OPTICAL RECEIVER CIRCUITS:Current to Voltage Converter Circuits, Post Signal Amplifiers

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Chapter Six

OPTICAL RECEIVER CIRCUITS

The overall task of the optical receiver is to extract the information that has been placed on the

modulated light carrier by the distant transmitter and restores the information to its original form.

The typical through-the-air communications receiver can be broken down into five separate

sections. These are: light collector (lens), light detector (PIN), current to voltage converter, signal

amplifier and pulse discriminator. There may also be additional circuits depending on the kind of

the signal being received. As an example, a receiver that is extracting voice information will need a

frequency to voltage converter and an audio amplifier to reproduce the original voice signal.

Computer data receivers will also need some decoding circuits that would configure the transmitted

serial data bits into 8 bit words. However, this section will concentrate on the circuits needed for

processing voice information. Volume II of this book will contain additional circuits for digital data

receivers.

Light Collector

For long-range applications it is essential to collect the weak modulated light from the distant

transmitter with a glass or plastic lens and focus it onto a silicon PIN photodiode. Although mirrors

could also be used to collect the light, glass or plastic lenses are easier to use and cost less. Plastic

lenses measuring from a fraction of an inch to six inches are available. For a system that demands a

large lens, the flat "Fresnel" lens is much less expensive than a solid lens. Forming special

concentric bumps in a clear plastic sheet makes Fresnel lenses. The bumps bend the light just as a

conventional thick lens would. Fresnel lenses are available with diameters of several feet.

For certain short-range applications it may also be possible to use a naked light detector without any

lens. Distances up to several hundred feet are possible with systems that don't rely on lenses at

either the transmitter or the receiver. Lens-less systems are especially useful when very wide

acceptance angles are required. Many cordless IR stereo headsets use two or more naked detectors

to provide acceptance angles approaching 360 degrees.

The lens chosen should be as large as possible but not too large. A lens that is too large can produce

a half angle acceptance angle that is too small. Acceptance angles less than about 0.3 degrees will

result in alignment difficulties. Building sway and atmospheric disturbances can cause signal

disruption with narrow acceptance angles. A rough rule-of-thumb might be that the lens diameter

should not be more than 100 times larger than diameter of the active area of the PIN detector. Also,

the receiver should never be positioned so sunlight could be focused onto the light detector. Even a

brief instant of focused sunlight will destroy the sensor. A north/south alignment for the transmitter

and the receiver will usually prevent an optical system from going blind from focused sunlight.

Light Detector

As discussed in the section on light detectors, the silicon PIN photodiode is the recommended

detector for most all through-the-air communications. Such a detector works best when reversed

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biased. In the reversed biased mode it becomes a diode that leaks current in response to the light

striking it. The current is directly proportional to the incident light power level (light intensity).

When detecting light at its peak spectrum response wavelength of 900 nanometers, the silicon PIN

photodiode will leak about 0.5 micro amps of current for each microwatt of light striking it. This

relationship is independent to the size of the detector. The PIN photodiode size should be chosen

based on the required frequency response and the desired acceptance angle with the lens being used.

Large PIN photodiodes will have slower response times than smaller devices. For example, 1 cm X

1 cm diodes should not be used for modulation frequencies beyond 200KHz, while 2.5 mm X 2.5

mm diodes will work beyond 50MHz. If a long range is desired, the largest photodiode possible that

will handle the modulation frequency should be used.

Stray Light Filters

Some systems can benefit from the placement of an optical filter between the lens and the

photodiode. The filter can reduce the effects of sunlight and some stray light from distant street

lamps. Filters can be especially effective if the light detector is going to be processing light from a

diode laser. Since laser light has a very narrow bandwidth, an optical band pass filter that perfectly

matches the laser light can make a light receiver nearly blind to stray sunlight.

If light emitting diode light sources are used, optical filters with a much broader bandwidth are

needed. Such a filter may be needed for some situations where man-made light is severe. Many

electronically controlled fluorescent and metal vapor lamps can produce unwanted modulated light

that could interfere with the light from the distant transmitter.

But, in all but a few rare exceptions, band pass filters produce few overall improvements if the

correct detector circuit is used. Since no optical filter is perfectly transparent, the noise reduction

benefits of the filter usually do not out weigh the loss of light through the filter. Also, if the detector

is going to process mostly visible light, no optical filter should be used.

Current to Voltage Converter Circuits

The current from the PIN detector is usually converted to a voltage before the signal is amplified.

The current to voltage converter is perhaps the most important section of any optical receiver

circuit. An improperly designed circuit will often suffer from excessive noise associated with

ambient light focused onto the detector. Many published magazine circuits and even many

commercially made optical communications systems fall short of achievable goals from poorly

designed front-end circuits. Many of these circuits are greatly influenced by ambient light and

therefore suffer from poor sensitivity and shorter operating ranges when used in bright light

conditions. To get the most from your optical through-the-air system you need to use the right front-

end circuit.

