Gate current in a JFET: The development of a very sensitive, speech-frequency optical receiver.

Back in 2007-2008 I was working on equipment for "new" ham band - for me at least - the one that is now labeled as "...above 275 GHz" in the FCC rules.  As you might expect the most accessible portion of this infinity of electromagnetic spectrum is that containing visible light, and that is where I was directing my interest.

At this time "high power" LEDs were starting to appear on the market at reasonable prices, and by "high power" I mean LEDs that were capable of dissipating up to 5 watts, each.  What this meant was that from a single emitting die of rather small dimensions one could pump into it enough current and, with the good efficiency of the device, obtain a quantity of light that was suitable for long-distance optical communications.

To be sure, I was building on the fine pioneering work of others, including that of two Australians, Dr. Mike Groth (VK7MJ) and Chris Long (now VK3AML) who had determined that it was the noncoherent light produced by LEDs that offered the greatest probability of practical, very long-distance atmospheric optical communications.  (As a primer as to why this is the case, read the article Optical Communications Using Coherent and Noncoherent Light - link).

Optical receiver needed:

In the midst of producing the various pieces of equipment required for experiments in optical communications (e.g. optical transmitters, modulators, receivers, support equipment, etc.)  I was investigating the different circuit topologies of practical optical detectors.  My goal was not to achieve extremely high data speeds, but rather to use audio-frequency signalling (speech, tones) to start with and, perhaps, work up from there.

One of most common such detectors is the phototransistor - but I quickly dismissed that owing to its very small photoactive area and the fact that the various pieces of literature relating to weak-signal optical detection noted that they are inferior in comparison to practically any other device owing to their intrinsic noise level.  (CdS cells - article here -  were not seriously considered because they are too slow to respond - even for audio frequencies.)

One option was the venerable Photomultiplier Tube (article here) and while this was technically possible and, in theory, the best choice, it was ruled out because of its fragility (electrically and mechanically), its large size, the limited response at the wavelengths of interest (more below on that) and the need for a high voltage supply (around 1000 volts).

While these technical difficulties are surmountable I could not overlook the fact that available literature on these devices - and advice from the Australians, who'd actually used them - pointed out that there were but a few photomultipier tube types that have good sensitivity in the "red" end of the optical spectrum where there is also good atmospheric transparency - and even fewer of these rare types, in known-usable condition, available for a reasonable price on the surplus market!

Figure 1: 
The transimpedance amplifier in its simplest form. 
This circuit converts the photodiode currents
into a proportional output voltage.

The Transimpedance Amplifier:

This left me with the photodiode (article here) and the most commonly-seen circuit using this device is the "TIA" - TransImpedance Amplifier (article here).  As can be seen from Figure 1 this is very simple, consisting of just an operational amplifier with a feedback loop with the photodiode connected directly to the noninverting input.  In this circuit the photodiode currents are converted directly to voltage (hence the name) with the gain set by the feedback resistor with the added capacitor being used to assure stability, compensating for photodiode and op-amp capacitance.

This particular circuit has the advantage that it is very predictable and the frequency response can be determined by the combination of the bandwidth of the op amp and the intrinsic capacitance of the phototransistor.  To a degree, one can even increase the frequency response for a given set of devices by reducing the feedback, but this comes at the expense of gain and ultimate sensitivity.

In other words:  With photodiodes you can have high sensitivity, or you can have wide bandwidth - but not both!

Figure 2:
 A practical, daylight-tolerant TIA optical receiver circuit.  This has good sensitivity in both darkness and light and does not suffer from "saturation" in high ambient light conditions because of a built-in "servo" that self-adjusts the phototiode's virtual ground to offset photon-induced bias currents.  Because of this "servo" action this receiver does not have DC response like the circuit of Figure 1 with the low-end frequency being limited by the values of R104 and C106.
While the LM833 is a reasonable performer, there are other (more expensive!) op amps that have lower noise.
Click on the image for a larger version.

While very simple (there are even single-chip solutions such as the "OPT101" that include the photodiode, amplifier, and even feedback resistor in a clear package) there are some very definite, practical limitations to the ultimate sensitivity of this sort of circuit if the goal is to detect extremely weak, low-frequency currents.  When you get to very low frequencies, "1/f" noise (a.k.a. "flicker noise") becomes dominant from a number of sources and there are various other types of noise sources (thermal, shot, etc.) that can be produced by the various components.

As it turns out, this circuit - with practical op amps - has very definite limitations when it comes to trying to divine the weakest signals at low-ish frequencies (audio, sub-audio):  For an article on why this is so - and some of the means of mitigation - see the January, 2001 Electronic Design article, "What's All This Transimpedance Amplifier Stuff, Anyway?" - link by Robert Pease.

