A 173 mile (278km) all-electronics, FSO (Free Space Optical) contact: Part 1 - Scouting it out

Nearly 10 years ago - in October, 2007, to be precise - we (exactly "who" to be mentioned later) successfully managed a 173 mile, Earth-based all-electronic two-way contact between two remote mountain ranges in western Utah.

For many years before this I'd been mulling over in the back of my mind various ways that optical ("lightbeam") communications could be accomplished over long distances.  Years ago, I'd observed that even a modest, 2 AA-cell focused-beam flashlight could be easily seen over a distance of more than 30 miles (50km) and that sighting even the lowest-power Laser over similar distances was fairly trivial - even if holding a steady beam was not.  Other than keeping such ideas in the back of my head, I never really did more that this - at least until the summer of 2006, when I ran across a web site that intrigued me, the "Modulated Light DX page" written by Chris Long (now amateur radio operator VK3AML) and Dr. Mike Groth (VK7MJ).  While I'd been following the history and progress of such things all along, this and similar pages rekindled the intrigue, causing me to do additional research - and I began to build things.

Working up to the distance...

Over the winter of 2006-2007 I spent some time building, refining, and rebuilding various circuits having to do with optical communications.  Of particular interest to me were circuits used for detecting weak optical signals and it was those that I wanted to see if I could improve.  After considerable experimentation, head-scratching, cogitation, and testing, I was finally able to come up with a fairly simple optical receiver circuit that was at least 10dB more sensitive than other voice-bandwidth circuits that were out there.  Other experimentation was done on modulating light sources and the first serious attempt at this was building a PIC-based PWM (Pulse-Width Modulation) circuit followed, somewhat later, by a simpler current-linear modulator - both being approaches that seemed to work extremely well.

After this came the hard part:  Actually assembling the mechanical parts that made up the optical transceivers.  I decided to follow the field-proven Australian approach of using large, plastic, molded Fresnel lenses in conjunction with high-power LEDs for the source of light emissions with a second parallel lens and a photodiode for reception and the stated reasons for taking this approach seemed to me to be quite well thought-out and sound - both technically and practically.  This led to the eventual construction of an optical transceiver that consisted of a pair of identical Fresnel lenses, each being 318 x 250mm (12.5" x 9.8") mounted side-by-side in a rigid, wooden enclosure comprising an optical transceiver with parallel transmit and receive "beams."  In taking this approach, proper aiming of either the transmitter or receiver would guarantee that the other was already aimed - or very close to being properly aimed - requiring only a single piece of gear to be deployed with precision.

After completing this first transceiver I hastily built a second transceiver to be used at the "other" end of test path.  Constructed of foam-core posterboard, picture frames and inexpensive, flexible vinyl "full-page" magnifier Fresnel lenses, this transceiver used, for the optical emitter and transmitter assemblies, my original, roughly-repackaged prototype circuits.  While it was neither pretty or capable of particularly high performance, it filled the need of being the "other" unit with which communications could be carried out for testing:  After all, what good would a receiver be if there were no transmitters?

On March 31, 2007 we completed our first 2-way optical QSO with a path that crossed the Salt Lake Valley, a distance of about 24 km (15 miles.)  We were pleased to note that our signals were extremely strong and, despite the fact that our optical path crossed directly over downtown Salt Lake City, they seemed to have 30-40dB signal-noise ratio - if you ignored some 120 Hz hum and the occasional "buzz" from an unseen, failing streetlight.  We also noted a fair amount of amplitude scintillation, but this wasn't too surprising considering that the streetlights visible from our locations also seemed to shimmer being subject to the turbulence caused by the ever-present temperature inversion layer in the valley.

Bolstered by this success we conducted several other experiments over the next several months, continuing to improve and build more gear, gain experience, and refine our techniques.  Finally, for August 18, 2007, we decided on a more ambitious goal:  The spanning of a 107-mile optical path.  By this time, I'd completed a third optical transceiver using a pair of larger (430mm x 404mm, or 16.9" x 15.9") Fresnel lenses, and it significantly out-performed the "posterboard" version that had been used earlier.  On this occasion we were dismayed by the amount of haze in the air - the remnants of smoke that had blown into the area just that day from California wildfires.  Ron, K7RJ and company (his wife Elaine, N7BDZ and Gordon, K7HFV) who went to the northern end of the path (near Willard Peak, north of Ogden, Utah) experienced even more trials, having had to retreat on three occasions from their chosen vantage point due to brief, but intense thunderstorms.  Finally, just before midnight, a voice exchange was completed with some difficulty - despite the fact that they never could see the distant transmitter with the naked eye due to the combination of haze and light pollution - over this path, with the southern end (with Clint, KA7OEI and Tom, W7ETR) located near Mount Nebo, southeast of Payson, Utah.

Figure 1:
The predicted path projected onto a combination
map and satellite image.  At the south end
(bottom) is Swasey Peak while George Peak is
indicated at the north.
Click on the image for a larger version.

Finding a longer path:


Following the successful 107-mile exchange we decided that it was time to try an even-greater distance.  After staring at maps and poring over topographical data we found what we believed to be a 173-mile line-of-sight shot that seemed to provide reasonable accessibility at both ends - see figure 1.  This path spanned the Great Salt Lake Desert - some of the flattest, desolate, and most remote land in the continental U.S.  At the south end of this path was Swasey Peak, the tallest point in the House range, a series of mountains about 70 miles west of Delta, in west-central Utah.  Because Gordon had hiked this peak on more than one occasion we were confident that this goal was quite attainable.

