This
receiver has been tested in the field. For an article about the
expedition during which it was used, see this blog entry:
Important:
Prior to delving into the content on this page, it is
highly
recommended that you take a look at the
following web pages about optical receivers:
The above pages provide reference material, rationale and
background for the designs described on this page.
Using LEDs as APDs?
As it turns out, many types of LEDs appear to exhibit
gain effects akin to those of an avalanche photo diode
when reverse-biased - an effect that has been observed
in the 25-300 volt area with the voltage, spectral
response, and overall sensitivity/gain depending on the
LED's construction and material. It is possible to
adapt some of the circuits on this page to be used with
an LED for experimentation of this gain mechanism.
Not surprisingly, the degree to which this effect occurs
and at what voltage tends to vary wildly from device to
device as this is not a mode in which LEDs were intended
to operate! The stability and long-term viability
of operating an LED this way is another factor that
should be considered.
While the results do vary, some LEDs have been noted to
offer reasonable performance as gain-enhanced detectors
when operated in this mode making it practical to have
at least moderate performance as a detector - a
particular attraction if one wants to build an optical
transceiver that uses just one diode for both receive
and transmit.
While an interesting phenomenon, tests thusfar seem to
indicate that, at least for baseband (audio) use, the
ultimate sensitivity of a typical LED operated in this
mode is worse than that of a
well-designed optical detector using standard PIN
photodiodes. Having said this, this avalanche
effect can be used to good effect to make an optical
transceiver that uses the LED for both transmitting and
detection and yield a single, compact package with
reasonable performance. I've not seen "ultimate
sensitivity" comparisons with PIN photodiode-based
receivers and avalanche LED receivers for subcarrier
(ultrasonic and up) operation.
The circuits described on this page can form the basis
of an optical receiver utilizing the avalanche
effects. If higher bandwidth is desired (with an
acceptance of poorer weak-signal sensitivity) then a
typical transimpedance amplifier circuit may be employed
with either a "real" APD or a reverse-biased LED.
An article describing
the use of the avalanche gain effect exhibited by LEDs
may be found at the bottom of this page in the article
"Optical Receiver
Operation With High Internal Gain of GaP and
GaAsP/GaP Light-emitting diodes" linked at
the bottom of this page.
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Why use an APD?
The
Avalanche
Photo Diode (APD) is, as its name implies, a diode that
electrically responds to light that impinges on its active
surface using the
photoelectric
effect, converting varying amounts of light to
correspondingly varying electrical current. "Normal"
photodiodes also do this, but have no "gain" in doing so - that
is, it takes the action of more than a single photon to cause
the movement of a single electron. As one might imagine,
at very low light level - where there are relatively few photons
- the currents produced by a standard photodiode can be very
small indeed! APDs have intrinsic to them the ability to
effectively amplify the signal due to the fact that one "event"
can loose a barrage
(or "avalanche") of electrons, the number
being related to the gain of the APD itself which is also
related to the voltage applied to the APD.
The tiny currents produced at very low light levels have to
compete with currents
(noise) from other sources such as the
thermal noise from the circuits attached to it, noise
contribution by components in that circuitry and other noise
sources related to the diode and transistors themselves.
As one can imagine, at these very low light levels these other
noise sources could easily dilute or drown out the desired
signal!
The APD helps combat the problem of these noise sources by
having its own internal amplification mechanism meaning that, on
average, a single photon can cause the movement of many
electrons and by having this additional amplification, the
signal coming out of the APD can be high enough to overcome much
of the noise of the following components and amplifier
stages. As with any circuit, the APD
does contribute some of its
own noise - particularly when it is run "hotter"
(e.g. higher
voltage and gain) - and this excess gain can, in fact, reduce
overall sensitivity when the device's own noise starts to drown
out the desired signal. Because of this intrinsic
amplification APDs are often used in very high-speed optical
detectors such as those used in fiber optic receivers by
allowing (comparatively) insensitive, fast circuits to be used
for amplification.
One of the difficulties associated with the receiver design has
to do with the fact that in order to be used with inexpensive
Fresnel
lenses, the active area of the photodiode needs to be
fairly large - on the order of 1mm diameter or larger - in order
to accommodate the
imperfect optics of these
molded lenses: Much smaller than this, efficiency is lost
as the "blur circle" from the lens would be larger than the
active area and some of the light from the distant source would
be lost. While it is possible that a conventional glass or
plastic "secondary" lens could be employed to further-reduce the
effective size of the blur circle, the added complexity and need
for precision starts to push the construction of such an optical
system outside the realm of the amateur experimenter!
