Increased Speed and Performance from an RF VNA

Increased Speed and Performance from an RF VNA

Hewlett-Packard Co. (HP),
Microwave Instruments Division
Santa Rosa, CA

The introduction of the original model HP 8753A vector network analyzer (VNA) in 1986 marked the first offering of a compact VNA with an integrated synthesized source and comprehensive capabilities such as direct measurement of group delay and time-domain characterization. While the model HP 8510 VNA served higher frequency applications, designers working at RF and low microwave frequencies didn't have a comparable product available to them. Not surprisingly, the model HP 8753A was a big success and later became the most popular network analyzer ever produced, a claim it retains after four generations of models. However, over the years, customers have required ever-faster test speeds and device throughput. The fifth-generation model HP 8753E RF VNA is introduced as a response to this challenge.

The model HP 8753E VNA is aimed at RF component manufacturers, who are producing large volumes of devices at high speed. The instrument is up to seven-times faster than the previous-generation model HP 8753D. In the near future, the model HP 8753E also will offer the ability to display all four S parameters (as opposed to two) simultaneously, a valuable feature for tuning duplexers, resonators and isolators. The new model has the same high level of performance as its predecessor with a dynamic range of 110 dB from 30 kHz to 3 GHz, and 105 dB to 6 GHz with 0.02 dB dynamic accuracy. Sensitivity is –100 dBm and power at the test port is –85 to +10 dBm. Table 1 lists the model HP 8753E VNA's key performance specifications.

The user now can connect an external VGA monitor to the analyzer for better visibility on the bench and, while lighter and shorter in depth, the model HP 8753E retains the same height and width for compatibility with rack-mount systems. Both the front-panel features and HP-IB programming commands are backwards compatible with the model HP 8753D, which makes upgrading to the new version simple.

The unit also is better suited to measuring noninsertable devices as a result of a new adapter-removal calibration technique. Measurement time can be reduced by the swept-list frequency mode, which allows the user to sweep segments of a user-defined frequency list instead of using a stepped mode as in previous models. In addition, the analyzer allows frequency segments with independent IF bandwidths and power levels to be selected. Data files are compatible with the TouchstoneTM S2P format so they can be imported directly into a PC for analysis and optimization. Finally, flash electrically erasable programmable read-only memory allows firmware to be updated from a floppy disk.

Speed Enhancement
The model HP 8753E employs a model 68040 microprocessor and overhauled firmware that produces major reductions in the time required to sweep, perform measurements, transfer data, recall instrument states, perform error correction and perform time-domain analysis. Typical examples of the speed improvement over the model HP 8753D are listed in Table 2 for a 201-point data measurement.

A firmware upgrade, which will be available in June to model HP 8753E customers, will increase the number of S parameters that are displayed simultaneously from two to four. This upgrade eliminates the need to recall instrument states manually when measuring each S parameter, as shown in Figure 1 . The improvement also allows the tuning effect on the overall performance of the device to be viewed, something that cannot be achieved without a four-parameter display. While this capability aids in testing all types of devices, it also allows for much faster and easier optimal tuning of duplexers and isolators. Any combination of reflection and transmission parameters can be displayed in magnitude, phase, group delay, Smith chart, polar, SWR or time-domain formats.

Improved Calibration
Several enhancements have been made to the analyzer's calibration capabilities, including the addition of the adapter-removal technique, which improves accuracy when measuring noninsertable devices or devices with connectors that are not supported by HP calibration kits. Adapter-removal calibration provides the most complete and accurate calibration procedure for noninsertable devices, and was found previously only on the model HP 8510 VNAs (recently in firmware revision 6.12 and above for the models HP 8753D and 8720D). This method uses a calibration that has the same connectors as the noninsertable device under test (DUT). The electrical length of the adapter must be specified within one-quarter wavelength at each calibration frequency. Type N and 3.5 mm calibration kits for the model HP 8753E contain adapters specified for this purpose.

Two full two-port calibrations are needed for an adapter-removal calibration. In the first calibration, the calibration adapter is placed on the analyzer's port 2 and the test results are saved into a calibration set. In the second calibration, the calibration adapter is placed on the analyzer's port 1 and the test data is saved into a second calibration set.

Pressing the adapter-removal calibration softkey causes the analyzer to use the two sets of calibration data to generate a new set of error coefficients that remove the effects of the calibration adapter. At this point, the adapter can be removed and the analyzer is ready to measure the DUT. In addition to the adapter-removal method, the model HP 8753E performs full two-port calibration with short-open-load-thru standards (the most accurate method for the model HP 8753E) and thru-reflect-line (TRL*) calibration (useful for in-fixture environments such as microstrip). The analyzer also is compatible with new RF ECal electronic calibration products, which calibrate the instrument automatically from 30 kHz to 6 GHz, reducing calibration errors, connector damage and maintenance. With the appropriate calibration module, RF ECal calibrates both insertable and noninsertable devices.

