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Патент USA US2124212

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July 19, 1938.
Filed Dec. 12; 1936
2 Sheets-Sheet 1
July 19, 1938.
Filed 11%. 12, 1936
2 Sheets-Sheét 2
$2203 ‘
‘ y'
. N \
Patented July 19, 1938
‘ _ ' UNITEof'fsTnTris"
PATENT gOFF/lfCzE : ~ .
‘ 2,124,212
‘ Noel Meyer RustQChelmsford, England, assignor
; to Radio Corporation of America,.a corpora
tion of Delaware
Application December, 12, 1936, Serial No. 115,475
‘In Great “Britain December 10, 1935
6 Claims. 1‘ (Cl. 250-20)
an output circuit is employed to diminish input
’ This invention relates to radio and like re
ceivers and more particularly to thermionic
Preferably in‘ carrying out this. invention the‘
valve circuit arrangements forcoupling a radio
receiving aerial to the remainder of the receiver.
reaction is obtained by means of a semiéaperiodic
network in the cathode circuit of the'
, The usual present day practice in broadcast impedance
coupling valve having the semi-aperiodic input
radio receivers is to connector couple the re-'
ceiving aerial to a tunable circuit which'feeds
into the ?rst valve of the receiver, this tunable
circuit being as ayrule gang controlled with, the‘
other tunable circuits. of the receiver.
A difficulty met" with in such arrangements is‘
that the aerial causes loss‘ of sharpness of tuning
in the tunable circuit ‘to'which. it is coupled or
connectedwhile furthermore, it is always neces
of signals to be received (for example an aerial)
is remote from the receiver proper and is con
nected thereto through a high frequency cable,
the coupling valve, instead of‘ having an input
circuit constituted by an ordinary semi-aperiodic‘ ‘
circuit, may be fed through 'a tapered line or a
the said tunable circuit was to avoid ‘disturbing
tion, Fig. 1 is a portion of an ampli?er circuit
avoid unduly damping the ?rst tunable circuit.
network equivalent thereto. w
the “ganging” of the tuning controls, and also to
sary to resort to a compromise in the adjustment
Mi of the‘ coupling oonditionibetween the‘a'erial and
In some cases, more especially where the source
In the drawings accompanying the speci?caa
serving to explain the invention,
Fig.2 shows a cathode circuit embodying a
part of the invention,
Moreover, _ the adjustment which‘ should ‘be
Fig. 3 shows a receiving circuit according to
adopted for best results when one receiving aerial
the invention utilizing a coupling tubebetween
is coup-led or connected to the receiver will not ‘ the
aerial and the receiver, proper, impedance
be the same as that for best results if a different
such as shown in Fig. 2 being connected
aerial is connected'or coupled to the receiver,
to the cathode and to the control grid,
and in practice, therefore, it is ‘necessary to pro
Figs. 4-7 are frequency characteristic curves
vide an aerial ‘ftrimmer” condenser to allow the
receiving set to be adjusted to ‘suit the various obtained with a superheterodyne circuit of which
di?erent aerials which may be encountered with
in practice. Another important disadvantage
Fig. 3 is a part,
met with in the usual known arrangement is that
aerial to the coupling tube, .
crossimodulation effects .tend to appear-particu
larly when it is. sought to vreceive a relatively
weak‘station operating 'onga WaVelength'adja-L
cent that of a more powerful station.
The object of. the. presentiinvention ‘is to ‘pro
vide improved arrangements whereinv‘the' above
defects and disadvantages are/‘substantially re
ducedor eliminated.‘ .More speci?cally the in
vention seeks to ‘provide, arrangements wherein
Fig. 8 is an alternative circuit for coupling the
onic valve having ‘a semi'faperiodic input circuit
and this valve is arranged to be subjected to
negative reaction applied insemi-aperiodic man
ner. Thevterm “semi-aperiodictis employed in
this ‘speci?cation to mean . the obtaining of
.55 aperiodic '_or approximately aperiodic action only
' over a predetermined frequency range-1-normally
the tuning range. The term “negative reaction”
is employed in .this speci?cation to mean'reaction
6.0 appliedin such manner that energy derived from
. '
of that shown, in Fig‘. 9 and ‘made up of high
12 and 13 the response . curves
I ob
tained with the circuit of Fig. 11,
I "
Fig. 15 is an input network combining elements
circuit of Fig. 15.
