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Патент USA US2408931

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Od. s, 1946.
» ' Filed June 14, 1945
2, 8,930
3 Sheets-Sheet l
Oct. 8, 1946.
Filed June_14, 1943
5 Sheets-Sheet 2».
v vâïAxî
Oct‘ 8’ 1946-
Filed June 14, 1943
3 Sheets-Sheet 3
Patented Oct. 8, 1946
Laurens Hammond, Chicago, and John M. Ha
nert, Wilmette, Ill.; said Hanert assigner to
Hammond Instrument Company, Chicago, Ill.,
a corporation of Delaware
Application June 14, 1943, Serial- No. 490,746
3 Claims.
(Cl. 172-239)
Our invention relates generally to frequency
nected to a suitable source of plate Voltage, indi
cated as a terminal +90 v., through load resistors
R28. The control grid 3i) of tube E6 has a series
grid resistor R32 in addition to the grid resistor
Our invention also includes a ground speed in
dicator for aircraft employing the improved fre
quency meter.
RI4. The screens 34 of the tubes It to I9 are
connected to a, suitable source of screen voltage
indicated as a terminal +45 v. The blocking con
The measurement of frequency is in general a
rather difiicult problem and usually requires a
great deal of expensive precision apparatus if
reasonable accuracy is to be attained. Further
more, frequency meters used at present which are
of the bridge type are not direct reading, while
other meters require that a standard frequency
densers CI2 couple the output plate circuits of
each of these tubes to the succeeding tube of the
cascaded series. The suppressor grids 35 of tubes
i6 to I9 are connected to the cathodes 22.
The tube 20, operating substantially as a tri
be supplied for comparison, usually by the beat
ode, forms a low impedance driver and has a
cathode 38 connected to ground through a self
The frequency meter of our invention may be 15 bias resistor R40, while its screen grid ¿l2 is con
utilized in the same manner as a voltmeter or am
nected to the plate 50, the latter being connected
meter and is »provided with an A. C. voltmeter
to a terminal 44, the potential of which is con
calibrated in frequency, and its indication consti
-trolled by potentiometer 46 connected between
tutes direct reading of the frequency of the signal
ground and a source of screen and plate voltage,
supplied to the apparatus.
20 indicated as a terminal +150 V. The adjustment
It is thus an object of our invention to provide
of the potentiometer 46 is made so as to have the
an improved frequency meter.
voltage upon the screen 42 and plate 5t at +135
A further object is to pro-vide an improved in
volts under the given conditions. Upon changes
dicator for the ground speed of aircraft, or for
in line voltage the slider of the potentiometer 46
indicating the lineal speed of any other object.
25 is adjusted to bring the screen voltage back to its
A further object is to provide an improved fre
proper value, and for convenience in making this
quency meter in which the range is readily ad
adjustment a voltmeter 48 is p'rei‘erably provided,
justable and in Which the full range of the indi
its terminals being connected ,between ground and
cations may be utilized to cover a very narrow
the slider o-f the potentiometer 46. Any other
frequency range near certain selected frequencies. 30 suitable means for controlling the voltage of the
Other objects will appear from the following
terminal 44, to compensate for changes in line
description, reference being had to the accom
voltage could, of course, be substituted for the
panying drawings in which:
manually adjusted potentiometer 49. Thesup
Figure 1 is a circuit diagram of the frequency
Figure 2 is a circuit diagram of a speed indi
pressor gridr54 of this tube 20 is internally con
35 nected to the cathode 38 thereof.
The plate 5B has connected thereto a plurality
of condensers C60, CGI, C92 and C83, the other
terminals of these condensers being c_ennected
to switch points of multi-contact switch Se. The
cater utilizing the novel form of frequency meter;
Figure 3 is a diagram of optical portion of the
speed indicator shown in Fig. 2.
40 switch arm $4 is connected to the plate 66 of a
Referring to Fig. 1 the frequency meter corn
prises a distorting and amplifying system having
input terminals I9, H which are resistance
coupled through a blocking condenser CIZ, across
a grid resistor RM, to the input of an electron 45
discharge device Hi forming the first of a cas
diode 68 and to the cathode 79 of a diode 12. Thev
plate 14 of the diode l2 is connected to a suitable
constant potential source indicated as -3 v.
cathode 16 of the diode E8 is connected through
a direct current milli-ammeter ‘I8 to ground.
