An Integrated Microwave Measurement Test Tool

Guy Purchon, Bryan Harber and Tom McConnell
IFR Ltd.
Stevenage, UK

The major instruments that form a microwave test bench have been combined into a single integrated test tool. The result is a one-unit solution to the problem of subsystem testing. This new integrated test tool can characterize microwave networks in terms of power, frequency, return and insertion loss, frequency conversion loss/gain, gain compression, intermodulation, and harmonic and spurious responses.

Functionally, the instrument is an integrated spectrum analyzer, scalar analyzer and fully synthesized signal source operating up to 24 GHz. This powerful combination is capable of measuring the prime performance aspects of passive devices (cables, loads, attenuators and filters), active devices (amplifiers, duplexers, oscillators and mixers) and combinations of these such as up- and downconverters. This article demonstrates that the combination of the three measurement facilities in one unit has greater test capability than the individual discrete test instruments, allowing measurements beyond the capability of stand-alone instruments.

An All-in-one Unit

The 6800 series microwave system analyzer contains three measurement systems in one unit: an independent low phase noise microwave source, a fully featured spectrum analyzer and a scalar analyzer. The choice of hardware architecture and flexible software structure enables the user to make a variety of useful measurements that previously would have required separate instruments in addition to a general-purpose interface bus (GPIB) controller and specially written software.

Network analysis measurements (gain, loss and match) using scalar detectors or the spectrum analyzer tuned input, distance to fault, signal distortion, power and frequency can be accomplished with the measurement system. The performance parameters of the source, including good phase noise and low harmonic content, mean that it also can be used as a stand-alone stimulus.

The flexibility of the microwave system analyzer enables several unique synergistic measurement combinations, including the measurement of up- and downconverters using the spectrum analyzer receiver as a tuned input in conjunction with the scalar analyzer, the gain of multipliers and dividers using the scaling ratio of the spectrum analyzer tracking generator and the use of the source as a spectrum analyzer tracking generator to 24 GHz in comparison to most other instrument combinations, which are restricted to a few gigahertz. In addition, the system can use different measurement modes (scalar, spectrum or fault location) on each of two independent measurement channels to elicit more information about the device under test (DUT) using the same input connector and measurement setup. For example, it is possible to measure the gain of a downconverter subsystem using the tuned input on one channel configured as a scalar analyzer while simultaneously measuring the subsystem LO leakage using the spectrum analyzer on the second channel.

The system also can employ different modes on each channel using different measurement system connectivity. For example, the system can monitor the spectrum on one channel while simultaneously measuring return loss on the other channel using the source output and scalar detector connections. The choice of measurement modes, depending on local circumstances, can allow interference rejection to be achieved when diagnosing faults in antenna feeders. Migration of features traditionally associated with one type of instrument (for example, functions employing several measurement markers on a scalar analyzer) to other types of measurement, such as spectrum analysis, also can be accomplished.

Broad Spectrum Analyzer Capabilities

Spectrum analysis provides the engineer with one of the most effective diagnostic tools available for analyzing signals in terms of power, frequency, distortion and modulation characteristics. A spectrum analyzer in combination with a fully controllable independent source extends the range of measurements further to the characterization of components and subsystems.

Contained in the microwave system analyzer is a fully featured RF and microwave spectrum analyzer covering the frequency range up to 24 GHz. The architecture is conventional; an up- and downconverter cover the 1 MHz to 4.2 GHz range and a preselected harmonic mixer is used for the microwave range. The changeover from RF to microwave ranges is accomplished with an electronic switch, enabling measurement of higher orders of harmonics without having to change bands and avoiding the lifetime issues associated with electromechanical switches that usually perform this task.

The LO uses a YIG-tuned oscillator phase locked to a low noise UHF fractional-N synthesizer. Two modes of operation are used. For frequency spans greater than 20 MHz, the first point in the sweep is locked and the rest of the sweep is free running. On spans of 20 MHz or less, the LO is locked to the UHF synthesizer at every point in the sweep for the best accuracy and phase noise characteristics. With a -90 dBc/Hz noise level at 20 kHz offset, the phase noise of the LO is capable of being used for identifying close-to-carrier spurious signals.

