He was troubleshooting, repairing and calibrating (at home) some U.S. transceivers.
I was 3 years old and I'm clearly remember U. S. soldiers coming home and distributing some chocolates or chiclets..
He was using SCR-211-M to retuning transceivers.
When I was young (and when my father was away) I was playing with it...
I was placing earphones , rotating dial, and searching for beat notes etc.
When my father was passed away, I have keep in charge this apparatus but never using it.
In 1986 I have obtained my Ham licence but I was only interested by Packed Radio and SCR-211-M was sleeping somewhere in my shack (ON1KCF-->ON4KCF-->ON4JCI).
Very recently, I have made a AC/DC power supply to replace the batteries and ,when powered up I have inserted earphone in audio jacks, some dial rotation and audible beat was still there... !
I have a lot of instruments (Spectrum analyser, frequency counter, generator etc) so I have been able to check it and this marvellous instrument, as old as I am, il still very precisely working !
Unhappily, the calibration book is now uncomplete.
1- Front sheet (N°1 & 2) is missing,
2- High Frequency sheets (49--> 74) have been lost.
If anybody, owning a complete Calibration Book MC-177 from a similar BENDIX Frequency Meter , be so kind to scan those pages, I could be able to complete the book after having re calibrated those missing pages.
My SCR-211-M has a Serial Number 6086 graved on identification plate (corresponding to the red number printed on the front face under this identification plate).
For example: I have Open the Calibration Book on page 44 (as shown on the picture)
Set the Dial on 1598.4 (2,6 MHz)
Spectrum and counter coupled to the antenna.
Perhaps some people would be happy to see my incomplete Calibration Book...?
etc... up to 71
FREQUENCY METER SET SCR-211-M User Manual (clic to read it).
HP Fcy meter is connected to antenna plug and we look, for example at the High Fcy BAND ( 2.0MHz-> 4MHz 50KHz stepping), we move the dial in order to read different frequencies reprinted on Excel Sheet as shown in the following figure.
A frequency in front of the Dial setting
or
A Dial value to be set in order to reach a required frequency.
With this table, the usage of (my) Calibration Book is no more required...and missing part is no more a problem except for ...Vintage valuation.
... and now, look on 1944 available tools used to realise Production & calibration of Thousands SCR-211 Frequency-Meters of all releases.
AUTOMATIC CALIBRATOR For Frequency Meters
ELECTRONICS (Mc GRAW-HILL) Vol.17 No. 5, pp. 98-107 (May 1944).
A 126-tube electronic calibrator combined
with adding machine records on paper tape the calibration data at 327 points
for Army SCR-211 two-band frequency meter interpolates between these points,
and automatically prints in the individual calibration book a five-digit dial
number for 3252 frequency values
By DAVID SUNSTEIN
Factory Engineering Division
Philco Corporation, Philadelphia. Pa. |
and
|
JOSEPH TELLIER
Research Engineering
Dept.
Philco Corporation, Philadelphia, Pa.
|
IN THE MANUFACTURE of highly precise measuring instruments, it is
sometimes found necessary, in order to obtain the required accuracy, to hand
calibrate each individual instrument. In the particular case at hand, a
two-band frequency meter known as the Army SCR-211 is required to maintain an accuracy
of the order of 0.01 percent in the field. This frequency meter as manufactured
by Philco Corporation consists of an electron-coupled variable-frequency
oscillator, which can be checked at certain points of the dial against an
internal fixed-frequency crystal oscillator.
Anyone who has had experience in the
production of receivers having dials which read directly in frequency can
appreciate the practical impossibility of making an oscillator track to a
predetermined dial scale within 0.01 percent.
 |
| FIG.1 - Block diagram showing the three basic units of the automatic calibrator and their relation |
The Army specifications call for a listing
of calibration points every 0.1 kc. on the low band and every 1.0 kc. on the
high band, or 3,252 calibration points in all. Each of these calibration points
is recorded as a five-digit dial number.
Fortunately, it is found that by proper
design of the tuning capacitor, the plot of dial reading versus frequency can
uniformly be made sufficiently close to a straight line that only every tenth
point printed in the calibration book need be hand calibrated, the remainder
being interpolated linearly. Thus, hand calibration points are required every 1
kc. on the low band, and every 10 kc. on the high band, or a total of 327
points must be recorded by hand.
Calibration Time Is Shortened.