The plate 50 is also connected by a conductor
caded series of distorting and amplifying tubes.
80, blocking condenser C32 and a decoupling re
In addition to the tube Iii the successive stages
sistor R84 to a multi-contact switch arm 86 and
of this distorting and amplifying system Íinclude
alsoto the control grid 38 of a pentode 99.
tubes IT, I8, and i9 and a power driver tube 20. 50
The lpentode 90, which may be oi the GKSGT
The tubes IG to I9 may be pentodes of the 6SJ'7
type, comprises a cathode 92 connected to a junc
type while the tube 20 may be of the 6K6GT
tion` S4 between voltage dividing resistors R96 and
type. The cathodes 22 of the tubes i6 to I9 are
R91. These voltage dividing resistors` are pref
respectively »connected to ground through seli
erably Wire Wound resistors of low value. The
bias resistors RM while their plates 23 are con 55 resistors R96 and R91 are connected in series be
the relatively sharp cutoff point oi the tube IÓ
tween ground and the terminal 44, which, as pre
viously noted, is adjusted to be maintained at
-will cause a corresponding flattening of the nega
tive portion of the signal wave. By selecting a
+135 volts. The screen grid 98 of the pentode 90
is likewise connected to the terminal «ill while the
4000 ohm self-bias resistor and 100,000 ohm plate
resistor R23, the distorted input Wave becomes
symmetrical about the horizontal axis.
The second stage of amplification comprising
suppressor grid |00 is internally connected to the
cathode 92. The plate |02 of the pentode 90 is
connected to the terminal M through a load re
the tube I?, operates in a similar' manner, but
sistor RIM and is also connected by a conductor
since the input signal to this tube is presumably
|06 to terminals of condensers C||l8, C409 and
CHD, the other terminals of these condensers 10 of greater amplitude, this tube will introduce
appreciable distortion in the signal, and its out
being respectively connected to switch points en
waves will be generally of the shape indi
gageable by the switch arm B4.
cated by the wave |22. Similarly tubes ||l and
A plurality of tuned meshes H2, H3 and H4
I9 forming parts of the third and fourth stages
are adapted to be connected selectively across
of amplification, will further amplify and dis
the input of the pentode 90 by operation of
tort the signal waves to more pronounced rec
the switch 86. Each of these meshes H2 to H4
tangular shapes, such as illustrated by the waves
comprises a condenser and inductance connected
|23 and |24.
in parallel to provide impedances varying with
The final stage of amplification, which includes
frequency. For example, the mesh H2 may be
tuned to resonate at 40 C, P. S., the mesh H3 20 the driver tube 2t, will further distort the wave
to a substantially perfect rectangular shape as
at 75 C. P. S., and the mesh H4 at 1250 C. P. S.
indicated by the wave |25.
It will appear hereinafter that these frequency
In the foregoing description oi the operation
responsive meshes are utilized when measuring
frequencies close to 32, 60 and 1000 C. P. S. re
oi the amplifier it was assumed that a sine wave
In utilizing the instrument the frequency to
be measured is impressed across the input ter
minals l0, ||. The input signal, with the degree
of amplification and distortion as shown, should
Because of the fact
that the tube 20 has its output limited in ampli
tude, and because the gain of the amplifier is
very high, a signal of very small amplitude sup
plied to the input of the tube IB will produce a
voltage above this value.
input wave 0f higher amplitude will likewise pro
25 signal was being amplified.
exceed 5 millivolts. Due to the non-linear prop 30 wave, such as the wave |25, of maximum ampli
tude in the output of tube 20. Furthermore, any
erties of the system, the signal may be of any
Presumably the fre
quency to be measured is a sine wave, or a wave
closely approaching a sine wave and such input
wave is indicated at |20.
The input wave may .
be of saw-tooth, rectangular, triangular or of
any other generally symmetrical shape.
