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Test Setup For Measuring Amplifier Distortion And Noise; Noise And Distortion Measurement - HP 450A Operating And Servicing Manual

Hp 450a amplifier serials prefixed: 010
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Section IV
Paragraphs 4-22 to 4-23
b. Set the oscillator output to any convenient fre-
quency between 100 and 10,000 cps and the output
voltage to exactly -20 db as read on the 4000 Volt-
meter connected to the oscillator output.
c. Set the amplifier to 40 DB and measure the open-
circuit output signal level with the 4000 Voltmeter.
d.
If
necessary adjust R6 to obtain a reading of ex-
actly +20 db on the voltmeter.
Vary line voltage
between 104 and 127 volts to be sure gain remains
within specifications.
e. Set the amplifier to 20 DB and increase the oscil-
lator output voltage to 0 db as read on the 4000 Volt-
meter connected to the amplifier input.
f. Measure the open-circuit output level from the
amplifier which must be within 1/4 db of the reading
obtained in step d.
g. Adjust R6 so that the difference between the 20
and 40 db gains is divided equally about +20 db on the
voltmeter scale.
The gaib tolerance is
±
1/8 db on
each range.
If
the gain difference is greater than
the specified tolerance, R3 in the amplifier or the
voltage range switch in the voltmeter is inaccurate,
or VI or V2 may be defective.
h. Repeat steps band c using an oscillator frequency
of 2 mc.
i.
If
necessary adjust Cll to obtain an output voltage
within
±
1 db of +20 db. Vary line voltage from 104
to 127 volts to be sure gain remains within specifi-
cations.
j. Set the amplifier to 20 DB, set the oscillator fre-
quency to 1 mc and increase the oscillator output
voltage to 0 db as read on the 400D Voltmeter con-
nected to the amplifier input.
AUDIO OSCILLATOR
205AG
AMPLIFIER
450A
TO
POWER
SOURCE -
ADJUSTABLE LINE
TRANSFORMER
Mode14S0A
k. Measure the open-circuit output voltage from
the amplifier.
If
necessary refine the adjustment
of Cll so the output level is within
±
1/2 db of +20 db
on the voltmeter.
m. Recheck the 40 db gain at 2 mc.
If
a satisfactory
compromise cannot be reached for these high fre-
quency gain measurements, the value of C12 may
require adjustment.
Increasing C12 increases the
gain of the amplifier at high frequency.
n. In the same manner, amplifier gain at low fre-
quency may be checked using a voltmeter such as
the Model 403A.
If
the low-frequency gain is below
that specified, check the coupling capacitors and
tubes.
p. The gain adjustment is completed; replace ampli-
fier cover and return amplifier to normal service.
4-23. NOISE AND DISTORTION MEASUREMENT.
4-24. Distortion in the amplifier output is measured
by applying a pure sine-wave signal to the amplifier
input and measuring the harmonics of this signal in
the amplifier output after rejecting the fundamental
frequency.
The Model 330B Distortion Analyzer is
an electronic ac voltmeter preceded by an electronic
frequency-rejection filter which is adjustable from
20 cps to 20,000 cps. After the fundamental frequency
is rejected by the filter, the total level of all re-
maining signals is measured by the voltmeter. This
level consists of random noise, line-frequency ripple,
and all harmonics of the applied signal frequency in-
cluding those in the applied signal. To measure dis-
tortion and noise, proceed as follows:
a. Connect the test equipment as shown in figure 4-2,
turn on and allow a ten-minute warmup of all instru-
ments.
DISTORTION ANALYZER
3308
~~~
G~~@~~=
3000
l'zWATT
COMPOSITION
RESISTOR
4-4
Figure 4-2. Test Setup for Measuring Amplifier Distortion and Noise
00001-1

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