With any photodiode comes device capacitance and taking the
BPW34 - a somewhat large-ish photodiode, when reverse-biased
with 10 volts its intrinsic capacitance is on the order of 20
pF. This may not sound like much at audio frequencies but
when one considers that the operating impedance may be on the
order of 10 Megohms, this correlates with a -3dB rolloff of
around 800 Hz - and this doesn't take into account the
capacitance of the attached amplifier or stray circuit
capacitance! It is, at least in part, this capacitance
that further attenuates the weak, higher-frequency AC currents
emerging from the photodiode.
Using an APD in a low-bandwidth optical receiver
For our application, we don't need the high speed for which an
APD may be capable, but rather its innate amplification
capability. In the "
Highly-Sensitive
speech bandwidth receiver" noted in the link above,
the ultimate limit of the sensitivity at mid speech frequencies
(200-2500 Hz) was not as much an attribute of the photodiode
itself, but due to the "noise floor" of the following amplifier
stages and because of practical considerations and the laws of
physics, the sensitivity could not be significantly improved
without drastic measures such as cooling - something that would
greatly complicate the design and implementation.
With the intrinsic amplification of an APD we can boost the
photon-induced signal to a level that may be sufficiently far
above the noise of the amplifying circuit to allow even weaker
signals to be detected and to do this, we need a source of bias
voltage to drive the multiplication effect within the APD
itself. At very low voltages the APD acts just as a
"normal" PIN photodiode would, but with increasing voltage an
impinging photon will be likely to loose an ever greater number
of electronics, effectively providing gain. At
still-higher bias voltage the gain continues to go up, but the
noise of the device increases
faster
than the gain and there is a point at which one starts to lose
actual sensitivity - even though the gain is still increasing!
Figure 1:
Top:
Schematic of the APD high voltage bias supply.
Bottom: The
as-built power supply in the enclosure.
Click on an image for a larger version
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This receiver uses a AD1100-8-TO52-S1 by First Sensor
(formerly
Pacific Silicon Sensor) an APD with a 1 mm
2 active
area
(circular, 1.128mm diameter) - a size that is a reasonable
match for the "blur circle" of good quality, molded Fresnel
lenses. This device is, unfortunately, rather expensive at
approximately $150 US each
(in 2009, listing closer to $210 each in 2015!) but when it is considered
that we spent
at least this much on food and fuel whenever we
mounted a major expedition to do some optical testing, the
device's cost can be put into perspective!
This device has a typical maximum bias voltage rating in the
130-150 volt region
(for maximum gain) so a regulated, variable,
low-current, high-voltage power supply was required. In
addition to the bias supply, an amplifier was needed to take the
output of the APD and amplify it to a level that was useful to
us. As it turns out, these devices were from the same
manufacturing lot and included individual data sheets indicating
that the maximum gain occurred at 134 volts at 25C.
The bias supply
Fortunately, only a few 10's of
microamps are required to adequately bias the APD so a very
simple boost-type switching supply was devised, seen in
Figure 1.
Op amp section U101a forms a simple 6.5 kHz oscillator which
drives the high voltage transistor Q101 which, in turn,
generates a high voltage every time it turns off due to the
magnetic field collapse of L101. The output from the
collector of Q101 is then rectified by D101 and filtered by C102
- and then filtered some more by C103 in conjunction with R106 -
to produce a fairly clean source of high voltage. The 6.5
kHz switching frequency was chosen by experiment: The
frequency of the oscillator was varied by substituting different
capacitance values at C101 and the output voltage and conversion
efficiency was noted and for the particular choke
(L101) used,
the best results happened to be obtained at 6.5 kHz.
Practically speaking, a choke in the range of 4.7 to 33 mH could
be used, but the individual characteristics
(DC resistance,
self-capacitance, etc.) will affect its efficacy in an
application such as this.
A sample of the high voltage is taken from R108 and R109 and
sent to the other section of the op amp where a comparison with
an on-board 5 volt supply
(from U102) is made. If the
voltage is too high, transistor Q102 is turned on, pinching off
the drive to the boost transistor, Q102, thereby regulating the
output. Without the regulator circuit it was observed
that the high voltage could soar to well over 275 volts - a
potential above the ratings of C103 which was probably starting
to exhibit leakage currents that ultimately limited the
voltage! R109 is used to adjust the "maximum" voltage when
R111's wiper is set at +5V and this should be adjusted to be
about that of the maximum ratings of the APD.
