Maihara, Toshinori

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RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Electronic Devices for InfraredAstronomical ObservationsMaihara, ToshinoriMaihara, Toshinori. Electronic Devices for Infrared Astronomical Observations. Memoirs ofthe Faculty of Science, Kyoto University. Series of physics, astrophysics, geophysics andchemistry 1971, 33(3): 251-2561971-03http://hdl.handle.net/2433/257518Memoirs of the Faculty of Science, Kyoto University, Series of Physics, Astrophysics,Geophysics and Chemistry, Vol . XXXIII, No. 3. Article 1, 1971.            ELECTRONIC DEVICES FOR INFRARED                ASTRONOMICAL OBSERVATIONS                                   By                                        '                          Toshinori MAIHARA                                          '      Department of Physics, Faculty of Science, Kyoto University, Kyoto, japan                          (Received May 7, 1970)                               ABSTRACT       A circuit system of electronics is presented along with its characteristics which    has been designed and used for the purpose of the infrared pho'tometry. The    system consists of a low noise preamplifier for infrarect detectors, a phase shifter    to adjust the phase of the reference signal and a synchronous rectifier having    an arbitrary tirrie constant. These units are described in detail on their resulting    properties such as the noise figure, the linearity or the frequency response etc.1. Introductien   An electronic system for the infrared measurement in astronomy is essen-tially a low noise amplifier whose circuit constitution is the same as that of a`lock-in amplifier', where an electric signal of alternating current, whichcorresponds to the intensiÅíy of the incident infrared radiatien, is rectified bymeans of a reference signal synchronized to the modu!ation frequency. Thislow level signal from the detector even buried under the apparent noise levelcan be picked up by selecting an appropriate time constant of the rectification,i. e. a sufficiently narrow band width of amplification.   In this paper we describe an electronic system outline and its characterlsticsof the device which has been designed and constructed for the purpose of infraredobservations in ekayama Astronomica! Observatory. We have carried outobservations several times since Decernber 1968 in the place, and celestialobjects such as Moon'}, planets, some infrared stars and many variable starshave been investigated by the photometric and polarimetric measurements. Inthese observations our electroniÅë system have shown a rather satisfactoryperformance in reliability and stability.2. Circuit Compositiens-Synchronous Rectification and Phase Shifter   An impedance of PbS detector is typically less than IMst at the roomtemperature, but it exceeds 10M9 when cooled to the temperature of liquid'nltrogen. So one must use the preamplifier of a very high 2nput impedanceand of a lower noise leve! compared to that of PbS detector itself. In our system 1) Matsumoto et al., 1969 COSPAR report, to be published.   S. Hayakawa et al., 1970 Space Research, le, 1007.252 T. MAMARA112A type pre-amplifier of P. A. R. Co. has been in use (Zin=100M9, gain Av-100. and noise figure N. F.S1 dB under the actual condition), which can satisfysevere conditons sufficiently.   After the pre-amplifier A. C. main amplifier of about 20 dB is connected andthen the A. C. signal modulated by the mecltanical chopper is again rectified bymeans of the synchronized referenÅëe signal and is converted to D.C. voltage.The obtained D.C. signal is recorded on the chart or tape recorder etc. afterbeing amplified by a D. C. amplifier. The whole scheme of our electronic circuitis shown by a block diagram of Fig. 1.                to loattei-yLR-C R, Ra dia tion   mu .s,•c Pi"e-Atnp.~100]Detectoi'Moter  LSynchnonousRleEe12S:-i         Em,lss,,. Pg'haifteti LIStiEtflsli-.ihapet D.c.Ampnvww:           Oiode Photo-Sensitgve                     CetL                                                     L8ecorg-fi-         Fig. 1. The block diagram of the electronic system for the infrared                astronomical measurement.(1) Refei'ence sig"al generator   There may be many ways to get the reference signal; for example from aspecial switching part which is sensiÅíive Åío metalic pieces of the chopper plate,or irom an A. C. generator coupled to the axis of the rnotor, and so on. Howeverwe use a pairing set of a Ga-As emission diode') and a silicon photo-sensitivecell iR order to obtain the reference signal in such way as they are confrontedeach other in both sides of the rotating chopper plate. So the light emitÅíed bythe Ga-As diode is recelved by the photo-sensitive cell, generating the corre-sponding reference signa12}.(2) Synchronous recetiL17cation   An elementary circuit of the transistor synchronous rectifier is shown inFig. ?". This is the same pattern as the phase detector3) which can yield theoutput voltage proportional to the cosine of the phase difference between twolnput signals. The germanium alloyed transistor can be used for these purpose 1) Ga-As emission diode: Hayakawa Electric Co. (Type GLE-502) 2) The emission spectrum of Ga-As diode is very narrow bandqd (930eAÅ}200A, at 25eC),   so its scattered light dose not interfere the infrared radiance. 3) Kawakami, M. ,Denshi Kairo V' 215.ELECTRON!C DEVICES FOR INFRARED ASTRONOMICAL OBSERVATIONS                Rt 7rr,                 Tr2 eo         eB@ .r . oD.C.OUTf)U7"          fNPUTB                          IAVPUT                           A•                          eA R, RTTc                      R2     Fig. 2. The elementary circuit of the synchronous rectification.                                