High Impedance Detector Circuit

One method that is often shown in many published circuits, to convert the leakage current into a

voltage, is illustrated in figure 6a. This simple "high impedance" technique uses a resistor to

develop a voltage proportional to the light detector current. However, the circuit suffers from

several weaknesses. If the resistance of the high impedance circuit is too high, the leakage current,

caused by ambient light, could saturate the PIN diode, preventing the modulated signal from ever

being detected. Saturation occurs when the voltage drop across the resistor, from the photodiode

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leakage current, approaches the voltage used to

bias the PIN device. To prevent saturation, the PIN

must maintain a bias voltage of at least a few volts.

Consider the following example. Under certain

bright background conditions a PIN photodiode

leakage current of a few milliamps may be

possible. If a 12v bias voltage were used, the

detector resistance would have to be less than

10,000 ohms to avoid saturation. With a 10K

resistor, the conversion would then be about 10

millivolts for each microamp of PIN leakage

current. But, to extract the weak signal of interest

that may be a million times weaker than the

ambient light level, the resistance should to be as

Figure 6a

high as possible to get the best current to voltage

conversion. These two needs conflict with each other in the high impedance technique and will

always yield a less than desirable compromise.

In addition to a low current to voltage conversion, there is also a frequency response penalty paid

when using a simple high impedance detector circuit. The capacitance of the PIN diode and the

circuit wiring capacitance all tend to act as frequency filters and will cause the circuit to have a

lower impedance when used with the high frequencies associated with light pulses. Furthermore, the

high impedance technique also does not discriminate between low or high frequency light signals.

Flickering streetlights, lightning flashes or even reflections off distant car windshields could be

picked up along with the weak signal of interest. The high impedance circuit is therefore not

recommended for long-range optical communications.

Transimpedance Amplifier Detector Circuit With Resistor Feedback

An improvement over the high impedance method

is the "transimpedance amplifier" as shown in

figure 6b. The resistor that converts the current to a

voltage is connected from the output to the input of

an inverting amplifier. The amplifier acts as a

buffer and produces an output voltage proportional

to the photodiode current. The most important

improvement the transimpedance amplifier has

over the simple high impedance circuit is its

canceling effect of the circuit wiring and diode

capacitance. The effective lower capacitance allows

the circuit to work at much higher frequencies.

However, as in the high impedance method, the

circuit still uses a fixed resistor to convert the

current to a voltage and is thus prone to saturation

Figure 6b

and interference from ambient light.

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Transimpedance Amplifier Detector Circuit With Inductor Feedback

A dramatic improvement of the transimpedance

amplifier with a resistor feedback load is shown

in figure 6c. This technique is borrowed from

similar circuits used in radio receivers. The

circuit replaces the resistor with an inductor. A

student in electronics may remember that an

inductor will pass DC unaffected but will

exhibit a resistance effect or reactance to AC

signals. The higher the frequency of the AC

signals the higher the reactance. This reactance

circuit is exactly what is needed to help extract

the sometimes small modulated AC light signal

from the large DC component caused by

unmodulated ambient light. DC signals from

ambient light will yield a low current to voltage

conversion while high frequency AC signals

Figure 6c

will experience a high current to voltage

conversion. With the right circuit, an AC vs. DC conversion ratio of several million is possible.

Such techniques are used throughout radio receiver circuits to process weak signals.

In addition, as the Q increases so does the

impedance of the LC circuit. Such high Q

circuits can also be used in a

transimpedance amplifier designed for

optical communications. To obtain the

highest possible overall impedance, the

inductance value should be as large as

possible and the capacitance should be as

small as possible. Since every inductor

contains some finite parallel capacitance

within its assembly, the highest practical

impedance occurs when only the

capacitance associated with the inductor

assembly is used to form the LC network.

In radio, connecting a capacitor in parallel

Figure 6d

with the inductor often produces high impedances and allowing the LC tuned circuit to resonant at a

specific frequency. Such a circuit can be very frequency selective and can yield impedances of

several mega ohms. The degree of rejection to frequencies outside the center resonant frequency is

defined as the "Q" of the circuit. As figure 6d depicts, a high Q will produce a narrower acceptance

band of frequencies than lower Q circuits.

You can calculate the equivalent parallel capacitance of an inductor based on the published "self-

resonance" frequency or you can use a simple test circuit to actually measure the resonance

frequency (see figure 6e on page 54) of a coil. Figure 6f lists the characteristics of some typical

coils.

