Amplitude Offset
Description
Amplitude offsets compensate for loss or gain between the test set's RF IN/OUT front panel connector and the mobile station's RF connector.
To access the amplitude offset feature, press the
SYSTEM CONFIG
key, followed by the
RF IN/OUT Amptd Offset
(
F5
) key.
Amplitude offset settings are preserved during power cycles or instrument preset.
Setting Up Amplitude Offsets and Frequency Points
Up to 60 frequency points can be assigned an amplitude offset. Negative amplitude offset values should be entered when there is a loss through the RF cabling and test fixtures and positive values should be entered when there is a gain.
The
RF IN/OUT Amplitude Offset
table displays the current (on/off) state of the amplitude offset feature. You can also enter frequencies and the corresponding offset values from the front panel.
To set up amplitude offsets remotely, one comma-separated string is sent to set up frequency points and another comma-separated string assigns the corresponding amplitudes.
Turning amplitude offsets on/off
When the RF IN/OUT Amplitude Offset State is on, all offsets that are not individually turned off are applied and the word "Offset" appears in the Instrument Status Area of the test set's display.
If the RF IN/OUT Amplitude Offset State is off, none of the amplitude offsets are applied.
GPIB Command
OUTPUT 714;"SYSTEM:CORRECTION:STATE ON" !Sets the RF IN/OUT Amplitude Offset State to On
Examples of Amplitude Offset Behavior
When the amplitude offset table entries accurately represent the loss in all components (cabling, connectors, and test fixturing) in the signal path between the test set and the mobile station, the test set will make the necessary adjustments in both receiver and transmitter measurements.
Mobile Station Receiver Example
When you set a transmit power level, the test set uses the amplitude offset value to adjust the power so that the test set's transmit power level refers to the power level received at the mobile station.
As shown in Amplitude Offset Mobile Station Receiver Example , with the test set's transmit power set to -85 dBm and a -3 dB amplitude offset the actual power level transmitted from the test set will be automatically offset to -82 dBm. With a 3 dB loss in the signal path the mobile station will receive -85 dBm, the actual setting.
Mobile Station Transmitter Example
When you measure power from the mobile station, the displayed and queried values are offset to show the level at the mobile station.
As shown in Amplitude Offset Mobile Station Transmitter Example , with the mobile station transmitting 12 dBm and a -3 dB amplitude offset is entered, the measured power at the test set would be 9 dBm. The displayed power level is automatically adjusted to 12 dBm to show the level at the mobile station.
If the expected power, which can be set manually or automatically is 12 dBm, the test set's internal hardware adjusts itself to receive 9 dBm which is the actual power from the mobile station after 3 dB loss in the network.
Amplitude Offsets Between Frequency Settings
If mobile station testing is performed at frequencies that do not have amplitude offsets assigned to them, the test set will estimate an amplitude offset based on the nearest settings. For example, the following screen shows five amplitude offsets for frequencies ranging from 890.2 MHz to 1710.2 MHz.
For test frequencies between the lowest (890.2 MHz) and highest (1710.2 MHz) frequency points that are not entered in the table, the test set will calculate offsets using piece-wise linear interpolation.
The graph shown in Amplitude Offset Interpolation is a conceptual representation of the test set's amplitude offset configuration using the settings from the RF IN/OUT Amplitude Offset table in RF IN/OUT Amplitude Offset Setup . Each of the five points are shown on a non-scaled frequency versus amplitude offset graph. At a test frequency of 890.4 MHz, which is midway between point number one (-1 dB) and point number two (-2 dB) the test set applies an offset of -1.5 dB. Be aware that since amplitude offsets are in units of dB, this piece-wise linear interpolation does not produce a linear transition from point to point.
If testing is done outside the range of frequencies bounded by the lowest and highest frequency entries, the test set simply uses the amplitude offset that is paired with the nearest frequency point.