Comment: I
grounded both ends of R109 only because it was convenient to
do so during construction. If I wanted to have a lower
"maximum" voltage I would have only grounded one end of the
500k pot.
Because a "standard" dual op amp was used, LED101, a standard
"non-high brightness" red LED was placed in series with the base
drive of Q101 as without it, the op amp's output voltage may not
go close enough to ground to fully turn Q101 off. Rather
than use a more expensive rail-to-rail op amp or just a couple
of ordinary diodes to drop the voltage, the LED was used to
insert a 1.5-1.8 volt drop and provide a "free" power-on
indicator and, since it is modulated with the 6.5 kHz tone, it
can also be used to quickly test the receiver for proper
operation simply by exposing the photodiode to its glow and
listening for the tone.
One of the properties of APDs is that as the bias voltage
approaches the maximum ratings of the diode, the gain will start
to increase exponentially and in the case of this circuit, that
would mean that over the majority of the rotation of the
voltage-adjust potentiometer, R111, there would be very little
effect. Near the top, however, most of the increase in
gain would occur over a vary small portion of the
potentiometer's rotation, "crowding" in at the high voltage end
and make fine adjustments very touchy. In lieu of a
logarithmic taper potentiometer - which can be difficult to find
- resistor R112 was placed across the potentiometer to help
"stretch" out the adjustments at the top end. Another
method of providing more fine adjustment capability would be to
add just another potentiometer that, when the "main" pot was
turned all of the way up, could provide reasonable "tweaking"
room.
P101 and P102 are pin-type test jacks that allow the connection
of the probes of a high-impedance voltmeter to measure the
precise bias voltage and because the contacts are recessed,
there is no significant shock hazard when nothing is plugged
into them. TH101, a self-resetting 300 mA thermal fuse and
D103 are used to protect the device from accidental polarity
reversal that might occur when connecting the battery -
something that it is easy to do when fumbling around in the
dark! R118 and C107 are included to provide a somewhat
filtered source of battery voltage to the remotely-mounted
optical front end.
It should be noted that in many applications an APD is run very
close to its maximum gain - which is also very close to the
maximum rated voltage. This bias supply was set, using
R109, to be capable of producing a few volts higher than the
maximum voltage rating of the APD, but because it is
current-limited by R202
(see
below) no harm is going to come to the diode:
Exceeding the maximum voltage causes a tremendous increase in
gain and, especially, noise, so this mode of operation isn't
very useful. "Fancy" APD bias circuits have provisions to
track the temperature of the APD and automatically adjust the
applied voltage to compensate for the APD's gain-voltage
characteristics to thermally drift but because we really don't
need to operate the APD at maximum gain, that was not done here
and I could have just as easily set R109 for a much lower
"maximum" voltage.
The optical front end
Figure 2 shows the optical front end. If you really
did
read the articles linked at the top of the page, you'll
recognize this circuit as an adaptation of the Version 3 (a.k.a.
"V3") receiver described on web page,
A Highly-Sensitive receiver
optimized for speech bandwidth noted above.
The obvious difference is the use of an Avalanche Photo Diode
instead of the standard PIN photodiode which, as in the case of
the V3 receiver circuit, has it's "hot" end
(cathode) connected
directly to the gate of Q201, a BF862 JFET. As with the V3
design, this looks - at first glance - to be a floating FET gate
(a "no no" in circuit design) but it is worth remembering that
as is the case with bipolar transistors, there is an intrinsic
diode across the gate-source junction, therefore that voltage
never exceeds approximately 0.6 volts, and since the "cold"
(anode) side of the photodiode is at a higher positive
potential, the FET's gate bias is always present -
see
the inset, below, for further discussion of this mode of
JFET operation.
Figure 2:
Top:
Schematic of the APD optical front end
Bottom: The
as-built optical detector electronics. In the
center, you will notice a smaller board that contains the
JFET, APD and a few other components.
Click on an image for a larger version
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In the case of the D201, the APD, it so-happens that instead of
just 7-10 volts of reverse bias as would typically be the case
for the PIN photodiode version of the V3 receiver, there could
be well over 100 volts - the maximum depending on the
characteristics of the APD itself. Under "dark" conditions
the leakage currents of the photodiode will be on the order of
nanoamps or 10's of nanoamps, increasing in proportion to the
luminous flux impinging its surface. If exposed to a lot
of light the photodiode will conduct more heavily, but the
current-limiting afforded by R202 will protect both the APD and
the FET and the bias voltage will necessarily decrease.