The sharp cutoff tubes I6 to I9 of the GSJ'I
duce a wave substantially identical with the wave
|25. Thus the output of the amplifier is substan
tially the same, particularly as to the lower order
harmonics of the output signal and the root
mean-square value, irrespective of the amplitude
of the input signal. As well known to those skilled
in the art, the harmonic series of a rectangular
operate in a class A manner with a gain of 40 40 wave shape such as the wave |25 may be repre
sented by a Fourier series of a fundamental and
only when their grids are supplied with very small
type, supplied with the potentials indicated,
signals. Thus it is apparent that for very minute
signals upon the control grid of the tube |ß,
the cascaded series of tubes I6 to 20 constitute
a very high gain amplification system. When
the input signal is increased this cascaded sys
tem operates non-linearly and limits the ampli
tude of the signal as delivered by the plate of
odd numbered harmonics. The third harmonic
has an amplitude one-third of the amplitude of
the fundamental; the fifth harmonic has an am
plitude one-fifth of the fundamental; the seventh
harmonic has an amplitude one-seventh ol the
fundamental; etc.
For a true rectangular wave
the harmonic series is thus of infinite extent,
which, of course, is not achieved in practice.
the tube 20 to a constant value.
This limiting action is made clear by reference 50 However, the values of the fundamental and low
er order odd harmonics and of the root-mean
to the representative output waves |20 to |25 of
square value are substantially independent of the
the different tubes, shown directly above the
exact degree of steepness of the sides of the gen
tubes. The wave |'2| is substantially a sine Wave
erally rectangular wave.
since it is assumed that the input signal is rela
tively small and the tube I6 is not driven far 55 As will be apparent by comparison of the out
put Waves of the successive stages of amplifica
beyond the point at which it operates non
tion, the wave becomes of more nearly true rec
linearly. However, in the event that an exceed
tangular shape as the signal is transmitted
ingly high input signal is present, series resis
through the successive stages. These changes in
tor R32, which is of high value, such as 2 meg
wave shape, after the wave has attained a gen
ohms, serves to limit the extent to which the sig
erally rectangular shape, e. g. wave |23, result
nal may swing the control grid 30 in a positive
As soon as the grid tends to go positive
with respect to the cathode, a voltage division
occurs between the resistor R32 and the cathode
mainly in an increase in the amplitudes of the
higher order harmonics of the Fourier series but
do not appreciably affect the amplitude of the
to-grid input impedance of the tube I6, which
lower order harmonics nor do they appreciably
will be much less than 2 megohms. Thus, further
affect the root-means-square value. Thus there
positive increases in the signal voltage have negli
will be no appreciable changes in their ampli
gible effect in driving the control grid 30 in a
tude in the output of the tube 20 provided the in
more positive direction.
put signal is above the predetermined minimum
The resistor R24 may be of 4000 ohms. Under 70 value of .005 volt. It is thus apparent that the
these conditions the average plate current will
signal as delivered by the last of these tubes is
remain constant regardless of the input signal
strikingly independent of the amplitude and wave
amplitude. When the signal drives the grid
shape of the input signal, and large changes
positive, as explained above, to produce a flat
have no appreciable effect upon the am
tening of the positive portions of the signal Wave,
plitude and root-means-square Value of the out
put signal.
the meter may be obtained by mov
. switch arm B5 to a position to connect mesh
Assuming that the approximate frequency of
the input signal is not known, the operator will
I I2 in the input circuit of the pentode‘gû and'atl
the same time moving switch arm 64 to bring the
appropriate condenser CIDB in the lmeter circuit.
As described above, the mesh I I‘2 is designed to
be resonant at 40 C. P. S. The condenser CI08 is
of such value that when it is connected in the Cir
cuit, and an input frequency of 35 C. P. S. ap
plied, the meter 'i8 reads its full scale value of one
move the switch arm $4 to the position such that
the condenser Cris is in the circuit. The output
wave #25 will thus be transmitted to the diodes
68 and 'if which operate as a half-wave rectifier
to transmit the energy of the positive portion of
the wave to the milli-amm’eter 1B. The ammeter
is preferably of good sensitivity such as provided
milliampere. For frequencies less than 35 C. P. S.