Thus, to calibrate a frequency meter accurately by hand, including
2.25 hours additional for miscellaneous operations, required a total of 16 man
hours. Furthermore, the fact that the frequency meter was under operating conditions
during the 2.5 hours manual calibration period necessitated a temperature-controlled
room for the calibration process, since ambient temperature changes, if permitted,
could put kinks in the calibration curve, thereby rendering the meter
inaccurate.
An automatic calibrating machine has been
designed and constructed at Philco which, together with semi-automatic
interpolating machines capable of typing the book directly, reduces the total
calibration time from 16 hours to 6.5 hours.
The actual direct time necessary to record the 327 calibration
points of each frequency meter has been reduced from 2.5 hours to 16 minutes.
and during these 16 minutes, the increments between adjacent calibration points
are also automatically tabulated, thereby eliminating the previous hour
required for manual computation. The short calibration time also eliminates the
need for a temperature-controlled calibrating room. The automatic method has
eliminated the human error, thus reducing the required checking time from 3.5
hours to 1.5 hours, which period is primarily devoted to insuring the stability
of the frequency meter. Overall, since the equipment is being used on a 24
hour-a-day basis, over 140,000 man hours were saved in 1943.
Automatic Equipment Used.
The semi-automatic interpolating machines
are similar to the adding machines used by finance companies for scheduling payments,
the only difference between the two being the size of type employed. These
machines are capable of carrying two totals, one of which may be added to the
other and the new total printed, thereby enabling interpolation. Since this
device is well known it will not be discussed further here.
The 126-tube automatic calibrating machine
has several unusual features. Essentially, it consists of three parts, shown as
a block diagram in Fig. 1.
The first supplies a source of standard
frequencies against which the meter is calibrated.
The second provides a means of mechanically
continuously driving the dial of the frequency meter and electrically
generating a sharp pulse every time the frequency meter is tuned through zero beat
with the standard signal.
The third unit records on a paper sheet the
dial reading of the frequency meter at that instant of time at which the pulse
is generated and also the difference between adjacent dial readings.
Standard Signal Source.
 |
|
FIG
2- A few of the infinite number of
zero beat wave forms which can occur, depending upon the random value of q.
|
As stated previously, calibration points must be taken every 1 kc. on the low band and every 10 kc. on the high hand of the frequency meter. This requires accurate signals at 16 kc. intervals for the low band (as will be explained later) and signals 10 kc. apart for the high band. The method used for generating these signals employs two multivibrators, each locked in with a crystal oscillator that is continuously monitored against Bureau of Standards radio station WWV. Since this system is conventional, it need not be treated here.
Zero Beat Detecting Problem.
Before describing the actual mechanism which was finally designed to detect, the instant of zero beat, a short description of the problem involved will be given. That there is a problem at all is a result of the continuous drive applied to the frequency meter. If the drive were stopped at each calibrating frequency, conventional circuits could be used to determine zero heat within a very few cycles. However, as will be seen, the constant rotation of the tuning capacitor of the frequency meter introduces factors which require special consideration.
Let it be assumed that a constant-speed
drive is being applied to the tuning capacitor shaft, so that the frequency meter
is generating a signal which is changing continuously in frequency. Assume also
a single standard signal whose frequency is constant and lies in the range of
the frequency meter. And finally, let the outputs of the two signal sources be
coupled into a mixer stage whose output circuit is responsive only to signals
in the audio range. Then, as is shown in Appendix I, the wave shape of the
audio signal developed across the output of the mixer will be as indicated in
Fig. 2, where the origin for time (t = 0) is taken to be the instant when the
frequency meter is exactly at zero beat with the calibrating
It will be noted immediately that more than
one form of the zero beat wave shape has been given. There are actually an
infinite variety. The exact one taken depends on a quantity indicated as q, and a constant a.
The equation of the zero be Voltage is
ezb = Ezb cos (a/2 t2 - q)
where the
amplitude Ezb, is determined by the amplitudes of the two signals,
a is
the rate of change of periodicity, wf, of the
frequency meter,
and q is
a random phase angle dependent on the phase angles of the original beating signals
Ezb and a can be held constant, but q may, and
probably will, have a different value each time that the frequency meter passes
through zero beat with a calibrating signal. Hence, any of the infinite variety
of which six are shown in Fig. 2 can be expected to appear at one time or
another.
 |
| FIG. 3- Zero beat waveform for which q =0°, and for which a calibration time of 6 minutes is assumed. Here t2 represents the maximum permissible limits for triggering |
It remains to be shown that the difference in zero beat waveform presents a problem. Figure 3 is quantitative picture of one particular wave which might occur.