Figure 3: 
The VK7MJ optical receiver using TIA and cascode techniques - used as the "reference" optical detector.
The optional "daylight" circuit provides AC coupling to prevent saturation of the circuit under high ambient
light conditions at the expense of low-light performance.
Click on the image for a larger version.

One can build transimpedance amplifiers using discrete components that outperform most of the integrated-circuit based designs and for a reference circuit I constructed and used one devised by Dr. Groth, VK7MJ and depicted in Figure 3.  In this circuit one may see the feedback path via R3/R4 with compensating capacitor Cf.  In this particular circuit Q1, the input FET, is rather heavily biased to increase its "bulk current" (a term used in the referenced Robert Pease article) with Q2 acting as a cascode circuit - link (e.g. current-based) amplifier with subsequent follower stages.  Additionally, the photodiode itself (D1) is reverse-biased, reducing its capacitance significantly and thereby improving high frequency response.  By hand-selecting the quietest JFETs one can obtain excellent performance with this circuit and since it is discrete, there is room for adjusting values as necessary to accommodate component variations and for experimentation.

This particular circuit is quite good across the audio range from a few 10's of Hz to several kHz, but above this range it is largely the capacitance of the photodiode (at least for devices that have square millimeter-range surfaces areas) that quashes the high frequency response.  Even though the photodiode's capacitance - and that of stray wiring and the JFET itself - may be only in the 10's of picofarads, at hundreds of k-ohms (or megohms) even small amounts of capacitance quickly become dominant - another good reason to implement the aforementioned cascode circuit and its tendency to minimize the "Miller Effect" - link to help optimize frequency response.

The K3PGP circuit and variations:

Figure 4: 
The K3PGP Optical receiver.
Click on the image for a larger version.

Building the above circuit as a "reference" I began testing on a "Photon Range" - a darkened room in my basement with a red LED affixed to the ceiling - where I characterized the various receiver topologies.  In this environment a small and adjustable amount of current (10's of microamps, typically) would be fed to the LED, modulated at an audio frequency, and the receiver under test would be placed on the floor below with its output connected to a computer in an adjacent room running an audio analysis program such as "Spectran" or "Spectrum Lab" to measure the signal-noise ratio at different frequencies.  Before and after each session I would measure the performance of my "standard" optical receiver - the VK7MJ circuit - and use it as a basis of comparison.

The receiver named after K3PGP (see his web site - link) was the next receiver to be tested.  This receiver is much more sensitive than the VK7MJ receiver - at least at very low audio frequencies (<200 Hz) and as may be seen in Figure 4 it is devoid of a feedback mechanism and the connection between the photodiode and JFET is made directly, with no external biasing components of any kind.

While a seemingly simple circuit, there is more going on here than one might first realize:  Without any feedback or any other components between the FET and photodiode the opportunity to introduce noise from such components or reduce the signal from the photodiode in any way is minimized.  In fact, when constructing this circuit there is the strong admonition that the photodiode-gate connection to the JFET be done in mid-air (and that one clean both components with alcohol to remove residue!) as leakage paths on circuit board material can cause significant signal degradation!

Effectively, the K3PGP circuit acts as a charge integrator with the energy slowly (in relative terms) bleeding off due to the leakage of the photodiode, its photoconductivity, and the gate-source leakage currents of the FET itself.  While extremely sensitive at low frequencies - specifically those below 200 Hz - above this, the sensitivity and output suffers due to the rather long R/C constant associated with the high gate-photodiode leakage resistance and capacitance and, to a lesser degree, the Miller effect.  This circuit also functions only in total and near-total darkness conditions:  More light than that, the voltage across the photodiode reaches equalibrium while turning the FET "on", effectively quashing the signal.

Inspired by the above circuit I made the modification indirectly depicted in Figure 5, below:

Figure 5:
 The version "2.02" optical receiver, used as a test bed for various circuit configurations - see text.
For the "K3PGP" configuration the photodiode would be reversed from what is shown
in the drawing above and the anode grounded with nothing else connected at point "C".
Click on the image for a larger version.

This circuit was devised as a "test bed" and although not shown in the diagram, it was configured by connecting the cathode of the photodiode to the gate and grounding the anode and having no other photodiode-gate connections present - just as in the K3PGP receiver.

