User's Manual

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RFI-148 & RFI-900 High Output Power Paging Transmitters User Manual Page 52 of 134

8.2.2 Pre-emphasis Filter

To minimise low frequency noise and distortion, a pre-emphasis filter can be engaged into the audio path

(“ATM32=1”). It is expected that a matching de-emphasis filter would be used in the receiving equipment,

accordingly, to restore the original audio. Disabling pre-emphasis results in a flat response across the supported

frequency band. Refer to Table 16 for filter specifications.

8.2.3 Deviation Limiter

To prevent adjacent channel interference, as might be caused by the injection of a signal louder than that used

to calibrate the audio circuit (by e.g. shouting into the microphone), a deviation limit can be configured

(“ATM30”). This value must lie between the currently configured deviation frequency and the maximum

supported deviation. Indication is provided when the limiter is being engaged via the Deviation Limiter Status

field (“ATM31”).

8.2.4 Audio Polarity

The PTX can be configured to swap the interpretation of the Audio+ and Audio- inputs (“ATM33=1”).

8.3 Calibration

Because the PTX can support a wide range of input amplitudes, its internal audio circuitry must be calibrated

when any of the following changes occur

• Deviation frequency

• Deviation Limiter frequency

• Audio input amplitude

Calibration is initiated by:

• Setting a new deviation value,

• Setting a new Deviation Limiter value or

• Pressing the labelled “Calibrate” push button on the PTX’s expansion plate (at the rear).

It is possible to install a PTX and successfully calibrate, with confirmation, without any serial or Ethernet

connection to the unit. This is achieved through the use of the front panel LEDs; specifically the Tx Power

LED array. On Calibration initiation, all green LEDs on the front panel Tx Power array will illuminate,

followed by a phase wherein each one extinguishes in turn. This “countdown” phase indicates that calibration

Note that changes to Carrier Offset, Pre-Emphasis engagement and Audio Polarity do not require recalibration.