At the north end of the path was George Peak in the Raft River range, an obscure line of mountains that run east and west in the extreme northwest corner of Utah, just south of the Idaho boarder.  None of us had ever been there before, but our research indicated that it should be possible to drive there using a high-clearance 4-wheel drive vehicle so, on August 25, 2007, Ron and Gordon piled into my Jeep (along with a 2^nd spare tire swiped from Ron's Jeep as recommended by more than one account) and we headed north to investigate.

Getting there:

Following the Interstate highway nearly to the Idaho border, we turned west onto a state highway, following it as the road swung north into Idaho, passing the Raft River range, and we then turned off onto a gravel road to Standrod, Utah.  In this small town (a spread-out collection of houses, really) we turned onto a county road that began to take us up canyons on the northern slope of the range.  As we continued to climb, the road became rougher and we resorted to peering at maps and using our intuition to guide us onto the one road that would take us to the top of the mountain range.

Luckily, our guesses were correct and we soon found ourselves at the top of the ridge.  Traveling for a short distance, we ran into a problem:  The road stopped at a fence gate that was plastered with "No Trespassing" signs.  At this point, we simply began to follow what looked like road that paralleled the fence only to discover, after traveling several hundred feet - and past a point at which we could safely turn around - that this "road" had degenerated into a rather precarious dirt path traversing a steep slope.  After driving several hundred more feet, fighting all the while to keep the Jeep on the road and moving in a generally forward direction, the path leveled out once again and rejoined what appeared to be the main road.  After a combination of both swearing at and praising deities we vowed that we would never travel on that "road" again and simply stay on what had appeared to have been the main road, regardless of what the signs on the gates said!

Looking for Swasey Peak:

Having passed these trials, we drove along the range's ridge top, looking to the south.  On this day, the air was quite hazy - probably due to wildfires that were burning in California, and in the distance we could vaguely spot, with our naked eyes, the outline of a mountain range that we thought to be the House range:  In comparing its outline and position with a computer-simulated view, it "looked" to be a fairly close match as best as we could guess.

Upon seeing this distant mountain we stopped to get a better look, but when we looked through binoculars or a telescope the distant outline seemed to disappear - only to reappear once again when viewed with the naked eye.  We finally realized what was happening:  Our eyes and brain are "wired" to look at objects, in part, by detecting their outlines, but in this case the haze reduced the contrast considerably.  With the naked eye, the distant mountain was quite small but with the enlarged image in the binoculars and telescope the apparent contrast gradient around the object's outline was greatly diminished.  The trick to being able to visualize the distant mountain turned out be keeping the binoculars moving as our eyes and brain are much more sensitive to slight changes in brightness of moving objects than stationary ones.  After discovering this fact, we noticed with some amusement that the distant mountain seemed to vanish from sight once we stopped wiggling the binoculars only to magically reappear when we moved them again.  For later analysis we also took pictures at this same location and noted the GPS coordinates.

Continuing onwards, we drove along the ridge toward George Peak.  When we got near the GPS coordinates that I had marked for the peak we were somewhat disappointed - but not surprised:  The highest spot in the neighborhood, the peak, was one of several gentle, nondescript hills that rose above the road only by a few 10's of feet.  Stopping, we ate lunch, looked through binoculars and telescopes, took pictures, recorded GPS coordinates, and thought apprehensively about the return trip along the road.

Figure 2:
The predicted line-of-sight view (top) based on 1 arc-second SRTM terrain data between the Raft River range
and Swasey peak as seen from the north (Raft River) side.
On the bottom is an actual photograph of the same scene at the location used in the simulated view.  As can be seen,
more of the distant mountain can be seen than the prediction would indicate, this being due to the refraction of
the atmosphere slightly extending the visible horizon.  Under typical conditions, this "extension" amounts to
an increase of approximately 10/9th of the distance than geometry would predict.  This lower picture was produced
by "stacking" multiple images using software designed for astronomy.
Click on the image for a larger version.

Returning home:

Retracing our path - but not taking the "road" that had paralleled the fence line - we soon came to the gate that marked the boundary of the private land.  While many of the markings were the same at this gate, we noticed another sign - one that had been missing from the other end of the road - indicating that this was, in fact, a public right-of-way plus the admonition that those traveling through must stay on the road.  This sign seemed to register with what we thought we'd remembered about Utah laws governing the use of such roads and our initial interpretation of the county parcel maps:  Always leave a gate the way you found it, and don't go off the road!  With relief, we crossed this parcel with no difficulty and soon found ourselves at the other gate and in familiar territory.

Retracing our steps down the mountain we found ourselves hurtling along the state highway a bit more than an hour later - until I heard the unwelcome sound of a noisy tire.  Quickly pulling over I discovered that a large rock that had embedded itself in the middle of the tread of a rear tire.  After 45 minutes of changing the tire and bringing the spare up to full pressure, we were again underway - but with only one spare remaining...

Analyzing the path:

Upon returning home I was able to analyze the photographs that I had taken.  Fortunately, my digital SLR camera takes pictures in "Raw" image mode, preserving the digital picture without loss caused by converting it to a lossy format like JPEG.  Through considerable contrast enhancement, the "stacking" of several similar images using an astronomical photo processing program and making a comparison against computer-generated view I discovered that the faint outline that we'd seen was not Swasey Peak but was, in fact, a range that was about 25 miles (40km) closer - the Fish Springs mountains - a mere 150 or so miles (240km) away.  Unnoticed (or invisible) at the time of our mountaintop visit was another small bump in the distance that was, in fact, Swasey Peak.

Interestingly, the first set of pictures were taken at a location that, according to the computer analysis, was barely line-of-sight with Swasey Peak.  At the time of the site visit we had assumed that the just-visible mountain that we'd seen in the distance was Swasey Peak and that there was some sort of parallax error in the computer simulation, but analysis revealed that not only was the computer simulation correct in its positioning of the distant features, but also that the apparent height of Swasey Peak above the horizon was being enhanced by atmospheric refraction - a property that the program did not take into account:  Figure 2 shows a comparison between the computer simulation and an actual photograph taken from this same location.


