Field Analyzer Advanced Commands

Field Analyzer Advanced Commands

AR RF/Microwave Instrumentation’s FA7000-series of Field Analyzers are unique tools. Traditionally, electric field measurements have been performed using probes designed to measure the root mean square (RMS) of a continuous wave (CW) field. Field Analyzers go further by utilizing a much faster sampling rate, allowing for measurements of pulsed fields and modulation envelopes. This sampling rate can provide a multitude of benefits, many of which are detailed in Application Note: Field Analyzers in EMC Radiated Immunity Testing. This application note will look at some advanced capabilities recently published in the FA7000-series Field Analyzer’s user manual.

Field Analyzer Data Utilization

Currently, there are two primary methods of utilizing Field Analyzer information. One is through the Field Analyzer’s Embedded Web Application. This gives an oscilloscope-like view of the measured field in time while providing a few post-processing options. The data is displayed graphically over a chosen time duration either in Free Run or Triggered modes. An example screenshot of this application is provided in Figure 1.

Figure 1: Screenshot of the Field Analyzer Web Application

The other is through remote control via the USB, GPIB, LAN (Ethernet), or serial ports. Remote commands allow the user to read the instantaneous min, max, or average composite field measurement. An example of where remote control is used is in AR’s emcware EMC test software. For instance, when running a radiated susceptibility scan with active field monitoring, the field analyzer is polled for an instantaneous field measurement at each frequency step.

Since the Field Analyzer's initial release, these have been the only two options for viewing or polling Field Analyzer data. When using the web application, the user can view packets of data but cannot download and use the raw data. Conversely, with remote control, the user can collect instantaneous field readings but cannot collect full packets of data. What has been lacking is a remote command for returning raw packets of data for the user to analyze themselves. With the Spring 2017 update of the FA7000-series user manual, AR has published these additional commands. This new capability allows the user to track trends in the measured field, such as modulation envelope, periodic fluctuations, and non-periodic anomalies.

Field Analyzer Advanced Commands

In total, three new commands have been published into the FA7000-series user manual. These commands can only be used through LAN (Ethernet) connection and are not accessible through any other communication ports. Specifics are provided in the manual, but a summary of the commands is provided here:

  • Read Un-Linearized Data – UDATA?
    • Returns a packet of data samples of field levels that have not been linearized per the Field Analyzer’s internal linearization correction table. This table is obtained through factory characterization of the individual probe. Each sample is an unsigned integer (16-bit) value that represents the digitally sampled value which is 12-bits (0 to 4095). The order of the bytes is least significant followed by most significant. The total number of samples depends on the time base and the trigger mode selected by the user.
  • Read Linearity Table – LTABLE?
    • Returns a sequence of information about the probe’s linearization. The first 32 bytes represent a string containing the probe model number, probe serial number, linearization date and the linearity correction table. The next 40 bytes are 10 A/D values (32-bit single precision floating point values) and the last 40 bytes are their 10-corresponding volt-per-meter values (32-bit single precision floating point values). Each value is represented by 4 bytes, from least significant byte to most significant byte. The total number of bytes for this command is 113 including the termination character.
  • Read Triggered Index – TI?
    • Returns a two-byte (16-bit) value that represents the index of the sample where the trigger occurred. The least significant byte is first.

The first command, Read Un-Linearized Data, provides packets of raw data to the user. Depending on the selected time base and trigger mode, the number of samples can be anywhere from 300 to 6300. Table 1 shows the number of samples for each time base and operation mode.

Time Base (uS/Div) Number of Samples (Free Run Mode) Number of Samples (Internal & External Trigger Mode) Offset to add to Trigger Index (Internal & External Trigger Mode)
400 6000 (12K Bytes) 6300 (12.6K Bytes) 3000
200 3000 (6K Bytes) 3300 (6.6K Bytes) 1500
100 1500 (3K Bytes) 2100 (4.2K Bytes) 900
40 600 (1.2K Bytes) 900 (1.8K Bytes) 300
20 300 (600 Bytes) 900 (1.8K Bytes) 300
10 300 (600 Bytes) 900 (1.8K Bytes) 300
4 300 (600 Bytes) 900 (1.8K Bytes) 300
2 300 (600 Bytes) 900 (1.8K Bytes) 300
1 300 ( 600 Bytes) 900 (1.8K Bytes) 300
Table 1: Un-Linearized Data Sample Sizes and Trigger Offsets
The table above is horizontally scrollable.

