Section 3 Operation
3-10
Selection of display units can be made by depressing
either the %, dB, kHz or Hz keys. On subsequent mea-
surement cycles the results will be displayed relative to
the original ratio reference. The RATIO key is an alternate
action key, therefore, depressing the RATIO key again
will deselect the ratio mode, extinguish the key’s LED and
return the display to the normal measurement mode. If
another measurement function is selected while the ratio
mode is active, the LED on the RATIO key will be
extinguished but the ratio reference is preserved for the
original measurement function and can be reactivated by
depressing the original measurement function key. The
ratio mode is limited to only one reference value and the
old ratio reference is lost if the ratio mode is activated in
an alternate measurement function.
3-32 Using Analyzer Filters. The audio filter keys are
alternate action keys which means they are “toggled” on
and off with each key stroke. The optional filters are
mutually exclusive; therefore, depressing on of the keys
will cancel the other. The same is true of the low-pass
filters, only one low-pass filter can be used at a time. The
DC filter, however, is mutually exclusive with all filters and
will also be canceled by selecting any measurement
functions other than Level.
3-33. The minimum bandwidth consistent with the mea-
surement bandwidth should be used to minimize noise
errors. For example, when measuring the distortion of a 1
kHz fundamental tone, the 30 kHz low-pass filter is
recommended. The DC low-pass filter is provided to
attenuate all AC components and measure DC level
directly. The DC low-pass filter can only be activated in
the analyzer level mode. Band-pass filters are combina-
tions of high- and low-pass filters and are used in some
measurements to simulate the sensitivity of the human
ear to the audible frequency spectrum. High-pass filter
selection is used to eliminate power line harmonics when
present. The 400 Hz high-pass filter typically provides
more than 80 dB of attenuation at 60 Hz.
3-34. The CCIR filter is a special case since its application
depends on whether the average or quasi-peak detector
is selected. The CCIR recommendation 468-3 specifies
the filter should be used with the quasi-peak detector.
Another application referred to as the CCIR/ ARM calls for
using the average responding meter (ARM) and applying
a gain correction value of -6.6 dB. The Model 1121A will
accommodate both applications. The average or
quasi-peak detector can be selected using special
function 71 or 72 respectively. The level readings will
also be adjusted by -6.6 dB automatically when the
average detector is selected.
3-35. Audio Oscillator General Description. The Model
1121A contains a variable frequency, low distortion audio
oscillator and a precision programmable amplifier/atten-
uator. The oscillator frequency tuning uses a frequency
lock technique under microprocessor control to yield high
accuracy and resolution. The output level can be varied in
very fine increments over a wide range of levels. Levels
can be set in either linear or logarithmic units to
accommodate existing test procedures and applications.
The oscillator incorporates a user configurable sweep
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