the meter reading will drop Very rapidly due to
by a Zero to one milli-ampere range. The value of
the condenser C63 is such that when the switch
the effect of the _threshold bias which is developed
Si connects this condenser in series in .the circuit
in the voltage divider resistor Râö. By increas
and the input signal has afrequency of 10,000
ing the amount of this threshold bias on the con
C. P. S. the milli-ammeter‘lß will indicate its full
trol grid of the tube 9|), the sensitivity of the fre
scale one milli-ampere reading. Assuming that
quency meter increases very rapidly. Inasmuch
the input frequency is 8 C. P. S., the deflection of
as the frequency meter in this condition is effec
the needle of the ammeter 18 will be extremely
tive to measure the fundamental component of
small under the given conditions. The operator 20 the distorted signal wave |25, changes in input
will therefore shift the switcharm E4 to bring
amplitude at the terminals I0, I I are substantial
the condenser CS2 in the circuit, this condenser
ly ineffective to change the amplitude of the
‘being of such value that an input of frequency of
fundamental of the complex wave |25. However,
1000 cycles will produce full scale deflection of the
small changes in signal frequency result in cor
needle of the ammeter lâ. .At the assumed input
responding changes in amplitude of signal oc
frequency of 8 C. P. S., the ammeter ‘I8 will still
curring across the mesh l I2 which is connected to
fail to provide an appreciable deflection and the
the control grid 88. This relatively small ampli
operator will therefore shift the switch arm 64 to
tude «change causes a very large >change in the
bring the condenser C6 I `in the circuit. This con
average plate current because of the presence of
denser is of such vaiuethat an input frequency of 30 the threshold bias as provided by the voltage drop
lili) C. P. S. will cause full scale deflection of the
occurring in the resistor R96. -In using the ire
meter 1S. Under these circumstances the 8 cycle
quency meteiyadvantage is taken of the steeply
input frequency will produce a deflection of th'e
rising portion of the impedance characteristic of
indicator of the ammeter which will be readable.
The operator will note thatthe reading is below
the mesh |I2 as the input frequency approaches
resonance. For instance, if the frequency range
is to lie about 32 C. P. S., the resonant frequency
of the mesh I I2 is set at a point such as 40 C. P. S.
under which conditions measurements may be
made of frequencies between 28 and 35 C. P. S.
the “0.1” mark of the ammeter scale and will
therefore make the final shift of the switch arm
|54 to the Contact which brings the condenser Cäil
in the circuit. The condenser C60 is of such value
that an input frequency of l0 C. P. S. will produce 40 Because of the threshold bias on the rectifier tube
full scale deflection of the meter 78.' .V‘Jhen the
frequencies below a predetermined minimum
apparatus has thus been brought to final ad
such as 28 C'. P. S. are ineffective to produce ap
justment the operator will read directly from the
meter the value of approximately 0.8 which will,
preciable plate current pulsations. Thus, by us
ing the correct amount of threshold bias, vthe fre
of course, be interpreted as a frequency of 8 45 quency range »of the meter may be compressed
C. P. S. Of course, the meter 'i8 will in practice
to any extent desired. For measuring commer
be frequency calibrated So as to be direct-reading.
cial power frequencies, such as 60 C. P. S., the
If the operator knows the approximate value of
switch arm Sli is set to connect with condenser
the frequency being measured, it will not be nec
CIDS and the switch arm 86 simultaneously set
essary for him to successively shift the switch 50 to connect mesh IIS (tuned to approximately 'I5
arin 64 as described but he may set it `immediately
C. P. S.) in the input circuit of tube 90. For
to the range within which the frequency lies.
measurement of frequencies in the 1000 C. P. Si.
range, the switch arm 64 may be connected to
When the switch Sil is in its most sensitive po
sition for a given input frequencyk (the setting
condenser HG and the Switch arm 86 set to con
with the condenser C60 in circuit in the above ex- '
ample) the current drawn through this con
denser, tl‘ie diodes »£8 and 12, and meter '18 con
stitutes a load upon the output Yof the driver tube
20 and this will result in rsome distortion of the
output wave |25. The diode‘lZ effects the dis
charge of the condenser CBS as well as providing
a low impedance shunt path to -3 V. for the
negative portion of the output wave.
When the input frequency is known to be very
5 neet mesh `iM (tuned to approximately 1250
C. P. S.) in the input circuit of tube 90.
The apparatus shown in Fig. l may be utilized
as a part of any mechanism inwhich anelement
is to be controlled in response to changing fre
0 quency. One such use of the apparatus is illus
trated in Figs. 2 and 3. In these figures the .ap
paratus forms part of a linearl speedindicator,
as for example indicating `the ground speed of
inps-wer line frequencies, theappropriate con
close to a certain frequency, such as isfthe case 65
In this use of the invention the pair of photo
tubes |30, |3| are mounted in a suitable case
denser Cliit, Clilil or Cliâì is connected inthe
circuit by the switch arm 64 and correspondingly
|32 having a dividing partition |33 therein. A
pair of light gratingkplates |34, |35 of glass' or
the .switch arm 86 is shifted to> connect the ap
’ prcpriate mesh H2, 'H3 or |I4 across the input '
circuit of the pentode s0. For this purpose the
switch arms 85 and til may be mechanically inter
other suitable transparent material are mount
70 ed in spaced relation in the case |32, the plates
being accurately positioned and clamped by any
suitable means so as to lie in parallel planes.