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Typical Inductor

Self Resonance Frequencies

Reactance at

Inductance

Frequency

Res. Frequency

200KHz

500K Ohms

100mH

200KHz

100K Ohms

47mH

250KHz

75K Ohms

27mH

300KHz

50K Ohms

15mH

500HKz

50K Ohms

10mH

700KHz

40K Ohms

4.7mH

800KHz

22K Ohms

2.2mH

1MHz

14K Ohms

1mH

2HMz

12K Ohms

470uH

3MHz

9K Ohms

100uH

7MHz

4.4K Ohms

Figure 6f

Transimpedance Amplifier Detector Circuit with Limited Q

The use of a LC tuned circuit in a transimpedance amplifier circuit does improve the current to

voltage conversion and does reject much of the

signals associated with ambient light. But, high Q

circuits are prone to unwanted oscillations. As

shown in figure 6g, to keep the circuit from

misbehaving, a resistor should be wired in parallel

with the inductor. The effect of the resistor is to

lower the circuit's Q. For pulse stream applications

with low duty cycles (short pulses with lots of time

between pulses), it is best to keep the Q near 1. A Q

of one exists when the reactance of the coil is equal

to the parallel resistance at the desired frequency. If

higher Qs were used, with low duty cycle pulse

streams, the transimpedance amplifier would

produce excessive ringing with each pulse and

would be prone to self-oscillation.

Figure 6g

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Figure 6h and 6i illustrate what happens

in a circuit with a low Q and high Q when

processing single pulses. If higher duty

cycle pulse trains are being transmitted,

higher Qs can be used. In near 50% duty

cycle transmission systems, Qs in excess

of 50 are possible with a careful design.

Table 6f lists the typical self-resonant

frequency of some inductors. If you don't

know the self-resonant frequency of a coil

you can use the schematic shown in figure

6e on page 52 to measure it.

In low duty cycle light pulse applications,

the inductor value should be chosen based

Figure 6h

on the width of the light pulse being sent

by the transmitter. The self-resonant period (1/frequency) of the coil should equal 2W, where W is

the width of the light pulse. Since the circuit layout, the amplifier circuit and the PIN diode will all

add to the overall circuit capacitance, some experimentation will be necessary to determine the best

inductor value for the particular application. The equation 2pFL should be used to calculate the

value of the resistor wired in parallel to the inductor to limit the Q to 1.

Figure 6j on page 53 is an example of a

complete transimpedance amplifier circuit

with inductive feedback. The amplifier

circuit shown in figure 6j on page 53 has a

light power to voltage conversion of about

23 millivolts per milliwatt (assuming 50%

PIN conversion) when used with 1

microsecond light pulses. Such an

amplifier should be able to detect light

pulses as weak as one nanowatt during

dark nighttime conditions.

Post Signal Amplifier

As discussed above, the transimpedance

amplifier converts the PIN current to a

Figure 6i

voltage. However, it may be too much to

expect one amplifier stage to boost the signal of interest to a useful level. Typically, one or more

voltage amplifier stages after the front end circuit are needed. Often the post amplifiers will include

some additional signal filters so only the desired signals are amplified, rejecting more of the

undesired noise. A general purpose post amplifier is shown in figure 6j on page 53.

The circuit uses a quality operational amplifier in conjunction with some filter circuits designed to

process light pulses lasting about 1 micro second. The circuit boosts the signal by a factor of X20.

Signal Pulse Discriminators

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Once the signal has been sufficiently amplified and filtered, it often needs to be separated

completely from any background noise. Since most systems use pulse frequency modulation

techniques to transmit the information, the most common method to separate the signal from noise

is with the use of a voltage comparator. The comparator can produce an output signal that is

thousands of times higher in amplitude than the input signal. As an example, a properly designed

comparator circuit can produce a 5 volt peak to peak TTL logic output signal from a input of only a

few millivolts.

But, to insure that the comparator can faithfully extract the signal of interest, the signal must be

greater in amplitude than any noise by a sizeable margin. For most applications, I recommend that

the signal to noise ratio exceed a factor of at least 10:1 (20db). Then, with a properly designed

comparator circuit, the comparator output would change state (toggle) only when a signal is present

and will not be effected by noise.

A complete signal discriminator circuit is shown in figure 6k on page 54. The circuit is designed so

a positive input pulse needs to exceed a threshold voltage before the comparator produces a

negative output pulse. A variable resistor network allows the threshold voltage to be adjustable. The

adjustment thereby provides a means to set the sensitivity of the circuit. The adjustment should be

made under the worst case bright background conditions so the noise produced by the bright

background light does not toggle the comparator.

Frequency to Voltage Converters

If the light pulses being transmitted are frequency modulated to carry the information, then the

reverse must be done to restore the original information. The pulse frequency must therefore be

converted back into the original amplitude changing signal. A simple but very effective frequency

to voltage converter circuit is shown in figure 6k on page 54. Each pulse from the pulse

discriminator circuit is converted into a well defined logic level pulse that lasts for a specific time.

