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CHAPTER IX

IR DETECTOR TEST SETUP

9.1. INTRODUCTION

The work reported in the earlier chapters of thethesis has been on the preparation and characterisationof PbS and SnTe thin films. These two, in the famikyof IV-VI compounds, have found applications as evapor­ated, polycrystalline film IR detectors and some pre­liminary work in IR detection such as the fabricationof a setup for the testing of IR detectors is reportedin this chapter. The characteristics of an IR detectorand how these quantities are measured arespelled outfirst and then the details of the fabrication aredescribed.

Using IR detectors, the quantities of energy tobe detected are usually very small either because thesource is at a considerable distance and subtends a

small solid angle, or because the band—width of spectrumbeing observed is very narrow, as in many spectroscopicapplications. A detector converts an incident radiationsignal into an electrical output. This output is made upof two parts, the response of the detector to the incident

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radiation (called the signal) and random fluctuationsknown as ‘noise’.

An IR detector is characterised mainly by itsblack body parameters, noise, and spectral response/1,2/. Blackbody parameters include blackbody responseand blackbody detectivity. Blackbody response indicatehow much voltage or current is obtained as output powerfor a given input blackbody spectral power in watts tothe detector. The noise equivalent power (NEP)of adetector is the incident power required to produce asignal output equal to the rms noise output. BlackbodyD* is a quantity which is the reciprocal of NEP normal­ized for detector area and noise bandwidth. This quantityallows comparison of different detectors having the samespectral response but different active areas and alsomeasured at different noise bandwidths.

Measurement of rms noise power as a function ofnoise frequency is important as noise determines howsmall an incident power can be detected. Spectralresponse measurements indicate the wavelength region inwhich the detector has measurable response. This meas­urement is necessary when the detector should have speci­fied response within a particular wavelength range.

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9.2. BLACKBODY RESPONSE MEASUREMENT

Fig.9.1 shows the block—diagram of the setupfor the measurement of blackbody response of the detectorunder test. This setup include a blackbed*, a lightchopper (the fabrication of which is described in 9.4),a specimen cell, a lock-in-amplifier etc. The specimencell is a whole metallic cell provided with a cold fingerto hold the IR detectors and has also been fabricated(fig.9.2) as part of this setup. This cell can be eva­cuated to 10-2 torr and the specimen can be cooled toliquid nitrogen temperature. The detector is properlybiased using a current source and it is irradiated witha known amount of radiation from a 500K blackbody.. The

detector output signal is measured as a function of biascurrent and chopping frequency. Noise is also measuredas a function of frequency (at constant band width) fordifferent bias currents. These measurements, togetherwith active area measurement, can be used to calculateresponsivity and D* as a function of chopping frequencyand bias current.

System Noise Measurement

The detector is replaced by a dummy low noiseresistance of approximately the same value-as that ofthe detector. The temperature of this low noise

Lock_m_ahp.

as 3 *­fi>~ .53 3 33Q8 9:; 2 *5 Detecfor’.CD3 .§CL «.1 "3___;F3

biassvpwy

Fig.9.1 : Block diagram of the setup for themeasurement of blackbody response.

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wilsonseal

Contact

Window

188

I1n—_¢-—-—- Feed through

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Fig.9.2 : Metallic cell to hold the IR detector.

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resistance is maintained such that the thermal noisegenerated in the resistor is small compared to thenoise generated in the amplifier. The rms noise

voltage VN (f) is measured as a function of frequency.a

fiystem Gain Measurement

The system gain as a function of frequencyg(f) is then measured, with the dummy resistance inplace of the detector, by injecting a signal of knownamplitude from an audio oscillator.

Q"

Blackbody Measurement

BOOK blackbody radiation is allowed to fall

on the detector and the detector output signal Vs(f,b)is recorded as a function of chopping frequency (f) and

bias current (IB). Then the blackbody radiation isremoved by closing the shutter and detector noise VN (b)as a function of frequency (at constant band width) 0for the same bias current as used in the blackbodymeasurement, is recorded.

Calculation of Responsivity

The responsivity is the ratio of output tothe radiant input. The blackbody responsivity is givenlmy

R(IB, r, T) = ‘,7; volts watt" (9.2.1)where V5 is the detector output in volts and ¢BB is the

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incident blackbody power in watts.

Calculation of D‘

The noise equivalent power (N5?) is theincident power required to produce a signal output equalto the rms noise output. It may be shown that

VNEP = —“ (9.2.2)B .where Vn is the rms noise voltage.

Th (D detectivity D of a detector is the reciprocalof NEP. D* normalise the detector area and noise bandwidth.

(AdAf)1/2 R(Ad Ar)“/213* = D (Ad Ar)“/9 = NE = V (9.23)B n

where Ad is the active detector area and Zbf is thenoise bandwidth.

9.5. SPECTRAL RESPONSE MEASUREMENT

Fig.9.5 shows the block-diagram of the setup tomeasure the spectral response of the detector. Thisincludes a source, chopper, double monochromator,radiometer, a Lock-in-amplifier etc. The detector isilluminated with monochromatic radiations. The centre

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chroma!’

bklssupp[y \1')’eEector­

Lock—h»

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Fig.9.} : Block diagram of the setup for the measurementspectral response of the detector.

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wavelength of the monochromator is varied over the

wavelength range of interest and the detector outputvoltage is compared with the value obtained by theradiometer.

