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Патент USA US3068437

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Dec. 11, 1962
M. WEINBERG
3,068,42 7
FREQUENCY MQDULATOR INCLUDING VOLTAGE SENSITIVE CA PACITORS
FOR CHANGING THE EFFECTIVE CAPACITANCE AND
INDUCTANCE OF AN OSCILLATOR CIRCUIT
Filed Feb. 11. 1960
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INCREASING BIAS VOLTAGE
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MORTON WEI/V5575
BY
Ah‘ys
United grates Patent G?frce
3,068,427
Patented Dec. 11, 1962
2
i.
ing bias voltage or decreasing capacitance in the equiv
alent circuit of FIG. 2.
3,668,427
FREQUENCY MCDULATOR INCLUDING VOLT
AGE SENSITIVE CAPACITORS FOR CHANGING
THE EFFECTIVE CAPACITANCE AND INDUCT
ANCE OF AN OSCILLATOR CIRCUIT
Morton Weinberg, Des Piaines, 111., assignor to Motorola,
Inc., Chicago, lill., a corporation of litinois
Fiied Feb. 11, 1960, Ser. No. 8,027
10 Claims. (Cl. 332-30)
In practicing the invention there is provided an elec
tron oscillator circuit including a quartz crystal operat
ing in a series resonant mode in the feedback circuit. A
transistor is used as the electron amplifying device in
the circuit. The quartz crystal is shunted by a neutraliz
ing coil, and is connected in series with a series resonant
circuit between the collector and emitter electrodes of
10 the transistor.
The resonant circuit includes a ?rst volt
age sensitive semiconductor diode serving as a capacitor
connected in series with an inductor which is shunted
by a second voltage sensitive semiconductor diode. The
of a crystal controlled oscillator circuit in accordance
?rst device forms a variable capacitor and the second
with a modulating signal through the utilization of back
15 diode varies the inductance of the inductor and diode
biased variable capacitance semiconductor devices.
combination. Each voltage sensitive diode may be shunt
In frequency modulation transmitters, as in other
ed by a temperature compensating capacitor, and a linear
radio transmitters, it is necessary to exercise precise con
ity coil may be connected across the entire resonant
trol of the frequency transmitted. It has been customary
This invention relates generally to frequency modu
lation systems and in particular to varying the frequency
to use, for frequency stability purposes, crystal controlled
oscillators which provide a high degree of frequency
circuit. A regulated bias voltage is applied to the junc
stability even when there are wide changes in ambient
lating voltage is superimposed on the bias voltage to
tion of the two diodes, and an audio frequency modu
change the capacitance of the diodes, and thereby change
temperature and supply voltage. Because crystals pro
the oscillator frequency. Thus, when the capacitance of
vide close control of the frequency transmitted, in fre
the diodes is increased by a signal which opposes the
quency modulation transmitters the problem of obtaining
the required deviation for frequency modulation has been 25 bias, the net effect in the feedback circuit is an increase
acute. Usually it has been necessary to use various cir
“a
in inductive reactance thereby causing a decrease in the
oscillator frequency. The two diodes are controlled by
cuitry to achieve phase modulation and then utilize phase
the same modulating voltage and the increased capaci
shifting devices and corrective networks to convert phase
tance thereof for example, decreases the capacitive re
modulation to frequency modulation. Although circuits
have been provided for achieving frequency modulation 30 actance of the series semiconductor diode and increases
the net inductive reactance of the parallel combination
with crystal controlled oscillators these circuits have
of inductor and semiconductor diode thus multiplying
been complicated and have had the disadvantage of being
the effect of a change in capacitance to a large change
unstable and have not provided the required frequency
in reactive impedance.
deviation.
Referring now to the drawings, in FIG. 1 there is
Thus, it is an obiect of the present invention to pro 35
illustrated a transmitter circuit including a modulation
vide an improved crystal controlled oscillator circuit the
source 10, an oscillator circuit 20, multiplier and ampli
frequency of which can be varied directly by applying a
modulating signal thereto.