In this circuit one has a FET input and a cascode circuit - just like that of the VK7MJ circuit - to reduce the Miller effect, but this particular cascode circuit has a modification:  Q3 forms a current source, in parallel with the cascode, that supplies the bulk of the drain current for the JFET - several milliamps.  Because the amount of current provided by the current source - which has a high operating impedance and is largely "invisible" - is fixed (but adjustable by varying R4 to suit specific characteristics of Q1) and it is left up to the cascode to supply the remaining drain current - which varies depending on the gate voltage.  In this particular circuit, due to the "cascode action" the voltage at the drain of Q1 and emitter of Q2 varies very little while the cascode - which is allowed to bias itself at DC, but is bypassed at AC with C3 - produces the recovered modulation at the collector of Q2, greatly amplified.  From the collector of Q2, noninverting amplifier U1a amplifies the signal further and presents a low-impedance output.

In other words, it is mostly the K3PGP circuit, but with a cascode amplifier and higher FET drain current:  By reducing Miller capacitance with the cascode the frequency response was to be improved somewhat and by increasing the drain current the noise contribution of the FET itself should be reduced as noted in the Pease article mentioned above.

In testing it was observed that this particular circuit was, in fact, several dB more sensitive than the original K3PGP circuit and also that the frequency response was slightly better - but not as much as one might first think, mostly owing to the fact that it is mostly the photodiode capacitance that is limiting the response rather than the Miller effect - but every little bit helps!

I then rewired the circuit using the "Standard Config" noted in Figure 5 which, if you draw in the lines, converts it into a TIA circuit like that of the VK7MJ design with both adjustable reverse bias of the photodiode and adjustable feedback.  In this configuration the performance at very low frequencies was reduced, likely due to the noise contribution of the feedback resistor, increased leakage currents from the photodiode at reverse bias and also signal attenuation caused by the feedback submerging the lowest-level, low-frequency signals into the noise.  At "speech" frequencies it was slightly better than that of the VK7MJ receiver - probably due to the higher JFET current or, perhaps, random component variances - and the frequency response was also comparable to that of the VK7MJ circuit, the parameters varying according to the amount of applied feedback and compensation.

Improving the receiver:

My goal was a circuit that offered the sensitivity of the K3PGP circuit, but usable speech response - the latter not being available from the K3PGP circuit due to the R/C rolloff.  A quick check revealed that this was the typical 6dB/octave rolloff so I reconfigured the circuit, again, as a K3PGP-like circuit and followed it with an op-amp differentiator circuit with a breakpoint calculated to compensate for the measured "knee" frequency (e.g. that at which the 6dB/octave rolloff of the K3PGP circuit) began - the result being that I now had a fairly flat frequency response.  Not unexpectedly, while the signal-noise ratio was quite good at the very low frequencies, it decreased fairly quickly as it went up as that energy was simply submerged in the circuit noise.

In staring at the circuit, with the grounded anode of the photodiode, I wondered about reverse-biasing the photodiode to reduce the capacitance - but if I did this, how would I keep the voltage at the gate from rising without needing to add another (noise generating, signal-robbing) component to clamp it to ground?  Knowing that the gate-source junction of a JFET was much like that of a bipolar transistor in that there would be an intrinsic diode present, I knew also that the gate-source voltage would limit itself to 0.4-0.6 volts, but how would the FET behave?

Using JFET Gate current for "good":

In doing a bit of research on the GoogleWeb when I derived this circuit I could not come up with any sort of useful answer to the "gate current" question, so I simply did it, constructing a "gate current amplifier":  The photodiode was reverse-biased with the minute leakage, dramatically reducing its capacitance, and photoconducting currents being sinked by the gate-source junction.  As expected, the drain current increased noticeably, but the circuit worked extremely well, with both frequency response and apparent gain increasing dramatically!

Putting this "new" circuit back on the photon range I observed that although its low frequency (<200 Hz) sensitivity was slightly worse than that of the K3PGP circuit (see comment below), the higher speech-range frequencies (300-2500 Hz) were, on average, 10-12dB better than the VK7MJ circuit and approximately 20 dB better than the best, low-noise op-amp based TIA circuit that I'd built to date!

In analyzing the circuit, there are several things happening:

• Reverse bias of the photodiode:  This reduces the capacitance - typically by a factor of 3-6, depending on the specific device and voltage applied.
• The photodiode will produce current in the presence of light.
• Being reverse-biased, the photodiode will also operate in a photo-conductive mode, passing current from the bias supply in response to light.
• With the gate-source junction conducting, the reverse bias across the photodiode is maintained since the gate-source voltage will never exceed 0.4-0.6 volts.
• As described above, the amplifier is connected in "cascode" configuration to minimize Miller effects.
• There are NO other components or signal paths connected to the photodiode-gate junction that can contribute noise or attenuate the signals.
• In parallel with the cascode circuit is a current source which provides a high-impedance current source to increase the JFET's bulk currents, further reducing its noise.