Building confidence - A retry of the 107-mile path:

Having verified to our satisfaction that we could not only get to the top of the Raft River mountains, but also that we also had a line-of-sight path to Swasey Peak, we began to plan for our next adventure.  Over the next several weeks we watched the weather and the air - but before we did this, we wanted to try our 107-mile path again in clearer weather to make sure that our gear was working, to gain more experience with its setup and operation, and to see how well it would work over a long optical path given reasonably good seeing conditions:  If we had good success over a 107-mile path we felt confident that we should be able to manage a 173-mile path.

A few weeks later, on September 3, we got our chance:  Taking advantage of clear weather just after a storm front had moved through the area we went back to our respective locations - Ron, Gordon and Elaine at Inspiration Point while I went (with Dale, WB7FID) back to the location near Mt. Nebo.  This time, signal-to-noise ratios were 26dB better than before and voice was "armchair" copy.  Over the several hours of experimentation we were able to transmit not only voice, but SSTV (Slow-Scan Television) images over the LED link - even switching over to using a "raw" Laser Pointer for one experiment and a Laser module collimated by an 8" reflector telescope in another.

With our success on the clear-weather 107-mile path we waited for our window to attempt the 173-mile path between Swasey and George Peak but in the following weeks we were dismayed by the appearance of bad weather and/or frequent haze - some of the latter resulting from the still-burning wildfires around the western U.S.

To be continued!

[End]

This page was stolen from "ka7oei.blogspot.com"

A novel APD-based speech bandwidth optical receiver

In a previous posting I wrote about a novel application of a JFET (Read about that in the article "Gate current in a JFET - The development of a very sensitive, speech-frequency optical receiver" - link) in which the flow of gate current was integral to the operation of a photodiode-based optical detector.  In testing this circuit, which included tests using an indoor "photon range" and out in the field, it was observed that the sensitivity of this circuit was, at "audio" frequencies, on the order of 8-20 dB better in terms of signal/noise ratio than any of the more conventional "TIA" (TransImpedance Amplifier - read about that circuit here - link) circuits that had been tried.

In the analysis of this circuit it was determined that several factors contribute to the ultimate sensitivity, including:

• The intrinsic noise of the JFET.  This can be minimized by hand-selection of the device itself for the lowest-possible noise as well as selecting a device that can operate at a higher drain current to reduce noise - or even the use of several JFETs in parallel.
• The contribution of noise by other circuitry.  In the design this was minimized through the use of a cascode circuit topology as well as the use of a low noise, high impedance current source to supply the bulk of the drain current and the complete avoidance of other components being connected to the photodiode-JFET circuit junction.
• The reducation of parasitic circuit elements, such as capacitance (including the Miller effect) that reduces the amplitude of the signals from the photodiode, particularly as the frequency increases, effectively reducing the signal-noise ratio.
• The noise contribution of the photodiode.

Of these factors, the majority of the noise would appear to be due to the JFET itself, particularly above the low audio frequencies frequencies (e.g. below 100Hz or so) where ¹/_F noise would dominate. One of the possible approaches to get better noise performance is to cool the circuitry, but this is fraught with difficulties related to condensation which would require that the device itself be sealed in an atmosphere (e.g. dry nitrogen) in a manner similar to that used to cool CCD imagers for astronomy.

Figure 1:
The outside view of the completed APD-based optical receiver.  Because
of its extreme sensitivity it must be well shielded to minimize the pick-up
of stray fields such as those from AC mains and radio transmitters/phones.
Click on the image for a larger version.

What else may be done to improve the performance?

Perhaps counter-intuitively, the use of a smaller photodiode can help a bit and provide at least as much signal output as a larger one, provided that the optics can focus the given amount of light from the distant source of light efficiently onto its active area:  A smaller device will have lower self-capacitance shunting a smaller amount of the AC currents being produced in response to the impinging, modulated light in addition to having a lower intrinsic noise contribution.  In the case of an optical receiver the active area of the device is less important than in some other applications as lenses and mirrors may be used to concentrate the light from the distant source onto the photoactive area.

When reducing the size of the device one must assure that the optics themselves will resolve the distant spot of light to an area that is not larger than the active area of the device as well as taking into account additional constraints with respect to the accuracy and stability of the aiming and pointing mechanisms.  For example, using reasonable-quality molded Fresnel lenses of common focal lengths (e.g. an f/D ratio of approximately unity) one can expect only to resolve a spot with a "blur circle" of approximately around 0.2mm at best while high-quality glass optics should be able to reduce this by an order of magnitude or better assuming a suitably-distant source, a corresponding small subtended angle and proper paraxial alignment and focus.  If the resolved spot of light is much larger than the active area of the device - perhaps due to the device being too small for the optics' ability to resolve or due to the quality of and/or misalignment of the lens(es) - there may be an additional loss of available optical energy and signal-noise ratio as some of the light from the distant source is being "wasted" when it spills beyond the active area of the photodetector.

For more information on "spot sizes" using inexpensive, molded plastic Fresnel lenses see the article "Fresnel Lens Comparison:  A Comparison of inexpensive, molded plastic lenses and their relative 'accuracy' and ability to produce collimated beams" - link.

Aside from the reduction of the size of the photodiode or cooling, where else may one eke out greater performance from this circuit topology?