' be suited to different receiving ‘aerials.
According to 'the main?feature, of this inve'né
~tion the ?rst sharplyftun'ed circuit of a radio
or like receiver is ‘preceded by a coupling thermi
Fig. 11 is a network that may be used in place
an aerial is avoided; wherein the aerial does not
not necessary to provide any “trimmer”v con
denser, ‘or thelike, for, enabling thereceiver vto
Fig. 10 shows the response curve obtained with
the circuit of Fig. 9..
from both Figs. 9 and 11, and
adverselya?ect the tuning of . the ?rst sharply
tuned circuit‘of the receiver; and wherein it is
Fig. 9 is a network equivalent to that of Fig. 8,
cross modulation ,e?ects' are largely reduced'or
141) eliminated; wherein disturbance of “ganging’fby
h .
Fig. 14 is the response curve obtained with the
In order that the invention may beityhe better" I
understood, consider the relations which ‘exist
in a valve in which negative reaction‘ is obtained 45
by means of an impedance in the cathode cir
cuit thereof. Referring to Fig. 1 suppose a sig
nal voltage e be applied between the control
grid l of a valve 2 (e. g., as shown a pentode)
and a point 3 which is connected to the cathode
4 through a cathode impedance 5 of ,value Zc
the said point 3 being also connected to the
anode 6 through an anode impedance 1 of value
Za from which output voltage (E) is taken. (For
the purpose of simplicity anode and grid poten
tial sources are ignored). Suppose the valve
be of high internal impedance 1. e‘. of high im
pedance relative to the impedance Za. Let 'u
be‘ the voltage magni?cation constant of the
valve; 11 the internal impedance of the valve;
y the mutual conductance of the valve; eg the
voltage applied between the control grid l and the
cathode end of the impedance 5; 3c the voltage
across the cathode impedance 5; ia the anode
current, and 6a the instantaneous anode voltages,
Then, of course,
u _ de a
V (is, being constant) and
(the mutual conductance):
applied to high frequency circuits, a limitation
has been found to be set by the'elfective cathode
earth capacity which acts as a by-pass.
limit the resistance value which can be employed
in the feedback impedance to about 800 ohms. 10
Consider the effect of this limitation in the case
of a 'valve for which‘g=1 milliampere per volt.
Then taking Zc as 800 ohms and inserting these
' values in the formula
it will be seen that the counter voltage will be
only .8 of the actual voltage.
E_ éZu
Since the valve is of high impedance it may be
assumed that
indirectly heated cathode valves are employed
this capacity is mostly made up of the capacity
between the cathode itself and the heater, and
in practice, in known high frequency circuits as
just described, this capacity has been such as to
To obtain any im
provement in diminution of cross modulation
The following relations then exist:
. effects it is necessary to increase the value of Zc,
and in carrying out this invention as applied to
a known circuit as shown in Fig. 1 the imped
ance 5 which in the said known circuit is a sim
ple resistance is constituted by a semi-aperiodic
the accompanying Fig. 2, i. e. it comprises an
inductance 5L,.a resistance 5B and a capacity 5C
all in shunt with one another, the inductance
and capacity resonating together at a frequency 30
This system is preferably as shown in
at about the middle of the desired tuning range '
and the resistance being of such value as to pro
5 remote from the cathode are reduced by the
counter volts applied across the said impedance
5 so that the voltage effectively acting on the
grid I due to the signal may actually be very
much less than the input voltage e. It is roughly
true that the cross modulating effect of a strong
vide an approximately ?at topped impedance~
frequency curve for the said tuning range. For
example where the invention is applied to a radio 35
receiver designed to tune from 200 to 550 metres,
the inductance 5L and capacity 5C may be so
chosen as to give resonance at about 300 metres,
the resistance 5R being so chosen that the Q
value for’ the whole network ‘consituting Zc is 4.0
signal on a weak signal when both signals are
impedance Zc will only alter by about :30% and
In other words, the input signals applied be
tween the control grid and the end of impedance
applied to the grid of the same valve, is depend
ent mostly on the amplitude of the grid swing
‘ produced by the strong signal and only to a
very small extent upon the anode loading condi
less than 1 so that over the tuning range the
with a relatively low phase shift. An inductance
value of about 2000 microhenries, a capacity
value of about 12.5 micromicrofarads'and a re
sistance value of about 12,500 ohms are suitable
tions so that cross modulation effects cannot be for this numerical example. The capacity 50,
prevented by sharply tuning the anode circuit of course, includes (and in some cases may even
to o?er low impedance to strong unwanted sig— be wholly constituted by) > the effective self
50 nals. If, therefore, cross modulation is to be rcapacity
between the cathode of the valve and
reduced the grid swing must be restricted at the earth, that is to saythe self-capacity of the coil
grid of the valve most liable to introduce it-— 5L plus any stray capacity. The best arrange
even at the expense of ampli?cation. Although ment is that in which the inductance of 5L is
at ?rst sight such reduction of grid swing could made as big as possible and tunes mainly with1
be obtained by feeding the grid through a poten
its own self-capacity and stray capacity, any
tiometer, this expedient has the serious disad
actual physical condenser provided at 5C being
vantage that, although the input signal is re
merely a trimmer condenser for fine tuning ad
duced, valve “noise” is not reduced.