StgnaLfnput                                    o                                &6K                                     sr-22v51JojrenceS9ktIOK/oOK50/z2K3.6KST8.2X2SD 6/X2     leOK D.C.CU7-   o  c253                                ST-22 Sve,S609              iM 25C 372@                                                 tK   Fig. 3. The synchronous rectifier. The irnpedance of         an emitterfollower. Two input          matching. The time constant T=RC is decided by a          itor C in the output terminal side.because it operates well with the exckange of itsA. C. signal is rectified by the alternating biasing of Åíhesignal. The practical circuit in use is shown in Fig. 3.to the input signal is also verified (see the next sectien).transformers for the impedence conversion make theespecially in lower frequencies (K50Hz), although it islow frequency reglon because of the higher optirnum(3) Phase shifler   The rectification efliciency of the synchronousthe product integration of the two input waves; thesignal. So one must adjust their phase angle of wavesdifference or 1800 pltase difference. It the phasezero phase difference, the D.C. output has positive sign,case, it becomes to have negatlve sign.   As these two slgnals are generated independently,be fixed arbitrarily. However it is possible in our systemeach other by shiftlng the phase angle by an electronic                      R           reference signal is reduced bytransformers (ST-22) are used for irnpedance                resistor R and a capac-          collector and emitter, so the                square waved reference                 The linear response                  But the existence of            frequency response worse,                 not necessary in such              frequency of our detector.           rectification is proportional to            reference and the intrinslc                to either zero phase            adjustment is carried out to                  and in the opposite               the phase difference may                 to adjust wave phases               circuit as in the next254 T. MAIHARAIN e569 /O,u         39Ki tKtOK iM++B47K   IOxL lSOO94. The phase ' thecollector in the      to the not inverted '   signal of from theusing for the cooled by   to beSi of the   are shown    of the PbS   itself. The  of the detector   eguals 200Hz The rightresistor is ': l>tL22ff11<1)LL1o tjC VRtKxK,/.bi/,K)'  l  IArffjus   XLLttO /< VRj,t KKf2-F)tuoDO-12V                                                               -6Y'              3SPrir22 ,uA70?C 25C372@>(3                Fig. shifter of the refer nce signal.figure, in spite of moving position of the reference signal generator.   The phase at the usual transistor circuit of the emitter:grounded type is inverted input wave phase. On the other hand thewave at the emitter is m phase as in the emitter follower type.So one can obtain the arbitrary phase lag to the input wave bysuperposing the waves collector and the emitter.3. Resulting Characteristics   The PbS detector te t o  the electronic set explained above isa high sensitive cell liquid nitrogen, whose sensitive proper area is1Å~lmm. The radiation det cted is collected by a Fabri lens of lcmÅë,so the effective area detector is about 3/4 cm2, Some characteristicsof the electronic system as follows in the use of this PbS cell.(1) Noise Level   The input noise level d tector which can be expressed by aconcept of noise equivalent power (N. E. P.) is shown in Table 1, comparing thenoise of the preamplifier noise of the detector is supposed to come    Table l. Noise level (PbS) unde  the conditions as the chopping            frequency fo and the band width Av is IHz (the left            column). column shows the noise of the pre-amplifier when            a leMst connected at the input.E PbS detector (without PbS) R=:10MstNoise (yV) 1.80 E 1.Z5N. E. P. (W) 2.2Å~lo-i2 IBias :M2 o'i::xfrom many typer of complicated noise sourcescurrent noise, radiation noise and so on), but inis expressed in the 1/f law as a general feature. (Johnson noise, fiicker noise,the low frequency regien itOn the other hand the cooled     ELECTRONIC DEVICES FOR INFRARED ASTRONOMICAL OBSERVATIONS 255PbS detector has a rather slow response time of the order of 10 msed, so onemust find out the optimum operating condition experimentally about the biasvoltage, temperature or modulation frequency, etc. The detailed report on theoptimum condition of the liquid nitrogen PbS detector is seen in some reference(see for example Potter and Eisenman, 1962 Appl. Optics, l, 567).(2) Linearity Checle   It is important to chec!< the linear response of the electric signal againstthe incident infrared radiance. Tke test is easily carried out by changing thediaphragm radius of the infrared radiator. In Fig. 5 it can be seen that thelinearity of Åíhe system ln the range from 10-'SW to 10-i'W is realized exactly.Above the incident radiance of 10-SW the curve bends down because of thesatulation effect of the main amplifier. However this satulation effect may beavoidable by an attenuator before the main amplifier.g      '      '     '     '    '    '   '  'vyc :twvvw ,•Voise LeveLtl                      -li -10 -9 -8.                         LOG Jncident Radiance (VV)Fig. 5. Linearity test of the electric signal against to the infrared radiance from the      radiator. The test condition is as follows; the b}ack body temperature is       1000"K ; the modulation frequency equals to 200 Hz ; the effective wavelength       is 2.2 pt.g             5d, ,vu 200 500 IK                       Ay'odutc?tiot) Freguency CHz)Fig. 6. The frequency response of the circuit system, measured by the PbS       detector in the room temperature.256 T. MAIHARA(3) Freqttency resPonse and other characteristics    As is described before, the liquid nitrogen cboled PbS cell has a very slowtime constant of the order of 10msec. So it mttst be necressary to use thePbS eell in the room temperature, where the time constant is less than onemMisecond, ln order to test the frequency response of the circult system.   The lower limit of Åíhe frequency response is restricted by transformersused in the synchronous rectifier in the Fig. 3. The transformers are necessaryfor the impedance matching in the transister circuit. (CI)he aRother type ofsynchronous rectification by the use of field-effect transistors will be developedwhich can be served in direct coupling because of their high imput impedance.)But in the usual case of PbS detector or other pboto-conductive detectors, suchas Hg-doped Germanium detector, the practical frequency o'f the optimum condi-tion is in the range from 100Hz to 1kHz.

Electronic Devices for Infrared Astronomical Observations

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Electronic Devices for Infrared Astronomical Observations

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