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H 52005-301D
counter-clockwise
position,
it enters upon
an approximately
linear
range
of
at
least 40
dB adjust-
ment.
Maximum output, when the
control is fully
clockwise,
is at
least +15 dBm into
a
600 fl
load, or up to more than
8.5
V
acrcss
a high impedance load.
Make
small
adjustments to the
level with
the
FINE OUTPUT control.
This control
has a
range of
approximately
0.
7
dB.
20.
The output impedance
of
the signal source with no attenuation will be
that
of the two
oscil-
lators in parallel, i.e
.
300 0.
If, however,
10
dB
or more
of attenuation
is
used, the
effective
output impedance will rise to
600 fl.
21.
If it
is
desired to use the instrument as a
single tone
signal
source
the oscillator not in
use
should be disconnected; either by pressing the OUTPUT switch to the
right,
or
by
breaking the
plug
and
socket connection at the rear
.
If
this precaution is not
taken, and
the unwanted
oscil-
lator simply switched off, it will remain as
a
non-linear load across the
output
of
the oscillator
in use, giving rise to distortion.
MONITORED ATTENUATOR
INPUT
22.
Set
the METER RANGE switch to a position suitable for the input to
be
applied; set it to
OFF
if the input will not be
a sine
wave.
The
input
should not exceed
25
V a.
c
.
or
d. c.
23.
The INPUT socket
on
the front panel
is a type
BNC
.
Cover
this
socket
with
the
cap
pro-
vided when the INPUT
soc~ets
on the
rear
panel are
being used.
All the
sockets
are
in parallel.
24.
The input impedance is
600
fl,
providing
tha
t
the output
impedance
is
matched
by
the load.
H
the output
is
mismatched,
the
input impedance is still
600
fl
provided
that
a substantial
amount
of attenuation
is
in
use;
if
the load has a
very high or
a very
low
impedance
relative
to
the
matching
value, the attenuation
can
be
reduced to
about 13
dB
or
less before a
1
O%
variation
arises
in the
input
impedance.
When the load has
a
very high impedance,
the
internal termi-
nation should normally be used,
and the output
will then
be approximately matched
.
READING INPUT SIGNAL
LEVEL
25
.
Turn the METER RANGE
switch
clockwise
to
the most
sensitive
range
that can
be
used
without the meter deflection exceeding
full-scale.
Read the
app
ropriate scale on the
meter
according
to the
setting
of the
switch.
Readings on the
scales for the
lower
two ranges are
direct
reading and for the higher two ranges require to
be multiplied
by
10
a
nd
5.
The
meter
reading is the
:r.m.s.
value of the input
voltage, assuming
that the signal is
a sine wave.
The
reading may be inaccurate
if
the signal is
not
sinusoidal,
or
if its
frequency
is less than
20
Hz
.
26.
The meter
also
has
a
scale calibrated
-6
dBm to
+6
dBm
.
The
0
dBm
mark
on
this
scale
corresponds
to the
voltage,
on the
0
to
1.
5
V scale,
that
will
produce
1
mW
input
to the attenua-
tor.
When the voltmeter is used
on
higher
voltage ranges,
the dBm
scale
is
effectively
exten-
ded
by
addition of the dBm increments marked
at the range
switch positions.
27.
When the OUTPUT IMPEDANCE switch
is set
to
one of
the
BAL positions, if the wanted
output is such that
less
than
10
dB of
attenuation
is
required,
first
switch
in 10 dB
a
nd
terminate
the
attenuator
internally or externally
before
setting the
input and
reading
the
meter.
Then
reduce the attenuation
as required, and
do not
compensate for any
rise
in meter reading.
28.
The dBm
reading
indicates the power
fed into the attenuator
networks
assuming that
the
input impedance is
600
fl.
This will not be
so if the
instrument
is mismatched at
its
outlet
and
the value of attenuation is low (para
.
24)
.
29.
The dBm scale
and
the dBm markings on the
METER RANGE switch
can
be used to relate
in
dB different levels
of
applied
signal without
regard
to the actual
values
of power concerned
.
Chap.
3
Page 6
Nov. 79
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