The grating plates |34, |35 have alternate
.opaqueand transparent portions _extending ’in
Assuming that the input frequency is known to
be approximately 32 C. P. S., greatly increased 75 parallel lines transversely thereof. `A ‘ simple
method of making these gratings is to coat sheets
of plate glass with an aqueous solution of col
loidal graphite. After this coating is dried and
radiation from adjacent portions of the tei-rain
vary considerably even though to the eye they
machine for accurately spacing the lines and as
continuity in the intensity of radiation scanned
by the phototubes, to produce a significant sig
nal therein. The illumination of the phototubes
may appear to be relatively uniform. Thus any
slight irregularities such as a bush, a tree, a
set, portions thereof are removed therefrom as by
scraping it, using a milling or other precision Cà fence or a whitecap may produce suiiicient dis
suring their parallelism.
For example the col
loidal graphite lines may be in the order of .020”
|30, |3| as they scan a source of illumination,
in width and may extend the full width of the
glass plates. In a particular embodiment of the 10 produces electrical signal waves which are gen
erally triangular shape, corresponding to the lin
invention the ruled portions of the plates |34, |35
ear increase and decrease in illumination as rays
were approximately 3.5" by 7" and the spacing
from the light source which have passed through
between the plates was 2". In Fig, 3 the opaque
the grating of the plate |35 traverse the grating
lines of colloidal graphite upon the plates |34 and
of plate |34. The signals produced by the two
|35 are indicated by the heavy dash lines |36.
phototubes will be 180 degrees out of phase be
The casing |32 is mounted on the airplane so
cause of the fact that the gratings |34 and |35
as to have a clear view downwardly therefrom,
in front of the phototube |3| are staggered, while
and as a result, rays of light from the ground
the gratings in front of the phototube |30 are in
will pass through the transparent portions of the
plates |34, |35. However, it will be noted that
in the lower half of the grating |35 the opaque
lines |36 are staggered with respect to the opaque
lines of the grating |34 in a manner such that
rays striking the grating |35 perpendicularly and
passing therethrough will be stopped by the
opaque lines of the grating |34.
On the other
hand, rays striking the upper portion of the grat
ing |35 perpendicularly and passing through the
transparent portions thereof will also strike and
As best shown in Fig. 2 the phototubes |30,
|3| are connected in series, the cathode of the
phototube |30 being connected to a suitable source
of negative potential, as a terminal w45 v., while
the anode thereof is connected to the cathode of
phototube |3|. The anode of the latter photo
tube is connected to suitable positive potential
source, indicated as a terminal +45 v. The anode
of phototube |30 and cathode of phototube |3|
pass through the transparent lines of grating |34. ., are connected through a blocking condenser C|45
and radio frequency filter resistor RMB to the
This is indicated in Fig. 3 by the fact that per
grid |50 of an ampliñer tube |52 which
pendicular rays |40 are stopped by the lower
of the 6J’1 type. A grid resistor R|54 of rela
portion of the grating |34 and thus do not have
tively high Value is connected in series with the
an opportunity to affect the phototube I3|, while
such rays striking the upper portion of the grat 35 resistor R|40 in the input circuit of the tube
|52. The cathode |56 of tube |52 is connected
ing |35 pass through the transparent lines there
to ground through a self-bias resistor R453. The
of and also pass through the transparent lines of
screen grid |59 and suppressor grid |50 are con
grating |34 and may thus fall upon the sensitive
nected to the plate |5| so as to cause the tube
surface of the phototube |30.