 About the gate-source conductivity of the JFET, two things surprised me:

• The "diode action" of the gate-source clamping seems not to be a significant contributor of noise - at least at "dark" currents of the photodiode.
• There is little or no documentation about using a JFET this way, anywhere else!

It is likely that the main reason that this doesn't perform quite as well at the K3PGP circuit at low (<200 Hz) frequencies is because of the intrinsic leakage current noise endemic to the reverse biasing of the photodiode, particularly in a "1/F" manner:  At higher frequencies where this sort of noise falls away it performed far better. 

Note:

In "photon range" testing it was difficult to tell at which frequencies the K3PGP receiver performed better.  My K3PGP exemplar receiver was certainly better at, say, 20 Hz, but even at 100 Hz or 60 Hz it was a difficult call to make.  At such frequencies and under such conditions careful selection of the "quietest" photodiode and FET can make a significant difference and with most of these circuits, reducing their temperature - while somehow managing to avoid condensation - can help even more!

Plotting Gate current versus Drain and Gate voltages:

Later, I constructed a test fixture to analyze the gate-source voltage and gate-source current response of a 2N5457 JFET and plot this against the drain current - see Figure 6 below.

Figure 6:
 Gate-source voltage and Gate current plotted against drain current for a typical, real-life JFET - not a simulation!  Note the logarithmic scale of the gate current and also that the drain current continues to increase linearly with gate-source voltage, even after the gate-source junction is conducting.
Click on the image for a larger version.

As can be seen, as the gate-source voltage increases, the drain increases linearly - even after the gate-source diode junction starts to conduct:  In fact, there does not appear to be inflection of the drain current curve when this happens!  Following the other line representing gate current we can see that once our gate-source "diode" starts to conduct, the gate current follows the classic logarithmic curve that one associates with diodes - which should not come as a surprise.

Equation 1:
The relationship between drain current and
gate-source voltage.
Vgs= Gate-source voltage
Vp=FET Pinch-off voltage
Idss=Zero gate voltage drain current

According to typical JFET models, in the saturation region the FET operates such that the drain current is generally independent of the drain voltage as can be seen in Equation 1 and the graphs in Figure 6 indicate that this seems to be true even when the gate-source junction is conducting.

So, now we know what is happening.  At first glance, one might presume that with this diode in conduction that the logarithmic response would make the circuit unsuitable for general audio recovery - but this is not so:  At very low light levels the detector has lower than 1% harmonic distortion.

Figure 7:
Test circuit used to derive the curves in Figure 6.
For measuring the voltage at "Vgate Monitor" it will be
required that the negative lead of the voltmeter be referenced
to a regulated, negative (with respect to ground)
voltage source.  Q1 is the device being tested and
Q2 is just another JFET which need not be the
same type as J1.

In case you are interested, Figure 7 shows the circuit that was used to derive the curves in Figure 6, above.  10.0 volts was used for V+ and the drop across source-follower Q2 was easily characterized so that the drop across R1 - and thus the gate current in Q1 - could be determined.  The drain current was determined by measuring the voltage across R2.  Different values of R1 were used to achieve the measurement range depicted in Figure 6 which accounts for the very slight bend in the "Gate Current" curve.

Putting this into practice:

The circuit depicted in Figure 8 was developed for speech-bandwidth optical communications use.

As can be seen, this looks very similar to the circuit of Figure 4 with the exception that the reverse-biased photodiode is connected to the JFET and that there is the added circuit, U1b, that forms a bandwidth-limited differentiator - the component values chosen to approximately correlate with the low-frequency "knee" of the BPW34 photodiode and also to cease its frequency boost above 5-8 kHz.  (The "Flat" audio output, uncompensated by the differentiator for the 6dB/octave rolloff, is provided for both very low frequency - below 200 Hz - and high frequency - above 5 kHz - signals to be applied to a computer for analysis.)

The circuit in Figure 8 - and minor variations of it - have been replicated many times over the years using different components.  The important considerations are that both Q2 and Q3 be low-noise, high-beta transistors such as the MPSA18 (or 2N5089) and that the JFET used for Q1 be capable of rather high drain current.  In the original design, the 2N5457 was specified as this device is better-characterized that many other, similar FETs and is capable of quite low-noise operation:  The more common MPF102, with its extremely wide variation of parameters, might be suitable if an appropriate device is "cherry picked" from amongst several based both on high zero gate-source voltage drain current and tested "noisiness".  A more modern JFET is the BF862 - available in surface-mount only (as are most JFETs these days!) - that is even better for this application than the 2N5457 and capable of much higher drain ("bulk") current to the point where utilizing its full potential might compromise 9-volt battery life!