justment. For the wave length range consid
_ In carrying out this invention the cross modu
ered (200-550 metres) Z0 is approximately equal
60 lation effects are reduced by reducing the effec
to 10,000 ohms. It will be seen that the ratio of
tive input by negative reaction coupling or feed
counter voltage, to actual voltage is now 10 so
back induced by a cathode impedance. It is pos
that the actual grid voltage will be'only l/llth
sible by adopting this expedient to reduce the of the applied input voltage.
noise level to a very low amount, and there is the
If a, negative reaction arrangement with a
65 added advantage that the negative reaction not
cathode impedance as above described were 'em
only reduces a stabilizing action upon the stage ployed alone, the valve having an ordinary ape
to which it is applied but also indirectly upon riodic input circuit constituted, for example by
the receiving system as a whole.
a simple resistance, there would still be the dis
The obtaining of negative reaction by means advantage that cross modulation effects could
70 of a cathode impedance is, of course, known per occur due to strong signals‘ outside the tuning
se; for example it has been applied to low fre
range 1. e. at frequencies'at which the impedance
quency circuits and also—though with less suc
cess—to high frequency circuits. In known ar
rangements wherein negative reaction by means
of a resistance in the cathode lead has been.
Zc is so low that the value 9 Zc is not large com
pared with unity. For example if, taking the
speci?c ?gures above given, there were a strong
input signal on a wave length of 1500 metres Z0
as sharply tuned-as possible and to follow it with a
a radio frequency ‘bandpass ‘circuit. The aerial
coupling arrangement with the sharply tuned
anode circuit and‘th‘e succeeding band pass cir
cuit may be arranged to give, in combination, side
would, for this frequency,_only be about 250 ohms
and g Zc only about .26 so that the back coupling
effect would hardly vreduce theWapp‘lied input
voltage at all.
, In carrying out this'inven-tion this di?icull'ty ‘is
avoided by constitutingthe input circuit of the
valve by another semi-aperiodic circuit ‘of the
same, nature as the semi-aperiodic cathode im
pedance, the aerial being preferably “tapped
10 down” upon'this semi-aperiodic input circuit so
that the aerial impedance is properly matched.
The ?rst sharply tuned circuit of the receiver is
an ordinary parallel tuned circuit in the anode
circuit of the coupling valve,‘ and it will be ob-'
that this tuned circuit may be directly
L15 vious
ganged for uni-control withthe other circuits of
band cuto?‘ effects which may be corrected for in
subsequent stages of the receiver-e. g. in the case
of a superheterodyne receiver, in the intermediate
frequency stages. Thus by following the sharply
tuned circuit in the anode circuit of the coupling
valve with a radio frequency band passrcircuit oi
the ‘.‘double hump” characteristic type (i. e._ one
with two ‘humps’ joined by a “dip” in the middle
of the bandpass) and‘ by utilizing an intermediate
frequency band pass‘ ampli?er with a “similar 15
, “double hump” type of characteristic a substan
the receiver without considering the aerial at all.