On the other hand rays of light |4| striking 40 to operate as a triode in a linear manner. A
plate load resistor RI 02 is connecte-:l between the
the lower portion of the grating |35 at certain
plate IBI and a suitable plate voltage source
angles pass through the transparent portions
indicated as a terminal +90 v.
thereof and also strike and pass through the
Because tube |52 is utilized in the circuit as a
transparent lines of grating |34 and thus may
triode it offers the advantage of low plate im~
reach the sensitive surface of the phototube |3|.
pedance, and because of its high impedance in
Rays |4| passing through the upper portion of
put circuit this tube is preferably located with~
grating |35 are stopped by the opaque lines of
in or immediately adjacent the casing |32 and is
the grating |34.
supported to avoid the introduction of
Thus, as the apparatus moves in the direction
represented by the arrow |44, or in the opposite 50 microphonic disturbances.
The output of the tube |52 is transmitted
direction, a stationary source of light would al-- `
ternately energize the phototubes |30 and |3|.
through a shielded conductor |64 to a blocking
condenser CIBG. The grid |58 of an amplifier
With the gratings of the dimensions indicated
and distorting pentode |10 is connected to the
above, a complete cycle of energization of the
phototube |30, |3| would take place as the light 55 condenser CIGS through a series grid resistor
R32 and is connected to ground through the grid
source and the gratings shift relatively through
resistor R|4. The pentode |10 corresponds in
an angle in the order of 1°10'. Consequently the
function and in its associated circuit. elements
relative speed of a light source and the photo
with the tube I1 of Fig. 1 and is coupled to a tube
tubes may be determined by measuring the fre
quency generated by the phototubes, provided the 60 |12 which corresponds to the tube I8 of Fig. l.
The coupling is through a sensitivity control po
distance from the source is known.
tentiometer R|14.
In utilizing the apparatus as a ground speed
Plate |16 of the tube |12 has voltage dividing
indicator for aircraft the altitude of the plane
load resistors R|18 and R|80 connected between
may be quite accurately determined by conven
tional instruments and thus the frequency at 65 the plate and +90 v. terminal. The junction of
the resistors R|1B and R|80 is connected through
which the phototubes |30, |3| are alternately
a blocking condenser C|82 and a, series resistor
energized will constitute a measure of the ground
R|84 to the grid |86 of a pentode |88 which may
speed of the plane. There will, in nearly all
be of the 6SJ'1 sharp cutoff type. The pentode
cases, be sufficient irregularities or discontinui~ties in the level of illumination of different por 70 |88 is provided with a grid resistor RM and a
self-bias resistor R24. The other electrodes of
tions Of the ground to differentially energize the
the pentodes |88 are connected t0 suitable fixed
phototubes |30 and |3|. Of course, if all portions
potential sources, and the output signal, which
of the terrain reñected light uniformly, no signal
is a wave rectangular shape corresponding to the
would be generated in the phototubes, but ex
perience has shown that the intensity of light 75 wave |25 of Fig. 1, is transmitted through a
blocking condenser C|90 tothe input circuit of a
pentode |92, through a current limiting grid re
sistor Ri94. A grid resistor R|96 in series with
the resistor R|94 is connected to a suitable bias
ing potential source indicated as a terminal
_22.5 v, which constitutes a large negative grid
bias for this type of tube, so that this tube oper
ates in the manner of the grid controlled recti~
~her having a high input threshold.
rlühe plate §93 of the pentode |82 is connected
through a plate load resistor R293 to a suitable
plate voltage source, indicated as +45 v., to
which the screen 28| of this tube is also con»
nected. A ,ny-pass condenser C232 is connected
in shunt with the load resistor R228. The plate
|98 is also directly connected with the grid 294
of a pentode 226. The cathode 288 of the tube
286, as well as its suppressor grid 299, is con
nected to a +45 v. terminal. The plate Zili ci
the pentode 286, which is a power tube and may
be of the GKSGT type, is connected through the
winding or a relay 2l2 to a plate voltage source
indicated as a terminal +90 v. The screen grid
2|3 of this tube 226 is also connected to the lat
ter terminal.
It will be seen that when the pentode |92 is not
conducting current the voltage on the grid 284 of
the pentode 2GB will be substantially +45 v1 and
tor 234, which is provided with a suitable speed
governor, and is shunted by an anti-spark re
sistor 236. The relay 2|8 has single-pole double
throw switches 238 and 239 which when in the
upper position shown connect a conductor 248
(normally connected to +12 v. through the switch
2|6) to the one terminal of the motor 234, and
connect the other terminal of the motor through
the switch 239 and conductor 24| to one terminal
of a limit switch 242, the other terminal of which
is grounded.