Here q is taken to be zero, and a calibrating time of six minutes on the 2 to 4-Mc. band of the frequency meter is assumed. With this calibrating time, a, the rate of change of periodicity, is 2p (4x104? -2x106?) divided by 360 seconds. Under these assumptions the time t, taken for the beat voltage to go through its first zero value is approximately 0.020 second (Appendix II).
Accuracy Required.
To achieve the overall accuracy of
calibration mentioned in the introduction, it was desired that the error
introduced by the zero beat detector itself should be unreadable on the dial of
the frequency meter. Since the latter is graduated into fifty thousand vernier
divisions, it was decided that an accuracy of better than plus or minus one
part in one hundred thousand (half of one vernier division) would be
acceptable. Assuming a linear scale, on the 2 to 4-Mc. band of the frequency
meter each vernier division represents 40 cycles change in frequency. Therefore
it was required that the zero beat indicator trigger when the frequency of the
signed under calibration was within 20 cycles of true zero beat. With a calibrating
time of 360 seconds the frequency meter changes in frequency 20 cycles in a
period of plus or minus 0.0036 second or a total of 0,0072 second. This is
indicated as t2 on Fig. 3
The problem of calibrating with sufficient
accuracy arises from the fact that the time interval t2, is relatively short
compared with t1. Over the latter interval the zero beat wave is marked by
distinctive characteristics which could be used to trigger the calibrating
equipment. But it might be difficult to make such triggering occur always in
the shorter interval t2.
Integration, A Step Toward Solution.
Evidently the best means of utilizing the
distinctive nature of the waveform near zero beat is an integrating network.
Such a circuit may be considered most simply as summing algebraically the area
under the beat wave curve, adding areas above the zero axis, and subtracting
areas below. At times remote from the instant of zero beat, alternate positive
and negative areas are relatively small and nearly equal, so that their
algebraic sum is small and builds up in amplitude very gradually. However,
through the period t1 (Fig. 3) a large positive area is added. The integral or
sum hence contains a large positive pulse, building up through the interval t2.
Such a pulse could be used to trigger the indicating equipment. Referring to
Fig. 2, if q had been 90 deg instead of 0 deg, a
very different pulse would have been developed, and in addition it would
 |
|
|
have begun to build up inch earlier in time. It is therefore highly possible that triggering would have occurred outside the Period t2 which has been specified. Further, for other values of the pulse might be negative, or there might even be no appreciable pulse at all.
The integrated output voltage obtained for
various values of q is shown in Fig. 4, with the
acceptable triggering period t2 again indicated. These curves are also based on
a six-minute calibrating time, and are in the form of Fresnel integrals. Tables
of this integral are available.
It is apparent therefore that the major
difficulty with the system outlined is the erratic nature of the pulse which
would be obtained, with respect to amplitude, shape, and time of occurrence.
Fortunately a means of overcoming this difficulty was found. The mathematical
basis for the method devised will be stated here, and a proof of this
particular case is given in Appendix III.
Principle of Zero Beat Detector.
 |
| FIG. 5 – Pulse form obtained at each zero beat by addition of squared integrals of quadrature zero beat waveforms. This curve is independent of random phase variations between the frequency meter and the standard signal. Note that practically the entire pulse occurs in the interval t’2 |
For certain types of functions, of which the integral of the zero beat wave given above is one, the sum of the squares of any two forms of the function which differ only in that they are 90° apart is always the same regardless of the absolute phase of the two forms. Moreover, the resulting function is the square of the envelope of the original function plotted for all possible values of its phase angle.
A simple example of this can be given. If
the original function is taken to be A sin (wt + q) where q is any phase angle, then a second form, differing in phase by 90°,
is A sin (wt + q + 90°).
But this is the same as A cos (wt + q), and the sum of the squares of these
two forms is A2. If a plot of the original function is made for all values of q, it is seen that the envelope is a straight line of amplitude A,
and the square of this envelope is a line of amplitude A2.
In Fig. 5 is shown the envelope of all the
integral curves in Fig. 4, with the dashed line representing the square of this
envelope. This dashed line is the pulse which can be derived from the zero beat
wave regardless of what value the random angle q may have. It is only required that a second zero beat wave be
produced which differs in the angle q by 90°.
This can be done readily, as will be described later. The squaring action
produces a sharper pulse than any of the integrals themselves. The acceptable
triggering time is again shown as t2.
This is the basic principle of the zero
beat detector. The manner in which it was incorporated will next be described.