?er stages 50, output stage 60, and antenna 70. Oscilla
Another object of the invention is to provide a simple 40 tor circuit 20 includes a transistor 21 used as the pri
mary electron discharge device which provides gain for
and inexpensive crystal oscillator circuit which allows
sustaining oscillation. Crystal 23 has choke 24 shunted
maximum frequency deviation while maintaining mini
therewith for neutralization. Collector 33 of transistor
21 is coupled to output transformer 34 which is tuned
tal oscillator having a frequency controlling resonant 45 by capacitors 35 and 36 connected in series and shunt
ed across the primary winding thereof.
circuit which may be modulated by variation of a reac
The frequency controlling circuit of the oscillator in
tance element in the circuit which provides stable opera
cludes capacitive diode 26- in series with coil 40 across
tion over a wide range of variations.
A feature of the invention is the provision of a crystal
which capacitive diode 27 is shunted, and all of which
is shunted by coil 25. Capacitor 43 connected in series
oscillator circuit utilizing a single transistor and a variable
with coil 40 serves as a blocking capacitor. Coil 25
resonant feedback circuit for modulating the frequency
produced thereby.
is variable to change the frequency versus bias variation
characteristic of the modulator. Capacitors 28 and 29
A further feature of the invention is the provision of an
are ?xed capacitors shunted across capacitive diodes
oscillator circuit utilizing a piezoelectric crystal in series
with a series resonant circuit controlled by the capaci 55 26 and 27 to provide temperature compensation and re
duce reactance change with voltage, when less than
tance variations of two voltage sensitive semiconductor
maximum deviation is desired. Coil 40 may be tunable
devices. One of the devices may form the capacitance of
and may be used to warp the oscillator to the desired
a series resonant circuit and the other device may coop
operating frequency. The resonant or frequency con
erate with an inductor to form the inductance of the
series resonant circuit.
60 trolling circuit is connected at 30 to the crystal-choke
and together therewith forms the feedback circuit for the
Another feature of the invention is the provision of
a series resonant circuit the frequency of which is con
oscillator. This is connected from the output circuit,
mum distortion of the output signal.
A further object of the invention is to provide a crys
trolled by back-bias voltage applied to variable-capaci
between capacitors 35 and 36, through capacitor 31 to
tance semiconductor diodes in the series resonant cir
cuit. The series resonant circuit may be used as the
emitter 32 of transistor 21.
The collector voltage supply is regulated by regulator
diode 37. This voltage is also used as the basis on the
variable capacitance diodes 26 and 27 which control the
FIG. 1 is a partial block diagram of a typical trans
frequency of the oscillator. This bias is fed to the junc
mitter with the oscillator circuit of the invention shown
tion of variable capacitive diodes 26 and 27 through re
in a schematic diagram;
FIG. 2 shows a simpli?ed equivalent circuit used to 70 sistor 33 and choke 39. Signals from modulation source
16 are also applied through choke 39 to the junction of .
explain the operation of the oscillator circuit; and
frequency controlling circuit of an oscillator.
-
FIG. 3 shows net impedance plotted against increas
diodes 26 and 27. Resistor 33 forms the audio load and
8,068,427
3
4
may be selected so that the audio is not substantially
loaded. Choke 41 is used as a DC. return to ground for
the diode bias.
As the modulation source voltage is superimposed on
they bias voltage, diodes 26 and 27 vary in capacitance to
change the tuning of the resonant circuit, and the oscilla
percent of the value of coil 4%. By combining the circuit
tor output frequency follows these changes. This may
appear'more clearly from a consideration of FIG. 2. As
the negative voltage across diode 26 increases, the capaci
with a proper choice of components, a frequency modula
tor results which has good linearity over the entire oper
ating range. The operating range will be determined by
the distance the varying bias can warp the crystal along
the frequency axis between the two anti-resonant points
and the range over which the impedance versus freqency
curve for the crystal modulating circuit compensate each
other to produce a linear change in frequency for a change
tance thereof decreases and the capacitive reactance in
in bias voltage.
creases. At the same time the increase in negative volt
Thus, the invention provides for an oscillator circuit
age across diode 27 will decrease its capacitance and in
which utilizes a transistor and variable capacitive diodes
crease its capacitive reactance. This will decrease the
to vary the frequency of a crystal and thereby permit
effective inductance of diode 27 and coil 40 in parallel.
direct frequency modulation of a transmitter. The circuit
The variations in capacitance and inductance will have 15 has been found to be highly stable and provides a rela
additive effects to greatly increase the oscillator frequency.
tively large variation in frequency from a modulation
When the capacitance of diodes 26 and 27 is increased by
signal of a given level.
applying less bias, the net effect in the feedback circuit is
I claim:
7
an increase in inductive reactance and decrease in capaci
1. An electronic oscillator circuit including in com=
tive reactance causing a decrease in oscillator frequency. 20 bination, a transistor having emitter and collector elec*
The inductive reactance of coil 40 must always be less
trodes, a feedback circuit connected between said emitter
than the capacitive reactance of the diode 27 to sustain
and collector electrodes and cooperating with said tran
operation of the series resonant circuit. Oscillator cir
sistor to form an oscillator, said feedback circuit includ
cuit 20 includes a transistor 21 used as the primary elec
ing a piezoelectric crystal unit series resonant at a prede
tron discharge device which provides gain for sustaining 25 termined frequency and a series resonant circuit connected
oscillations.
in series with said crystal unit, said series resonant circuit
The capacitance of all semiconductor diodes is essen
including a ?rst voltage sensitive capacitor and an in
tially proportional to the reciprocal of the square or cube
ductor connected in series, a second voltage sensitive c'a
root of the applied back-bias voltage. In the circuit, care
pacitor shunted across said inductor, bias voltage means
must be taken to prevent the sum of the bias voltage, 30 applying a regulated bias voltage to said voltage sensitive
audio peak voltage from the modulation source, and radio
capacitors, input signal means applying a varying input
requency peak voltage across the diode from becoming
signal voltage smaller than said bias voltage to said volt
positive in value thereby causing the diode to conduct.
age sensitive capacitors thereby forming a composite sig
Since diodes 26 and 27 are operated at all times with
nal voltage, variations in said composite, signal voltage
back-bias, the impedance of the modulator is very high 35 causing variations in the capacitance of said voltage sensi
(since it is determined almost entirely by the value of
tive capacitors, to thereby change the tuning of said series
resistor 38) and the audio ampli?er used to drive the
modulator need not supply a great amount of power.
It is to be pointed out that the bias and modulating
signals are applied to the junction of the back to back
diodes so that the full bias and modulating voltage is
applied across each. This causes maximum capacity
variations from a given modulating signal. As one diode
is also connected in parallel with an inductance the change
in capacitance is multiplied giving a much larger change
in reactance than would occur by simply connecting one
resonant circuit, said ?rst capacitor producing a change in
the capacitive reactance and said second capacitor and
said inductor in parallel providing an opposing change
in the inductive reactance for an additive effect, the fre
quency of said series resonant circuit varying from said
predetermined frequency in response to the input signal
voltage applied to said voltage sensitive capacitors, and
the frequency of oscillations being determined substan
tially entirely by said feedback circuit.
2. An electronic oscillator circuit including in combi
nation, a transistor having emitter and collector elec
trodes, a feedback circuit connected between said emitter
and collector electrodes and cooperating with said tran
The diodes 26 and 27 need not be equal but must have 50 sistor to form an oscillator, said feedback circuit including
a piezo-electric crystal unit series resonant at a predeter
values which provide resonance at a normal bias position.
It may be desired to use a diode 27 having a greater
mined frequency and a series resonant circuit shunted
capacitance than diode 26 to provide a greater change in
by a ?rst inductor and connected in series with said crys
net series reactance with modulating voltage.
tal unit, said series resonant circuit including a second in
FIG. 3 shows the net series reactance of the network 55 ductor and ?xed capacitor connected in series, ?rst and
second voltage sensitive capacitors connected in series
in FIG. -2 as a function of the ratio of the initial diode
capacitance at quiescent voltage to the capacitance at a
with each other, each of said voltage sensitive capacitors
new voltage at a ?xed frequency. Going to the right on
being shunted by a temperature compensating capacitor,
said ?rst voltage sensitive capacitor being shunted across
the abscissa is in the direction of increasing bias or nega
tive voltage or decreasing capacitance. The upper ordi 60 said second inductor and said ?xed capacitor, said second
voltage sensitive capacitor being connected in series with
nate is inductive reactance and the lower ordinate is
said second inductor and said ?xed capacitor in said series
capacitive reactance. The operating range is shown equi
diode in series with an inductance.
