In the absence of sound, the detector is continuously recording into the memory. When a sound
exceeding the chosen trigger level is detected, the detector interrupts the recording and automatically
replays the stored signal once. A 50% pretrigger function is applied, which means that the detector
stores the signal starting from a time “50%” prior to the triggering instant. 50% relates to the chosen
memory size. E.g. with a memory size of 3.4 seconds, the pretrigger time is 1.7 seconds. That way the
onset of the pulse that triggered the detector, will not be cut off.
The trigger level can be continuously varied with the LEVEL ADJ control. You need a small
screwdriver or similar to change the LEVEL ADJ control. Turning the control clockwise increases the
threshold level. Setting the control maximum counter-clockwise will cause the detector to trigger all
the time, since the noise in the input amplifier then will exceed the threshold level. There is also a
switch (TRIGGER LEVEL) to enable quic kly changing the threshold. The LOW position will trigger
the time expansion system at a low signal level, i.e. the detector will capture weaker signals. Changing
the LEVEL ADJ will affect the sensitivity in both the LOW and HIGH positions.
Normally, all sounds exceeding the threshold level within the frequency range of the detector will
trigger the time expansion system. It is, however, also possible to activate the frequency selective
triggering mode. This is done by setting the TRIGGER SOURCE switch to the HETerodyne position,
and the output from the heterodyne system will be used as trigger signal instead. That way the setting
of the heterodyne frequency control (the frequency shown on the display) will determine which
frequencies the time expansion triggering system will respond to. Setting the heterodyne control to 30
kHz will capture signals between approximately 25 and 35 kHz. This is useful to reduce the influence
from background noise or to make the detector capture only certain species. It should be noted that
even if the detector has its highest sensitivity near the tuned heterodyne frequency, strong signals
outside of this range can also trigger the system. In particular, if the detector’s input amplifier is
overloaded, harmonics will be created, which may cause the triggering system to trigger although the
fundamental frequency is not within the tuned (heterodyne) frequency range.
Heterodyne analysis of the stored signal
The signal stored in the time expansion memory is normally replayed at one tenth of its original rate. It
is, however, also possible to play it through the heterodyne system at its original rate. That way the
user can analyze the stored sequence carefully, e.g. checking for max and min frequencies etc. This
mode is also useful to demonstrate the heterodyne principle in front of an audience.
To feed the stored signal through the heterodyne system, the NORMAL/ TE
→
HET switch should
be in the TE
→
HET position and the HET/TIME EXP switch in the HET position.
This mode is intended for use in the manual triggering mode. If you wish to use it in the automatic
triggering mode, you should change the TRIG switch to MAN immediately after the signal you want
to study has been captured, or the detector will start recording again after replaying the sequence once.
Using the relay output
In some situations, you may want to automatically dump the time expanded calls onto a tape recorder.
This can be achieved using a voice activated tape recorder. However, the D240x also has an output
which can be used to control a relay, which in turn can turn on/off a tape recorder. A separate relay
unit will be available from Pettersson Elektronik.
The relay control output should be used only in the automatic triggering mode. It will be activated
when a signal exceeds the triggering threshold level, and remains activated while the sequence is
replayed once.