A common requirement is to identify the spectral characteristics from a transmitter or subsystem. Measurements such as power output, harmonic content, modulation and carrier frequency are made. Figure 1 shows fundamental and harmonic power being measured utilizing the instrument's marker table. The peaks of the signals are identified automatically using the peak identification function. This function can also be used to examine the side bands in a modulated carrier, as shown in Figure 2 .

In addition, the measurement system has several other useful features that assist in making these measurements. The instrument can be set up in a dual-channel mode with a wideband sweep on one channel and a narrowband sweep on the other to measure both parameters simultaneously. In addition, the use of limit line masks and marker tables enables a comprehensive survey of leakage signals and adjacent-channel power to be made. The frequency of a signal can be measured either directly from the marker readout (since the narrowband sweep is known to be locked) or, for greater accuracy, using the built-in frequency counter function in the marker menu. This function halts the sweep at the marker position and uses a counter built into the IF system to measure the frequency.

The spectrum analyzer has an instantaneous dynamic range of greater than 80 dB, which enables useful high dynamic range measurements to be made on components when operating in conjunction with the source as the stimulus. Thus, the receiver is made available as an input to the scalar analyzer, which provides the appropriate measurement functionality such as path calibrations and marker features that are appropriate to network analysis. Component and subsystem gain and return loss can be measured in this mode.

The gain of amplifiers, filters, attenuators and cables can be measured with the source set to sweep the same range as the spectrum analyzer receiver. Mixers and up- and downconverters can be measured using the source set to sweep with a constant offset from the spectrum analyzer range, as shown in Figure 3 . Multipliers and dividers also can be measured with the source set to the appropriate ratio of the spectrum analyzer frequency range.

With the source set to CW mode, additional signal characteristics of these devices also can be measured with the same measurement setup by using the second channel settings, as shown in Figure 4 . Return loss of a component or subsystem can be measured with the addition of a bridge or coupler on the output of the source and the spectrum analyzer connected to the coupled arm.

Frequency Translation Components and Subsystems

The measurement of components and subsystems where the output frequency range differs from the input presents a significant measurement challenge. Although an experienced user may feel comfortable thinking about the measurement setup in terms of the frequency offsets and scale factors required between the source and the receiver, most users would prefer to let the instrument do the work.

A special conversion measurement user interface has been developed within the scalar network analyzer that allows the user to specify the known characteristics of a mixer or up-/downconverter, thus dealing with the complexities caused by the choices of conversion required. Figure 5 shows the frequency translation entry screen. The user may select an up- or downconverter and enter its input, output and LO frequencies, and select the upper or lower side band. Usually, it is only necessary to enter three of these parameters; the remainder are determined by calculation and automatically entered for the user.

In addition, the entry interface automatically determines whether the selected output signal will be sweeping down in frequency as the input signal sweeps in the conventional upward direction. That being the case, the signal source is instructed to reverse its normal sweep direction in order to preserve a conventional low to high frequency sweep at the output of the DUT. Once the entry table is complete, the offset sweep system is configured automatically and the mixer under test can be inserted, allowing both frequency response and output power to be determined, as shown in Figure 6 .

A Calibration Process for Conversion Gain/Loss Measurements

After selecting the calibration key and connecting the two measurement ports, as shown in Figure 7 , the user is offered a choice of two regimes: a default selection where the source is directed to sweep over the selected analyzer input frequency range or an alternative that allows the analyzer sweep range to be moved to that of the source. The correct choice is determined largely by the expected influence of the interconnecting cables and their relative position when the DUT is connected into the circuit. The DUT then can be inserted into the measuring circuit and the output trace, and its response is shown. A second trace with no calibration applied can provide a measurement of the output power of the DUT as measured through the spectrum analyzer input. The two traces are displayed on the same screen and the sweeps are multiplexed.