As the frequency increases and decreases, the time between the pulses will change. The changing

frequency will therefore cause the average voltage level of the signal produced by the converter to

change by the same proportion. To remove the unwanted carrier frequency from the desired

modulation frequency, the output of the converter must be filtered.

Modulation Frequency Filters

A complete filter circuit is shown in figure 6l on page 55. The circuit uses a switched capacitor

filter (SCF) integrated circuit from National Semiconductor. With the values chosen, the circuit

removes the majority of a 10KHz carrier signal, leaving the wanted voice audio frequencies. The

filter's cutoff frequency is set at about 3KHz that is the minimum upper frequency needed for voice

audio.

Audio Power Amplifiers

The final circuit needed to complete a voice grade light pulse receiver is an audio power amplifier.

The circuit shown in figure 6l on page 55 uses a single inexpensive LM386 IC. The circuit is

designed to drive a pair of audio headphones. The variable resistor shown is used to adjust the audio

volume. Since the voice audio system described above does not transmit stereo audio, the left and

right headphones are wired in parallel so both ears receive the same audio signal.

Light Receiver Noise Considerations

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One of the most difficult problems to overcome in an optical through the air communications

system is ambient light. Any stray sunlight or bright background light that is collected by the

receiver optics and focused onto the light detector will produce a large steady state DC level

through the detector circuit. Although much of the DC is ignored with the use of an inductive

feedback amplifier method in the front-end circuit, the large DC component in the light detector

will produce some unwanted broadband noise. The noise is very much like the background static

you may hear on an AM radio when tuning the dial between stations. As discussed in the section on

light detectors, the amount of noise produced by the detector is predictable.

The equation shown in figure 6m

describes how the detector noise varies

with ambient light. The relationship

follows a square root function. That means

if the ambient light level increases by a

factor of four, the noise produced at the

detector only doubles. This characteristic

both helps and hurts a light receiver

circuit, depending on whether the system

is being used during the light of day or

during the dark of night. The equation

Figure 6m

predicts that for high ambient daytime

conditions, you will have to dramatically reduce the amount of ambient light striking the detector in

order to see an significant reduction in the amount of noise produced at the detector circuit. The

equation also describes that under dark nighttime conditions, the stray light has to dramatically

increase in order to produce a sizable elevation in noise. If the system must work during both day

and night, it will have to contend with the worst daytime noise conditions. Conversely, some light

receivers could take advantage of the low stray light conditions found at night and produce a

communications system with a much longer range than would be otherwise possible if it were used

during daylight.

As mentioned above, inserting an optical filter between the lens and the light detector can reduce

the effects of ambient light. But, as shown by the noise equation, the amount of light hitting the

detector needs to be dramatically reduced to produce a sizable reduction in the induced noise. Since

most sunlight contains a sizable amount of infrared light, such filters do not reduce the noise level

very much. However, very narrow band filters that can be selected to match the wavelength of a

laser diode light source, are effective in reducing ambient light and therefore noise.

Other Receiver Circuits

The circuits described above were designed for a voice audio communications system that received

narrow 1uS light pulses. An experimenter may wish to use other modulation frequencies. In

addition, untuned broad band receiver circuits are handy when monitoring modulated light signals

where the frequency is not known. I have included some additional circuits below that you may find

helpful.

A very simple and inexpensive broad band light receiver circuit is shown in figure 6n on page 56.

The circuit uses a CD4069UB C-MOS logic integrated circuit. Make sure to use the unbuffered UB

version of this popular device. The first section of the circuit performs the current to voltage

conversion. The other section provides voltage gain. The overall conversion is about 2 volts per

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

microwatt. With the values shown, the circuit will work with light modulation frequencies between

1KHz and 200KHz.

A similar circuit is shown in figure 6o on page 57. It uses a much faster 74HCU04 device instead

of the CD4069UB. The circuit should be operated from a 3v supply. For real flexibility, I have

shown how a Motorola MFOD-71 optical fiber photodiode module can be used. The circuit's 2MHz

bandwidth is great when monitoring light pulses with fast edges. A section of inexpensive plastic

optical fiber can be attached to the detector and used as a light probe to inspect the output from

various modulated light sources. Keep in mind, that since both broad band circuits do not use an

inductor in the feedback circuit, they should only be operated in low ambient light conditions.

A very sensitive light receiver circuit, designed for detecting the 40KHz signal used by many

optical remote control devices, is shown in figure 6p on page 58. The circuit shown uses a one inch

plastic lens in conjunction with a large 10mm X 10mm photodiode. With the values chosen, the

circuit will detect light from a typical optical remote from several hundred feet away. If the remote

control circuit also used a small lens the separation distance could extend to several miles.

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Figure 6e

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

Figure 6j

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Figure 6k

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

Figure 6l

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

Figure 6n

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

Figure 6o

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

Figure 6p

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Optical Through-the-Air Communications Handbook -David A. Johnson, PE

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