Just to be sure, I did some testing of the APD receiver in total
darkness with a very dim, modulated LED as a weak signal source
where I set off a xenon photoflash unit only inches away from
the APD: No damage or change in sensitivity was noted
indicating that both the APD and JFET were more than capable of
handling the worst-case scenario - a brief impulse that would
dump the charges of C202/C203 through the APD and into the FET!
On the drain of Q201 is bipolar transistor Q203, the two forming
a
cascode
circuit. Because of the very nature of the cascode
circuit, the voltage change at the junction of Q201's drain and
Q203's emitter is very small with the detected modulations of
light being largely that of current, instead. It is with
these variations in current - rather than voltage - that the
cascode circuit shines as this greatly reduces the
Miller
Effect - that is, the tendency of the gate-drain
capacitance to be multiplied by the voltage gain of the circuit,
something that would adversely impact frequency response.
This particular cascode has been modified in that it's
self-biasing at DC, but it response to AC signals by virtue of
C205 and R207/R208 allowing it to auto-adapt to voltage changes.
Another departure from the standard cascode circuit is the use
of a bipolar current source in the form of Q202, diodes D202 and
D203 and resistors R204 and R205. This circuit establishes
the majority of Q201's drain current thus allowing a larger
proportion of the remainder to be "seen" by the cascode
transistor Q203. Doing this has the advantage of
increasing the FET's drain current - which will reduce its noise
contribution - but unlike a low ohmic-value resistor that might
be used to establish the same amount of current, the impedance
of the Q202 current source is quite high, typically in the 10's
of k-ohms for this type of circuit.
It is worth noting that both Q202 and Q203 should be
"high-Beta", low noise transistors for optimal
performance. Q202, the PNP current source uses a 2N5087 or
BC560C transistor but this circuit could be re-worked to use an
NPN as depicted in the "Version 3" receiver described
here.
Q203 is nominally an MPSA18 transistor, but a 2N5089 or BC550C
could be used instead.
On the collector of Q203 is attached an op amp wired as a
unity-gain follower. One section of an LM833 dual
low-noise op amp
(U201b) is used and it provides a low-impedance
buffer for the comparatively high impedance signal found at
Q203s collector, effectively isolating it from the load.
Following the unity-gain buffer is a simple differentiator stage
consisting of U201a, C207 and R209 that counteracts the
frequency rolloff caused by the photodiode's capacitance,
flattening the frequency response in a manner appropriate for
baseband operation with a gradual rolloff above about 6 kHz to
minimize "ear fatigue" to the lightener. On this circuit
there is also a high/low gain switch
(SW201) that, when opened,
increases the differentiator's gain by approximately 20dB.
The feedback resistors and capacitors
(C208/R210 and C209/R211)
form additional low-pass elements and set the gain that, in
conjunction with the values of C207 and R209, cause the high
frequency to be rolled off above 6 kHz: Clearly, the U201a
circuit is intended for baseband operation!
It should be noted that a sample of the output of U201b, the
unity gain buffer, is made available as a "flat" frequency
response output. While not truly "flat" owing to the
capacitance of the photodiode, this output is intended to be
used for those applications where frequency response outside the
normal speech band is required, such as very low
(<200 Hz)
narrowband signalling or high frequency (>6 kHz) systems
involving subcarriers. In the case of the latter, the
-6dB/octave slope of the photodiode's output at higher
frequencies is not particularly problematic when fairly narrow
band
(<10%) signals - such as an SSB signal at 25 kHz - are
involved as the amplitude slope across such a narrow frequency
range is not likely to cause any problems. In testing, it
was found that at relatively high APD gain and voltage settings
that there was usable output in the 1-2 MHz area from the "flat"
output: Because of the circuit configuration and the high
APD gain involved the sensitivity at these frequencies was not
particularly good, but the observation was interesting
nonetheless!
Because of the
extreme
sensitivity of the receiver it is necessary to make sure that as
much of the 6.5 kHz energy from the bias voltage generator is
removed from both power supply leads as possible. On the
high voltage lead, R201 and C201 are placed right at the point
where the bias voltage enters the enclosure and final filtering
and current limiting
(to protect the APD) is provided by R202
and C202. Because a slight amount of 6.5 kHz energy also
appears on the battery line, L201, a 100 millihenry choke, and
C211 provide additional filtering along with R215 and C212.
As can be seen from the diagram and picture in Figure 2, the
APD, C203, the input JFET Q201, R203 and C204 are mounted on a
small board by themselves. This board is connected to the
main enclosure - which is constructed from glass-epoxy circuit
board material - using short, bent pieces of #18 AWG wire.
