Nortech PD27 Series - Automa�C Frequency Selec�On

307UM0200_01 May 2016 PD27x User Manual Page 13 of 30
The level of the detect must be suciently greater (at least 0.8%) than the sensivity level or else the maximum
potenal temperature dri from one time of day when the power fails to another when the power returns mustn’t
exceed 20°C.
The level of the detect should be no less than 0.15% and a minimum of 0.8% above the sensitivity level.
With AFS acvated, none of the channels must be in fault since this will cause the unit to retune on recovery.
If the presence of a vehicle should occur less than a second before power is lost, there is a chance that the unit won’t
retain the detect and the outputs could toggle on restoration of power.
If these condions are not met, it is possible for the detect condion to be lost on recovery of power.
Powerfail cannot be guaranteed to work if a user interferes with the detector in any way when the power
is OFF. No sengs are allowed to be changed. The enclosure housing the detector shall NOT be opened
when the unit is OFF and Powerfail is turned on, as any drastic change in temperature could result in
Powerfail starting up in the incorrect state. Nortech accepts no responsibility or liability in the event of
incorrect behaviour caused by user interference.
4.6 Automatic Frequency Selection
AFS allows the detector to briey evaluate all five frequency bands and select the best operang frequency available. It
weighs up each selecon based on where the frequency is located within the operaonal range, the signal strength, and
the level of detected noise. AFS allows the detector to evaluate all the frequency selecons.
Due to the increased processing required, AFS takes longer to tune than when it is deacvated. The tune me with AFS
on can range from 5 to 20 seconds. If aer this period of time, the detector sll has not tuned, it is also possible that
none of the frequency selections are suitable for the loop. If this occurs, manual frequency selecon should be used.
Due to the sporadic nature of noise, the channel may seem quiet during the evaluaon but sll suffer from cross-talk.
AFS can be toggled on or off via the DIP switch. While the detector is tuning, it is recommended to allow the AFS tuning
to complete.
On both manual and automatic frequency selecon, once a valid frequency offset has been selected, the detector waits
for that seng to sele below the sensitivity setng before allowing normal operaon to connue. If there is sufcient
dri from an extreme temperature change in the installaon locaon or vehicles driving over the loop at the me, it is
possible that the tuning will timeout, invalidate that frequency selecon and aempt to tune again. If the situaon is
very close to the limit, it is possible that it might tune as the dri or noise goes back over the threshold and the unit
goes into detect just as it tunes. In a noisy environment, this can be prevented by retuning and if the situation persists
to decrease the channel sensivity.