A preferred aerial coupling network in accord
ance with this invention is shown in the accom
‘20 panying Fig. 3.. , Here the aerial 8 is connected to
, a tapping point lowdown upon an inductance
tially ?at topped over-all characteristic can be
obtained. This is graphically shown in. the ac
companying Figs. 4, I5, 6 and’! where curves .A and
B in Fig. 4 are respectively band pass and sharply 20
tuned high frequency characteristics; the curve
A B of Fig. 5 is the resultant; the curve C of Fig.
is the intermediate frequency band pass char
l of a high impedance coupling valve 2 (e. g. as
acteristic; and, the curve ABC of Fig. '7 is the
shown a high frequency pentode) and earth.
overallresultant characteristic.‘ In Figs. 4, 5, 6 .
:25 The inductance IOL is shunted by a resistance
and "7', :1: represents the required band pass range.
IBR. and ‘also by a capacity lllCso that the grid Substantially‘ constant selectivity ‘circuits of the '
circuit of the coupling valve is constituted by a variable inductance type, wherein tuning is ob
semi-aperiodic network generally designated ll) tained by movement of a portion of the core of
ML which is connected between the control grid
having a sub'stantiallyconstant response 'for the
intended tuning range, they response vfalling‘ away
outside this range.' The cathode 4 of the ‘coupling
valve—which may be indirectly heated-is con
nected to earth through another semi-aperiodic
network generally designated 5 and consisting of
.35, an inductanceSL shunted by a resistance 5B, and
by a capacity 5C, the dimensioning of this semi
aperiodic network 5 being substantially. the same
a'ferro-magnetica‘lly cored inductance or induct- -
ances (the core being, for example, of the mate
rial known in England under the registered trade
mark “Ferrocart”) may advantageously be em
ployed for the tuning circuit in the anode circuit
of the coupling valve and for a “double hump”
7 radio frequency band pass circuit succeeding said .
tuned circuit. Although in carrying out this in
vention a high degree of immunity from cross
as that of thesemi-aperiodic gridcircuit network I modulation effects can be obtained, quite a rea
Ill. The screen grid H of the valve is positively
biased, as in the usual way, andthe suppressor
grid l2 thereof is connected to the cathode also as
in the usual way. ‘The anode 6 of the valve is
coupled, for example, through a condenser 13 to
the next stage (not shown) of the receiver and
is also connected through a sharply tuned par
allelntuning circuit ‘1' (constituting the first
sharply tuned circuit of the receiver) to a source
of anode potential (not shown). The tuning re
actance in this tuned circuit 1' (e. g. .a variable
condenser) is gang controlled with the tuned re
50 actance or'reactances of the other variably tun
able circuits of the receiver such as circuit 20.
Preferably, though not necessarily, the circuits
l0 and 5 arescreened; for example, as shown,
they'may be mounted in separate screening boxes
With the arrangement of Fig. 3 the input volts
applied to the coupling valve from the aerial
are, within the tuning range, reduced due to the
negative back coupling, and as respectsfrequen
60 cies outside the tuning range (where the'cathode
impedance due to network 5 becomes reduced and
accordingly the back coupling effect is also re
duced) the appliedv input voltage from the aerial
is reduced owing to the drop in impedance of the
semi-aperiodic grid circuit l-O. With similar
semi-aperiodic circuits at It] and 5 it is possible
to maintain the ratio of the net voltage applied
between grid l and cathode 4 of the coupling
valve 2 to the actual voltage supplied fromthe
70 aerial at a roughly constant‘ quantity both :as re
spects frequencies in the tuning range and outside
It is preferred to make the parallel tuned cir
75 cuit 1' in the anode circuit of the coupling valve
sonable ampli?cation'fcan also‘be obtained in the
coupling valve by the use of a coupling valve of
high impedance with a sharply tuned circuit in
cluded in its anode circuit." By'employing good
quality coil's‘the value of ‘Za at resonance. may be
made as high as 200,000 ohms and with g=1 and .45
> ’
'Zc=10,000 an ampli?cation of 18.2 is obtained
from the coupling valve. Similarly the aerial in
put e?iciency can readily be made higher than
would be the case were a purely aperiodic grid cir
cuit employed.
If desired,‘ the semi-aperiodic grid circuit may
be so designed in a manner known per se. that
its response instead/‘of‘being constant or approxi
mately consta'ntover the tuning range, rises, or
alternatively falls somewhat with increase in wave
length. This-‘may beof- advantage in some cases
since it may assist ‘in the obtaining of a desired
over-all frequency-sensitivity curve for the receiv
er as a whole, e. g.'to compensate for an undesired
shape of characterisic presented by some other
' stage of the receiver.