Not only does this provide a ground return, but it allows the
precise adjustment of both focus and paraxial alignment of the
APD when mounted in the optical assembly: The wire allows
movement along all three axes, but it also holds it in place
once adjustments are complete. A short length of small
gauge wire gently twisted with another wire connected to the
ground make the connection to the emitters of Q202 and Q203 on
the main amplifier board. Because of the cascode nature of
the circuit, the small amount of capacitance added by this wire
does not appreciably affect performance in the frequency range
of interest.
Finding the optimal drain
current for Q201:
"No, the gate isn't
floating!"
At first glance of
Figure 2 it may appear that the JFET's gate is
floating: IT IS NOT!
Note that the "cold end" (non-gate side) of the
photodiode may be biased to a rather high voltage and
were the FET of an insulated gate
type the potential would try to rise to roughly match it
- at least until it broke down! Since it is
a junction FET, the "gate-drain diode" junction will
conduct and keep the "hot end" of the photodiode to
within about a "diode's drop" of the drain voltage which
- for most practical purposes - is at drain (ground)
potential.
This does several things:
- This allows a bias to be established across the
APD, both reducing its capacitance and allowing its
internal amplification properties to be realized.
- The FET is turned "on." As expected, the
channel resistance of the FET drops with increasing
gate-drain voltage but what is not commonly realized
is that with most JFETs, the channel resistance will
continue to decrease even as the gate-drain voltage
goes positive. Once the gate-drain junction
"diode" begins to conduct, the device's resistance
will continue to decrease as the voltage will still increase
although you now have a diode there with its
expected curve! If you are skeptical of this
observation, the construction of a simple test jig
using almost any common JFET will bear this out as
demonstrated in the graph below:
Figure 3:
Gate current versus drain current for a
typical JFET using measured values in a test
fixture.
Click on image for a larger version.
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As can be seen in figure 3 the current increases
exponentially with gate-source voltage in a "diode-like"
manner. Like bipolar transistor, the drain current
(akin to collector current) increases with gate
current (akin to base current), but it's in
linear proportion to the gate voltage rather
than the gate current! This feature is due
to the fact that the "gate-source" junction is
conducting and is doing so in a classic "diode-like"
manner.
For our purposes the JFET operates in this mode in a
manner much more "quietly" than a bipolar transistor
would if we were to simply drop one in its place in this
circuit, mainly due to the fact that noise currents are
a small portion of the FET's overall drain current
whereas they would be comparatively large in the case of
a bipolar tranasistor.
Although this graph doesn't extend far enough, this
"semi bipolar-like" property of JFETs is exhibited only
for very low gate currents as the FET itself is "mostly"
saturated at the point that a significant amount of gate
current (e.g. gate current >> gate-source
leakage current) begins to flow and there is
a limit as to how much drain current will flow and still
exhibit any resemblance to the curve above!
Under low-light conditions, the operational and leakage
currents of the APD aren't enough to "saturate" the JFET
and it continues to operate "normally" - even with a
high (>100 volt) bias on the APD.
If operated under conditions with higher ambient (or
incident) light, the bias voltage should be reduced as
much as necessary and R202 will provide ample protection
to the APD and FET to prevent either from being
damaged. It should be remembered that if there is
plenty of "extra" light, the extremely high sensitivity
of an APD-based isn't going to be required, anyway and
one might be better off using a different (and
less-sensitive) detector! |
One of the trickier aspects of this circuit is finding the best
operating current at which Q201 should be biased. Because
there is actually gate-source current flowing in Q201, it is
"turned on" very heavily and it will sustain quite a bit of
current - typically 20-30 milliamps for the rather "substantial"
BF862 JFET specified here. In the source lead of Q201 is
R203, a 10 ohm resistor that may be used to monitor the drain
current at a ratio of 10 millivolts of drop per milliamp.
Once the proper device current has been found it is suggested
that this resistor be jumpered
(JP201) to take it and its
voltage drop and possible noise contributions completely out of
the circuit.
For truly optimal performance one would experiment with the
value of the current-setting resistor R205 and find the value
that increased the current just to the point where the
sensitivity of the receiver - as tested in a dark room "photon
range" using very weak test signals - started to decrease, and
then back off a bit
(say 10-20%) from there to assure that the
current would stay within the proper range over a wide variety
of operating temperature and supply voltages. However, it
is rather safe to run Q201 with a "lower than optimal" current
and have only a slight decrease in ultimate sensitivity - and it
is much easier to do this than running a lot of dark-room tests!
