RF Path Characterization

Transcription

1 RF Path Characterization RF path characterization is the process of determining the RF offsets in each RF path, storing those offsets in a symbol table, and correcting RF measurements and generator levels based on the stored offsets. RF Offsets RF Paths Compensating for Offsets RF Path Software Tools RF offsets are the RF losses or gains in the RF path between an RF instrument and unit under test. RF losses vary with frequency and are caused by cable resistive losses, losses of connectors, switches, and so on. RF gains will occur when using RF amplifiers in the RF path. In the TS-5550 system, an RF loss is expressed as a negative (-) offset, and an RF gain is a positive (+) offset. RF offsets affect the accuracy of RF measurements made using the RF path. For accurate measurements, it is important to compensate measurements for the RF offsets. Some systems may incorporate RF switching to allow RF devices such as attenuators to be switched in or out of the measurement. This results in multiple RF paths from an RF instrument to the test fixture. In the HP TS-5550 system, RF offsets are automatically compensated for by the software. You store an array of offsets vs. frequencies for each RF path in a special FixtureCalibration symbol table. Whenever an HP-supplied RF measurement- or stimulus-related action is executed, the measurement or generator output will be automatically compensated based on the stored data. Two sets of software tools are provided with your system to help you determine, manage, and manipulate RF offsets: RF Path Characterization Software FixtureCalibration Table Editing Tools. The RF Path Characterization Software partially automates the process of determining offsets and storing them in the FixtureCalibration table. We recommend that you use this software as it greatly simplifies the characterization process. This software can be used by the system programmer to initially define RF paths and offsets and by production operators to periodically update the RF offsets. This software is available from the HP TS-5500 Utility Software interface and instructions for using it are contained in its on-line help. The FixtureCalibration Table Editing tools allow you to directly edit symbols and to merge data from another table. These tools are generally used for development and debugging purposes.

2 RF Path Characterization in Detail Measuring the RF Characteristics of each Path The following discussion explains the details of how RF offsets are characterized. Most of these details are performed automatically when using the RF Path Characterization Software. The setup shown in <Blue>Figure 1 could be used to characterize an RF path (Offset = 10Log 10 Measured Power/Source Power). When measuring the RF path characteristics, be sure to cover the complete range of both transmit and receive frequencies. HP 8924C RF Test Set RF Power Meter Configured to output a specific power at each frequency being measured Measuring power at each frequency Path being characterized Figure 1. RF Characterization Example For example, assume the RF amplitude vs. frequency is as shown in <Blue>Figure 2. Frequency (MHz) RF Loss (db) Figure 2. Example RF Amplitude vs. Frequency

3 Storing the Offset Data Each RF path is represented as a unique symbol in the FixtureCalibration symbol table. Each symbol is a two-dimensional array of offset vs. frequency data. The first array column (0) contains the frequencies and the second column (1) contains the offsets, in db. Frequency data must be entered in ascending order (lowest to highest). For example, the following table shows the frequency vs. offset data from the previous example. Table 3. Frequency vs. Offset Example Data Array Position Frequency Array Position Offset e e e e e Default Symbol Names User-Defined Symbol Names <Blue>Figure 4 on page 4 shows a typical FixtureCalibration table. Notice the default symbol names in the left-hand column of symbols. For example, CDMA RFIn/Out is the default symbol name for an RF path connected to the HP 8924C RF In/Out port. These default names are automatically used whenever your testplan is initially opened. If you have only one RF path per instrument connector, you can use these default names and will not have to create any additional symbols. If you have multiple RF paths per instrument connector, you will need a unique symbol for each additional path. For example, assume you have two RF paths connected to the HP 8924C RF In/Out port. The first RF path is already accommodated by the default symbol CDMA RFIn/Out. For the second path, you need to create another symbol with a different name (for example, CDMA RFIn/Out2). After creating a new symbol, you can then instruct RF measurement-related actions to use this new symbol by including one of the following actions in your testplan: cdmaacellconfrfcal--specifies the symbol name to use with an RF path connected to the HP 8924C s RF In/Out port. cdmaacellconfduplexcal--specifies the symbol name to use with an RF path connected to the HP 8924C s Duplex port. cdmaacellconfantennacal--specifies the symbol name to use with an RF path connected to the HP 8924C s Antenna In port. Notice that whenever a testplan is initially opened or the system is rebooted, the testplan will look for the default symbol name (for path #1 in this case) until one of the above actions is executed to specify another symbol.

4 FixtureCalibration Symbols in fixture.ust file: Actions that Specify SymbolName to use: Figure 4. Typical FixtureCalibration Table CDMA RFIn/Out CDMA DuplexOut CDMA AntennaIn SymbolName1 SymbolName3 SymbolName5 SymbolName2 SymbolName4 SymbolName6 cdmaacellconfrfcal Specifies SymbolName to use with an RF path connected to the HP 8924C RF In/Out connector cdmaacellconfduplexcal Specifies SymbolName to use with an RF path connected to the HP 8924C Duplex Out connector. cdmaacellconfantennacal Specifies SymbolName to use with an RF path connected to the HP 8924C Antenna In connector. Default Symbol Names User-Defined Symbol Names for Additional Paths

5 Important Points Relating to RF Paths RF offset symbols are stored in the fixture.ust topology file. HP TestExec SL, upon opening a testplan, reads the fixture.ust topology file and dynamically creates the symbol table named FixtureCalibration. HP TestExec SL, upon running a testplan, examines the FixtureCalibration table for a case-sensitive match to the default symbol names (e.g., CDMA RFIn/Out). The offsets in the default symbols (if found) are used until an action is executed specifying different symbols to use. If the default symbols are missing, empty, or not a two-dimensional array, a NULL value is stored and the path offset is assumed to be 0dB. These actions specify which symbol(s) to use: cdmaacellconfantennacal, cdmaacellconfduplexcal, or cdmaacellconfrfcal. Typically, an RF offset symbol should cover the entire expected bandwidth (both transmit and receive frequencies). Both the Calibration Table Editing Tools and the RF Path Characterization Software can be used to create/modify/delete symbols in the FixtureCalibration symbol table. When using both the editing tools and the characterization software, be sure to use identical symbols/names. If you use different names or have differing numbers of symbols, the calibration software will write-over any symbols that do not match.

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