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Section IV
Model 654A
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 current from
the Average Detector. S2C14 is switched in parallel with
A3C10 on the X10 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 (A2CR21 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 A2CR21) flows to the meter
circuits. A fixed part of this current flows into the Meter
Offset Current Reference and the remainder flows through
the Meter and its shunt resistors A3R17 and A3R18. The
Meter (M1) 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 - 1 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.
4-31. 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, A3R8, A3R9 and A3CR2. A3CR2 is a
special
temperature
compensated
Zener
diode
which
maintains a constant voltage across A3R7 and A3R8 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 A3R9
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
Meter Differential Amplifier. A3C6 is connected across the
Meter to damp the meter movement, and A3C7 is switched
in parallel with A3C6 on the X10 RANGE so as to further
improve damping of the Meter at very low frequencies.
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 C10 to
the Attenuators (S4). The attenuator assembly (S4) consists
of four attenuators; a 1 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 1 dB steps, controls both of
the 1 dB step attenuators simultaneously.
4-35. IMPEDANCE SELECTOR. ( Schematic 3)
4-36. The front panel IMPEDANCE switch (A4S1) selects
the
required
output
impedance
of the
654A.
The
impedance networks and the switching connections 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, J3 and J4, are
used; in the UNBAL mode, only J4 is used.
4-37. REGULATED POWER SUPPLIES. ( Schematic 4)
4-38. The regulated power supplies provide all voltages
required by the 654A circuits. The power supplies consist
of a (nominally) + 31 volt series regulated supply and a
(nominally) - 26 volt series regulated supply. The - 26 volt
supply is referenced to the + 31 volt supply.
4-39. The + 31 volt regulated supply is of the conventional
series regulator type. Q1 and Al Q1 are connected in the
Darlington Configuration to increase loop gain of the
circuit, thus improving voltage regulation. Al R14 allows
the voltage to be adjusted to + 31 volts (± 0.5 ); it also
affects the - 26 volt supply (making the plus supply more
positive, makes the negative supply more negative).
4-40. The - 26 volt regulated supply operates in a manner
similar to the + 31 volt supply. Al Q5 is a current limiter
which conducts only when the load current exceeds the set
value. Conduction of AlQ5 causes the series regulator Q2
to reduce the output voltage until the load causing the
excessive current is removed. Diodes Al CR6 and Al CR7
protect the control transistor Al Q4 from short circuits
between the two supplies and short circuits at the output of
the - 26 volt supply.
4-4
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