The 120 ohm value shown here for R205 should be quite safe when
using a BF862 FET which is both fairly well characterized
compared to many FETs and is generally capable of far more
current than Q202 and Q203 will source. If one substitutes
a different transistor for the JFET, Q201, adjustment of R205
will likely be necessary. The 2N5457 is a good substitute
and it may work well with the 120 ohm value specified for R205 -
depending on the individual device, but keep in mind that
individual devices can vary. If using a
poorly-characterized device like the MPF102 - which can have
drain currents all over the map - it will likely be necessary to
try different values of R205 and/or a number of different
devices to find the one that works best.
When the BF862 is used with the values noted in the diagram, it
should be noted that the drain voltage will be very low -
possibly well under a volt. In the two prototype
receivers, the drain current worked out to be around 7-8
milliamps
(to moderate current consumption and improve
battery life) and the drain voltage was on the order of
0.21 volts with an APD bias of 12 volts and about 0.155 volts
with an APD bias of 135 volts - the point at which the APD was
beginning to break down and its gain
(and noise floor!) was the
highest - both of these readings taken with the APD in complete
darkness. Compared to a garden-veriety JFET such as the
MPF102, the BF862 has quite low "on" resistance so it will
exhibit a significantly lower drain-source voltage drop than
many other JFETs when turned fully "on" as would be
approximately the case with this circuit during normal
operation.
It is important to remember that it is the
variations
in current that the cascode circuit sees rather than voltage so
a fairly low voltage can be expected
and still
have the circuit work properly. It is also important to be
reminded that because it's the drain
current that is
changing with applied light, you will not easily see signals if
you were to put an oscilloscope probe on the drain of Q203!
Finally, it should be noted that because the APD itself
does have gain, eking every
last dB of performance out of FET Q201 is arguably less important
here than it is on the "Version 3" PIN-diode based optical
receiver as it is more the noise contribution of the APD than
the FET that is likely to determine the receiver's ultimate
weak-signal performance.
Additional
comments:
As can be seen from the pictures, the high voltage bias supply
and optical front end are built into separate enclosures - and
for a number of good reasons:
- Isolation of the high voltage generator. Because it
uses a high voltage boost circuit, it is important that both
the electric and magnetic fields produced by this circuit be
kept away from the receiver. Were they co-located
within the same enclosure, it would likely be very difficult
to keep it out of the extremely sensitive optical front end.
- Mechanical and practical reasons. By having the
optical detector separated, connected only by a single
cable, the size and weight of what is to be
mounted on the back end of the optical enclosure is reduced. This
also means that if adjustments are to be made to the bias
voltage - or if the battery is to be replaced - this may be
done on using the box containing the high voltage supply
rather than having to touch the optical front end which may
be very mounted to a lens assembly that is delicately
pointed and easily disturbed if touched!
- As shown in the pictures, the optical front end is
constructed inside a box made from double-sided glass-epoxy
circuit board material. In all cases, both surfaces of
this material are connected together and to the receiver's
ground to provide good shielding and EMI/RFI
protection. Because I had them available, I used
solder-in feedthrough capacitors for both the low and high
voltage inputs as well as the baseband audio output and a
BNC connector was used for the "Flat" audio output. If
feedthrough capacitors are not available, put a 0.001-0.01
uF capacitor at the point where each wire enters the
enclosure.
- This circuit was tested in broad daylight over an optical
path of about 21km (13 miles) and it was found to work
reasonable well. Because it was daylight the diode
was saturated, requiring that the bias voltage set to
minimum. Even at that, the recovered audio was
somewhat distorted due to the diode and/or JFET operating in
a nonlinear range, but this could have been mitigated had
optical attenuation been applied to reduce the overall light
levels reaching the APD. See the page Daylight Optical
Testing for a few more details about this
experiment.
- As noted above, if either low (<200 Hz) or high (>6
kHz such as for subcarriers) operation is intended, use the
"Flat" output for best results.
- When an APD is operated near its maximum gain, care must
be taken to assure that the bias voltage is compensated with
device temperature to assure that the device gain is
consistent over the expected range. Since this
receiver would normally be operated so that the diode at far
less than maximum gain, and since we are also not really
concerned with absolute gain levels (we are just
detecting audio!) this aspect is of no real
importance. The bias supply is adjusted (using R109)
so that the maximum voltage that may be applied to the APD
is on the order of 140 volts, but because if the
limited-current nature of the supply and the presence of
R202, a series current-limiting resistor, the APD cannot be
damaged by the application of excess voltage.