The Avalanche Photodiode:

The Avalanche Photodiode (APD) is a type of photodiode that contains an internal mechanism for amplification.  Simply put a single photon has a given probability of mobilizing a single electron when it impinges the active area of a standard PIN photodiode.  In an APD, what might have been a single electron being loosed as in a normal PIN diode, that same single-electron event can cause the mobilization of many electrons via an "Avalanche" effect, providing amplification of the optical signal and hence the name.  The result of this intrinsic amplification is that the output signal from this diode from a given photon flux can be much higher than that of a standard PIN photodiode.

Because the signal from the Avalanche photodiode itself is amplified internally it is more likely to be able to overcome the effects of the capacitance on frequency response as well as the noise intrinsic to the JFET amplifier, support circuitry and components, providing the potential of producing a greater signal/noise ratio for a given signal. Typically an Avalanche photodiode is incorporated into a TIA (TransImpedance Amplifier) with good effect, but what about its use in the previously-described "Version 3" photodiode receiver circuit that utilizes JFET gate current?

The basic design:

From the previous article (link) one can see the basic topology of the "Version 3" circuit using a "normal" PIN photodiode depicted in Figure 2, below.

Figure 2:
A diagram of the "Version 3" optical detector that utilizes JFET gate current.  In this circuit Q1 and Q2 comprise a cascode
circuit with Q3 providing the majority of Q1's drain current while U1b is configured as a differentiator to compensate
for the low-pass effects of the intrinsic capacitance of D1, the photodiode and Q1.  Resistors R1 and R2 along with
C1 provide a filtered reverse bias for D1 which not only decreases its capacitance, but it also biases Q1 to
its operating state where it is drawing maximum drain current.  In this circuit the connection between the Photodiode (D1)
and the gate of the JFET is made in air and not on a circuit board to minimize capacitance, stray signal pickup and
most importantly a source of leakage currents and related noise.  The fundamental circuit around Q1 and its art was described in a 2008 article in an SPIE journal (Vol. 6878) written, in part, by the author.
Click on the image for a larger version.

In this design PIN photodiode D1, a BPW34, is reverse-biased via R1 and R2.  One of the main benefits of doing this is that the capacitance of D1 decreases from approximately 70pF at zero volts to around 20pF at the operational voltage, reducing the degree to which high frequency signal are attenuated by this capacitance.  A somewhat less tangible benefit of this is that in addition to photovoltaic currents produced by the impinging light, the bias also allows photoconductive currents to flow from the bias source, through the photodiode and into the gate of the JFET.  As noted in the original article it is the presence of the gate-source junction of the JFET (Q1) and its conduction that limits the gate-source differential to around 0.4-0.6 volts, permitting D1's reverse bias to become established without the need of any additional noise-generating or lossy components.  In this configuration the drain current of the JFET is still proportional to the gate-source voltage (but with an offset of drain current greater than the "zero bias" drain current) and like a bipolar transistor's base voltage and current, the relationship between gate voltage and gate current is logarithmic.

A question now comes to mind:  What about replacing D1 with an avalanche photodiode?

Testing with an Avalanche Photodiode:

Like its more-sensitive distant cousin, the Photomultiplier tube, the avalanche photodiode requires a rather high bias voltage in order to function at maximum gain.  Rather than requiring a kilovolt or so as is needed for a photomultiplier, typical photodiodes may operate with up to "just" a few hundred volts.  Like the photomultiplier, the current required for "dark" operation is minuscule - a few hundred microamps in these "dark" conditions is more than enough.

In perusing the various component catalogs I noted that Mouser Electronics carried some avalanche photodiodes - but as expected, there was a price - literally:  Around US$150 at the time for just one APD.  In a compromise between size, availability and cost I chose the AD1100-8-TO52-S1 by First Sensor  (previously known as "Pacific Silicon Sensor") - a device with a round, 1mm² (1.128mm diameter) active area - a reasonable compromise between cost, size, and the practical limit of the Fresnel-based optics.  This device, which came with its own test sheet, indicated a maximum gain ("M" factor) of approximately 1000 occurring at 134 volts at a temperature of 25C.

In most ways using an APD is just like using a reverse-biased PIN photodiode - except that the reverse bias voltage will be much higher.  Perusing the literature and manufacturer's specifications one will note that many designs depict a temperature-compensated bias voltage supply, but further investigation reveals that this is necessary only if the device is being used at/near maximum gain (and maximum voltage) and/or if it is necessary to precisely maintain the gain over a wide temperature range.  For our application we don't really care if the gain changes with temperature, so an arbitrarily adjustable high voltage supply is fine - and actually preferred.

In my initial research I noted that the internal action of any APD suffers an inevitable, but expected, effect:  As the gain goes up with increasing bias voltage, the intrinsic noise of the device itself increases at a faster rate than the gain.  What this means was that there is going to be a point above which a further increase of device gain will cause the signal to noise ratio to decrease even though the actual signal level continues to increase with bias voltage.  With this in mind, the question is "At what voltage might this happen, and would this 'crossover' point occur at a point where we can expect the overall 'gain+noise' to offer a net advantage over a PIN photodiode?"

Building a prototype receiver similar to that depicted in Figure 2 I substituted an APD for D1 using a string of sixteen 9 volt batteries and a 1 megohm potentiometer with a 100k resistor in series with the wiper (and some bypass capacitors to ground on the "hot" side of the diode) in lieu of R1 to set the bias voltage.  Placing this prototype in my "Photon Range" - a windowless room in my house where there is an LED mounted to the ceiling that may be modulated - I compared the sensitivity of this prototype to both my "standard" TIA receiver (the VK7MJ design) and an operational exemplar of my "Version 3" design.

