11-12-2012, 01:10 PM
A Versatile Test Bench for Wireless RF/Microwave Component Characterization
[attachment=43337]
Introduction
Wireless product development requires many types of RF/microwave characterization measurements. These
measurements cover the gamut from on-wafer testing to final system performance verification of a completed wireless
product, such as a pager, cellular telephone or two-way radio. A well-equipped RF/microwave characterization
laboratory would include several test stations with a cost of over $100K each. However, if measurements below 3 GHz
are the focus, these needs can be satisfied using the few basic microwave instruments configured in the low cost test
bench described in this article.
Here, the primary interest in a versatile low cost test bench is for undergraduate and graduate wireless-related
education. However, the described techniques and equipment are equally applicable to design and manufacturing
applications. Assisted by grant funding from the National Science Foundation and Hewlett-Packard Co., the University
of South Florida (USF) has recently established an innovative new Wireless and Microwave Instructional (WAMI)
laboratory.1 The described equipment is used on six identical benches in this new learning environment for a required
undergraduate course entitled "Wireless Circuits and Systems Design" as well as senior-/graduate-level electives,
including a course entitled "RF and Microwave Measurements."
The goal of these courses, and the purpose of this article, is to provide exposure to a wide range of measurement
techniques of interest to the wireless RF/microwave circuit and system designer. Specific test configurations are
described for several example measurements of interest, including basic S-parameter measurements, swept amplifier
compression, mixer port match, isolation and conversion loss, and third-order intercept (TOI).
Test Bench Description
The basic USF WAMI lab test bench equipment is shown in Figure 1. The three main
microwave instruments are a 0.3 to 3000 MHz model HP8714 (B or C) RF VNA, a 9 kHz
to 2.9 GHz model HP8594E SA and a 100 kHz to 3200 MHz model HP8648C
synthesized signal source. Significant cost savings can be realized if measurements to 1
GHz (rather than 3 GHz) are determined to be sufficient, as listed in Table 1. Further cost
reductions are possible by replacing the VNA with a scalar network analyzer.
Basic S-parameter Measurements
The basic VNA calibration consists of connecting short-open-load (SOL) standards on port 1 to establish a reflection
calibration and a thru connection between ports 1 and 2 (usually desired at the ends of a pair of test cables) to establish
a transmission response calibration. This procedure provides a full three-term reflection model accounting for
imperfect directivity source match and reflection tracking, and a one-term transmission error model accounting for
transmission tracking errors.
With additional effort, source match errors can be corrected for in transmission calibrations but not load match.
Although the VNA provides a more limited error model than used in full two-path/two-port VNAs (such as the HP8753
or Anritsu 37200A), it usually provides more than adequate measurement accuracy up to its 3 GHz upper frequency
limit. The effects of (uncorrected) source/load mismatch in transmission measurements
can be reduced by using attenuators (or pads) on either side of the DUT.
An example two-port S-parameter measurement made with the VNA for the case of a
Piezo Technology lumped-element 915 MHz bandpass filter is shown in Figure 3. The
filter has a center frequency of 914.3 MHz, 3 dB bandwidth of 31.1 MHz and in-band
insertion loss of 4 dB. These measurements are determined easily using a soft-key option.
Power Compression Measurement
The power for P1dB measurement is a commonly used figure of merit for amplifiers and mixers (in which case P1dB is
the power for 1 dB conversion gain/loss compression). The swept frequency test configuration also can be used for
compression testing. In this case, the VNA is set in CW mode and the power sweep feature is used to sweep input
power (instead of frequency) as the x-axis variable in the display. Once the desired input power sweep range is set
through a combination of internal (if available) and external attenuation, calibration is achieved with a thru connection
in place of the DUT. The output pad must provide sufficient attenuation to maintain linear VNA receiver operation at
the maximum expected amplifier output power. In the case of a high power amplifier measurement, additional
precautions, such as the use of a directional coupler and high power load, may be necessary. For amplifier
measurements, the tuned receiver (normal VNA mode) is used. For mixer measurements, the broadband (diode
detector) mode is used along with some other test accessories and considerations.