- As noted above, in testing the APD receiver was operated
at maximum bias voltage (about 140 volts) and a photoflash
was repeatedly set off inches away from the APD in otherwise total darkness. No damage to either the
APD or the input JFET (Q201) was observed, this being a
worst-case scenario for abuse!
The BF862 is available only in surface-mount format, a fact that
slightly complicates the mounting of the device. No matter
which JFET is used it is
important that one
NOT
mount the anode lead of the APD
or the gate
terminal of the JFET on circuit board material as this will
cause a degree of leakage - particularly important with the high
voltages involved - plus the fact that any circuit board trace
will increase both the capacitance and the surface area of a
conductor that might pick up noise. The circuit shown in
Figure 2 depicts the APD to
gate connection being made
in the air with the
body of the APD being grounded and some of the other components
being mechanically stabilized using clear epoxy with care taken
to avoid getting that epoxy near the gate/APD lead. After
it was constructed and the epoxy cured, any stray bits were
scraped away and then the APD and JFET carefully cleaned of
solder flux, oils and other contaminants with denatured alcohol.
The operating current of the entire APD receiver and bias supply
is less than 35 milliamps which means that it will last for well
over 10 hours on a fresh 9 volt alkaline battery. but it is
strongly recommended that one always brings at least
two
fresh batteries for spares!
Operation
Figure 4:
Top: The
APD bias supply - which also contains the battery that
powers the front end.
Center: The
rear side of the APD optical front end showing the cover
shield. On the right edge one can see the white
audio cable carrying baseband, the gray cable that carries
both the low and high voltage and a glimpse of the BNC
connector for the "Flat" output. The wires visible
above and below the box are used to hold it in rough
paraxial alignment and focus during initial setup with the
"fine" adjustments being made to the small board that
holds the APD and JFET.
Bottom: The
"photodiode end" of the APD optical receiver. The
black ring is used to provide optical alignment when
installed in the lens box.
Click on an image for a larger version
|
|
|
Before turning on the APD receiver it is always a good idea to
first turn the voltage down to minimum, mostly to avoid being
blasted by noise. As noted above, the LED is not only
useful as a power indicator, but because it is also modulated
with the 6.5 kHz switching tone from the voltage converter it
may be used as a convenient optical test signal for this and
other optical receivers. As noted above, with little or no
bias voltage, the performance of the APD - operating as just an
ordinary photodiode - is comparable to any other photodiode.
For narrowband speech operation, it has been determined that
fairly low bias
(around 45 volts) is appropriate for the best,
low-signal performance under dark conditions and in this voltage
range, the amplification factor, "M", of the APD is quite
modest (a signal voltage in the range of 10-30). In this
range there is enough amplification provided by the APD itself
to overcome the noise floor of Q201, the JFET, and an
improvement of 6-10dB is observed when compared with a standard
photodiode receiver.
At higher voltages
(>45 volts) the gain continues to
increase, but the noise generated by the APD itself increases
even faster! As the voltage continues to go up the
detected signal continues to increase in amplitude, but a weak
one may soon overtaken by APD's own noise and may be lost altogether!
Note:
Different APDs have different voltage/gain
curves, but in general, the best S/N for low-frequency
(audio range) operation occurs in the range where "M", the
amplification factor, is in the range of 10 to 30,
regardless of the actual bias voltage.
As the bias voltage is increased, the usable bandwidth of the
receiver also increases, both owing to continued reduction in
the APD's device capacitance, but also due to the APD's
intrinsic gain boosting the higher-frequency components
(and
noise!) above the noise floor of the amplifier circuitry as well
as increasing "gate-source diode" currents of the JFET causing
its apparent impedance to drop and increasingly quash the
effects of JFET gate and APD capacitance.. As you have
probably already guessed, there is a tradeoff between bandwidth
and sensitivity here! As with the V3 receiver, very high
levels of light (ambient or from the distant optical
transmitter) also affect the frequency response owing to the
fact that the APD's photoconduction and the JFET gate current
will shunt the capacitance and raise the "knee" frequency of the
R/C-caused lowpass filtering.
Continued increases in the bias voltage turn what was a weak
background hiss into a loud roar as the APD's internal noise
generation becomes dominant. For the APDs that I used,
their maximum gain occurs just below their maximum voltage
rating which, for these devices, is around 135 volts. As
noted previously, the last several volts yield very rapid and
dramatic increases in both gain and device noise, making
adjustments of the bias voltage control rather tricky!
When decreasing the voltage rapidly the receiver may momentarily
go dead - particularly under very low light conditions.
What is happening is that with a decreasing bias voltage, the
charge on the Q201's gate goes negative, turning it off.