Scalar Analysis and Fault Location with a Synthesized Source

Component characterization is achieved in terms of insertion and return loss and power, which are the most common measurements made at microwave frequencies. Where phase information is not an issue, scalar analysis is a cost-effective means of obtaining this information. Amplifier and oscillator performance parameters also can be determined in this way.

The flexibility of scalar analysis is due to the range of microwave accessories that can be used to construct the measurement system. The system requires detectors for signal amplitude measurements (either absolute power or as the ratio of signal levels) and directional devices such as autotesters or microwave bridges for return loss.

To achieve maximum accuracy, the scalar detection system has been optimized several ways. The main contributions to measurement error arise from mismatch uncertainty, nonlinearity, dynamic range and frequency response. Two detector types are utilized. One detector type has attenuator pads incorporated into the front end to provide lower SWR without significantly affecting noise floor. This configuration provides a better mismatch uncertainty and enables more accurate characterization of low loss devices such as cables and small value attenuators where mismatch ripple can severely mask the true response. The integral pad also enables higher power handling - advantageous for amplifier testing. The second detector type has no integral pad and is optimized for measuring devices with wide dynamic range such as high value attenuators or high performance filters where a low noise floor is required to see the true characteristics of the rejection band.

Wide dynamic range measurements also require accurate linearity correction, which is incorporated by means of correction data stored in an electronically erasable programmable read-only memory (EEPROM) within the detector. When the detector is connected to the instrument, the data are downloaded from the EEPROM and used in all subsequent measurements. The detector is characterized in terms of linearity and frequency response during manufacture so each detector stores its own unique set of correction data.

An integrated source and scalar analyzer offer a distinct advantage, namely single front-panel control of both the source output and display parameters. This capability may not be an issue for simple measurements, but for complex measurements it relieves the user of the need to understand the background technical detail.

Fault Location

Experience in the detection of faults in antenna feeder systems shows that the main cause of degradation and failure of RF and microwave communication systems is the transmission line from the transceiver to the antenna, whether waveguide or coax. To ensure minimum downtime, an easy-to-use monitoring system is required to check for degradation or, where failure has already occurred, to locate the exact position of the fault. Fault locators were designed specifically for this purpose. A fault locator contains a microwave bridge network and fault locating detector network integrated into a single housing, thus allowing the simultaneous measurement of return loss and distance to fault by a single test port. When used with the microwave system analyzer, the complex task of measurement and data storage of antenna feeder characteristics is made simple.

In the case of coax feeders (a nondispersive transmission medium), the source provides a normal linear frequency sweep as the stimulus to characterize the reflection response of the line with distance, as shown in Figure 8 . When dispersive transmission media (such as waveguide) need to be tested, the distance information would be incorrect if a linear frequency sweep was used. To compensate for this condition, a warped sweep is generated where consecutive data points conform to a nonlinearly increasing frequency.

Antenna measurement presents a further complication as it is often necessary to establish antenna match performance in the presence of airborne signals. These signals are registered by the detectors and make the results meaningless. However, by using a modulated signal as the stimulus for the measurement, unwanted signals can be rejected. This AC detection mode maintains scalar measurement accuracy regardless of the environment.

Finally, the close integration of source and analyzer produces a power sweep. This facility, where the x-axis is source power (rather than frequency) and the y-axis is power, enables amplifiers to be characterized not only in terms of gain, but also with regard to frequency and gain with power level. Thus, measurements of output power, gain, return loss and gain compression (1 dB gain compression point) can be made simultaneously using the setup shown in Figure 9 . By configuring the other independent measurement channel as a spectrum analyzer, the amplifier's harmonic distortion can be ascertained at the same time. Figure 10 shows the results of a simultaneous power and gain compression measurement.

Conclusion

The integration of a spectrum analyzer, scalar analyzer and synthesized signal source into one unified microwave test system has produced measurement capabilities that exceed those of the individual test instruments and provides size and portability advantages that are particularly advantageous for production test and field service applications. This single test measurement system is capable of fully characterizing most passive and active components and subsystems operating to 24 GHz with ease and accuracy.