Amplitude Offset

Last Updated: August 29, 2007

The Amplitude Offset window allows you to configure the test set's amplitude offset feature. You can configure the frequency value in MHz and the offset value in dB for up to 100 amplitude offsets. You can configure different tables for the RF IN/OUT and RF OUT ONLY connectors.
Amplitude offsets compensate for loss or gain between the test set's RF IN/OUT and RF OUT ONLY connectors and the wireless devices's RF connector. The amplitude offset settings are preserved during power cycles or instrument preset.

How to access the amplitude offset feature

To access the amplitude offset feature:

from the display's Main Menu.
On the Main Menu select the Screens menu, select Configuration, and then select the Amplitude Offset softkey, or

using the front panel.
Under SCREENS, press the Config key, and then select the Amplitude Offset softkey.

Amplitude Offset Parameters

Amplitude Offset Table Usage

Sets the usage of the amplitude offset table. When set to:

Single only the RF In/Out amplitude offset table and state are used. These settings are used on both the RF IN/OUT and RF OUT ONLY connectors.
Multiple both the RF In/Out and RF Out Only amplitude offset tables can be used if their respective state's are set to On.

This parameter is available on both the RF In/Out and the RF Out Only tabs, and changing it on one tab automatically changes it on the other. When this parameter is set to Single the RF Out Only Amplitude Offset State and the table associated with the RF OUT ONLY connector are not applicable, therefore you cannot select them. To use different amplitude offset tables on each connector you must set this parameter to Multiple.

SCPI command: SYSTem:CORRection:TABLe[?]

RF In/Out Amplitude Offset State

Sets the amplitude offset state of:

the RF IN/OUT connector when Amplitude Offset Table Usage is set to Multiple, and
both the RF IN/OUT and RF OUT ONLY connectors when Amplitude Offset Table Usage is set to Single.

SCPI command: SYSTem:CORRection[:RFIOut]:STATe[?]

RF Out Only Amplitude Offset State

Sets the amplitude offset state of the RF OUT ONLY connector when Amplitude Offset Table Usage is set to Single. When Amplitude Offset Table Usage is set to Multiple the state for the RF OUT ONLY connector is controlled by RF In/Out Amplitude Offset State.

SCPI command: SYSTem:CORRection:RFOut:STATe[?]

Number/Frequency/Offset Table

Allows you to configure the frequency value in MHz and the offset value in dB for up to 100 amplitude offsets. You can configure different tables for the RF IN/OUT and RF OUT ONLY connectors.

To use:

different tables on each connector you must set Amplitude Offset Table Usage to Multiple and ensure both the RF In/Out Amplitude Offset State and the RF Out Only Amplitude Offset State are set to On.
the same table on both connectors you must set Amplitude Offset Table Usage to Single and 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 wireless device, the test set makes the necessary adjustments in both receiver and transmitter measurements.

Wireless Device 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 wireless device.

As shown in the following graphic, 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 is automatically offset to -82 dBm. With a 3 dB loss in the signal path the wireless device receives -85 dBm, the actual setting.

Wireless Device Transmitter Example

When you measure power from the wireless device, the displayed and queried values are offset to show the level at the wireless device.

As shown in the following graphic, with the wireless device transmitting 12 dBm and a -3 dB amplitude offset is entered, the measured power at the test set is 9 dBm. The displayed power level is automatically adjusted to 12 dBm to show the level at the wireless device.

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 wireless device after 3 dB loss in the network.

Amplitude Offsets Between Frequency Settings

If wireless device testing is performed at frequencies that do not have amplitude offsets assigned to them, the test set estimates 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 calculates offsets using piece-wise linear interpolation.

The following graph is a conceptual representation of the test set's amplitude offset configuration using the settings from the above table. 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. Note 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.

It is highly recommended that amplitude offsets are set up for each test frequency. This eliminates inaccuracies due to the mismatch between the test set's linear interpolation and the actual frequency response of the RF path between the test set and wireless device.

W-CDMA Cal Application (E6832A)