Superhet Circuit Is Used.
 |
| FIG 6 – Block diagram of complete zero beat detector |
For the same fundamental reasons ordinary radio receiver, it was decided to use the superheterodyne type of circuit for the zero beat detector.
There are two channels, one for the
standard signal and one for the frequency meter, as shown in the block diagram
in Fig. 6. The variable tuned portions of these and the frequency meter are all
ganged together, on one motor-driven shaft. This ganging is indicated on the
block diagram by the dashed lines, and is pictured in Fig. 7.
Each channel is fed from its own source,
converted to an intermediate frequency by a common oscillator, and passed
through its own i-f system. Both are then coupled to each of two mixers, the
outputs of which are zero beat forms differing from each other in phase by 90
deg. These are each integrated and passed through square law stages, and the
sum of the two taken. At this point the required constant-shape pulse has been
obtained. The remainder of the detector is made up of circuits for obtaining a
large-amplitude pulse suitable for operating the printing mechanism.
These channels will now be examined stage
by stage, and those circuits of a unique nature described in some detail.
R-F Section.
Channel 1 is fed from a standard signal
source, which in this case is a multivibrator held in synchronism with WWV as
heretofore mentioned. Since for high band calibration (which will be discussed
first) signals are generated every 10 kc. from 2 to 4 Mc, it was found
desirable to introduce a tuned r-f stage to remove all but a few signals in the
vicinity of the one desired. These selected signals are applied to one grid of
the first mixer in channel 1.
Channel 2 is fed from the frequency meter
through a dual-purpose stage, which for high band operation is effectively a
unity gain untuned stage. It will be described in more detail later. The signal
is then applied to one grid of the first mixer in Channel 2.
The second grid of each mixer fed from a
common oscillator stage, but from isolated points to prevent channel
interaction.
These stages comprise the r-f section of
the unit, and as mentioned above are ganged on a common shaft. All except the oscillator
are adjusted to tune together and are so geared that their curve of frequency
versus shaft rotation are the same as an average frequency meter. The
oscillator is adjusted to track 480 kc. higher at all Points, a frequency which
is entirely optional and determined only by design considerations. Because of
the type of circuit used to obtain the quadrature signal, however, it is
necessary that this frequency once determined, be maintained exactly. To this
end a conventional automatic frequency control circuit is used, operating from
channel‑2
First Mixers.
 |
|
FIG. 7-
Overall view of zero beat detector with r.f section open. Note ganging of
tuning capacitors to the motor at left, and to revolution counter and frequency
meter at right. The i.f. channels and pulse forming stages are in the rack at
the rear, together with regulated power supplies for each channel.
|
The signals developed in the plate circuits of the two first mixers may now be considered. Since the frequencies of the oscillator and the frequency meter are varied together, and held to a constant difference. of 480 kc. the signal in the plate of the mixer in channel 2 is course constant at 480 kc. In channel 1 this is not the case because the signals against which the oscillator is beating are fixed, while the oscillator itself is varying at a uniform rate with the motor drive.
Assume that the oscillator is at 3,48 Mc.
at some instant. The mixer and r-f stages of channel 1 are then tuned to the
particular standard signal which is at 3.0 Mc. and the i-f is 480 kc. As the
oscillator changes to 3.485 Mc., the beat from the 3.0 Mc. signal becomes 485
kc. But at this time the beat from the text standard signal, which is 3.010 Mc.
becomes 475 kc., and as the oscillator approaches 3.49 Mc., this beat
approaches 480 kc.
Thus, as the oscillator is tuned through
each standard signal, the i-f signal in channel 2 sweeps through the frequency
480 kc. If the bandwidth of this i-f channel is held to less than plus or minus
5kc., there will be only one signal present at any time. This is necessary to
prevent spurious beats.
Limiters and Triplers.
Proceeding through the two i-f channels, it
will be noted that the final stage in each case is a combined tripler and
limiter. In order that the calibrating pulse may be of the same amplitude as
well as the same shape for each zero beat, it is necessary that the two signals
which are actually being combined be of the same amplitude (though not
necessarily the same as each other) for each beat.
Limiting is therefore incorporated in the
plate circuits of the two final i-f stages, and sufficient gain is provided in
the i-f systems so that on the weakest standard signal and the lowest output
frequency miter these stages are driven well past their limiting levels. Thus considerable
latitude may be permitted in the amplitudes of the harmonics generated by the
multivibrator and the output of the frequency meter without impairing in the least
the accuracy of the equipment.
