STATUS REPORT FOR PERIOD 1 NOVEMBER THROUGH 30 NOVEMBER 1970 U.S. GOVERNMENT CONTRACT(SANITIZED)

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP79B00873A001300010010-9
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 95
• Document Creation Date: December 28, 2016
• Document Release Date: August 29, 2012
• Sequence Number: 10
• Publication Date: November 1, 1970
• Content Type: REPORT

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STATUS REPORT
for period
1 November through 30 November'1970
U. S. GOVERNMENT
File NO. 11038
STAT
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STAT
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ii
to,
This document is presented as the Monthly
Status Report under Contract to the U. S.
The report period represented herein covers the
period 1 November through 30 November 1970.
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STAT
STAT
STAT
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fl 30 November 1970
STEREOCOMPARATOR
INDEX
Page No.
EProgram Summary 1, 2
Task 22 interferometer, Measuring Assembly T22-1 through 12
] Task 24 Image Analysis System, Correlation System T24-1 through 5
ETask 43 Computer Programming and Services T43-1 and 2
Task 45 Acceptance Test in Fabrication Plant T45-1 through 17
Appendix
Acceptance Test Part I Revised November 25, 1970
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STEREOCOMPARATOR
PROGRAM SUMMARY
Scheduled Percentage of Completion 98.9%
Actual Percentage This Date 96.0%
This report period includes the performance of the
Part I In-Plant Acceptance Test. In this report (see Task 45)
is the acceptance test data, a summary, and conclusions.
All the acceptance test values were achieved or
exceeded, with the exception of the maximum stage speed
and the minimum film clamping time, feels that these
two parameters are not consequential in terms of Stereocom-
parator performance, and that the values achieved are fully
adequate for the purpose.
The significant parameters, such as resolution, are
substantially exceeded, and the Part I tests show that the
Stereocomparator is performing very satisfactorily.
The Part I Acceptance Test, amended to reflect the
actual work performed and results achieved during the testing,
are included in thq Appendix to this report:
The revised interferometers perform extremely well
(see Task 22), and the correlator performance is excellent
(see Task 24).
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The computer program work must be completed
before the Part II Acceptance Tests can be run. It is /
presently anticipated that the we?k of December 14 5
schedule for final in-plant acceptance testing will be
achieved.
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STEREOCOMPARATOR
Task 22
Interferortieter, Measuring Assembly
Scheduled Percentage of Completion -100%
Actual Percentage This Date 95%
During the last report period significant changes
in. the interferometer system were implemented which
yielded greatly improved performance.
As stated in previous reports, it was found that
the original Twyman-Green interferometer configuration
did not yield performance considered sufficiently accurate
and trouble-free for use in the Stereocomparator. Speci-
fically, the problems encountered were:
Mirror non-flatness.- caused phase shifts
of the fringes with attendant counting errors.
Return beams into the lasers caused the laser
servo locks to become unstable - consequently,
it was necessary to deviate the return beam
T22-1
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from the incident beam axis, which caused
a measuring scale error and DC shifts in the
electronic detecting circuitry.
Differential phase-shifts between the two
channels in each interferometer which are
quadrature-analyzed to determine direction
of motion.
The combination of the above problems produced
a highly unsatisfactory system performance. The three
problem areas have been remedied, and an excellent oper-
ating system has resulted. Specifically,
a) The solution to problem (1) above was to obtain
mirrors of much heavier and more precise
construction, and to mount them in an improved
manner.
b) The solution to problem (2) above was to re-
design the interferometer assemblies to in-
corporate optical arrangements which extinguish
the return beam by means of selective polar-
ization devices and by deviating the return beam
T22-2
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30 November 1970
with a Rochon prism to a point where the beam
could meet the reference beam Ca nd yet not
return to the laser.
Earlier attempts to mitigate the return beam
problem by extending the beam path length
? were unsuccessful due to the large amount
of beam jitter introduced by air Currents causing
refractions near the laser. These refractions
became significant due to Lthe long, path*
length, and the peculiarities introduced by
the folded optical ph (never fully explained)
combined to make thd system less than satis-
factory.
Happily, however, the present interferometer
system eliminates all of the difficulties ex-
perienced with the laser return beams and
off-axis operation.
The solution to problem (3) above is discussed
below.
T22-3
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A certain amount of phase shift variation
between the interferometer channels is permitted,
with the criticalness being a function of speed.
+ o
Generally speaking, a variation of - 45 at
1
top stage speeds wip not cause counting errors.
Now, various mechanical factors, such as
stage pitch and yaw, can cause phase vari-
ations by tilting the fringes. A yaw change
of only 2 arc-seconds will tilt the fringes
about 45 degrees. Since one half this mag-
nitude of yaw is experienced during acceler-
ations and due to non-perfect way straightness,
it can be seen that the permissible variation
in the phase between the interferometer fringe
detecting electronics is on the order of only
20 degrees.
Now, as explained in past reports, photosen-
sitive field-effect transistors were used for
detectors. These devices showed high gain
and low noise characteristics combined with
quite good risetimbs (1,vsec). These devices
were 'incorporated into circuits which were
T22-4
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30 November 1970
highly stable with time and temperature, and
yielded a good wide-band system. However,
certain unexplained phase shifts existed which
could not be checked by electrical means; i.e.,
the response of the system to light input appeared
different than for a dummy electrically simu-
lated signal applied at the photosensitive
FET gate.
When, as a result of the change in mechanical
configuration being made, it became necessary
to re-lay-out the interferometer circuit boards,
It was decided to attempt an investigation of
the phase-shift phenomena.
