Output Attenuator; Regulated Power Supply; Simplified Schematic Of Current Limiting Circuitry - HP 652A Operating And Service Manual

Model 652A
components which might be present on the detector output.
In
addition, A3C3 is switched in on the XIO range to
dampen the meter by reducing low frequency ripple.
4-23. The differential amplifier, A3Q2 and A3Q3, amplifies
the difference between the fIltered output of the detector
and the reference voltage from the variable reference power
supply. The output is taken from the collector of A3Q2, is
further amplified by A3Q5, and is applied to the emitter
follower, A3Q6. The emitter follower has a low output
impedance and is used to drive the monitor. A3Q4 acts as a
current source to provide approximately 50 microamps of
current to the differential amplifier.
4-24. The reference power supply is a conventional series
regulator type that has a variable output. Zener diode
A3CR6 maintains a constant bias on the emitter of the
reference amplifier, A3Q8. The base bias of A3Q8 is changed
whenever the REF SET coarse or VERNIER controls are
varied. This change is amplified and applied to the base of
the series regulator, A3Q7. A3Q7 will change the reference
supply output in direct proportion to the amount that the
REF SET coarse or VERNIER controls are changed.
4-25. Two adjustments are provided for the expand monitor
circuit: A3Cll and A3R14. A3CI l adjusts the expand
monitor circuit response at 10 MHz, and A3R l 4 calibrates
the monitor at 10 kHz.
4-26.

OUTPUT ATTENUATOR.

4-27. The output attenuator provides a means of attenuating
the signal level applied to the 50-ohm and 600-ohm output
connectors. The OUTPUT ATTENUATOR switch, S2,
selects a combination of four resistor networks to produce
the desired level of signal attenuation. Each step provides an
attenuation of 10 dB. The �'I1PLITUDE controls, R2A and
R2B, vary the level of attenuation in increments between
each 10 dB step selected by the OUTPUT ATTENUATOR
switch.
4-28. Output impedances other than the standard 600-ohms
can be obtained by changing the value of resistor S2R13. The
value of the resistor replacing S2RI3 is added to the 50-ohm
oscillator output impedance to obtain the new output
impedanc e level at the 600-ohm connector.

4-29. REGULATED POWER SUPPLY.

4-30. The regulated power supply provides all de voltages
required by the 652A Test Oscillator circuits. The power
supply consists of a + 30 V and· 25 V series regulated
supply. Each power supply is protected by current limiting
and foldback current limiting.
4-31. The + 30 V and· 25 V power supplies are func·
tionally identical. Both use operational amplifiers for out·
put voltage error amplification. A l R40 adjusts the + 30 V
supply voltage and A I R 4 l adjusts the· 25 V supply volt·
age.
� 2:0V
;:! g
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a
IOV
FOLD BACK
CURRENT LIMIT
IOOmA
OUTPUT CURRENT
Figure 4-2. Regulated Power Supply Output
Voltage vs. Current.
4-32. Conventional current limiting is used in both supplies
to limit the output current to approximately 300 mAo
Foldback current limiting further limits the output current
if the output voltage is pulled below approximately 16 V
by a malfunction in the 652A circuitry. A direct short to
ground of either supply will result in an output current of
approximately 10 rnA as shown in Figure 4-2.
4-33. Figure 4-3 is a simplified schematic of the current
limiting circuitry used in the 652A power supplies. The
Current Limiting Transistor A IQ4(+) or AIQ7(·) is a vari·
able shunt to the series regulator drive current. It is first
switched on by the voltage drop across the Current Limit
Sensing Resistor when the power supply output current
reaches approximately 300 rnA. The power supply will
remain in this Conventional Current Limit condition until
the output current decreases allowing the supply to return
to normal operation, or until the power supply output volt­
age drops below the Foldback Reference (16.2 V). If the
lalter occurs, the Diode Switch is effectively closed and the
power supply goes into a Foldback Current Limit condi­
tion. In this condition the Current Limiting Transistor is
controlled by the power supply output voltage. As the out·
put voltage decreases, the shunt current is increased.
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EuOtf��K I
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LIMITER
, I SWITCH
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I L _ {I�Y]._J
SE�ES REGULATOR
DRIVE CURR£NT
Figure 4·3. Simplified Schematic of C urrent
Limiting Circuitry.
Section IV
OONVENTlONAL
CURRENT
liMIT
200mA
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SHUNT
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4-3