UDATA? response and correction

When operating in Free Run Mode or when Triggered, the Field Analyzer gathers raw data in the form of un-linearized, digitally sampled A/D values from the field sensor. This data is collected and grouped into 300-sample segments. Once a UDATA? command is received, a set of concatenated segments with total number of samples equal to those shown in Table 1 is added to the output buffer. For this data to be valuable, though, it must be corrected per the Field Analyzer’s stored linearity correction table. Table 2 shows an example linearity correction table.

A/D Field Strength (V/m)
70 (0x428C0000) 0.0 (0x00000000)
81 (0x42A20000) 20.5 (0x41A40000)
121 (0x42F20000) 42.3 (0x42293333)
217 (0x43490000) 78.8 (0x429D999A)
400 (0x43C80000) 138.2 (0x430A3333)
707 (0x4430C000) 240.0 (0x43700000)
1182 (0x4493C000) 392.9 (0x43C47333)
1870 (0x44E9C000) 616.8 (0x441A3333)
2823 (0x45307000) 931.2 (0x4468CCCD)
4095 (0x457FF000) 1350.8 (0x44A8D98D)
Table 2: Example Linearity Correction Table

These linearity correction tables contain 10 A/D values and their corresponding field strength values in volts-per-meter. The A/D values for field strengths that lie in between the listed values can be linearly interpolated. The LTABLE? command allows the user to read the table that is stored in the Field Analyzer. Once the linearity correction values are known, the conversion must be performed manually by the user on each raw data sample. Equation 1 gives an equation for linear interpolation for use with the linearity correction table and raw data.

Equation 1

$$ y_o=\left(\frac{y_2-y_1}{x_2-x_1} \right)\\(x_o-x_1 )+y_1 $$

From the linearity correction table, y1 and y2 values are from the volts-per-meter column and x1 and x2 values are from the A/D column. Select the two rows that straddle the measured A/D value (xo). Input all these values into the equation and it computes the interpolated volts-per-meter value (yo).

Internal / External Trigger Mode Data Processing

In Internal / External Trigger Mode, when a trigger occurs, the segment where the trigger occurred becomes the ‘center’ segment. The Field Analyzer then gathers enough segments on either side of the ‘center’ segment to encompass the required pre and post data according to the selected time base. This includes the required number of segments to properly represent the selected time base, plus an additional one or two segments of pre and/or post data to account for the trigger location and to gather enough data to allow a full 50% pre and 50% post data. Figure 2 gives an illustration of data collection for a time base of 100 μsec.

Figure 2: Internal / External Trigger Data Collection for a 100 μsec Time Base

Due to the extra data provided, further steps are needed to size the total data packet properly. Otherwise, the timing in relation to the trigger will be out of sync. Keep in mind that the time base and trigger mode are set independent of the process described here. First, it must be determined if a trigger has, in fact, occurred. This information can be obtained using the System Status (STATUS?) command. Details of this command are provided in the FA700-Series User Manual. If a trigger has occurred, the data has stopped flowing into the buffer. The data buffer and trigger index remain stationary until the data is read. Additionally, the user needs to determine where the trigger has occurred. This determination is done with the Trigger Index (TI?) command. The value returned from this command is the index of the sample that crossed the trigger threshold level.

The value returned from this command is the index of the sample that crossed the trigger threshold level.

Once the trigger index is known, the user can now read the stored buffer of data using the UDATA? command. Once UDATA? is called, the trigger and stored data buffer are automatically cleared and the field analyzer is ready to read the next trigger. It is then up to the user to adjust the trigger offset and linearize the data. The trigger index value is within only the Center Segment of data, not an index of the entire data packet itself. After collecting a full Internal & External Trigger packet of data using UDATA? the user must then add the offset value shown in Table 1 to the trigger index value to get the exact sample within the packet where the trigger occurred. The appropriate packet of data can then be centered around this point. Finally, the data can be corrected using the information found in the Linearization Table the same as described above. See Figure 3 for a process flow for Internal / External Trigger Data Collection.

Figure 3: Internal / External Trigger Data Collection Process Flow

Conclusion

With these newly published commands, new capabilities of the FA7000-series Field Analyzers are available. Now the user can utilize remote commands to view raw data over selected time segments, rather than just graphical representations or instantaneous measurements. With the raw data, the user is free to perform all means of analysis and post-processing. If you would like to learn more, feel free to contact one of our applications engineers at 800-933-8181 or visit our website at www.arworld.us.