An avc system is also employed in the
standard signal channel for the same purpose, inasmuch as this channel is
particularly subject to wide variations in signal level. With these circuits
included in the i-f systems, the amplitude of the signal at each mixer grid is
maintained extremely constant.
The use of tripling in the final i-f stages
is interesting in that it provides a simple means of tripling the accuracy.
Recalling the equation for the zero beat wave, it is seen that the time taken
for the beat to go through the distinctive period t, is proportional to a. If a be tripled, this time interval is reduced
to one third. But is proportional to the number of cycles of change in
frequency per second, and hence if we triple the changing frequency and
therefore triple the number of cycles of change per second, the time interval
within which a triggering pulse is generated is reduced to one third.
The effect of this is shown in Fig. 5 by
expanding the time scale 3 times so that the acceptable triggering interval is
represented by Channel 2 is tripled of course merely to keep its frequency the
same as the center frequency of channel 1. It is obvious that quadrupling or
even higher multiplication could be used.
Second Mixers.
The output of the final i-f stage is now
fed to one grid of each of the two second mixers. The latter are the stages in
which the zero beats are actually developed. The actual frequency meter signals
and standard signals are not being used directly to produce the zero beat, but
since a common oscillator is used, when the i-f signals are identical in
frequency it follows that the two r-f signals are also identical.
Note that a slight deviation of oscillator
frequency from its proper value, such as might be caused by drag in the afc
system, does not impair the accuracy. Such a deviation will of course change
both intermediate frequencies by the same amount. Note further that this
stepping down from r-f to i-f does not reduce the accuracy in the same way that
tripling was seen to increase it; because in this case the reduction in
frequency is obtained by subtracting from another frequency, and not by
division. The rate of change of frequency, as represented by a, is unchanged.
The output of the tripler stage in channel
2 requires one more operation. It is necessary to obtain not only the third harmonic
of the constant frequency, but also a second signal identical except shifted in
phase by 90 deg. This is readily obtained by introducing a loosely coupled
double-tuned transformer, and taking one signal from the primary and one from
the secondary of the latter. With the input at the proper frequency, these
signals are in quadrature with each other. It is to maintain input at proper
frequency that afc is utilized. The quadrature signals are applied to the
second mixers in channels 1 and 2.
The signals found in the plates of the two
second mixers are now two identical zero beats between the fixed and varying
i-f signals, except that the angle q is
different by 90 deg. ( Appendix IV). Each is now ready for integration.
Checking quadrature at Mixers.
Before discussing integrators, a simple
means of checking quadrature at this point might well he mentioned. If the
outputs of the two second mixers be connected each to one pair of plates of a
cathode-ray tube, and two r-f signals of approximately the same frequency applied
to the inputs of the system, then an ellipse with axes at right angles should
appear on the screen of the tube. The secondary tuning of the quadrature transformer
may in fact be adjusted by this method. This should preferably be done with the
two r-f signals as nearly of the same frequency as possible since this is the
condition which is of interest.
Integrating Circuits.
 |
FIG. 8—Conventional integrating network
FIG. 9—Schematic of actual integrating circuit used
|
A standard integrating circuit consisting of a large series resistance
and low-reactance capacitor is given in Fig. 8, while the actual circuit used
is shown in Fig. 9. The difference is occasioned by the fact that it is
necessary to incorporate a blocking capacitor between the mixer plate and
amplifier grid, and it is also necessary to supply a d-c return for the latter.
The actual constants used are determined by two factors. In Fig. 8, the higher
the value of R and the lower the reactance of C, the more nearly will the
output approach a true integral. However, it is necessary in Fig. 9 that the
time constant in the amplifier grid be short enough to permit the grid to
return to is normal no-signal bias between pulses. The values given proved to
be a satisfactory compromise.
Square-Law Stages end Adder.
The two integrals thus obtained are passed
through amplifiers to the square-law stages and the adder, shown in Fig. 10. Each
square-law stage consists of two type 7B7 pentodes biased to a point where the
second harmonic distortion is greatest. They share a common plate resistor, and
their grids are driven with opposite polarity from the plate and cathode of a
type 7A4 tube serving as phase inverter. The fundamental and third harmonic are
balanced out and the voltage across the common plate resistor is predominantly
second harmonic, or the square of the input voltage.
 |
|
FIG. 10
Schematic of square-law stages and adder
|
 |
|
FIG. 11 Schematic of blocking oscillator which is
triggered by pulse
from adder and provides final pulse used to actuate printer.
|
Since the r-f signals are fixed, the
signals at the inputs to the square-law stages are sinusoidal and are the same
except for differing in phase by 90 deg. They may be represented by A sin (wt + q) and A cos (wt + q) , and the sum of their squares is A2,
or in other words. a d-c component only.