Varying the voltage from 10 volts to around 140 volts I noted that at a bias voltage comparable to the reverse bias applied in Figure 2 (approx. 8 volts) the apparent sensitivity was roughly on par with that of the Version 3 unit using a normal PIN photodiode after the signal levels were corrected to compensate for the smaller area of the APD as compared with the BPW34 (e.g. 1mm² of the APD versus 7mm² of the BPW34 - the larger size gathering proportionally more light in this lens-less system).  At around 130-135 volts, the output of the APD-based prototype was very high, but the weak, optical signals from the test LED were lost in the noise.  In the area of 35-45 volts I observed that the overall signal levels, while significantly higher than they were at 8-10 volts, were a fraction of what they were at 130 volts but the signal/noise ratio was roughly 6-10dB higher than it was at the lowest voltage when the differences in active area of the APD versus the photodiodes in the test receivers were taken into account.  As expected, even though the signals were much "stronger" at the higher voltage, the signal noise ratio at that voltage was very poor and would have submerged a much weaker signal completely.

Comments:

• The test receivers used BPW34 PIN photodiodes with an active area of 7mm² while the APD has an active area of just 1mm².  Because there were no optics in front of the photodiodes in this test there was 7 times as many of the LED's photons hitting the larger device, resulting in an approximate 8.5 dB difference in signal/noise - assuming all other parameters being equal.  It is when using the device in this "lens-less" configuration that this factor must be accommodated.
• While it is theoretically possible to use a photomultiplier tube (PMT) in lieu of an APD, there are several practical concerns.  Even though an "S-1" type of photocathode has a peak in the red-NIR area, its low quantum efficiency makes it a rather poor performer overall.  The "931A" PMT - widely available surplus - has a more typical blue/violet peak response (type "S-4") in which the longer red wavelengths suffer greatly in terms of quantum efficiency.  Field testing of these devices by British amateur radio operators has shown that they offered no obvious advantage over the "Version 3" PIN photodiode design for "red" wavelengths.  As of the time of this writing the use of PMTs with more exotic photocathodes (such as multialkalai and GaAs) that are better suited for "red" wavelengths (but much more difficult to find surplus!) have not been field-evaluated.

A practical design:  The high voltage APD bias supply:

First, a few weasel words:

Even though the currents are very low, there is some risk of injury with the voltages involved (e.g. several hundred volts) and it is up to you to educate yourself about high voltage safety!

If you wish to construct these circuits, be aware of possible hazards and always assume that any capacitors are charged, even after power is removed.

You have been warned!

Because it is not convenient to carry around a lot of 9 volt batteries connected in series, a simple high voltage converter was designed to provide the  microamp-level current required for the APD bias supply and it is depicted below in Figure 3.

Figure 3:
High voltage supply for the APD receiver.  U101a is an oscillator that drives Q101 to produce a high-voltage, low-current bias for the APD.  The output is regulated via U101b and associated components to the voltage set by potentiometer R111.  R109 is used to set the highest voltage that may be obtained when R111 is adjusted for "maximum".  R109 is shown with the two "ends" grounded only because it was convenient to wire it this way when the prototype was built.
Click on the image for a larger version.

 This design is a simple "boost" type switching converter using a high voltage transistor and an inductor to produce the needed bias.  In this circuit U101A forms an oscillator that drives the high voltage transistor Q101, and when Q101 switches off, the magnetic field of L101 collapses, producing a high voltage spike that is rectified by D101 and filtered and stored by C102, R106 and C103.  To regulate this high voltage a sample is divided-down by R108 and R109 and compared with a 5-volt reference from U102 that is made variable with R111:  If the output voltage is too high, U101b turns on Q102 to pinch off the drive for Q101.  Because I used an "ordinary" op amp with an output that could not go all of the way to the negative supply rail, LED101 was put in series with the transistor's base to provide a drop of around 2 volts to assure that Q101 could be shut completely off.

Figure 4:
Inside the high voltage (bias) supply for the APD receiver.  Potentiometer
R111 and the indicator, LED101, are mounted in the front of the
case.  Both the high voltage generator and the receiver itself are powered
from a single 9 volt battery.  The typical combined current consumption
for the both sets of circuits is less than 35 milliamps.
Click on the image for a larger version.

LED101 also provides two other features:  It functions as a "power on" indicator, and since it is in series with Q101's base drive it is modulated at approximately 6.5 kHz (determined by experiment to be the frequency at which Q101 and L101 produced the highest voltage with the best efficiency) and can be used as an optical signal source to verify that the receiver is working.  Worth noting is that R112 is placed across the "hot" end and the wiper of R111 to "stretch" the high voltage end of the linear potentiometer's adjustment range a bit to compensate somewhat for the fact that near the maximum voltage, the gain goes up exponentially with the bias voltage, making fine adjustments at this setting easier.

The APD (optical) receiver:

The optical receiver section is depicted in Figure 5, below:

Figure 5:
The optical receiver which works in a manner very similar to that depicted in Figure 3.  In this implementation
the high voltage bias is applied to the cathode of D201, the APD, which has its anode connected to the gate of the JFET,
Q201.  Q201 and Q203 comprise a self-biasing, AC-coupled cascode amplifier while Q202 provides the a high-
impedance source for the bulk of Q201's drain current.  The components in the sections marked "HV Filter"
and "LV Filter" are used to keep the residual switching frequency energy from being conducted into these circuits.
As with other circuits of this type, the connection from the photodiode to the JFET's gate is made in air and not via a
circuit board trace - this, to minimize capacitance, leakage currents and noise.
Click on the image for a larger version.

