Operating Instructions; Introduction; Controls And Indicators; Adjustment Of Mechanical Zero - HP 333A Technical Manual

Model 333A/334A

3-1. INTRODUCTION.

3-2. The Models 333A and 334A Distortion Analyzers
measure total harmonic distortion from 5 cps to 600
Kc. Harmonics up to 3 Mc are included. The sharp
elimination characteristics, >80 db, the low level of
instrument induced distortion, and the meter accuracy
of the 333A and the 334A result in accurate measure-
ment of low level harmonic content in the input signal.
3-3. An RMS voltmeter is inherent in the 333A and
334A, The voltmeter provides a full scale sensitivity
of 300 u volts rms (residual noise <25 u volts). The
voltmeter frequency range is from 5 cps to 3 Mc ex-
cept on the 0. 0003 volt range, which is from 20 cps to
500 Kc.

3-4. CONTROLS AND INDICATORS.

3-5. Figure 3-1 illustrates and describes the function
of all front and rear panel controls, connectors, and
The description of each component is
indlcators.
keyed to a drawing included within the figure.
3-6. ADJUSTMENTS OF MECHANICAL ZERO.
3-7. The procedure for adjustment of mechanical
zero is given in Section V, Paragraph 5-25.

3-8. GENERAL OPERATING INFORMATION.

3-9. INPUT CONNECTIONS.

3-10. Signal source can be connected to the 333A and
334A through twisted pair leads or a shielded cable
with banana plug connectors. Keep all test leads as
short as possible to avoid extraneous pickup from
stray ac fields,
When measuring low-level signals,
battery operation is recommended to avoid ground
loops. Another method for avoiding ground loops is
by connecting only one instrument in a test setup
directly to power line ground through a NEMA (three-
Connect all other instruments to
prong) connector,
the power source through a three-prong to two-prong
adapter and leave the pigtail disconnected. Both the
333A and 334A have a dc isolation of ±400 vdc from
the external chassis with the shorting bar, (item 16 ,
Figure 3-1), disconnected.

3-11. VOLTMETER CHARACTERISTICS.

9-12. The RMS VOLTS markings on the meter face are
based on the ratio between the average and effective
(rms) values of a pure sine wave. The ratio of aver-
age to effective values in a true sine wave is approxi-
mately O. 9 to 1. When the meter is used to measure
complex waves, the voltage indicated may not be the
rms value of the signal applied. This deviation of
meter indication exists because the ratios of average
to effective values are usually not the same in a com -
plexwave as in a sine wave. The amount of deviation
depends on magnitude and phase relation between
harmonics and fundamental frequency of the signal
SECTION III

OPERATING INSTRUCTIONS

applied. Table 3-1 shows the deviation of the meter
indication of a sine wave partly distorted by harmonic..
As indicated in the table, harmonic content of less
than approximately 10% results in very small errors.
Table 3-1
Effect of Harmonics on Voltage Measurements
Input Voltage
Characteristic RMS Value
Fundamental = 100
Fundamental +10%
2nd harmonic
Fundamental +20%
2nd harmonic
Fundamental +50%
2nd harmonic
Fundamental +10%
3rd harmonic
Fundamental +20%
3rd harmonic
Fundamental +50%
3rd harmonic
NOTE
This chart is universal in application
since time errors are inherent in
all average-responding type voltage-
measuring instruments.
3-13. In distortion measurements where the fundamen-
tal frequency is suppressed and the remainder of the
signal is measured, the reading obtained on an average-
responding meter may deviate from the true total rms
value. When residual wave contains many inharmoni-
cally related sinusoids, the maximum error in the dis-
tortion reading is about 11% low for distorilon levels
below 10%.
Measured Maximum Error
Distortion In Meter Indication
2. 5% +0. 11 X O. 025 =
0.00027
This example represents the maximum possible error,
and in most cases the error is less. In distortion
measurements, the reading of an average-responding
meter is sufficiently close to the rms value to be
satisfactory under most measurement conditions.

3-14. USE OF OUTPUT TERMINALS.

3-15. The OUTPUT terminals provide a O. 1 v rms
output for full scale meter deflection
minals can be used to monitor the output signal with
an oscilloscope, a true rms voltmeter, or a wave
analyzer. The combination of the distortion meter
and oscilloscope provides more significant information
TM 11-6625-1576-15
Section III
Paragraphs-3-l to 3-15
Meter
True
Indication
100 100
100. s 100
102 100-102
112 100-110
100.5
9 6 - 1 0 4
102 94-108
112 90-116
Total
Distortion
0.025 +0. 0027 =
0.0277 or 2.8%
These ter-
3-l