A red LED as both the optical tx and rx active device
 
With thanks to Stuart G8CYW for all his knowledge and experience on this subject



Separate Tx and Rx heads obviously complicate the building and alignment of an optical transceiver, so to be able to use the transmit LED as the receive detector very much simplifies such a system. The advantage applies all the more for an entry level transceiver and is what was originally intended for the Finningley nanowave transceiver. However, the best I could achieve in terms of receiver sensitivity was a performance 20 dB down on my standard SFH-213 based front-end, and as time ran out, the opto-diode/LED pair arrangement was reluctantly reverted to. Since Stuart and colleagues up in the North East successfully use the LED-as-rx approach, all new LEDs that appear here are still tested for sensitivity and eventually one was found that was only 10 dB down compared to the SFH-213 when tested in my standard light tight receiver test box. That 10 dB reduces to 6 dB when you take the die area into account and are using a lens ahead of the front-end.

This current page details the work done on the specific LED found. For general background reading, Google 'G8CYW LED as receiver', inspired by this link:

http://een.iust.ac.ir/profs/Sadr/Papers/omd7.4.pdf

Also, the following link contains further interesting information:

http://makezine.com/projects/make-36-boards/how-to-use-leds-to-detect-light/

There's plenty of other similar articles out there too.

The LED in question

During an ebay  purchase of more10mm LEDs used in the Finningley optical transceiver, it was noticed that the type supplied this time were not as shown in jeledhk's advert. The difference can be seen opposite. Although the die area on the type sent (rh side opposite) is smaller and the heat-sink blade on the main LED body is also smaller, a sample device did survive being run at full continuous power for 24 hours, the spec summary shown below right opposite.

The 'full power' rating referred to does need some interpretation, as all Chinese devices seem to. Although sold as a 1W device, the maximum continuous current rating is quoted as 300mA, which at 2.3 volts is of course only 690 mW. They are 1W only at the quoted short term peak current (450 mA). In any case, this is the data for the original device, and although promised, no data sheet for the replacement item has materialised to date.

Incidentally, these LEDs first came to my attention via G3XBM, so thank you Roger - the devices have proved reliable enough and the light output is certainly high, plus the inherent lensing reduces the beamwidth to 40 degrees which is about optimum to properly illuminate an A4 size Fresnel lens.

sidetop

spec sheet


LED performance as a detector

As an initial test, the photo-diode on a Finningley receiver was removed and directly replaced with the red LED to be tested. Additionally, the 8v reverse bias supply for the photo-diode was swapped for a 0 - 100v variable supply to reverse bias the LED. Thus, the only loading on the live side of the LED remains the same FET gate that the photo-diode experienced. With a noisy (ie, low level) optical signal and 1 kHz amplitude modulation applied, the bias was increased to the point were further improvement in signal to noise ratio stopped, ie reverse breakdown was beginning to occur - in this case, about 64 volts (interestingly, the sensitivity does increase a little beyond this voltage, but the noise increases at a greater rate). The measurements were done in the light-tight test box described here.

A straight comparison of sensitivity between the original SFH-213 and the LED under test is shown below:

relative sensitivity

Further measurements and a bit of thought throw up one good and one  bad characteristic! The good point is that since the light box is applying an even illumination across the detector, and the equivalent die area of the LED is no more than a quarter of that of the photo-diode, the true difference in sensitivity once the light is optimally focused onto either device, should only be about 6 dB.

The bad characteristic of the LED is its much greater self capacitance (about 55 pF). This shows up as a deterioration in sensitivity as the modulation frequency is increased at an earlier point than does the photo-diode, and sadly, it occurs immediately above 1 kHz. Capacitance was measured using a 20 Mohm bias feed resistor:

LED self capacitance
area comparison





Once reverse breakdown occurs, the graph becomes meaningless, of course, but it's useful just to confirm the onset of breakdown non the less.


 
The S/N reduction with increasing frequency response is shown below, together with that of a receiver using an SFH-213 photo-diode front-end:

Rx frequency reponse

Since the reverse breakdown noise is less 'bitty' with increasing frequency, a 5 dB improvement in S/N can be obtained for the LED front-end above 10 kHz by slightly increasing the reverse bias voltage.


Outdoor testing

The acid test for the LED-as-receiver configuration is to try it out in the field, I suppose - so here's the result of my first tests down the garden.

