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Section V
Model 654A
EXAMPLE: Suppose
that
,
in
step a of
this
paragraph
,
the amplitude is found to be
too high for one of the impedances; then
set
the 654A AMPLITUDE
control
for an
ac voltmeter
indication
below 0.707V
nns,
and vice
versa.
c.
Repeat the
steps of
this paragraph
as
necessary to
ensure
that the level is
correct
for all impedances.
5-62. BALANCE ADJUSTMENTS.
5-63. 135 BA L.
a.
Perform steps a and
b
of
the Balance Check
of
Paragraph 5-30
.
b_ Set the 654A frequency to 10MHz and adjust
A2C32 (High Freq. Balance) for
optimum
balance
(lowest indication on
the
ac
voltmeter .)
c.
Set the
654A
frequency to
5MHz;
the ac voltmeter
indication
should
be below I I
.6mV
rms indicating
a balance
of greater
than 40dB_ If it is not then
select a
value for A2C21'"
or
A2C36* (but not
both) to give the required balance
at 5MHz.
(Typically, balance of at Icast
46dB, 5
.8
III
V
rillS
,
can
be
achieved).
- - - - - - NOTE
-
-
---
- -
There
is no
way of
predetermining which
side
of
the
balanced
amplifier
is
unbalanced
or
what value
of capacitance
is
required;
A2C21
*
or A2C36* must be
selected
by trial and error. Select the
lowest
value of
capacitance possible.
d. Perform
ste
ps
c
and d of Paragraph 5-30 (135 BAL
CHECK) to ensure
that
limits
are
met at
all
frequencies
of
the
654A.
5-p4_ 150AND600BALCHECK_
3.
Perform the
checks of
Paragraphs
5-31
through
5-34
to ensure
that balance is within
specifications
for the 150
ohm
and
600 ohm
impedances
b. If it is
not, repeat
the procedure of Paragraph
5-63
but
slightly
degrade the balance for the 135
ohm
impedance (be
careful
to remain within the
requirements
of
40dB from lMHz to
5MHz and
50dB
from
10Hz to
I
MHz) then repeat
step
a
of
this paragraph (5-64) until balance specifications
are met for all
three
impedances.
5-18
----------NOTE--
- - - - - -
This
situation
is not likely to arise;
however,
if
it
does,
then
it
is
recommended that
careful
notes be made
during the procedure
as
there is no way,
other
than
by
trial
and
error,
of
determining if the 135
ohm
balance is
being degraded in
a
direction
which will
improve
the
balance
for
the other
impedances,
or
if it is being degraded in a
direction
which
will worsen the balance
for the
other
impedances.
5-65. LEVEL FLATNESS ADJUSTMENTS.
5·66. 75 UNBAL FLATNESS.
a. Perform steps a
through e
of
Paragraph
5·16
(50
UNBAL FLATNESS CHECK)
with
the following
exceptions:
in
step
a.
Set IMPEDANCE to 75 UNBAL:
in
step
b. Use the 75
ohm
thermal
converter
(see
Table
5·3).
b. Set the
654A
frequency to 10MHz and adjust
A2C43 (Frequency Response) for null indication,
+/-t.E (calculated in
step
a
of
Paragraph
5·16) on
the dc null
voltmeter.
c.
Sweep the
654A
frequency
slowly
down to I ivlHz;4
the null
voltmeter
indication should remain with.in
~
+/-t.E of null
over
the whole
frequency
range.
----------- NOTE-----------
It may be necessary to Slightly
offset
the
adjustment of
A2C43 at 10MHL
so
that
the flatness is
within
tolerance
across
the
XIM range.
d.
Set the 654A frequency to 10Hz; if the dc null
voltmeter indication is not within
+/-t.E
of Ilull
change
the value
of
A2C40* (Increase the value
of
A2C40* if the 654A level is too low at 10Hz,
and
vice-versa .)
e.
Sweep the
654A
FReQUENCY dial
slowly
from
I
to 10 for
all settings of
the FREQUENCY RANGE
switch to
ensure
that the 75
ohm output is
flat
over
the
entire
frequency range
of
the instrument.
5-67. 50 UNBAL FLATNESS.
----------NOTE----------
The
adjustments
of
Paragraph
5·66
should
be completed
before making these
adjustments.
a. Perform
steps a
through e
of
Paragraph
5·16.
b. Set the 654A
frequency
to 10MHz; the dc null
voltmeter indication
should
be
within +/-t.E
or
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