the sequence once, it is replayed repeatedly until the push-button is pushed again. The frequency
control and the displayed frequency does not have any effect on the time expansion system.
The time expansion system also has several advanced operation modes, which are described below.
If the volume control is turned up too high, acoustic feedback (a howling sound) may occur when the
detector is working in the heterodyne mode. To avoid this, simply turn down the volume control or
use headphones instead.
The D240X comes with a soft carrying case. For protection of the detector we recommend you keep it
in this case when not in use. The detector should be placed with the display upwards in the case.
THE HETERODYNE SYSTEM
There are several different principles to convert ultrasound into audible sound. As mentioned above,
one of the systems in the D240x is based on the heterodyne principle. This technique means that a
limited frequency range is selected for conversion into the audible range. If the frequency control is set
to 30 kHz, the range from appr. 25 to 35 kHz will be transformed. This is illustrated in the figure
below.
Let’s assume an ultrasonic signal with constant frequency is emitted. When the frequency control of
the D240x is turned from low frequencies to higher, a high-pitched tone will be heard in the
loudspeaker when the frequency setting is appr. 5 kHz lower than the frequency of the ultrasound
source. The closer the tuned frequency comes to the ultrasound's frequency, the lower the frequency of
the loudspeaker tone gets. When both frequencies are identical the resulting output frequency becomes
zero, i.e. nothing is heard in the loudspeaker. If you continue to turn the frequency control towards
higher frequencies, a tone will again be heard in the loudspeaker, however this time the frequency will
increase as the tuned frequency increases. By tuning the frequency control up and down it is possible
to locate the frequency resulting in a zero Hz output frequency (the tuned frequency then equals the
frequency of the ultrasound).
NOTE: The example above is only intended to illustrate the heterodyne principle. Constant frequency
sounds are very rare in practice, so in most cases you will only be able to get an approximate measure
of the signal frequency.
