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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
rms, 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 BAL.
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 11.6mV 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 least 46dB, 5.8mV rms,
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 steps c and d of Paragraph 5-30 ( 135 BAL
CHECK) to ensure that limits are met at all
frequencies of the 654A.
5-64. 150 AND 600 BAL CHECK.
a. 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 1MHz to 5MHz and
50dB from 10Hz to 1MHz) then repeat step a of
this paragraph (5-64) until balance specifications
are met for all three impedances.
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,
+/-AE (calculated in step a of Paragraph 5-16) on
the dc null voltmeter.
c. Sweep the 654A frequency slowly down to 1MHz;
the null voltmeter indication should remain within
+/-àE of null over the whole frequency range.
NOTE
It may be necessary to slightly offset the
adjustment of A2C43 at 10MHz so that
the flatness is within tolerance across the
X1M range.
d. Set the 654A frequency to 10Hz; if the dc null
voltmeter indication is not within +/- àE of null
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 1
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 +/-àE of
5-18
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