Not surprisingly this circuit looks very similar to the "Version 3" optical receiver of Figure 2.  Notable features include an R/C filter consisting of R201, R202, C201 and C202 to remove traces of the 6.5 kHz power supply ripple from the high voltage supply while L201, C211, R215 and C212 do the same for the 9 volt supply that the receiver circuitry shares with the high voltage generator.  The two sections - high voltage supply and optical receiver sections - are separate, connected by a 3 foot (1 meter) umbilical cable, both to provide isolation of the extremely sensitive optical receiver from the electrostatic and electromagnetic fields of the high voltage converter and also to remotely locate the controls on the high voltage supply away from the lens assembly on which the receiver portion is mounted so that adjustments can be made without disturbing it.

Figure 6:
Inside the receiver portion of the APD receiver.  This section is physically
separated from the high voltage converter to prevent the switching energy
from getting into these extremely sensitive circuits.  In the center is
a small sub-board with the APD and JFET that is mounted on short pieces
of 18AWG wire to allow its position to be adjusted in all three dimensions
to provide both paraxial alignment and focus.
Click on the image for a larger version.

The APD itself is mounted on a small sub-board along with Q201 (the JFET) and the other capacitors noted in the box in Figure 5.  Most of Q201's drain current is provided by Q202's circuit, a current source, that operates at high impedance while Q203 is the rest of a cascode amplifier circuit that is designed to be self-biasing at DC and to provide gain mainly to AC signals.

The output of the cascode amplifier is passed to U201b, a unity gain follower amplifier.  This signal then passes to the circuit of U201a, a differentiator circuit that is designed to provide a 6dB/octave boost to higher frequencies to compensate for the similar R/C low-pass roll-off intrinsic to the APD and JFET itself:  Without this circuit, higher frequency audio components of speech would be rolled off, reducing intelligibility.  By design the frequency range of the differentiator and its surrounding circuitry is intentionally limited so that low frequencies (below several hundred Hz) are strongly rolled off to prevent AC mains related hum from urban lighting from turning into a roar as are very high frequencies - above 5-7 kHz - which would otherwise become an ear-fatiguing "hiss" were the differentiation allowed to continue to frequencies much higher than this.

An interesting property of the photodiode circuit is that the "knee" related to this 6dB/octave roll-off occurs varies somewhat with the bias voltage and thus amount of device capacitance and, to a certain degree, its gain.  Because of this the frequency response of the APD/JFET circuit and the differentiator don't match under all operating conditions but experience has shown that it is better to have a bit of extra "treble boost" than not when it comes to making out words when the distant voice is immersed in a sea of noise.

A sample of the output from U201b, before differentiation, is also passed to J20, the "Flat" output.  The audio taken from this point, lacking differentiation, will sound a bit muffled under normal low-light conditions as it is not subject to either the high or low pass effects of the U201a differentiator which means that it will pass both subsonic and ultrasonic components as detected by the APD amplifier itself.  On the low end, the sensitivity is limited by ¹/_F noise which becomes increasingly dominant below a few 10s of Hz while on the high end it is again the capacitance associated with the APD and JFET circuits.  In testing it was observed that at this "Flat" output it was possible to detect signals from an LED modulated up to several MHz, albeit with significantly reduced sensitivity.  The main purpose of this output is to provide a signal point suitable for both subsonic digital communications as well as ultrasonic for experimentation with low/medium rate data, FM carriers and SSB signals.

In this circuit the amount of drain current in the JFET will vary depending on the individual properties of the JFET itself, the bias voltage, and the amount of impinging light.  Under "dark" conditions the "standing" JFET current was set to approximately 7-10 milliamps by the current source and the drain-source voltage varied from around 0.21 volts when the APD bias was just 12 volt to around 0.155 volts when the APD was operating at its maximum rating of 135 volts.  The specified JFET, the BF862, is typically capable of handling more drain current than this - and to do so would likely reduce its noise contribution slightly - but it was set at this level (with R205) to moderate battery current consumption.

Circuit testing:

Although it may have risked component damage, the APD circuit was "torture tested" to check ruggedness.  In a completely dark room a xenon photo flash was set off just inches/centimeters away from the photodiode with the bias set at 135 volts.  While the receiver was deafened for a second or two - the time it took for the various circuits to recover (e.g. power supply, re-equalization of various capacitors, etc.) - repeated tests like this did not do any detectable damage to the receiver sensitivity or its noise properties indicating that the APD and JFET were more than rugged enough to handle any conceivable event that might happen in the field, aside from directly focusing the sun on the photodiode!

This circuit has also been successfully used in broad daylight.  While the receiver worked, the background thermal noise from the sunlit landscape was the limiting factor for sensitivity and the recovered audio had quite apparent nonlinearity (distortion) with an altered frequency response (e.g. "tinny") because the ambient light and resulting photodiode conductivity effectively shunted the high voltage bias and device capacitance.  In short, in such high ambient light conditions this circuit has no advantage over other optical receiver topologies such as the original "Version 3" or even a more conventional TIA (TransImpedance Amplifier) but its ability to be useful under such conditions is indicative of its versatility.

The results of in-field testing:

This receiver was first field-tested on a 95+ mile (154km) optical path during the September 2012 segment of the ARRL "10 GHz and up" contest:  For detail on this communication, read the blog entry "Throwing One's Voice 95 Miles on a Lightbeam" - link
 

Figure 7:
My end of the 95+ mile optical path during the session where the APD-
based optical receivers were first field-tested.  As seen in the picture
the optical path passes over urban lighting which tends to slightly raise
the noise floor due to both Rayleigh and lens-related scattering
effects.
Click on the image for a larger version.

During this test the optical (voice) link was first established using the "Version 3" PIN Photodiode receiver depicted in Figure 2.