Similarly when the relay 2|8 is energized, an
energizing circuit for the motor 234 is established
through a circuit including the conductor 248
(at +12 volts), the switch 239, motor 234, switch
238, conductor 243 and limit switch 244 to ground.
The limit switches 242 and 244 are operatively
connected to the armature of motor 234 and open
its above described energizing circuits when the
l motor is driven in one direction or the other be
yond predetermined limits.
The motor 234 is connected through a suitable
speed reduction gearing to the adjustable element
of a variable condenser C246, the latter being
connected between ground and the current lim
iting resistor RI 94 associated with the input grid
of tube |92. The condenser C246, in conjunction
with the current limiting grid resistor Rl94,
hence the latter tube will be conducting and
forms a frequency responsive element for control
- maintain the relay 2l2 energized. When, how~ 30 ling the amplitude of the signal impressed upon
ever, the pentode 292 is conducting the signal
the input circuit of the pentode |92.
impulses the voltage drop across the load resistor
To explain the function of the motor operated
R220, due to the resultant increase in plate cur
variable condenser C246, it will be assumed that
rent flow through the pentode |92, will be suiii
the input frequency initially generated in the
cient t-o cause the grid potential bias on the tube
phototubes |30, |3| decreases in Value. Such de
236 to drop to substantially a cutoff value and
creased frequency signal, after passing through
thus cause the deenergization of the relay 2l2.
the amplifying and distorting system comprising
The relay 2 | 2 upon energization closes a switch
the tube |52, |18, |12 and |88, will cause an in
2|4 in a circuit which includes a single-pole dou
crease in the amplitude of the signal across the
ble throw relay switch 2 I6 and a suitable operat~
input of the tube |92. This increase in the input
ing current source indicated as a terminal +12 v.,
signal amplitude to the tube |92 will usually ex
and also the winding of a relay 2i8, which is
ceed the threshold determined by the _22.5 v.
shunted by an anti-spark resistor R222.
bias, and the pentode |92 will thus be rendered
The switch 2|9 is operated by a cut-«out relay
conducting, and as a result, increasenegatively
222. The relay 222 operates, as will hereinafter 45 the bias upon the grid of the power tube 206, ef
appear, to prevent effective utilization or the ac~
iectively blocking this tube. As a result the re
tua-tion of the relay 2l2 whenever the amplitude
lay 2l2 is deenergized and the relay 2|8 likewise
of the signal output of the pentode |`|2 falls be
deenergized. Deenergization of the relay 2|8,
low a predetermined minimum value. This mini
through the positioning of the switches 238 and
mum value is determined by the minimum ampli 50 V239, reverses the polarity of the motor 234 and
tude signal which will cause the formation oi" a
true rectangular wave shape in the output of the
Unless such wave is of substantiaily
causes it to drive the condenser C246 in a direc
tion to cause its capacitance to increase. As a
result of the increased capacitance of the con
denser C246, there is a corresponding decrease in
true rectangular shape, the signal for actuating
the relay 2 l2 will not be thoroughly reliable and 55 the amplitude of the signal at the input of tube
it is therefore desirable to prevent the effective
|92. The resulting decrease in current ñow
operation of this relay under such circumstances.
through this tube |92 causes an increase in the
The voltage in the output signal of the pentode
potential of the grid 294 and the tube 285 is
|`|2 is transmitted through a blocking condenser
rendered conducting and energizes relay 2l2.
C224 to the grid 226 of pentode 228, A biasing 60 1Ptelay 2l2 then energizes relay 2l2», and the lat
threshold determining resisto-r R223 connects the
ter, by moving switches 238, 239 to their dotted
grid 226 to a suitable biasing potential source in
line positions, causes the motor 234 to rotate in
dicated as a terminal _22.5 v. The output oi the
a reverse direction, i. e., in a direction to decrease
pentode 228 is coupled to the input of a power
the capacitance of condenser C246. Whenever
pentode 238 which operates, in a manner similar 65 the amplifier is receiving a significant signal the
to the tube 228, to conduct whenever the pentode
motor 234 is operating either in one direction or
228 is not conducting an appreciable signal and
the other. This is of advantage in that the con
to be cut ofi when the pentode 228 is drawing
denser C246 is at all times increasing or decreas
substantial plate current.
ing its capacitance, such increase and decrease
The lower contact of the switch 2|6 is connect 70 being eiïective to tune the input circuit for the
ed through an indicator lamp 232 to a +6 v. ter
pentode |92 to a frequency alternately slightly
minal, and a visual indication of the operation of ‘ above and slightly below that of the input signal.
relay 222 thus provided.
