Amber 5500 - Minimum Load Impedance

Programmable
Audio
Measurement
System
Section 2
Amber model
5500
Owner’s
Manual
Version
07
The
values can be
changed by
using
a
special
function.
In the dBm
case,
the
assumed load
can take values
from 1 to
9999
ohms.
In dBV
and
Volts,
the load can
be 1 to
9998 ohms or
infinity
(open
circuit).
Note that
dBm,
being
a unit
of
power,
cannot assume an
infinite
load. For
dBm the
SPCL FNCT is
5,
for Volts
and
dBV it is
SPCL FNCT 4. To
enter a
value,
key
in the SPCL
FNCT number
(4
or
5)
followed
by
a
decimal
point
followed
by
the
value
in ohms
from
1
to
9999
and
terminated
by
the
SPCL
key.
(In
the
dBV or Volts
case,
the
value
of
9999
will
represent infinity
or
open
circuit).
For
example,
to
set the dBm
external
load to
150
ohms,
key
in:
'
SPCL
J”
M
mmm
'Q
Note
that
in the
dBm case the load
im-
pedance
is also
the dBm
reference im-
pedance,
that
is,
the
output
level will be set
using
the
true
meaning
of
dBm,
not
using
dBu or
dBm
referenced to
600 ohms.
To
determine
the
currently
stored
load im-
pedance,
key
in
4.0 SPCL or
5.0 SPCL. The
left hand
display
will show the
stored value.
2.2.6.1 MINIMUM
LOAD
IMPEDANCE
The
power
amplifier
used
in the
generator
is
subject
to
certain
constraints. One
of these
is
the
maximum
voltage
it can
produce.
A
second is the
internal
dissipation
it can
hand-
le. To
provide
protection
for the
latter,
the
power
amplifier
has
a current
limit
feature.
This
insures
that no
damage
will
occur
even
if the
output
is
short
circuited.
The
combination of the
voltage
constraint
and
current limit will
define
the maximum
output
capability
for various
source and load
impedance
combinations.
Conditions that
exceed
these limits will
cause
the
amplifier
to
current
limit
(to
clip)
as a
dissipation
protec-
tion but
without
warning
the
user.
The user must
specify
the
value of the exter-
nal
load
impedance,
the
generator
source
impedance
and the
desired
signal
level
delivered to
the
external
load. The
5500
computer
then
calculates the
power amplifier
drive level
required
to
produce
this
desired
signal
level in
the extemal
load. If the
drive
level exceeds
the
power
amplifier
voltage
capability,
an
error
condition is
generated.
However,
if
the
voltage
is
within limits but the
current is
exceeded,
no
error is
generated.
Similarly,
if
an
external load
impedance
below the
specified
load
impedance
is
used,
the
amplifier
may
current
limit
(clip).
Certain extreme
conditions of
overload can
actually
shut
down the
entire
5500 instru-
ment due to
power
supply protective
fault
monitoring.
Table 1
illustrates the
minimum
value of ex-
ternal
load
impedance
that
can be used
without
causing
the
power
amplifier
to
cur-
rent limit.
As a
single power
amplifier
is used
to
drive both the
A and
optional
B
output,
the
limit
changes
depending
on whether
one
or
both
outputs
are
being
used.
Table
2 illustrates the
maximum
output
in
dBm that'is
possible
under
various
smirce
and load
impedance
variations. Note that the
dBm value follows
the
strict
definition as
power
relative to
1 mW
-
not
dBm/600
ohms
2-20
f
b