The signal at the arm of the
adding potentiometer should thus be a straight line, and of zero amplitude if
the oscilloscope does not pass direct current. The vertical plates of the
cathode-ray tube are therefore connected to the arm of the potentiometer and the
latter adjusted for a minimum of audio ripple. Some of the latter will be
observed, since a certain amount of fourth and higher even harmonics will be
present in the output of the square-law tubes.
Blocking Oscillator.
At each zero beat there appears at the arm
of the adding potentiometer a pulse which is constant in shape and amplitude.
It is, however, negative in polarity because of the nature of the square-law
stages, and is not of sufficient amplitude to actuate the printing mechanism,
To obtain a sufficiently large pulse, the blocking oscillator shown in Fig. 11
is used. The negative pulse is applied to the latter through a polarity inverter,
which is merely a resistance-coupled amplifier stage.
 |
|
FIG. 12—Schematic of dual purpose input stage for
channel 2. This stage accepts a signal from either band of the frequency meter
without switching, and supplies a 2 to 4-Mc signal to channel 2. The latter
signal is the fundamental of the high band or the 16° harmonics of the low
band.
|
When the positive driving
pulse appears at its first grid for a short instant, it begins to pass through
one cycle of a normal blocking oscillation, and the usual sharp pulse of great
amplitude (over one hundred volts) is developed across the grid winding of the
transformer. The second grid immediately blocks and remains cut off for the
remainder of the natural period of the oscillator.
The system is thus unresponsive to spurious
pulses of any sort during the greater part of the interval between true pulses.
By the time the oscillator would normally begin its second cycle the driving
pulse has long since disappeared, so that only one output pulse is obtained for
each zero beat. This is the final output of the zero beat detector.
Low Base Calibration.
 |
| FIG. 13—Block diagram of printing system |
 |
|
FIG. 14—Open view of revolution counter switch.
Note the S decks of 10-point switches and the “disconnect” section at front. |
 |
| FIG. 15 Schematic of one column of number storage bank. The dashed rectangle at the right encloses a representative section of the revolution counting switch. |
Such a system was actually
constructed, and was found to be unsatisfactory simply because of the
difficulty in eliminating phase modulation from the source of 1Kc. signals. In
order to synchronize such a source on the 100-kc crystal oscillator, it was
necessary to use at least one intermediate multivibrator at 10 kc. As a result
it was found that while the total number of cycles per second remained correct,
the starting time of each cycle was subject to slight variations. This is the
equivalent of erratic phase modulation of the 1-kc signals. Because of the
enormous multiplication involved tin the worst case, from 1 kc. to 250 kc. and
from 30 kc. to 1440 kc.) the phase modulation became several cycles of
frequency modulation. The zero beat detector is of course very sensitive to
frequency modulation, so that spurious beats were obtained in nearly every
case.
After considerable investigation a satisfactory means of making the
calibration was devised. It will be noted that the sixteenth harmonic of the
low band varies from 2 to 4 Mc. By multiplying the signal from the frequency
meter by sixteen, therefore, it may be run through the high band calibrator
directly. Since calibration points are required every 1 kc. of the fundamental,
the sixteenth harmonic must pass through zero beat every 16 kc. on the
calibrator and a multivibrator producing standard signals every 16 kc. from 2
to 4 Mc. is required. It will be seen that 126 calibration points will thus be
obtained, which is the number required.
To obtain synchronization for the 16-kc.
multivibrator, an 80-kc. vibrator was locked on the fourth harmonic of the
100-kc standard. The fifth harmonic of the 16-kc. multivibrator was then
synchronized on the 80kc. signal.
Dual-Purpose R-F Stage.
The dual-purpose two-band stage located
between the frequency meter and the first mixer of channel 2 is shown in Fig.
12. It was designed to accept signals from either band of the frequency per
without switching passing the high band directly, and multiplying the low band
by 16. The first 7G7 provides considerable gain on the low hand, but relative attenuation
on the high band because of the capacitor shunting the plate load. The second
7G7 is a straight band-pass amplifier on the high band, and a multiplier to
select the sixteenth harmonic of the low band. As a multiplier, the gain of
this stage is of course much less than as an amplifier. At the output the
signal from either band is of the same order of magnitude, and the overall gain
is therefore approximately one.
