CHARACTERIZATION AND SIMULATION OF A 915MHZ WIRELESS RECEIVER

Lawrence P. Duuleavy Paul G. Flikkema

Anbuselvan Kuppusamy

Department of Electrical Engineering University of South Florida

4202 E. Fowler Ave., ENB 1 18 Tampa, FL 33620

dunleavy@eng.usf.edu http://www . eng.usEeddWAMI

OFF.: (813)974-2574 FAX: (813)974-5250

Abstract - A versatile test bench and CAE environment are employed to characterize and simulate a 915MHz receiver subsystem. The subsystem, which down-converts to a 70MHz IF frequency, is constructed with coaxial components and characterized at several difFerent levels. The 3GHz test bench used includes a vector network analyzer, a spectrum analyzer, and a synthesized signal source. Measurements are compared to system simulations enabled by HP EEsof s Series IV: Ch"mcations Design SuiteTM (CDS). Simulation accuracy is improved through the use of measured component level characterization data. This receiver case study provides powerful insight into the capabilities of modem measurement and simulation tools for assessment of system- level linear and non-linear performance. The revealing case study, developed for a new undergraduate course in Wireless Circuits and Systems, would be easily duplicated for in-house company training, short course seminars, as well as RF/microwave courses at other universities.

Key Word - Wireless systems, RF measurements, microwave measurements, receiver, 1dB compression, system simulation, downconverter, CAE.

I. Introduction

Knowledge of the relationships between component and system performance is fundamental to wireless RFImicrowave product design. Interestingly, there are few if any courses that treat both RF/microwave systems and components, much less do so with a hands-on hardware approach with direct linkage to a modem simulation tool set. class that attempts to fill this void [l]. Presented is a 915MHz receiver subsystem developed in the framework of this new class. The subsystem, shown in Figure 1, is comprised of coaxial components that can readily be characterized individually and assembled together to function as a receiver/downanverter. The subsystem is studied in detail herein through a variety of component and subsystem level characterization measurements, as well as system simulations using a modem CAE tool set. The information presented is relevant to both industrial as well as academic/training settings.

Recently, the University of South Florida has developed a revolutionary undergraduate laboratory

The equipment shown in Figure 1 is contained in an exciting new learning environment at USF called the Wireless and Microwave Instructional Laboratov, a.k.a. "WAMI" Lab (see http://www.eng.usf.eduNAhQ. In the work described herein, characterization measurements were made using primarily the HP8714 vector network analyzer (VNA), which features both broadband and narrow-band detection modes and an upper frequency of 3GHz. Also utilizes was an HP8594E spectrum analyzer (SA) with an upper kquency of 2.9GHz. Referring to Figure 1 , during system evaluation and testing the input is eitha connected to port 1 of the

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VNA or to an antenna, for reception of lab-generated (via an HP8648C signal source) as well as unwanted "real-world" signals. The output is either connected to the receive port (port 2) of the VNA for swept power or swept frequency measurements, or to the SA for analysis of signal frequency and amplitude content under non- swept CW conditions. The test bench was described in more detail, along with test codigurations and techniques, at a recent conference [2].

In this work, measurements and simulations were made of two complete receiver assemblies, mering only in component serial #'s, denoted Sys 1 and Sys 2. The simulations are enabled by the system analysis capabilities of HP-EEsof s Series lV (ver. 6.0) Communications Design Suitem (CDS), formerly known as Omnisysm. Sys 1 and Sys 2 measurements were made on merent test benches containing essentially the same instrumentation. The scope of this paper includes small signal analysis, power budgeting, and power compression characteristics; consideration of amplitude and phase noise is left for future treatments. The component and subsystem measurement and simulation techniques should be of interest to anyone involved in development of RFlmicrowave fiont-end systems.

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Figure 1. Basic test bench used for component and subsystem characterization. The bench includes a 2.9Ghz spectrum analyzer (SA), a 3.2Ghz synthesized signal source, and a 3Ghz vector network analyzer (VNA) . Also shown is the 915MHz reeeiver subsystem studied. TP1, "2, TP3, and TP4 are test points.

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II. ComDonent Characterization

Our analysis of the subsystem of Figure 1 begins with a characterization of individual components. Shown in this section are the measurements for the hardware setup denoted Sys 1. Included are characterization results for the two amplifiers, the mixer, and the two filters. The pads were also measured and found to be very close to the specified attenuation values.

