the LED could have been driven to more than four times the output available from this circuit (theoretically doubling its range) there is the matter of the effect of atmospheric scintillation on any amplitude-modulated signal.

Because the video signal was directly amplitude-modulated onto the LED, critical signals such as horizontal and vertical synchronization are present as amplitude variations.  Also, in order for a video signal to work properly with any typical monitor, it must be held to fairly close tolerances plus or minus 1dB or better - for acceptable viewing.  (Commercial and even normal consumer video levels are held to much tighter standards than that!.)

Even with medium-sized lenses on both ends it doesn't take propagation through much of an atmospheric path to experience scintillation that greatly exceeds 1dB and it is likely that typical atmospheric conditions along a near-to-ground path length of a mile or so (1.6km) would probably be pushing the limit.  Why?  With the fluctuating light levels, the recovered AM signal will also be fluctuating wildly:  These changing video levels would play havoc with the TV synchronization signals - not to mention affecting apparent brightness of different parts of the video picture.

While it is theoretically possible to implement various schemes to normalize signal levels at the receiver (such as sync-keyed AGC circuits, etc.) it would still require at least 25dB of signal-noise ratio of the video signal in order to maintain anything closely resembling a noise-free picture and it is likely that such circuits would be problematic in the presence of higher scintillation levels, anyway.

A way around this problem would simply be to modulate the LED with a video signal carried on an FM subcarrier:  A variation of this method is used in consumer-grade videotape recorders for the same reason that we would need to use it:  The amplitude variation of the signal recovered from a videotape would cause unacceptable artifacts in the playback video.  With FM, the absolute amplitude of the signal is irrelevant - as long as it is above the noise threshold of the detector.  Another advantage that FM has is that with a signal-noise ratio of better than 10-12dB (depending on system parameters) the recovered signal is nearly noise-free.

One example of this is the system used by the German Laser ATV experimenter group (the website is in German) where video is modulated onto a 20 MHz carrier using an NE564 PLL.  Likewise, demodulation is accomplished using an NE564 as well:  Those familiar with FM-ATV demodulator circuits (as well as those used in the early days of analog satellite TV) will immediately recognize these circuits.

Another example, already mentioned, is the method (often called the "color under" system) used to record video onto tape as used in consumer-grade videotape recorders.  In these systems, the luminance (black and white) portion of the signal is modulated onto an FM carrier in the 2-8 MHz range (the frequency and amount of deviation depending on the recording system) while the chroma (color) portion is heterodyne downconverted to something in the 500-900 kHz range (the precise frequency also depending on the recording system) and both signals are put onto the tape.  On playback, the chroma upconverted and re-united with the demodualated luminance signal, mostly recreating the original video signal.  It is interesting to note that even though the chroma signal is subject to significant level variations upon being read from the videotape, the human eye is generally insensitive to such variations so relatively little effort is made to correct this effect.

It is likely that the "color under" system would eventually be limited in its range by the amount of scintillatory amplitude variations that the video monitor could tolerate, although this effect could be somewhat mitigated by keying an AGC circuit to the colorburst level of the received signal.  The main advantage of the "color under" system is that it would not require as high a frequency response as, say, the 20 MHz German system:  Using photodiodes, reduced frequency response translates directly to better achievable sensitivity.  If photomultiplier tubes are used, however, the frequency response limitation is not as much a problem, but this comes at the cost of the added complexity, fragility, and expense of the use of the tube.

---

Other related links:

• RONJA (Reasonable Optical Near Joint Access) is a system developed, at least in part, by Twibright Labs in the Czech republic.  This system uses high-brightness LEDs and reasonably-sized optics that is rated to provide reliable 10 megabit links at distances of up to 1.4 kilometers (almost a mile.)  Click here for more technical information on RONJA.  Because this is a purely binary system (on/off) it is immune to the effects of scintillation - provided that the minimum amplitude of the scintillatory troughs is above the receiver's threshold.
  

Return to the KA7OEI Optical communications Index page.

If you have questions or comments concerning the contents of this page, or are interested in this circuit, feel free to contact me using the information at this URL.

Keywords:  Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector