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4-4
Section IV
differential at the base of A3Q6 is equal to its quiscent
value. The ALC circuit contains an integrator for fast
response without overshoot and without sacrificing the
ability to reject ripple
superimposed
on the curr.ent from
the Average Detector. S2C14 is
switched
in parallel with
A3Cl0 on the XIO RANGE for required response of the
Amplitude Control Integrator at low frequencies.
4·29
.
METER CIRCUITS. (Schematic 2)
4·30.
The meter circuits consist of the Meter Differential
Amplifier, the Meter Offset Current Reference and the
Meter. As
explained
in Paragraph 4·22, the Average
Detector (A2CR2] and A2CR22) monitors the Balanced
Amplifier output and produces two dc currents, equal in
amplitude but opposite in polarity, proportional to the
Balanced Amplifier output. The positive output of the
Average Detector (from A2CR2l) flows to the meter
circuits. A fixed part of this curren t flows into the Meter
Offset Current Reference and the remainder flows through
the Meter and its shunt resistors A3Rl7 and A3RIS. The
Meter (M]) is calibrated to indicate center scale when the
654A output into rated load (the attenuators set at 0 dBm)
is 0 dBm. The total range of the meter scale is +/·1 dBm so
that when the Meter indicates·] dBm, no current is flowing
through the Meter and all of the current from the Average
Detector is flowing through the Meter Offset Current
Reference circuit.
4·3 I. The action of the Meter Offset Current Reference
and the Meter Differential Amplifier is very similar to the
action
of
the Amplitude Current Reference and the
Amplitude Control Integrator (described in Paragraphs 4·22
through 4-25)
.
Apart from a few minor differences the
circuits are identical.
4-32. The Meter Offset Current Reference consists of
A3R6, A3R7, A3RS, A3R9 and A3CR2. A3CR2 is a
special temperature compensated
Zener diode
which
maintains a constant voltage across A3R7 and A3RS in
series. Thus. the current flowing into the circuit is
determined essentially by the voltage across A3R9
.
This
current must always be a fixed amount so as to offset the
Meter scale correctly
;
therefore, the voltage across A3 R9
must always be fixed; this is achieved by means
of
the
Meter
Differential
Amplifier. The Meter Differential
Amplifier consists of A3Q2 through A3Q5. A3Q2 and
A3Q3 form a differential pair; since the base
of
A3Q3 is
connected directly to ground, the base of A3Q2 will be
held
at
a virtual ground. Any difference between the two
bases causes an output change from the
collector
of A3Q3
which is amplified by A3Q4 and applied to A3Q5 so as to
return the base of A3Q2 back to virtual ground. One side
of
A3R9 is connected to the base of A3Q2, which is clamped
to a virtual
ground;
the other side of A3R9 is connected to
a constant voltage point (set by A3R7); thus the voltage
across A3R9 is held constant as required,
and
the Meter
Offset Current Reference always takes a fixed amount of
the
current
from the Average Detector to offset the Meter.
A3C5 serves to improve the frequency stability of the
Model 654A
Meter Differential Amplifier. A3C6 is connected across the
Meter to damp the meter movement, and A3C7 is switched
in paraUel with A3C6 on the XIO RANGE so as to further
improve damping of the Meter
at
very low frequencies.
1\
4·33. ATTENUATO RS. (Schematic 3)
4·34. The balanced sine wave signal, developed across
A2R74 and A2R75 in the Balanced Amplifier, is fed
through A2R76 and C9, and through A2R77 and CIO to
the Attenuators (S4). The attenuator assembly (S4) consists
of four attenuators; a ] dB
step
and a 10 dB step attenuator
connected in series for
each
of the two halves
of
the
balanced signal. Each attenuator consists of four resistive
networks which are switched in various combinations to
give the required attenuation. The front panel controls
consist of two concentric rotary knobs labelled OUTPUT
LEVEL dBm; the outer control, marked in 10 dB steps,
controls both of the 10 dB step attenuators simultaneously;
the inner control, marked in ] dB
steps,
controls both of
the 1 dB step attenuators
simultaneously.
4·35
.
IMPEDANCE SELECTOR. (Schematic 3)
4-36. The front panel IMPEDANCE switch (A4S]) selects
the
reqUired
output
impedance of the 654A. The
impedance networks and the switching connect ions are
shown on schematic 3; the switch connections are shown
with the 50
ohm
UNBAL button pushed. In the BAL
mode, both front panel output connectors, 13 and 14,are
used; in the UNBAL mode,
only
14 is used.
,r
4·37. REGULATED POWER SUPPLY.
4-38
.
The regulated power supply provides
all
dc voltaged
required by the 654A Test Oscillator
circuits.
The power
supply consists of
a
+31 V and·
26
V series regulated
supply. Each power supply
is
protected by current limiting
and foldback
current
limiting
.
4·39
.
Tile +3] V and - 26 V power supplies are functially
identical. Both use
operational
amplifiers for
output
voltage
error amplification
.
A
I
R40 adjusts the +3
I
V supply
voltage and A
I
R41 adjusts the .
26
V
supply
voltage
.
4-40. Conventional current limiting is used in both
supplies
to limit the output curren
I
to approximately 300 mA
o
Foldback
current
limiting further limits the output current
if the output voltage is pulled below approximately 16 V
by a malfunction in the 654A
circuitry.
A direct
short
to
ground of
either supply will result in an output
current of
approximately 10 mA
as shown
in Figure
4-2.
441. Figure 4·3 is a simplified schematic of the
current
limiting
circuitry
used in the 654A power supplies. The
Current Limiting Transistor A
I
Q4( +) or A
I
Q7( - ) is a
variable shunt to the series regulator drive
current.
It is first
switched on
by the voltage drop across tile Currellt LimIt
Sensing Resistor when the power supply
output
current
reaches approximately
300
mAo The power supply will
remain in this Conventional Current Limit
condition
until
the output current decreases
allowing
the
supply
to return
"
to normal
operation,
Of
until the power supply output
~J
voltage drops below the Foldback Reference (]6.2V).
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