This results in a
large frequency variation of the oscillator from a given
level of modulation signals.
distant from either side of the quiescent point where
resonant circuit, bias voltage means applying regulated
Co/C’0=l. C0 is the capacitance of diodes 26 and 27
bias voltage to said voltage sensitive capacitors, input sig
at the quiescent or non-modulated bias voltage. C’o is 65 nal means applying a varying input signal voltage smaller
the capacitance of diodes 26 and 27 at any voltage within
than said bias voltage to said voltage sensitive capacitors
the range of the curves shown. At the quiescent point
thereby causing variation in the capacitance thereof, the
Co is equal to C0 and the reactance is zero. When the
frequency of oscillation being determined substantially
back bias voltage is increased, C’o is less than C0 and the
entirely by said feedback circuit whereby the frequency
circuit becomes capacitive. When Co is greater than 70 of said oscillator circuit varies from said predetermined
C0 or the bias voltage is approaching zero, the circuit
frequency substantially linearly with input signal voltage
becomes inductive. ‘
The solid curve of FIG. 3 was plotted for a very large
applied to said voltage sensitive capacitors.
3. An electronic oscillator circuit including in com
value of inductance in coil 25. The dotted curve of FIG,
bination a transistor having emitter and collector elec
3 was plotted when the value of coil 25 was seventy-?ve 75
trodes, a feedback circuit connected between said emitter
5,058,427’
5
6
and collector electrodes and cooperating with said tran
sistor to form an oscillator, said feedback circuit includ
by ?rst inductor means and connected in series with said
crystal unit, said series resonant circuit including second
inductor means, ?rst and second voltage sensitive semicon
ing a crystal unit series resonant at a predetermined fre
quency and a series resonant circuit shunted by a ?rst in
ductor and connected in series with said crystal unit, said
series resonant circuit including a second inductor, and
?rst and second voltage sensitive diodes connected in
series and for backward conduction with respect to each
other, said diodes having capacitance which varies with
the voltage applied thereacross in the direction opposite
to the direction which causes conduction, said ?rst voltage
sensitive diode being connected in parallel with said sec
ond inductor, said second voltage sensitive diode being
connected in series with said second inductor in said series
resonant circuit, bias voltage means connected to the
ductor diodes connected in series and for backward con
duction with respect to each other, said diodes having a
capacitance which varies with the voltage applied there
across in the direction opposite to the direction which
causes conduction, said ?rst voltage sensitive semicon
ductor diode being connected in parallel across said sec
ond inductor means, said second voltage sensitive semi
conductor diode being connected in series with said sec
ond inductor means in said series resonant circuit, bias
voltage means connected to the junction of said ?rst and
second semiconductor diodes for applying a regulated
?xed bias voltage thereto in said opposite direction, modu
lating signal means applying a varying input voltage
smaller than said bias voltage to said junction of said
voltage sensitive diodes thereby forming a composite sig
nal voltage, variations in said composite signal voltage
voltage sensitive diodes thereby forming a composite sig 20 thereby causing variations in the capacitance of said volt
age sensitive diodes, said composite signal voltage having
nal voltage, variations. in said composite signal voltage
junction of said ?rst and second voltage sensitive diodes
and applying a regulated ?xed bias voltage thereto in said
opposite direction, input signal means applying a varying
input signal voltage smaller than said bias voltage to said
thereby causing variations in the capacitance, thereof of
said voltage sensitive diodes, said composite signal voltage
a maximum excursion of such nature that said voltage
sensitive diodes remain non-conductive, said composite
signal voltage being effectively applied to said diodes in
having a maximum excursion of such nature that said volt
age sensitive diodes remain nonconductive, said ?rst diode 25 parallel whereby overall modulation is linear with re
spect to applied voltage, said ?rst semiconductor diode
producing a change in the capacitive reactance and said
producing a change in the capacitive reactance aided by
second diode in parallel with said second inductor pro
an opposing change in the inductive reactance caused by
ducing an opposing change in the inductive reactance
said second semiconductor diode in parallel with said
which ‘is additive in e?ect to said capacitive reactance
change, the frequency of oscillation being determined sub 30 second inductor means, the frequency of oscillation being
stantially entirely by said feedback circuit whereby the
determined substantially entirely by said feedback circuit
frequency of said oscillator circuit varies from said prede
termined frequency in response to input signal voltage
applied to said voltage sensitive diodes.