A bias battery pack, with some voltage adjustment, was made up using PP3's from Poundland. For the half dozen or so LEDs tried so far, the onset of reverse breakdown has been fairly consistent, being about 64 +-1 volt, so voltage adjustment was made by using a 500k pot across only one of the PP3s. 

bias battery pack

The output voltage is variable between 60 and 70 volts, PP3s seemingly being 10v these days. Since I don't fit the tilt switch on my Finningley transceivers, I use pin 12 on the 15 way D connector to input the bias supply.

An A4 size Fresnel lens was used to focus the test source onto the detector, refocusing between the LED and photo-diode, depending on the option. The test source was located some 50m down the garden. To hand were two micro-power test sources, one modulated with a single 1 kHz tone and the other with a pair of near 15 kHz tones in warble mode. A lot of effort was spent ensuring that the focusing was optimum for both the LED and photo-diode receivers, resulting in the following:

outdoor testing results

The bottom two pictures (2 tones at 15 kHz) are so noisy as part of the tone switching that I thought it best repeat the 15 kHz tests with a single tone:

single tone senstivity at 15 kHz

No matter how I adjust the focusing on the two front ends, the sensitivity difference remains within 1 dB of the figures shown in the graph. Though the relatively high LED self capacitance should result in poorer sensitivity at 15 kHz, the sensitivities remain stubbornly similar. I'll re-measure these on another evening, but I think the explanation is that the 1 kHz and 15 kHz test sources, using different red LEDs, were of slightly differing wavelengths and the 15 kHz one was closer to the peak receive wavelength of the detector LED.

update on relative sensitivities

A few weeks later, the set-up was taken out to test over a 66 km long path between Manton in North Linconshire and High Bradfield, near Sheffield. I joined Richard G0RPH (shown opposite) at Manton, whilst Barry G8AGN and Gordon G0EWN operated from High Bradfield.

With Barry using his phlatlight source into an A4 size fresnel lens, there was no problem in identifying his signal, it being a decimal place brighter than anything else on the horizon. His morse ident was audible before I had even got the transceiver tube into its mating section of the light box (right hand box in the picture). Turning his beam away from from us until his signal became noisy but still readable resulted in binoculars being required to see his signal by eye.

The signal received by Richard's  photo-diode receiver was showing a similar background noise level, confirming that the LED based receiver sensitivity is indeed on a par with that of a conventional SFH-213 photo-diode one.

Manton end


One further thing I did do was to swap the Fresnel lens for a 4" circular convex lens. With this set-up, the signal was 10 dB lower which is slightly high given the difference in area of the two lenses.

The small die size result in a beam-width of +- 0.22 degrees when used with an A4 size fresnel lens (of focal length = 350mm).


Narrow optical bandwidth

As implied above,  the receive optical bandwidth of the red LED is quite narrow. This is most noticeable when using my switched red/infra-red mini beacon. When switched to IR, the beacon has to be placed immediately in front of the receiver to hear anything at all. I'll take a measurement at some point, but it has be at least 60 dB down on the red output LED.

The graph opposite is taken from the link content at the top of this web page and shows how narrow the receive optical bandwidth is for the red LED sample they were talking about. It is also apparent that there is a difference in wavelength peak for the receive and transmit situations. At the moment, I have no test equipment that can produce the same graph for the HK LED.
optical frequency
                          response, red LED

Removing the need for a separate bias battery

Whilst it is fine in the short term using a few series connected PP3s to generate the 63v LED bias, long term it is nicer to generate it from the existing 12v supply, and if possible, to locate it on the transceiver pcb. The later requirement runs the risk of pick up of the bias oscillator onto the high impedance receiver input. For that reason, a much higher frequency oscillator frequency was sought and the following arrangement using a 1 MHz ceramic resonator was used. It also takes up minimal board space and the repeatability between resonators is not too bad, particularly if you have a bag full...

LED based circuitry

Temporary control pcb:

So that headphones can be used, the original agc chip and emitter follower were added to a revised plug-in control pcb. No agc has been added to the mic/tx path.

Stopping the SL6270 from 'pumping' seems to be an impossible task, but perhaps it simply needs a more competent designer defining the circuitry. The version opposite is the best I can manage so far.

It would be useful to have put the 1 kHz tone oscillator on this board too, but it just didn't happen.
control pcb


Update as of July 2017:

The agc chip on the control board has been changed to a standard audio driver IC preceded by a volume control - more output, level controlled and no pumping!. Also, a tone generator consisting of a 4060 divider IC driven from the 1 MHz ceramic oscillator has been added, giving a reasonably stable 980Hz tone.

To be continued....