The mean of these two frequencies will be that
The relay 2 I8 is adapted to control the opera
of the input signal with a high degree of ac
tion of a reversible permanent magnet field mo 75 curacy. Since the motor is operating at all times,
there is no lag in the response of the instrument.
This is because the circuit elements, particu
larly the condenser C246 and series grid resistor
Rl94, may be so designed that the two frequen
cies, between which the frequency response is
varied, may be relatively close to one another
with the result that the motor 234 will reverse its
direction of rotation at very short intervals, in
the order of a second or two.
The drive from
While we have disclosed particular embodi
ments of the invention, it will be apparent to
those skilled in the art that numerous varia
tions and modifications of the invention may be
made without departing from the underlying
principles thereof. We therefore desire, by the
accompanying claims, to include within the scope
of our invention all such variations and modifica
tions by which substantially the results of our
the motor to the movable part of the condenser 10 invention may be obtained through the use of
substantially the same or equivalent means,
C246 will ordinarily be such that the movable
We claim:
part of the condenser will oscillate through such
1. In a frequency responsive system, the com
a small arc that the oscillations will hardly be
bination of an amplifier operable to amplify and
noticeable. Thus it will be seen, the position of
the movable part of the variable condenser C246 15 distort into a substantially rectangular wave iorm
of constant amplitude an input signal of sym
will constitute an indication oi' the frequency
metrical wave shape and exceeding a predeter
generated by the phototubes |30, |3'|.
mined minimum amplitude, a frequency respon
Any suitably calibrated indicating or recording
sive apparatus coupled to receive the output oi
device may be driven by the motor 234 to pro
vide a direct reading indication or record of the 20 said amplifier, a reversible electric motor, con
trol means operated by said frequency respon
frequency generated by the phototubes |30, |3|,
sive apparatus to cause rotation of said motor in
or of the factor which combined with the altitude
one direction when the signal frequency from
will show the ground speed. Such indicator is
said ampliiier exceeds a set value and to rotate in
diagrammatically shown as comprising a pointer
243 cooperating with a frequency graduated scale. 25 the opposite direction when the signal frequency
is below said Set frequency, said control means
Whenever the signal generated by the photooperating to maintain the motor in operation in
tubes is of such low amplitude as to lack signi
one direction or the other at all times when the
?lcance, the tube 228 becomes biased substan
input signal to the amplifier is of signiñcant am- .
tially to or beyond cutoiï and the pentode 230
is thereby rendered conducting and energizes re 30 plitude, and means operated by said motor to
lay 222. Energization of relay 222 completes
change the frequency response characteristics of
the circuit to the warning signal lamp 232, and
said apparatus in a direction to cause the set fre
opens the circuit by which power is supplied to
quency of said apparatus to change in a manner
the motor 234, The motor 234 can therefore no
to cause it to become the saine as the signal
longer be controlled by the relay 2 | B and the con 35 frequency.
denser 2|4, and any frequency indicating means
2. The combination set forth in claim l in
operated by the motor 234 will remain stationary
which means are provided to render said fre
until the phototubes |30, |3| again supply a sigquency responsive apparatus ineffective to con
nal of significant amplitude.
trol said motor whenever the amplitude or the
When the airplane on which the apparatus is 40 signals supplied thereto decreases below a prede
mounted is iiying over an ordinary terrain, the
termined minimum value.
apparatus will provide a continuous indication
3. The combination set forth in claim l in
of the angular speed at which the ground is pass
which said motor is reversible, and in which said
ing directly beneath the plane. To obtain in
apparatus includes a relay controlling the direc
creased accuracy, the casing |32 containing the 45 tion of rotation of said motor, said relay being et'
phototubes |30, |3| may be mounted upon a gyro
fective to condition said motor for operation in
stabilized support, so that the phototubes always
one direction or the other at all times without
receive light from an area directly beneath the
any appreciable time of rest upon reversals.
airplane and so that minor irregularities in the
flight path or attitude of the airplane will not
appreciably aiîect the operation of the apparatus.
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