Ericsson LBI-39128 manual Alarms, Power Measurements, SWR Calculations

transmitters, make all the required power and SWR calculations, and compare all measurements and calculations to the applicable high and low alarm limits. Only alarm conditions that are measured 750 milliseconds or more after the associated transmitter is keyed are used to send an alarm to the Site Controller computer. This gives the transmitter time to reach full power and avoid false alarms.

These messages also tell the PMU when each transmitter is about to be unkeyed (about to stop transmitting). This advance warning will prevent the PMU from making measurements while or after the associated transmitter is unkeyed, again to avoid false alarms.

Power Measurements

The PMU makes power measurements by measuring the dc voltages from the power sensors connected to its Analog Inputs. Each dc voltage measurement is translated into a power level by using a table of equivalents that was empirically derived from the typical characteristic of a 1000 watt power sensor. This table, shown in Table 7, contains only certain discrete power levels. Therefore, power measurements made by the PMU can only be one of these discrete power levels, nothing between.

Any specific power level (P) given in the table represents a range of dc voltages (V) from the sensor. The lower limit of this range is the voltage shown in the same row of the table as the power. The upper limit of this range is the voltage shown in the next lower row of the table. Because voltage ranges and a look-up table are used to determine a power level (as opposed to using an actual mathematical calculation), a voltage measurement that is teetering on the dividing line between two voltage ranges will appear to be jumping as much as 2.5 watts at the 100 watt level (98.8 to 101.3) and give the impression that the power level is unsteady when it isn’t. If one of these power levels is within the alarm limits and the other is out, you may see this as an intermittent alarm.

When the PMU is told by the Site Controller computer that a specific transmitter is keyed, the PMU measures the dc voltage from the power sensor installed in that transmitter’s output circuit, determines the power using the values shown in Table 7, and compares the power to the high and low alarm limits for this transmitter. If the power is within the limits, nothing more happens. If the power is not within the limits, an alarm for this transmitter is sent to the Site Controller computer (see the Alarms heading).

When the PMU is told by the Site Controller computer that a specific transmitter is keyed, the PMU also measures the forward power dc voltage from the power sensor installed in the input circuit for the antenna used by this

transmitter, determines the power using the values shown in Table 7, and compares the power to the high and low alarm limits for this antenna. If the power is within the limits, nothing more happens. If the power is not within the limits, the alarm is sent to the Site Controller computer (see the Alarms heading).

SWR Calculations

Antenna SWR calculations are based on the two dc voltages (one for forward power and one for reflected power) from the bi-directional power sensor installed in the antenna’s input circuit. The dc voltages are translated into power levels as described under the Power Measurements heading. The PMU then calculates SWR using the following formula:

SWR = (1 + p) / (1 – p)

Where: SWR = standing wave ratio p = the square root of (Pr/Pf) Pr = reflected power

Pf = forward power

When the PMU is told by the Site Controller computer that a specific transmitter is keyed, the PMU measures the forward and reflected power dc voltages from the power sensor installed in the input circuit for the antenna used by this transmitter, determines the forward and reflected power using the values shown in Table 7, calculates the SWR, and compares the calculated SWR to the high and low alarm limits for this antenna. If the SWR is within the limits, nothing more happens. If the SWR is not within the limits, the alarm is sent to the Site Controller computer (see the Alarms heading).

ALARMS

A transmitter alarm indicates that the upper or lower alarm limit for the output power has been exceeded. An antenna alarm indicates that the upper or lower alarm limit for the input power, or the upper limit for the SWR, has been exceeded.

When the Site Controller computer receives an alarm from the PMU, it only receives the transmitter channel number or the antenna number for the alarm - not which limit was exceeded. (To know which limit has been exceeded, look at the Alarm History Report screen on a terminal connected to the PMU. For more information, look under the Diagnostic Screens heading in the Maintenance section.)