Recording Problems.
The zero beat detector serves to generate a
pulse at the instant that the frequency meter is tuned to a calibration
frequency. The remaining problem is that of accurately recording the dial reading
at this instant. Since the dial is driven through 50,000 vernier divisions in
approximately 6 minutes, 139 vernier divisions are passed in a single second.
To record accurately the dial reading to the nearest vernier division while the
dial is rotating at this speed requires special care. An ordinary revolution
printer which momentarily presses a piece of paper against a set of revolving
drums carrying the dial reading would cause appreciable blur.
Such a printer could he modified by having
the drums stand still while the imprint is being made, after which the
revolutions lost by such standstill would have to be made up through a
differential or spring storage system. The mechanical complexity of such a
recorder, however, is such that frequent maintenance would be required. A
Strobotac photograph might he employed as an alternative but fast emulsions
would have to be used, either necessitating production delay in development by
established photographic concerns, or requiring that the radio firm
manufacturing the frequency meter go into the photographic business.
Recording System Used.
To overcome these objections, it was
decided to use an adding machine as the printer by providing a means of setting
up the dial reading of the calibration point on the keyboard.
The complete recording system is shown in
the block diagram in Fig. 13. The frequency meter dial is mechanically coupled
through gear drive of suitable ratio to an electrical revolution counter. This
counter ( see Fig. 14) consists of 5 decks of 10-point selector switches, each
deck being geared to its adjacent deck by a 10:1 intermittent stepdown so that
every time the "tenths" rotor rotates between 9 and 0, a gear link
advances the "units" rotor by one stator segment. Every time the
"units" rotor passes between 9 and 0 another gear link advances the
"tens" rotor by one segment, etc.
The stator contacts selected by the rotors at any given instant,
therefore correspond to the numerical dial reading at that instant. If at the
particular instant at which a reading is to be taken, the "tenths"
rotor happens to be between contacts, then the next point to be contacted will
be recorded.
For this reason and because of the
practical impossibility of adjusting the "tenths" rotor so that it
will break contact with the “9” point exactly simultaneously with the breaking
of the rotor contact in the units section, a "disconnect" section is
added. The rotor of this section is directly coupled mechanically to the
"tenths" rotor and the stator segment is so arranged that the rotor
and stator are in contact throughout approximately 330 deg of rotation. The
remaining 30 deg is a little greater than the angle throughout which the
"tenths" rotor is not in contact with either the "9" stator
contact or the "0". Electrically this "disconnect" section
is in series with the voltage supplied to the remaining rotor sections, so that
the switch is rendered electrically inoperative during the mechanical
throw-over period.
Each stator contact of the revolution
counter is connected to the starter anode of a cold-cathode gas discharge tube
(OA4G) as in Fig. l5, which shows the schematic of one column of the number
storage bank, The circuit is so arranged that when switch S is open, all voltages
are removed from the tubes, leaving them de-ionized.
When the reading on the counter is to be
recorded, switch S (which is actually a thyratron ionised by the pulse from the
zero beat detector) is closed, supplying 185 volts to the anodes of all the tubes.
Resistors R4 and R5 supply approximately 100 volts to the starter anode of the
first tube to be selected by the rotor of the counter switch.
Under these conditions, the tube selected
will ionize and reduce its anode-cathode potential to 70 volts. Since R1=R0,
57.5 volts will appear across R0 and will act as a positive bias on the
cathodes and starter anodes of the remaining tubes.
Further rotation of the counter switch
rotor will then supply an effective starter anode-to-cathode voltage of only
42.5 volts to any subsequent tubes selected, and this value is insufficient to ionize
any of them.
After S is closed, the first tube to be
contacted by the counter switch becomes ionized and will remain ionized until
switch S opened. (This "opening" will actually be a deionization of
the thyratron being employed for S, after storage of the number is no longer
required).
Thus the number storage bank constitutes an
electrical means of storing the dial reading of the frequency meter at any
calibration point for any desired length of time. This reading is transferred
from the storage bank to the keyboard of an adding machine by having the plate
current of each ionized OA4G tube operate a relay (coil represented by R1 in Fig.
15) which in turn operates a corresponding solenoid that presses the proper
number key of the adding machine. A view of the adding machine and solenoids
with cover off is shown in Fig. 16, and the entire number storage bank with the
front door open is shown in Fig. 17.