4. An electronic oscillator circuit including in combina 35
tion, a transistor having emitter and collector electrodes,
whereby the frequency of said oscillator circuit varies
a feedback circuit connected between said emitter and
from said predetermined frequency substantially linearly
with said modulating signal applied to said voltage sen
sitive diodes.
6. An electronic oscillator circuit including in combina
tion a transistor having emitter and collector electrodes,
collector electrodes and cooperating with said transistor
a feedback circuit connected between said emitter and
to form an oscillator, said feedback circuit including a
collector electrodes and cooperating with said transistor
piezo-electric crystal unit series resonant at a predeter 40 to form an oscillator, said feedback circuit including a
crystal unit series resonant at a predetermined frequency
mined frequency and a series resonant circuit shunted by
?rst inductor means and connected in series with said
and a series resonant circuit having ?rst inductor means
for maintaining frequency linearity shunted thereacross
crystal unit, said series resonant circuit including second
connected in series with said crystal unit, said series res
inductor means, ?rst and second ?xed capacitor means,
and ?rst and second voltage sensitive capacitor means, 45 onant circuit including second inductor means, a ?xed
capacitance connected in series between said crystal unit
said ?rst ?xed capacitor means and said ?rst voltage sen
and said second inductor means, ?rst and second tempera
sitive capacitor means being connected in parallel with
ture compensating capacitors connected in series, and ?rst
each other and in series with said second inductor means
and second voltage sensitive diodes connected in series‘
in said series resonant circuit, said second ?xed capacitor
and for backward conduction with respect to each other,
means and said second voltage sensitive capacitor means
said diodes having capacitance which varies with the
being connected in parallel with said second inductor
voltage applied thereacross in the direction opposite to
means, bias voltage means including a circuit having re
the direction which causes conduction, said voltage sen
sistance and inductance applying a regulated ?xed bias
sitive diodes and said ?rst and second temperature sensi
voltage to said voltage sensitive capacitor means, input
tive capacitors being connected in parallel with said ?rst
signal means applying a varying input signal voltage
inductor means, said second diode having said second in
smaller than said ?xed bias voltage to said voltage sen
ductor means and said ?xed capacitance shunted there
sitive capacitor means thereby causing variations in ca
pacitance thereof said ?rst voltage sensitive capacitor
across and connected at one end to said crystal unit, bias
voltage means including a regulator diode and a resistor
means producing a change in the capacitive reactance
which cooperates with an opposing change in the inductive 60 and a choke connected in series to the junction of said ?rst
and second voltage sensitive diodes and applying a regu
reactance produced by said second voltage sensitive ca
lated bias voltage thereto in said opposite direction, a
pacitor means in parallel with said second inductor means,
direct current return to ground being provided through a
the frequency of oscillations being determined substan
choke connected to the conducting end of said ?rst diode,
tially entirely by said feedback circuit whereby the fre
quency of said oscillator circuit varies from said prede 65 input signal means applying an input signal voltage smaller
than said bias voltage to said junction of said voltage sen
termined frequency in response to input signal voltage
sitive diodes thereby causing variation in the capacitance
applied to said ?rst and second voltage sensitive capacitor
thereof, the frequency of oscillation being determined
means.
substantially entirely by said feedback circuit whereby the
5. An electronic oscillator circuit including in com
bination, a transistor having emitter and collector elec 70 frequency of said oscillator circuit varies from said prede
termined frequency in response to input signal voltage
trodes, a feedback circuit connected between said emitter
applied to said voltage sensitive diodes.