Once the bias voltage stops changing, the minute leakage
currents across the APD gradually bleed off the capacitance of
the APD and FET, turning the JFET back on and restoring
operation - a process that can take several seconds. This
particular phenomenon is normal, expected, and there is really
nothing that can be done about it that wouldn't degrade the
receiver's operation!
Final comments and in-the-field observations:
It should be stated once again that the goal was to produce
a highly-sensitive radiometric optical detector that was
optimized for speech range
(up to about 3 kHz) frequency
response. Additionally, being self-funded hobbyists, there
was the additional goal that such a detector be built - as much
as possible - using inexpensive, readily available,
off-the-shelf components and construction techniques that were
well with the capabilities of the advanced electronic
hobbyist: With the designs outlined above, we believe that
we have largely achieved that goal.
If, on the other hand, the goal is to achieve optimal
weak-signal detection capabilities at very low (sub-speech)
frequencies or higher bandwidths
(above speech, including the
use of high-speed data, video, or multiple carriers) then
careful consideration is warranted when deciding whether or not
the methods outlined elsewhere on this page are entirely
appropriate.
At the time of original design the use of an APD offered an
apparent 6-10dB improvement in sensitivity at baseband
(audio)
frequencies was noted on the "photon range"
(an indoor, darkened
room where signal levels from an LED driven at low current can
be used to analyze receiver performance) when compared with a
typical, similar-sized PIN photodiode. When an A/B
comparison was done in the field on the "dark" end of a 150+ km
optical path, this same degree of improvement was also noted -
although variations in signal levels owing to atmospheric
conditions, etc. tend to make precise numbers more difficult to
nail down. During these same field tests it was observed
that, in fact, best overall sensitivity was noted with the APD
bias set in the 45 volt range as had been noted on the photon
range.
One question that is yet to be answered about the performance of
the APD optical receiver is that of usable bandwidth. With
the PIN Photodiode receiver, the primary factory limiting
sensitivity at higher frequencies
(e.g. >2 kHz and up) was
that of the capacitance of the photodiode itself. To a
degree, a smaller-area photodiode could be used but much smaller
than 1 mm
2 makes both mechanical alignment (e.g.
focus and paraxial) more critical and too much smaller than this
one starts to approach the limit imposed by the size of the
"blur circle" of the Fresnel lens.
(For a discussion
on "blur circle" size for typical Fresnel lenses, see the page
Fresenel
Lens Comparison
at this web site.
Because of its intrinsic self-amplification, an APD should be
able to provide even better higher-frequency performance than a
similarly-sized PIN photodiode as the self-amplification will
overcome the noise floor of the amplifier itself to a degree,
plus the higher reverse voltage will also do more to
further-reduce the device's capacitance. As noted,
however, an "A/B" comparison of similar-sized APD and PIN
devices has not been carried out at ultrasonic and higher
frequencies.
Related pages:
- Fresnel Lens
Comparison - On this page one may find a discussion
and comparison of the optical qualities of various molded,
plastic Fresnel lenses. In particular, note the size
of the "blur circle" of such a lens when it is used for
receiving a distant signal with respect to the size of the
detector's active area.
- The K3PGP
Pages:
- A Low Noise PIN Diode Laser Receiver - Part 1
and Part 2
Note that some of these pages may not render
properly on some browsers.
- F1AVY's
pages - Yves describes many aspects of
detection (and methods of using lasers to generate
signals.) Please note: The
hosting web site of Yves' pages has changed and most
of the pages at the new site are in French - click
here for a Google translation of the main
page into English.
- Photodiode
Amplifiers - Changing Light to Electricity
(archived in 2006) - From National
Semiconductor, an online seminar about various aspects
of using photodiodes and how to amplify their
output. This page links not only to some .PDFs of
slides and transcripts of the seminar, but it also has
an online video of the original presentation.
Related to this topic is National Semiconductor's
application note AN-1244
(from the Texas Instruments site) which also
contains information about this same topic.
- Hamamatsu
Photonics has, on its website, a number of
papers about the "what and how" of many types of optical
components. For more info, look at:
- The Technical
Notes page. This page describes the
general theory behind the operation of many types of
optical devices, such as photodiodes, photomultiplier
tubes, and many more devices.
- The Application
Notes page. This page has a number
of articles describing how optical devices are used in
the real world.
- The Hamamatsu Document Libraray
- This links to a search for documents on various topics, but it does
not seem to have a "browseable" list of titles with descriptions that I could
find...
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Keywords:
Lightbeam
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