With the reasonably clear air and the moderately long path we noted that we could reduce the LED current to a tiny fraction of the maximum before significant signal/noise degradation was noted.  At this lower LED current each station at opposite ends of the path switched from the PIN photodiode to the APD receivers and after tweaking our pointing and reducing the LED current even more we observed what turned out to be between 6 and 10 dB improvement in the signal-noise ratio - about what was observed on the indoor "Photon Range" with the initial prototype circuit.  It is likely that the actual improvement in sensitivity was greater than this, but because our respective optical paths passed directly over populated areas (see Figure 7) our ultimate noise floor was degraded by light pollution which included a thermal "hiss" from the urban lighting and a low-level, harmonic-rich 120 Hz hum.

As was determined in the lab, the best signal-noise ratio in the field occurred with the APD biased in the 35-45 volt range where the "M" (amplification) factor was in the area of 3-10 (approximately 10-20dB gain).  At this rather modest bias voltage the "Gain+Noise" from the APD itself was sufficient to overcome much of the intrinsic noise of the JFET.  At higher voltages the gain continued to increase but the signal-noise ratio decreased at a faster rate until the APD's own avalanche noise drowned out the desired signal.

* * *

For more information about (speech bandwidth) free space optical communication, check out these links from my "Modulated Light" web site (link):

• Using Laser Pointers for voice communications -  This page describes in more detail the methods by which one may successfully use inexpensive laser pointers to cast voice through many miles of the ether!
• A Highly-Sensitive Optical Receiver Optimized for Speech Bandwidth - This is one of the most sensitive speech-range optical receivers yet devised that uses standard photodiodes and is much more sensitive than the simple receiver depicted in figure 1, above.
• Receiver for low-bandwidth optical (through the air) communications using an Avalanche Photo Diode (APD) - This optical receiver is an enhancement of the one above, achieving another 6-10dB of ultimate sensitivity using an APD.  This receiver is about as sensitive as you can get without resorting to an exotic red-sensitive photomultiplier tube!
• A "Cheap" Optical Transceiver lens assembly - If you really want to improve your receive sensitivity, the best way to do this is with a large lens.  This article describes how one could use inexpensive foam-core poster board to make an assembly that will focus the distant light on the detector diode of an optical receiver.
• A "Mini" full-featured Pulse Width Modulator for high-power LEDs and laser diodes -This describes a simple, computer-based PWM modulator that will not only transmit audio, but generate test tones as well.

Be sure to check out the "ModulatedLight.org" web site's other pages as well!

[End] 

This page stolen from "ka7oei.blogspot.com". 
 

Throwing one's voice 95 miles on a lightbeam

For more information about long-distance optical communications, go to the modulatedlight.org web page  (link)

This past weekend (September 15-16, 2012) was the 2nd part of the annual ARRL 10 GHz and up contest and we decided to use one of the highest-available amateur bands - the one known in the FCC rules as "275 GHz and up."  Actually, this covers a lot of territory including submillimeter radio frequencies and far infrared wavelengths, but the part that we are more interested in is that for which most of us are equipped to detect directly - light.

We've done this before, managing to have spanned 107 miles (173 km) on several occasions and even 173 miles (278 km) (read about those efforts here - link) so we weren't going to break any of our own DX (distance) records, but it's fun to do this, anyway - and it gave us an excuse (as if we really needed one...) to go out and test some new gear that had not yet been tested over anything but relatively short (20 km or so) paths.

The two locations for the stations were about 96 miles (154 km) apart with Ron and Elaine Jones (K7RJ and N7BDZ) being at the far end at an elevation of about 5600 ft (1700 m) ASL near Park Valley, Utah in the extreme northwestern corner of Utah, a few miles from where the U.S. Transcontinental Railroad was joined for the first time in 1869 and only a few hundred meters away from the historic stagecoach route that paralleled part of that later railroad.  Along with friends Gordon (K7HFV) and Gary (AB1IP), I  was closer to home at about 9300 feet (2830 m) near a minor protuberance known as "Bountiful Peak" about 10 miles (16 km) north of Salt Lake City.  As it turns out, the path is a grazing one and were it not for the slight refraction of the Earth's atmosphere, it may not even quite be "line of sight."

We'd tried this same path during the first weekend of the 10 GHz and up contest but the thick veil of smoke from wild fires elsewhere in the western U.S. prevented a successful contact - although our light beam was occasionally just visible to the binocular-aided eye in Park Valley.  This time, however, the air was reasonably clear, only somewhat hazy from the still-burning fires:  Since we "almost" made contact a month ago we were confident that this time we would have no problems.

Figure 1:
The high-power red LED shining to the north-northwest.
The lights of Layton, Utah and surrounding communities
may be seen way below, in the background! The dot at the terminus
of the red shaft of light is the light from Ron's end of the path.
Click on the image for a larger version.

Soon after we arrived on site Ron shone a 500,000 candlepower halogen spotlight in our direction and immediately we noted a lone, flickering, yellow-red dot in the blackness "above" the last ribbon of visible lights from the populated areas of Layton and Ogden about a mile (1600 meters) in elevation below us.  Using this as a visual reference I swung my high-power LED in his direction, using the Rayleigh-scattered shaft of red light as a guide, and immediately Ron reported that it was easily the brightest light visible:  Considering that there were only a small handful of lights visible from his dark, rural location, anyway, this wasn't saying much, but if anyone where to have dropped by and looked in that direction they would have seen the bright, red light and asked, unprompted, "What's that?!?"

Using our light as a guide Ron immediately fine-tuned his pointing and soon, a very obvious red light appeared in the darkness.  Initially starting out with the lower power 3-watt LED he soon switched to the much higher-powered 20-30-ish watt LED and the red dot in the distance was even more striking than before.  The dot at the end of the red shaft of light in the above picture was from Ron's LED.