9.4. A VARIABLE SPEED LIGHT CHOPPER FOR USE IN TH£ IR

DETECTOR TEST SETUP

For use in the test setup described above asimple mechanical light chopper has been designed andfabricated. Although these may appear to be relativelysimple device, there are many problems associated withthem. Maintaining a constant frequency and eliminatingthe transient noise spikes generated by motor speedcontrollers are only two of the problems. Variablespeed motors have a speed stability of only about 3 1%and in many applications this may not be sufficient,especially when photon detectors are being tested. Manymotor speed controllers use silicon controlled rectifiersas regulators which may generate all kinds of transients.The simple light chopper described, below has been designedto have a chopping frequency ranging from 10 to 2000 Hz,using two chopping blades. The circuit is simple,reliable and inexpensive. A small DC motor whose speedcan be varied ten fold is employed and its speed is main­tained constant by feedback control.

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Design of the chopping blade

In optical experiments the limiting appertureat the source is always circular. when a circular beamis chopped with a series of straight sided slots cutin the rim of a disc it gives excellent approximation

to a square wave, provided that eb/Gt is approximatelyequal to zero (fig.9.4) where ab and at are the anglessubtended by the light beam and a tooth—slot pair,respectively.

The maximum number of teeth in a chopper blade

that will have an RMS conversion factor differing by lessthan 1% from that of a true square wave chopper is givenby

0.251 (D _ D )n = ° b (9.4.1)Db

where n is the number of teeth, DC is the diameter of thechopper wheel and Db is the beam width.

Description of the circuit

The speed of the motor is regulated by generat­ing a square wave reference signal with a photodiode(SI 100, BEL, India) looking through the chopper blade,and sending this reference signal into a frequency tovoltage converter to control the current through the motor.

wave chopper’6try °f4 : GeomeFig.9­

194

195

0 PER ATIONALAMPLIFIER DIFFERENTIATOR MONOSTABLE INTEGRATORIOK ‘+I2v

I I 50 K,O,( 8 4 IO-047 R’

5552 5 IOK3 Vc,__L IOCO00:] 1'0-t

REGULATOR Vt+r2v

2N 3055

I0 K 6.5HELIPOT

Fig.9.5 . Schematic diagram of speed control.

196

The photodiode is connected to an operational amplifierand its output is differentiated and the negative goingpulses are used to trigger a monostable multivibrator.The monostable pulses are integrated by feeding them intoa RC integrator. Thus the chopper frequency is convertedinto a proportional analogue voltage.

The value of the timing capacitor and resistor

are selected such that for a given chopper blade whenthe motor is running at its maximum rated speed, the RCintegrator develops 7V, which is the typical Zenerreference voltage of the 725 voltage regulator. Thefeedback voltage is compared to the reference voltageapplied and depending on the speed, the motor current iseither increased or decreased to make it equal to the setpoint value.

Analysis of the circuit

Let Vr be the reference voltage and Vf thefeedback voltage. The voltage applied to the motor is

vs = (vr - vf) A (9.4.2)where A is the amplification of the voltage regulator­

transistor combination. The feedback voltage Vf isgiven by

vf = 0.695 ctntvr = kf (9.4.5)

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where Ct is the value of the timing capacitor, Rt isthe value of the timing resistor, V is the pulse heightof the monostable pulses and f is the frequency of themonostable pulses.

For a DC motor we have, neglecting the brushresistances

VS = Vback + I (Rm + RS) (9.4.4)

where Vback is the back EMF of the motor, Rm is themotor winding resistance and RS is any series resistanceassociated with the motor.

The back EMF of the motor is given by

Vback ‘ Kf (9'4'5)where K is a constant which includes the magnetic fluxdensity at the air gap, number of teeth in the chopperblade, etc.

Ak (fr - r) — Kf = I (Rm + RS) (9.4.6)

where fr is the frequency corresponding to the referencevoltage Vr and f is the actual chopping frequency.

Solving this, the steady state error is

Kf I R RE = If — f = -—'£' + ( m + S)S r kA K + kA

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bince kAj$>Kfr, the first term in the righthand side of (9.4.7) may be neglected giving,

I (Rm + RS)ES =K + kA

This equation determines the steady state errorand since the voltage regulator-transistor combinationhas an amplification factor of 104, it can be seen thatthe steady error is a small quantity.

Performance of the chopper

A resistance R8 was put in series with themotor and was used as a current sense for the 725 voltageregulator to limit the maximum current through the motorto what is necessary to just start the motor under load‘conditions. This eliminated overshooting of the motorfor small errors. The effect of this limiting resistance

RS on the accuracy of speed regulation is shown infig. 9.6. It can be seen that a proper selection of RSwill give very high accuracies in speed regulation.The chopper blade was not directly driven by the motorbut for increased transient stability it was driventhrough a flywheel. This greatly reduced transientinstabilities associated with both line voltage trans­ients and motor current variations.

0-1»0-06

0-03

-003

-O-IO

Fig.9.6 : Effect of the sensing resistance RS, on

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ADVHOODV 39V.l. N33ti3d

499

percentage accuracy O-5OOHz, 13-1000 Hz,and 0 --1500 Hz.

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The regulation of the chopping speed wasexcellent and was tested with a universal electronic

counter with 0.4 Hz resolution. After a warm up periodof half an hour it gave a frequency stability betterthan 0.1%. This is better than commercially availablechoppers. Drift due to temperature variation can bemade negligible by using a high stability resistorand capacitor in the timing circuit.

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REFERENCES

1. R.K. Willardsen and A.C. Beer (eds.), Semiconductorsand Semimetals, Vol.12 (Academic Press, New York,1977).

R.H. Kingstan, Detection of Optical and Infraredradiations, Springer series in Optical Sciences,Vol.10, D.L. Mac Adam (ed.), (Springer-Verlag,Berlin, 1978).