Output Impedance; Noise Spike Test Setup; Output Impedance, Test Setup - HP BENCH Series Operating And Service Manual

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TERMINATION
POWER SUPPLY
CASE
T-CONNECTOR
AC-
ACC-
6ND-
0.01 uf
+ O-
50 '
f'
Li^
n OSCILLOSCOPE
CASE
(t 1
O.OIuf
— d).
V
<—
% ^
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VERTICAL
INPUT
V„
VERTICAL
INPUT
AC
ACC
-GND
T-CONNECTOR
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TERMINATION
Figure 5-7. Noise Spike Test Setup
power supply would be zero at all fre
quencies, while the output impedance
for an ideal constant current power
supply would be infinite at all fre-
quenc ies.
The output impedance of a power supply is normal
ly not measured, since the measurement of tran
sient recovery time reveals both the static and
dynamic output characteristics with just one meas
urement. The output impedance of a power supply
is commonly measured only in those cases where
the exact value at a particular frequency is of
engineering importance.
5-36 To check the output impedance, proceed as
follows:
a. Connect test setup shown in Figure 5-8.
cables, must be substituted for the shielded two-
wire cable.
2. Impedance matching resistors must be
included to eliminate standing waves and cable
ringing, and the capacitors must be connected to
block the DC current path.
3. The length of the test leads outside the
coax is critical and must be kept as short as pos
sible; the blocking capacitor and the Impedance
matching resistor should be connected directly
from the inner conductor of the cable to the power
supply terminals.
4. Notice that the shields of the power sup
ply end of the two coax cables are not connected
to the power supply ground, since such a connec
tion would give rise to a ground current path
through the coax shield, resulting in an erroneous
measurement.
5. The measured noise spike values must
be doubled, since the impedance matching resis
tors constitute a 2-to-l attenuator.
6. The noise spikes observed on the oscil
loscope should be less than 0. 5mV p-p.
5-34 The circuit of Figure 5-7 can also be used
for the normal measurement of low frequency ripple
and noise; simply remove the four terminating re
sistors and the blocking capacitors and substitute
a higher gain vertical plug-in in place of the wide
band plug-in required for spike measurements.
Notice that with these changes, Figure 5-7 be
comes a two-cable version of Figure 5-6C.
5-35 OUTPUT IMPEDANCE
Definition: At any given frequency of
load change, AEqut/ AIqUT- Strictly
speaking the definition applies only for
a sinusoidal load disturbance, unless,
of course, the measurement is made at
zero frequency (DC). The output im
pedance of an ideal constant voltage
VOLTMETER
hp403B
INDICATES Eo
POWER SUPPLY
UNDER TEST
VOLTMETER
hp403e
INDICATES Ein
OSCILLATOR
hp 200 CD
IK
IVWVJ
100 OHM
Figure 5-8. Output Impedance, Test Setup
b. Set METER SELECTION switch to VOLTS
position.
c. Turn on supply and adjust VOLTAGE con
trols until front panel meter reads 20 volts.
d. Set AMPLITUDE control on Oscillator to
10 volts (Ein), and FREQUENCY control to 100 Hz.
e. Record voltage across output terminals of
the power supply (Eq) as indicated on AC voltmeter.
f. Calculate the output impedance by the
following formula:
Zout = g.
t-in - tiO
Eq = rms voltage across power supply output
terminals.
R = 1000
Ein =10 volts
g. The output impedance (Zout) should be
less than 0. 030 ohms.
5-6