DSP222H_Manual_A
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setting. If a channel is not used, it may be switched off (disabled) by setting it to
Sensitivity 0 (all three switches off).
f) Frequency setting needs to be changed only if crosstalk occurs between nearby
loops. Crosstalk usually manifests itself as chattering of the call output. Change the
frequency if crosstalk is encountered. Four frequency positions are available on each
channel to assist in alleviating interference. It may be necessary to reset the channel
after the frequency has been changed.
g) Check the front-panel indicators. If the Fault LEDs on either channel are flashing
there is a problem with the loop system on that channel. Observe the flash sequence to
determine the type of fault. A single flash followed by a pause indicates an open loop
system. A double flash indicates a shorted loop system. Check the loop connections
carefully. Reset both channels by momentarily selecting a different sensitivity or mode
and then return to the original setting.
h) Monitor operation and make adjustments to the sensitivity and frequency as
necessary.
4. Theory of Operation
The DSP-222 works on the principle that loop frequency is directly related to loop
inductance. However, the change in loop frequency is very small -- perhaps as little as
one hertz. It is easy to measure a change of one hertz simply by counting the number of
cycles in one second. Unfortunately, the minimum response time using this method is
easily greater than one second. To improve the response time, the measurement is
done by gating a high-speed stable oscillator with the loop frequency. This scheme,
called period measurement, provides high-resolution in a short period of time. This
measurement is compared to a previously established reference to determine whether
or not the frequency of the loop oscillator has changed sufficiently to indicate the
presence of a vehicle.
The high speed of the reference crystal oscillator ensures that the sample can be taken
within a very short period of time. For example, at sensitivities less than 4, a sample can
be taken within 2 ms which allows the sensor unit to respond to a vehicle presence (or
loss of presence) in the detection area with an accuracy of 1 ms. Note: This is true only
if the ALTERNATE jumper J2 is installed.
Sensitivity is a function of how long a sample is used for the determination of a vehicle
presence, i.e. the longer the sample time, the more sensitive the sample. Since the
sensor unit measures the minute changes in loop oscillations, it follows that low
frequency loops are innately more sensitive than high frequency loops. In order to keep