Though not limited to its application thereto
the invention is particularly ‘advantageous when
the coupling valve is also-called .“electron beam”
valve, that is to say, a valve such as is described,‘
in the British Patent Speci?cation No. 403,973,
wherein the electron discharge partakes more of
the nature of the discharge in a cathode ray tube
than of the discharge in an ordinary thermionic
valve as at present in common use.
In some‘ cases,‘ especially Where the source of
signals to be received‘is remote ‘from the receiver
proper, the coupling valve grid may be fed
through a tapered‘ line or ‘equivalent network;
for example the circuit lilof Fig. 3 may be re 75
placed by a tapered line or equivalent network.
Where the receiving aerial is connected to are
mote receiver through a high frequency cable
there arises the problem of how to transform the
relatively low impedance conditions of the cable
to the high input impedance conditions of the
valve while still retainingleffective action over
a considerable wave length range.
The use of
a tapered line or equivalent network, suitably
10 designed, goes far towards solving the‘ problem.
The accompanying Figure 8v shows a suitable de
sign, of tapered line. Here the tapered line con
sists of a central conductor of spiro-helical form
each other .at one frequency.
frequency other than the matching frequency and
hence the larger will be the re?ection loss.
sheath. The tapered line extends from B to C,
the length A B representing a high frequency
cable of normal constants of inductance L1 per
unit length and capacity C1 per'unit length.
.20 The input impedance to the cable is represented
by the resistance Z01 the input voltage being e.
As indicated in Fig. 8 the characteristic imped
ance changes gradually over the length of the
tapered line. If L1, C1, be respectively the in
ductance and capacity per unit length at B; ‘L2 C2
the corresponding quantities at D; and L3 C3
the corresponding quantities at C; then at B the
factor a (seelFig. 9) is a function of the ratio
of the matching frequency to- the cut-off fre
quency. Thus the performance of the arti?cial
taper line, owing to intersection “mis-matching”
eifects, can approach the ideal only if t ap
proaches unity; hence, for level response condi
tions, the ratio of output to input impedance
of simultaneously increasing diameter and closer
15 pitch centrally positioned in a bell like outer
The‘ larger the ta
per factor the larger will be the “mismatch” at a
must'b'e small if a smallnumber of sections; is to
For a large value of
the number of sections (11.) must be large.
In practice the cut-'o? frequency may be chosen
above the highest frequency in the band to be
passed and the sections may be designed to match
at this frequency. The number of sections is
then chosen from consideration of the require
ments of (1) input impedance to the coupling
valve (2) taper factor (3) and actual value of
thei?nal inductance Ln in relation to the highest
‘frequency in the band (Ln must not. be such as to
resonate anywhere near this highest frequency) ;
inconjunction with the ‘fact that the value
must not be less than the value given by stray
35 and at C the impedance
capacities; grid capacity to earth, wiring capacity 13.5
to earth ., . .and so forth.
With suitable de
sign, overall response curves of the general nature
‘Z03 represents the characteristic impedance look
4,0 ing into the end at C and, by the choice of a cor
rect terminating impedance correct matching of
impedance is obviously. obtainable. Further
there is a voltage step up. effect for the ratio
E/e (E is the output voltage) is equal to
W Z01
An actual tapered line as shown in Fig. 8 is dif
?cult to design and construct but an equivalent
50 network made up of a number of sections as
shown in the accompanying Fig. 10 will be ob
tained. ‘In this ?gure response is plotted along
the ordinate line and frequency along the ab 40
scissa line. t1 t2 t3v are progressively larger
values of taper factor; fm is the matching fre
quency; and fa the cut-off frequency. Thus a
practical compromise may be reached which will
give a reasonable step-up ratio where it is most 45
required with’a permissible drop in response at
the lower frequencies.
By employing an arti?cial taper line made up
of high pass sections-for- example, as shown in
shown in the accompanying Fig. 9 may be used.