Soon after we brought our transmitters up to full power we reduced them again to 1/4-1/15th as each other's signals were strong enough that there was noticeable distortion in the received audio - and it also allowed us to run full-duplex (e.g. both sides being able to send and receive simultaneously) without intercepting as much of our own, scattered transmit light and causing acoustic feedback between our speaker and microphone.

This was the first actual "long distance" test of the Phlatlight-based optical transmitter - these using CBT-54 LEDs and permitting a 20dB improvement on the audio received at the far end.  This also was the first test of some APD (Avalanche PhotoDiode) based optical receivers that I'd built some time ago (see the link at the bottom of the page) so we set about reduce each other's LED currents to do a sort of "limbo" dance - that's to say we wanted to answer the question "How low can we go?"

It immediately became apparent that even though we could read the Phlatlight modulators' current with a resolution of 0.1 amp, this was still too coarse when we got down to the lowest readable current and were still able to hear each other, so Ron switched to the older 3 watt Luxeon on which the LED current could be measured and adjusted down to the single digits of milliamps.  As it turned out, speech was copyable - with some difficulty - down to the 40-50 milliamp range with the old receivers but the APD receivers extended this down to around 20 milliamps - an approximately 6-10dB improvement, a number that agreed reasonably well with what had been calculated using similar measurements done at home on my "Photon Range" using a very dim LED and test receivers.

Practically speaking this meant that at full power with the Phlatlight LEDs we had about 50dB of  excess signal at the output of the receivers as compared to the minimum possible signal level using baseband speech and the "naked" ear.  Switching to MCW (tone-modulated Morse code) we could extend this by another 6-10dB and the the use of narrowband digital signalling techniques (such as WSPR or QRSS CW - very slow Morse) could have extended this by even another 20 dB or so.  The implication of this is that, in theory, we could communicate over that distance with only a milliamp or two of LED current!

Figure 2:
This time, a high-power green LED!
Click on the image for a larger version.

Satisfied with our tests I switched to a green CBT-54.  Interestingly - but not too surprisingly - Ron reported that subjectively the green LED wasn't really any brighter than the red had been.  On previous tests at much shorter distances (a few 10's of miles/km) the green far outshone the red owing to the fact that the human eye is at least 5 times as sensitive to green than the wavelength of red LED that we were using.  For these distances the atmospheric attenuation was sapping the vast majority of our light since the shorter (green) wavelengths are attenuated at a far higher rate than the longer ones, a fact that explains red sunsets and that we observed, at the beginning of our testing, that his white, halogen spotlight appeared to us as distinctly yellow/red in color.

The silicon photodetector didn't fare any better since it had far less sensitivity at green than red, the two factors (atmospheric and the Si sensitivity) adding up to between 20 and 30dB in degradation - assuming that the subjective measurement of "equal" brightness between red and green was correct.  As it turns out the degradation was probably far greater than that as the APD-based receiver could hardly detect voice at all, but this may have been also due, in part, to the fact that the gain of a standard APD drops off precipitously with shorter wavelengths and that it was likely not focused properly for green light due to chromatic aberration of the Fresnel lens!  In retrospect we should have switched to a receiver with a larger, "non-APD" detector - and thus less sensitive to misfocusing due to chromatic aberration.

In addition to using high-power LEDs, we also exchanged 2-way communications using plain, ordinary, cheap low-power red LED laser pointers.  The signals were far weaker - mostly owing to the lower optical power of the laser pointer - but each other's lasers were visible to the naked eye over the distance.  Because of the combination of the laser's (relatively) coherent light and its small exit aperture (small beam diameter) the scintillation (fading) on the laser-based link was terrible while on the LED-based link it was only just noticeable.  Some of the methods and techniques to communicate using laser pointers may be found in the September 5th entry of this blog.

After several hours of standing around in the dark on the mountain, we decided that it was getting early (approaching 2 AM!) and packed things up and made our way down the mountain.

Overall, it was a fun little jaunt giving us a healthy dose of nerdiness... enough to last for a few weeks, anyway!

Afterward:

While we run these tests, we'll often play something from portable audio players so that we have a continuous source of sound.  In this case, one of the audio sources that I used was from a Soldersmoke podcast.

For the heck of it, I emailed Bill, N2CQR, who produces this podcast and he put it on his blog page (link) as well as commenting on it in his next Soldersmoke podcast (link)!  This may have had something to do with this post appearing on Hack-A-Day (link)!

Links from the "Modulated Light" (link) web site:

• Using Laser Pointers for voice communications -  This page describes in more detail the methods by which one may successfully use inexpensive laser pointers to cast voice through many miles of the ether!
• A Highly-Sensitive Optical Receiver Optimized for Speech Bandwidth - This is one of the most sensitive speech-range optical receivers yet devised that uses standard photodiodes and is much more sensitive than the simple receiver depicted in figure 1, above.
• Receiver for low-bandwidth optical (through the air) communications using an Avalanche Photo Diode (APD) - This optical receiver is an enhancement of the one above, achieving another 6-10dB of ultimate sensitivity using an APD.  This receiver is about as sensitive as you can get without resorting to an exotic red-sensitive photomultiplier tube!
• A "Cheap" Optical Transceiver lens assembly - If you really want to improve your receive sensitivity, the best way to do this is with a large lens.  This article describes how one could use inexpensive foam-core poster board to make an assembly that will focus the distant light on the detector diode of an optical receiver.
• A "Mini" full-featured Pulse Width Modulator for high-power LEDs and laser diodes -This describes a simple, computer-based PWM modulator that will not only transmit audio, but generate test tones as well.

Be sure to check out the "ModulatedLight.org" web site's other pages as well!

[End]

This page stolen from ka7oei.blogspot.com