and collector electrodes and cooperating with said tran
7. An electronic oscillator circuit including in com
sistor to form an oscillator, said feedback circuit includ'
bination a transistor having emitter and collector elec
ing a pieZo-electric crystal unit series resonant at a pre
determined frequency, a series resonant circuit shunted 75 trodes, a feedback circuit connected between said emit
8,068,427
7
ter and collector electrodes and cooperating with said
transistor to form an oscillator, said feedback circuit in
cluding a crystal unit series resonant at a predetermined
frequency, a series resonant circuit connected in series
8
ductive reactance caused by said second ‘diode in parallel
with said inductor, the frequency of oscillation of said
oscillator circuit being determined substantially entirely
by said feedback circuit whereby the frequency varies
with said crystal unit, and ?rst inductor means having 5 from said predetermined frequency in a substantially
?rst and second ends shunted across said series resonant
linear relationship with said modulation voltage applied
circuit for frequency linearity, said crystal unit including
to said voltage sensitive diodes.
a piezo-elcctric crystal shunted with a coil, said series
9. A series resonant circuit including ?rst and second
voltage sensitive semiconductor diodes connected in series
nected to said crystal unit, ?rst and second temperature 10 and for backward conduction with respect to each other,
compensating capacitors connected in series, and ?rst
said diodes in the non-conductive state having a capacit
and second voltage sensitive diodes connected in series
ance which varies with the voltage applied thereto .in a
and for backward conduction with respect to each other,
direction opposite to the direction providing conduction,
said diodes having capacitance which varies with the
an inductor shunting said second diode and forming there
resonant circuit including second inductor means con
voltage applied thereacross in the direction opposite to 15 with a net inductance which resonates with ‘the capacitance
the direction which causes conduction, said voltage sensi
tive diodes and said ?rst and second temperature com
of said ?rst diode, and means for applying a voltage to
said diodes to vary the capacitance thereof, said ?rst
pensating capacitors being connected in parallel with
diode producing a change in the capacitive reactance and
said ?rst inductor means, said second diode having said
said second diode in parallel with said inductor pro
second inductor means series connected with a capacitor 20 ducing an opposing change in the inductive reactance
and shunted thereacross, one end being connected to
which is additive in effect to said capacitive reactance
said crystal unit, bias voltage means including a regula
change and thereby vary the tuning of said resonant
tor diode and a resistor and a choke connected in series
circuit.
to the junction of said ?rst and second voltage sensitive
10. A series resonant circuit including a ?rst voltage.
diodes and applying a regulated bias voltage thereto in 25 sensitive semiconductor diode, an inductor connected in
said opposite direction, a direct current return to ground
series With said diode and a second voltage sensitive,v
being provided through a choke connected to the con
diode connected in shunt across said inductor, said ?rst
ducting end of said second diode, input signal means
and second diodes being connected for ‘backward con
applying an input signal voltage smaller than said bias
duction with respect to each other and having capacitance
voltage to said junction of said voltage sensitive diodes 30 values in the non-conductive state which vary inrespo-nse
thereby causing variation in the capacitance thereof, the
frequency of oscillation being determined substantially
entirely by said feedback circuit whereby the frequency
of said oscillator circuit varies from said predetermined
to a voltage applied, thereto in the direction opposite to
the direction of conduction thereof, the resonant fre
quency of said series resonant circuit being controlled by
a modulating voltage applied to said voltage sensitive
frequency in response to input signal voltage applied to 35 diodes, said ?rst diode producing a change in capacitive
said voltage sensitive diodes.
reactance which is additive in effect to an opposing
8. In the feedback circuit of an electronic oscillator
change in inductive reactance caused by said second’
having a crystal unit series resonant at a predetermined
frequency, a series resonant circuit connected to said
diode in parallel with said'inductor.
crystal unit including ?rst and second voltage sensitive
References Cited in the ?le of this patent
semiconductor diodes connected in series and poled for 40
backward conduction with respect to each other, and an
UNITED STATES PATENTS
inductor shunting one of said diodes, said diodes having
2,925,561
MacDonald __________ __ Feb. 16, 1960
a capacitance which varies with a modulation voltage
Firestone _____ ___ _____ __ Feb. 16, 1960
applied thereacross in the direction opposite to the direc- 45 2,925,563
tion which causes conduction, said modulating voltage
OTHER REFERENCES
having a maximum excursion of such nature that said
voltage sensitive semiconductor diodes remain non-con
ductive, said ?rst diode producing a change in the capaci
tive reactance aided by an opposing change in the in
Voltage Variable Capacitors-State of the Art, by Mac
Mahon et 211., Electronic Industries, December 1959,
pages 90-93.
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