Sequence of Printing Operations
After the dial reading of the calibration
point is set up on the adding machine, it is required that the reading be
printed and that the difference between adjacent readings also be tabulated.
This necessitates the following sequence of operations:
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|
FIG. 16- Open view of adding machine, showing
solenoids for actuating keys
|
(a) Press the "non-print" button.
(b) Press the "+" button if the
frequency meter dial is revolving to increasing numbers.
(c) After the machine starts running,
release all pressure.
(d) When the machine comes to rest, press
the total button.
(e) After the machine starts running,
release the pressure. (During the ensuing cycle of the machine, the difference
between the present calibration point and the previou s one will be printed.)
(f) When the machine comes
to rest, again press the "number keys”, as still determined by the number
storage bank.
(g) Press the "—" button.
(h) When the machine starts running, open
switch S of Fig. 15 by deionizing the thyratron used as S, thereby deionizing
the cold-cathode tubes of the number storage bank so as to prepare it for a new
calibration point; also remove pressure from all keys. (During this cycle of
the adding machine, the dial reading of the calibration point will be printed.)
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FIG.17- Number storage bank, showing 45 of the OA4G
tubes,
the associated relays, and the monitoring lights and meter
at upper
left.
|
If the dial of the frequency meter is being
driven from high numbers to low (on one hand it is driven in one direction, and
in the opposite direction on the other), then operations (b) and (g) above are
reversed.
All of the above operations
are performed by means of conventional relay switching practice, with each
operation initiated by the closing or opening of a contact placed on the adding
machine in such a manner that the contact is closed while the machine is
cycling and the contact is opened when the machine comes to rest. The entire
sequence system is shown with cover open in Fig. 18.
Contact-Failure Detector.
A safety device is incorporated in the
sequencing system to insure against incorrect calibrations in case of failure
of any one of the contacts of the revolution counter. (Such failure might be
expected to be rather frequent since the fastest revolving rotor of this switch
makes over 100 contacts per second. In actual use 24 hours a day, the
particular design employed in the revolution counter gives a contact failure
about once every two months, or after about 500 million contacting.)
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FIG. 18- Sequence system, showing thyratron S at left
relays, and monitoring
lights for rapid indicating of correct sequence of
operations.
|
This safety device operates upon the time
interval between the instant that the pulse is received from the zero beat
detector and the instant that the one OA4G tube in each column of the number
storage bank becomes ionized. If this interval is less than approximately 0.01
second, then the calibration point is printed in the normal manner; but if,
through a contact failure of the revolution counter, the interval exceeds 0.01
second, then no calibration point is printed. This is arranged by a suitable
relay system consisting of one relay coil of a fast-operating relay inserted at
R0 of Fig. 15 for each column of the number storage bank, and a
slower-operating relay connected from the plates of the OA4G to ground.
If the slow-operating relay closes after the five fast relays have
operated (the difference in operating time between the two types of relays is
0.01 second) then the contacts of the revolution counter are apparently in
proper working order and the circuit is arranged to permit the normal printing
sequences previously described to be executed. If, however, the revolution
counter skips a contact point at the instant that a dial reading is to be
recorded, the OA4G which should have beep ionized does not become ionized and
hence the slow-acting relay will be operated before the fast-acting one.
Through suitable additional relay circuits, this causes the sequence system to
omit the calibration reading entirely and to leave two blank spaces on the
recording tape.
Use of Calibration Tape.
In this manner, then, the dial readings at
the calibration points are recorded. The actual frequency corresponding to any
given dial reading is determined from the known frequency at the start of the
calibration tape and the number of readings intervening. In practice, after the
calibration tape has been completed by the machine, it is placed upon a ruled
table which has marking lines corresponding to certain reference frequencies.
In this manner, key frequencies (such as those which start each page of the finished
calibration booklet) can be marked off.
The ruled calibration tape is then used to
set up manually the interpolating machines, which supply ten interpolated
calibration points for each one appearing on the tape and which simultaneously
print the pages of the completed calibration book.
Credit is acknowledged to Mr. D. B. Smith
and Mr. E. S. Brotzman who directed the development of the zero beat detector
and the recording system respectively, and to each of the following for their
invaluable contributions: W. Frear, F. Reed, W. Newbold, H. H. Harris: M. Ames,
H. Brough, M. Riccardi, E. 0. Thompson, I. Stephan, At Mattison, A. F. Nickel,
D. Karcher, C. Woll, W. Miller, A. Miller, R. Seher, W. Bareiss and F. Kotulka.
