Section 3 Operation
3-6
3-13. Analyzer Measurement Description. The Model
1121A contains an independent distortion analyzer which
can measure frequency, AC and DC level, distortion,
SINAD and signal-to-noise. In addition, ratio measure-
ments can be made with all analyzer measurement
modes. A wide range of special functions enhance the
basic measurement modes without sacrificing the sim-
plified operation of the analyzer. Standard and optional
audio filters are provided to aid in harmonic distortion
analysis and weighted noise measurements. Finally, the
ability to store and recall specific measurement combi-
nations aid in configuring measurement applications for
manual and remote use.
3-14. Analyzer Input Description. The input configura-
tion of the Model 1121A can be selected for single-ended
or balanced/differential operation. The input mode can
be enabled using the front panel FLOAT key or over the
IEEE-488 bus interface.
3-15. Frequency Measurement Function. The Model
1121A measures wide ranges of audio frequency with
high accuracy and resolution. Microprocessor control of
the reciprocal counter results in automatic selection of
frequency ranges for maximum resolution. Measure-
ments are referenced to an internal 10 MHz timebase
accurate to 0.0001% and external reference capability
is also provided.
3-16. Frequency Measurement Display Units. Fre-
quency measurements can be displayed in Hz or kHz for
values above 199.999 Hz with automatic selection of Hz
units below this limit. To select the Frequency mea-
surement functions simply depress the FREQ key which
illuminates both the key’s LED and the KYBD legend in
the ANALYZER display. Display units can then be
selected by depressing the Hz or kHz keys.
3-17. Special Frequency Measurement Modes.
Special function 11 is provided to preset and hold spe
cific level ranges for frequency measurement. Refer to
paragraph 3-65. The frequency measurement mode can
function to input levels 14 db below the selected level
range. For example by setting the 3.000 volt level range,
measurements can be made with signal levels as low as
600 mV.
3-18. Level Measurement Function. The Model 1121A
measures both AC and DC voltage with high dynamic
range and selectable AC bandwidth. Resolution at full
scale is 3000 counts with an additional 33% overrange
capability. The AC rms detector is true rms responding
for signals with crest factors less than 3. Average and
quasi-peak responding detectors (rms calibrated) can
also be selected. Refer to paragraph 3-74. A period
sampling measurement technique is employed which
results in adaptive measurement rates optimized to the
period of the dominant AC component of the input signal.
This technique yields fast settled measurements in the
AC level mode while effectively filtering large AC com-
ponents in the DC level mode. Measurement bandwidth
is selectable using the standard and optional filters to
reject out-of-band noise or provide industry-standard
weighting characteristics.
3-19. Level Measurement Display Units. AC and DC
level measurements can be displayed in linear or loga-
rithmic units. Linear measurements are displayed in mV
or V with V automatically selected for levels above 750
mV and mV automatically selected for levels below 0.300
V. Logarithmic measurements are displayed in various
forms. The default mode uses dBV units (dB relative to
1.000 V rms). Power in dBm units (dB relative to 1 mW)
can be selected for various impedances as described in
paragraph 3-74. To select the Level measurement
function simply depress the LEVEL key which illuminates
both the key’s LED and the KYBD legend in the
ANALYZER display. The various display modes can then
be selected by depressing the appropriate units
associated with the desired display mode. For example,
to select AC level in logarithmic units depress the dB key
and to return the display to linear units depress the mV or
V keys.
3-20. Special Level Measurement Modes. Special
function 11 is provided to preset and hold specific level
ranges to achieve faster first measurement rates and to
eliminate possible range-to-range nonlinearity. Refer to
paragraph 3-65. Special function 17 extends the mea-
surements sampling period to provide a more consistent
reading in the presence of noise. AC and DC calibration
is performed through the use of special function codes 20
through 24. The rms, average or quasi-peak AC
detector type can be selected using special function 70,
71 or 72, respectively. Special functions 80 through 86
select logarithmic display modes in dBV or dBm units.
3-21. Distortion Measurement Function. The Model
1121A measures total harmonic distortion and noise over
a
wide range of frequency. The notch filter is automatically
tuned to reject the fundamental frequency and pass only
the harmonic and noise content. The AC measurement
techniques are similar to those used in the level mea-
surement function yielding fast settled measurements.
Measurement bandwidth is selectable to reject noise
while accurately preserving harmonic components.
Measurement results can be displayed in several forms.
The combination of harmonics and noise can be
displayed as an absolute level in mV, V, dBV or dBm units
or as a ratio in % or dB units to the total input signal
consisting of fundamental, harmonics and noise.
3-22. Distortion Measurement Display Units. Dis-
tortion measurements can be displayed in linear or
logarithmic units. Linear ratiometric measurements are
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