Time-saving Automation
Since the model HP 8753A VNA was introduced, the RF components industry has changed dramatically due, in large measure, to the explosion of wireless communications services. Even devices that do not readily lend themselves to high speed manufacturing need to be fabricated faster and at a fraction of their previous cost.

The new central processing unit has improved the measurement speed directly and is the most obvious enhancement to the RF VNA's performance, but numerous other improvements have also been made. For example, pass/fail testing is much faster, as shown in Figure 2 . By choosing any combination of single-point, horizontal or sloping-line limits from the front panel, the instrument performs measurements automatically and determines if the value obtained falls within the limits set by the user. Pass/fail indication is shown on the display and external monitor, or indicated with a beep. To part handlers, pass/fail is indicated via a TTL output. When the instrument is part of a large automated test equipment (ATE) system, this information is passed over HP-IB to the central instrument controller.

Creating test routines is simple because no previous programming ability is required. The model HP 8753E simply records the keystrokes entered by the user to perform a given set of measurements. The measurement sequence then is recalled by pressing a button on the front panel. The display prompts the user for tuning and other manual adjustments and for input on go/no-go decisions during execution of the test sequence. Once the routine is created, it operates without operator intervention. The analyzer also can control other HP-IB-compatible instruments as well as part handlers.

The model HP 8753E features interpolate error correction, allowing a broadband calibration with up to 1601 points to be performed. The frequency span and measurement points can be adjusted later to suit a specific DUT. The instrument recalculates the error terms automatically based on the new values of frequency and measurement points. This capability reduces the time required to evaluate a device's performance under specific conditions, while maintaining full display resolution.

The instrument allows a device to be measured at up to 30 discrete frequencies or frequency sweep segments, each containing a specific test-port power and IF bandwidth setting. This technique can dramatically reduce the amount of time

required to test a device because performance is measured only at frequencies of interest. Individual frequency segments also can be swept separately without calibration loss.

Device-specific Measurements
When measuring narrowband devices such as filters, resonators and duplexers, the high resolution, stability and wide dynamic range of the unit ensure the validity of the results. The instrument's synthesized source provides 1 Hz resolution, and dynamic range and measurement speed can be optimized by choosing IF filters with a bandwidth from 10 Hz to 6 kHz, as shown in Figure 3 .

Multiple marker values can be used simultaneously while the display shows the performance of the DUT at its 3 dB point, along with pass band ripple and maximum or minimum values. The instrument also allows surface acoustic wave (SAW) devices to be characterized fully for return loss, insertion loss and group delay, as shown in Figure 4 . Pass band ripple can be measured with 0.001 dB magnitude resolution and 0.01 ps delay resolution. In addition, RF leakage and triple transit time can be measured when the instrument is configured with the time-domain option. By applying gating to the main lobe response, the device's response of matching networks and connectors can be determined independently.

In addition to its wide dynamic range and high sensitivity, the VNA provides the ability to set test-port power with precision over a 95 dB range for amplifier measurements. Swept power simplifies gain-compression testing. Power meter calibration provides an accurate setting of the source output power. For mixer measurements, the model HP 8753E offsets its receiver frequency from its internal synthesized source, allowing transmission characteristics of mixers (conversion loss, amplitude, phase tracking and group delay) to be evaluated by stimulating the device over one frequency range while viewing its response over another.

The instrument stores ASCII data files that conform to the CITIFILE standard. The instrument can also create files that are compatible with the S2P format used by Touchstone design software and other design automation packages. In this way, measurement data can be input directly into the design process for circuit simulation.

Multiple Configurations
The model HP 8753E can be tailored to specific user needs through a comprehensive package of options. The base instrument operates from 30 kHz to 3 GHz (optionally to 6 GHz). The front-panel display can now be deleted, reducing costs for users who integrate the instrument into an ATE system. The time-domain option helps locate and resolve discontinuities, and a harmonic-measurement capability allows swept second and third harmonic levels down to –40 dBc to be displayed directly or relative to the fundamental carrier. The integrated test set can be deleted as well.

The base model HP 8753E (3 GHz) with front-panel display and test set is priced at $32,250. The instrument with the 6 GHz option is priced at $37,250. Availability is eight weeks. Additional product information can be obtained from the company's Web site at http://www.tmo.hp.com/tmo/datasheets/English/HP8753E.html.

Hewlett-Packard Co. (HP), Microwave Instruments Division,
Santa Rosa, CA (800) 452-4844, ext. 5661.