A light-emitting diode (LED) was obtained
which has a turn on/turn off time of about
5 nanosec. This was mounted in a block
so as to radiate into the FET window. A
current driver for the LED was fashioned and
the system was driven by a signal generator.
Using a wide-band X-Y oscilloscope, a
Lissajous figure showing the LED current
T22-5
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versus FET output voltage was obtained.
The FET showed considerable phase shift
within the frequenc+ range of interest,
amounting to more tikan 360 degrees at the
frequency corresponding to the higher rates
Lzi
of stage travel. Moreover, tests on several
units showed this phase shift versus frequency
to be variable from FET to FET. An analysis
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of the equivalent circuit of the FET showed
that this complex phase shift was dtie to non-,
fl linear division of displacement currents between
the drain and source at higher frequencies.
These effects were shown to be dependent
upon device parameterls which have a significant
spread from unit to unit. In fact, the only
reason the system worked at all is that the phase
shifts seem to track to a certain degree.
C=3
It was then decided that a search should be
made for a better photo detector. Various
types of phototransistors (bipolar) and photo-
diodes were tested. The best unit was deter-
mined to be a PIN diode (Schottky barrier.
T22-6
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30 November 1970
device) which exhibited very low phase shifts
with low load resistances. Unfortunately, the
output level with low load resistances is so
small as to be virtually useless for our purposes.
However, it was found that with higher load
resistances, the output level increased but the
capacitances in the diode, input amplifier,
and wiring caused a roll-off commencing at
about 50kHz. It was determined, however,
that the roll-off was a simple pole, with an
equation of the form
eo
h? I + jw't
whereao is output voltage
i is light input
k is a circuit constant
jw is frequency
A" is the RC time constant of the diode
circuit
Thus, the phase shift at high frequencies is
-90 degrees maximum. This immediately suggested
a feedback network as a means of holding down
T22-7
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30 November 1970
? phase shift. Accordingly, a circuit was con-
structed which uses a FET source-follower
(good to 100MHz) driving an MC1509F video
amplifier (good to 40MHz). A voltage divider
on the output of the video amplifier is tied to
the load resistor on the diode to provide the
feedback. The Use of extremely wide-hand
amplifiers guaranteed that no additional poles
would appear at loop gains of more than 1,
thus assuring closed-loop stability. A matched
pair of FET input amplifiers was used to allow
adjustment of the DC operating point and to
provide temperature-drift immunity. The resulting
circuit is shown in figure T22-A.. It will be
noted that the PIN diode is a dual device con-
taining two sensors in one package. This allowed
elimination of the 90o wedge mirror formerly
used, with its attendant losses. Also, since
the two devices are fabricated on a single sub-
strate chip simultaneously, excellent matching
between channels is assured. The active areas
are rectangular and are separated by .005 inch, with
a differential output linearly related to fringe
T22-8
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30 November 1970
displacement, which was precisely what was
needed. To complete the design, the whole
assembly was fabricated into a cordwood module
of very small size with both channels laid out
perfectly symmetrically to balance and minimize
stray capacitances. The outputs are differential.
also, using a twisted pair to eliminate noise
pickup in each channel. A nickel-plated copper
case completes the assembly, providing electrical
shielding.
This unit was tested and found to have perfectly
flat response to 3.5 MHz with no phase shift,
about 10 times as high as encountered in the
system. There was no phase.difference between
channels to 5MHz, which is as high as our
signal generator goes. It was found that the
unit has a 1-volt output and exhibited a 50dB
(300:1) signal-to-noise ratio and absolutely no
parasitic oscillation or insthbility. Thus, we
now have an interferometer fringe detector which
is completely satisfactory in every respect.
T22-9
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In order to provide sufficient signal power to
traverse the cables to the logic rack, a new
circuit was designed which fulfills this need.
(See Figure T22-13.) This unit provides several
features which are explained below.
As has been explained in previous reports,
the laser uses a phase-lock loop to maintain
the output wavelength constant. This system
contains a movable mirror which modulates the
cavity length to change the laser frequency. The
system uses a 12kHz carrier and slope 'detects
the output of the photocell which monitors the
output level of the laser as the mirror is modulated,
adding a DC component to the modulation to main-
tain a precise cavity length. This 12kliz carrier
naturally appears in the output at a level of
approximately 10% of the "DC" output level.
This carrier must, of course, be ignored by
the interferOmeter, and this has been accomplished
by merely setting the detecting circuitry threshold
above this level. However, any drift in DC
output level is reflected in the interferometer
circuits and the 12kHz carrier does appear as a
4.
T22-10
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30 November 1970
noise source to the system, although the system
can be adjusted to ignore it. However, any
drift in the threshold adjustment may throw
the system to a point where the carrier could
be mistaken as fringe counts. The circuit de-
scribed below 'greatly reduces this possibility.
The circuit consists of a transistor level shifter
for the differential output from the interferometer
detector assembly described above, followed
by a variable-gain (AGC) video amplifier and
a cable driver. Also included is a ? 6 volt
power supply regulator which drops the t? 15
volt power used for the cable drivers to a highly
stable - 6 volts for the video amplifiers and
interferometer detectors.
The AGC is a relatively wide-band circuit (20kHz)
which is controlled by an auxiliary photosensor
whi-oh receives light from a beam splitter ahead
of the interferometer (i.e., this photosensor
monitors the laser level only.). Thus any 12kHz
cagier or DC shift app`earing in the laser beam
T22-11
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These drawings belong ith the Status Report
for period 1 November through 30 November 1970
STAT
File No. 11038
STAT
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REVISIONS
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