[attachment=43337]
Introduction
Wireless product development requires many types of RF/microwave characterization measurements. These
measurements cover the gamut from on-wafer testing to final system performance verification of a completed wireless
product, such as a pager, cellular telephone or two-way radio. A well-equipped RF/microwave characterization
laboratory would include several test stations with a cost of over $100K each. However, if measurements below 3 GHz
are the focus, these needs can be satisfied using the few basic microwave instruments configured in the low cost test
bench described in this article.
Here, the primary interest in a versatile low cost test bench is for undergraduate and graduate wireless-related
education. However, the described techniques and equipment are equally applicable to design and manufacturing
applications. Assisted by grant funding from the National Science Foundation and Hewlett-Packard Co., the University
of South Florida (USF) has recently established an innovative new Wireless and Microwave Instructional (WAMI)
laboratory.1 The described equipment is used on six identical benches in this new learning environment for a required
undergraduate course entitled "Wireless Circuits and Systems Design" as well as senior-/graduate-level electives,
including a course entitled "RF and Microwave Measurements."
The goal of these courses, and the purpose of this article, is to provide exposure to a wide range of measurement
techniques of interest to the wireless RF/microwave circuit and system designer. Specific test configurations are
described for several example measurements of interest, including basic S-parameter measurements, swept amplifier
compression, mixer port match, isolation and conversion loss, and third-order intercept (TOI).
Test Bench Description
The basic USF WAMI lab test bench equipment is shown in Figure 1. The three main
microwave instruments are a 0.3 to 3000 MHz model HP8714 (B or C) RF VNA, a 9 kHz
to 2.9 GHz model HP8594E SA and a 100 kHz to 3200 MHz model HP8648C
synthesized signal source. Significant cost savings can be realized if measurements to 1
GHz (rather than 3 GHz) are determined to be sufficient, as listed in Table 1. Further cost
reductions are possible by replacing the VNA with a scalar network analyzer.
Basic S-parameter Measurements
The basic VNA calibration consists of connecting short-open-load (SOL) standards on port 1 to establish a reflection
calibration and a thru connection between ports 1 and 2 (usually desired at the ends of a pair of test cables) to establish
a transmission response calibration. This procedure provides a full three-term reflection model accounting for
imperfect directivity source match and reflection tracking, and a one-term transmission error model accounting for
transmission tracking errors.
With additional effort, source match errors can be corrected for in transmission calibrations but not load match.
Although the VNA provides a more limited error model than used in full two-path/two-port VNAs (such as the HP8753
or Anritsu 37200A), it usually provides more than adequate measurement accuracy up to its 3 GHz upper frequency
limit. The effects of (uncorrected) source/load mismatch in transmission measurements
can be reduced by using attenuators (or pads) on either side of the DUT.
An example two-port S-parameter measurement made with the VNA for the case of a
Piezo Technology lumped-element 915 MHz bandpass filter is shown in Figure 3. The
filter has a center frequency of 914.3 MHz, 3 dB bandwidth of 31.1 MHz and in-band
insertion loss of 4 dB. These measurements are determined easily using a soft-key option.
Power Compression Measurement
The power for P1dB measurement is a commonly used figure of merit for amplifiers and mixers (in which case P1dB is
the power for 1 dB conversion gain/loss compression). The swept frequency test configuration also can be used for
compression testing. In this case, the VNA is set in CW mode and the power sweep feature is used to sweep input
power (instead of frequency) as the x-axis variable in the display. Once the desired input power sweep range is set
through a combination of internal (if available) and external attenuation, calibration is achieved with a thru connection
in place of the DUT. The output pad must provide sufficient attenuation to maintain linear VNA receiver operation at
the maximum expected amplifier output power. In the case of a high power amplifier measurement, additional
precautions, such as the use of a directional coupler and high power load, may be necessary. For amplifier
measurements, the tuned receiver (normal VNA mode) is used. For mixer measurements, the broadband (diode
detector) mode is used along with some other test accessories and considerations.