Figure 8:  
Version "3" of the optical receiver.  This receiver must always be operated on its own, completely isolated power supply to avoid feedback.  V+ is 8-15 volts and is typically a 9-volt battery.  D4 and TH1 prevent damage should the applied polarity of the power source be accidentally reversed.  After Q1's drain current has been measured and adjusted, jumper "J1" is closed.
A version of this circuit by the author of this page also appeared in an article published in the SPIE proceedings (#6878) which was presented at the 2008 "Photonics West" conference by another one of the paper's co-authors, Chris Long.
Click on the image for a larger version.

In a circuit such as Figure 8, above, the drain-source voltage will be much lower than one might initially expect - on the order of 0.2-1.0 volts for a JFET such as a 2N5457 and between 0.1 and 0.5 volts for the BF862 - but this is normal operation.  While the setting for Q3 current, adjusted via R5, (in Figure 8) at 120 ohms is suitable for most 2N5457 devices, the current may need to be reduced (e.g. R5 increased in value to 180 or 220 ohms) for some "lower 0 Vgs" current devices such as the MPF102.  In general, the higher the drain current, the lower noise contribution from the FET - but if you exceed the "magic" value and attempt to force too much current, the circuit will suddenly stop working:  Overall it is better to have a bit lower drain current than optimal and have a little bit more noise than to have too much drain current!  (Don't forget that the properties of the current source and the JFET itself will also change with temperature - but they generally seem to track.)

Interestingly, the circuit depicted in Figure 8 also works in daylight, albeit with some caveats.

When very high levels of light are present, the photoconductivity will shunt the reverse bias to the gate-source junction, and the frequency rolloff "knee" associated with the photodiode capacitance will shift upwards due to photoconductive shunting causing the audio to become "tinny".  The audio will also become somewhat distorted owing to the different light-to-audio transfer curve that occurs under such conditions, in which case the frequency response of the audio on the "flat" output is more suitable than otherwise.  In such situations one does not really need the high sensitivity of this type of receiver, anyway, and a typical TIA circuit with AC coupling such as that depicted in Figure 2 or Figure 3 could be used or one could apply optical attenuation in front of the detector to reduce the light level.

Practical use:

Figure 8:
An as-built "Version 3" optical receiver, constructing using
prototyping techniques and enclosed in a shielded, light-tight
enclosure using pieces of printed circuit board material.  For this
unit "feedthrough" capacitors are used for power and audio
connections to prevent the incursion of RF energy on
the connecting leads.
Click on the image for a larger version.

Entire web pages could be written (and have been - see the Modulated Light web site - link) about through-the-air, free-space optical communications over long distances (well over 100 miles, 160km) using both LEDs and low-power lasers, but even the most sensitive receiver - no matter the underlying technology - requires supporting optics (lenses!) in order to function properly:  It is through such lenses that 10's of dB of noiseless signal gain may be achieved, not to mention directionality and the implied rejection of off-axis light sources.

The circuits described on this page are likely to be suitable only for speech frequencies and low-rate data but this is, in part, due to the medium involved (the atmosphere) and method of transmission.  At the extreme distances that have been achieved with the above equipment (>173 miles, 278km) the signals are weak enough that only low-rate signalling techniques would likely be feasible under typical conditions at safe, practical optical power levels.

Additional web pages on related topics:

• Modulatedlight.org - This web site has a wide variety of information related to amateur, free-space optical, through-the-air communications.
• Optical Receivers for Low-Bandwidth, Through-the-Air Communications - This is a reference article that gives additional detail about the design of the circuits discussed in this article.
• Using Laser Pointers for Free-Space Optical Communications - This describes how one might use low-power laser pointers for low-rate optical communications, the practicalities of various circuits, the methods and the realities.
• The Modulated Light DX page - This page has several articles describing the practical aspects of free-space, through-the-air optical communications including various atmospheric effects.
• A description of this circuit appeared in the SPIE Conference Proceedings, Volume 6878, “Atmospheric Propagation of Electromagnetic Waves II” in the article "Dollars versus Decibels:  Long-Range atmospheric optical communications on a tight budget".  This article was presented at the January, 2008 Photonics West conference and a copy of this article may be read here.

The above web pages also contain links to other, related pages on similar subjects.


[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!

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This page stolen from ka7oei.blogspot.com