Coupler Measurements 69
In the past, FONIX analyzers have not measured harmonic distortion when
the signal level was set to 90 dB SPL or higher. This is called the “90 dB har-
monic distortion rule.” Since hearing aids in recent years have become more
sophisticated and able to handle loud signals, we have turned this off this
rule by default. However, if you would like to use it, first set the USER LEVEL
to ADVANCED in the Default. See Section 2.3.2 for details. Then, go into the
Custom Menu (press [MENU], [NEXT], [NEXT]) and turn ON the DIST 90 dB
RULE.
Figure 3.5.1—Measuring harmonic distortion
3.5.2 Measuring Intermodulation Distortion
Intermodulation (IM) distortion occurs when more than one frequency is pres-
ent in the source signal and those frequencies combine to create new frequen-
cies not actually present in the source.
In the static IM distortion test, the FP35 analyzer produces a signal consisting
of the primary and secondary frequencies, and shows the resulting response as
a composite type display. The measurement is updated in real-time until you
turn it off. See Section 2.5.2.2 for more information on the FP35’s IM distortion
measurement.
1. Press [MENU] from the Coupler Multicurve screen to enter the local menu.
2. Make sure DISTORTION is set to OFF, and make sure the REF MIC is OFF. If
necessary, use the arrow keys to make these selections.