Fig. 9 shows what is really an arti?cial tapered
line. In this ?gure the inductances in thesuc
cessive sections increase and the capacities de_
the accompanying Fig. 11 and designing on the .
same principle, astep-up ratio combined with a
high pass effect is obtainable. Typical response
curves for a ?lter as shown in Fig. 11 are repre
crease, in a de?nite ratio as indicated on the
?gure, where L1, L2,}L3 . . . are the inductances
sented in the accompanying Fig. 12 in which the
references correspond to those of Fig. 10. For
and the capacities are of‘the values marked. -
the line of Fig. 11
L1 ‘L2 fLs I
the taper factor (f)
(the taper factor):
L3_ Ln
Or, more generally expressed
_ Ln
_Ln—1 On
The larger the number of sections the smaller
will be the taper factor it.
By selecting fm further away from fc the gen
eral shape of the response curve becomes as shown
in the accompanying Fig. 13. The combination
of a curve such as the curve t2 of Fig. 13 with a
curve such as the curve t2 of Fig. 10 produces an
overall curve such as that shown in the accom
. panying Fig. 14 and the accompanying Fig. 15
and this product
will de?ne a cut-off fre
shows a double arti?cial taper line (consisting
quency in ordinary ?lter technique. The tapered
obviously of a low pass. portion succeeded by a
high pass portion) whereby a curve such as that
arti?cial line differs from an ordinary ?lter in
that adjacent sections can only be made to match
of Fig. 14 is obtainable. In practice, as is indi
cated in Fig. 14 a satisfactory aperiodic action is
obtainable over a range such as that extending
from 150 kc. to 1500 kc.
Having described my invention, what I claim
as novel and desire to secure by Letters Patent
1. A receiving'system comprising a coupling
tube which constitutes the ?rst tube of the sys
tem, a relatively highly damped resonant circuit
connected between the input grid electrode and
10 the cathode of said tube, a similar circuit con
nected to the cathode of said tube and included
7 in ‘a path common to both the grid-cathode cir
cuit and the anode-cathode circuit, and a reso
nant circuit which constitutes the ?rst variably
15 tuned circuit of the system connected between
the anode and cathode of said coupling tube.
2. A receiving system according to claim 1
wherein ‘said relatively highly damped resonant
circuits each comprises an inductance, a resist
20 ance and a capacity all connected in parallel.
3. A receiving system according to claim 1
wherein said relatively highly damped resonant
circuits each comprises an inductance, a resist
ance and a capacity all connected in parallel and
25 an antenna circuit having a connection to an
intermediate point on the inductance included
in the relatively highly damped resonant circuit
constituting the coupling tube input circuit.
4. A receiving system comprising a coupling
30 tube which constitutes the ?rst tube of the sys
tem, a relatively highly damped resonant circuit
connected to the input grid electrode and the
cathode of said tube, said circuit comprising an
inductance, a resistance and a capacity all in
35 parallel, said inductance and capacity resonating
with one another at a frequency at about the
middle of the desired tuning range and said re
sistance being of such value as to provide an
approximately ?at topped impedance-frequency
40 curve for said tuning, range, a second relatively
highly damped resonant circuit connected'to the
-cathode of said tube and included in a path com- I
mon to both the grid-cathode circuit and the
anode-cathode circuit, and a variably tuned cir
cuit connected between the anode and cathode of
said coupling tube.
5. In a receiving system, a plurality of tunable
circuits, the variable condensers of which are
ganged for unicontrol, an antenna, and means
interposed between said antenna and the ?rst of
the tunable circuits for preventing a change in 10
antenna constants from disturbing the tracking
of the gang condensers, said means comprising
a vacuum tube which is provided in a path com
mon to its grid-cathode and anode cathode cir
cuits with a network for producing degeneration, 15
a similar network connected between ‘the grid
and cathode of said tube, and a direct connection
from the antenna to said last named network.
6. A receiving system of the superheterodyne
type, comprising a coupling tube which consti~ 20
tutes the?rst tube of the system, a relatively
highly damped resonant circuit connected be
tween the input grid electrode and cathode of
said tube, a similar circuit connected to the
cathode of said tube and included in a path com 25
mon to vboth the grid-cathode circuit and the
anode-cathode circuit, said circuits‘of the cou
pling tube being designed to- give a double
humped band pass curve, a resonant circuit
which constitutes the ?rst variably tuned circuit 30
of the system connected between the anode and
cathode of said coupling tube, said anode reso
nant circuit being sharply tuned to give a re
sponse characteristic with a peak between the
two above humps, and an intermediate frequency 35
ampli?er also designed to give a double humped
characteristic, the three characteristics combin
ing to give a. substantially flat-topped band pass
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