A. Amplifier Characterization

The HP8714 VNA was used to measure the small signal characteristics of the two amplZers in the subsystem [2,3]. One issue involved with amplifier characterization is that the input signal must be low enough to ensure linear operation of the DUT. The standard configuration for the HP8714 VNA does not include an intemal attenuator, limiting the power range from -10dBm to +loam. Hence depending on the VNA options external attenuation may be needed to achieve the proper DUT input level. The output signal level must also be controlled (e.g. again with extemal attenuation, if necessary) such that it is within the linear range of the VNA's receiver.

Figure 2 shows the measured small signal responses for the two amplifiers. Reflection calibration for this, as well as the filter measurements shown later, was achieved using a 3.5" calibration kit's short, open, and load standards. Transmission calibration is achieved with a simple response calibration, using a thru Connection [3]. The measurements show that the Miteq amplifier (Figure 2a), although rated fiom 2-8GHz, has flat gain performance (3 1-32dE3) down to 150h4Hz. In our receiver subsystem it provides a gain of about 3 1.6dB at 915MHz. The input return loss is better than 12dB &om 150h4Hz to 3GHz. The Mini-cKcuits amplifier (Figure 2b) is rated to lGHZ and has a measured gain of about 22dB fiom 0.3MHz to 1GHZ. The return loss is better than l5dB across this fiequency range. With respect to the receiver fiequencies, both of these amplifiers can be practically considered as broadband gain blocks as they do not frequency limit the performance of the receiver. In fact, the same Mini-Circuits amplifier could be used in both the RF and IF amplifer positions (of course, subsystem performance would change).

(a) RF Amplifier - Miteq. (b) IF Amplifier - Minicircuits.

Figure 2. Broadband (0.33000MHz) measured forward reflection and transmission (S11 and S21) response for Miteq (a) and Mini-Circuits @) amplifiers. Marker 1 is at 915MHz in both plots, and Marker 2 is at 70MHz in (b). The input signal level was approximately 30 dBm for both (a) and (b).

The 1dB gain compression level, PldEt, of the two amplifiers will play a role in determining the saturation characteristics of the receiver. Figure 3 shows the VNA measured results used to determine PldE3 for the RF

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and IF amplifiers (see [2] for test setup). In each of the two plots, the gain is plotted as a function of corrected input power. The input power was corrected by directly measuring it using the VNA's power measurement mode [3], with the input cable directly connected to the VNA test port 2 connector. For the RF amplifier Figure 3a) the fiequency was 915MHz during the swept power measurement, while for the IF amplifier (Figure 3b) the fiequency was 70MHz. The results show an input PldB level of -17.3dBq or approximately +13.ldBm output PldB, for the Miteq ampliGer at 915MHz. Circuits amplifier, produced a PldB, referenced to the input of -16.3 dBm, corresponding to an output PldB of 4.8dBm.

The similar measurement, made at 70MHz on the Mini-

34

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-25.16 -22.69 -19.96 -17.38 -14.97 Corrected Input Power (dBm)

(a) RF Amplifier at 915MHz.

Figure 3. Measurement of input 1dB gain compression power for RF amplifier and IF amplifiers using power sweep mode of VNA Input power was corrected by a direct measurement using the VNA power measurement mode

@) IF Amplifier at 70MHz.

6. Mixer Characterization

There are many parameters of interest for a mixer; however, we will restrict our attention to mixer conversion loss. Conversion loss is defined as the difference in dB between the power presented to the mixer RJ? port (at the RF fiequency) and the power exiting the IF port (at the IF fiequency). Conversion loss measurements were made in two different ways for the present application. In the first method, the broadband detection mode of the HP8714 VNA [2,3,4] was used. The RF signal was swept from 785 to 1485 MHz, with the LO frequency held at 985MHz. This corresponds to IF fiequencies ranging fiom 0 to 500MHz. Accordingly a 500MHz low pass filter was used in the IF cable during mixer measurement so that the VNA's broadband detector only responded to the desired IF signal. In the second method, the spectrum analyzer was used to make conversion loss measurements at a subset of the same fiequencies. With both methods, corrections have been applied to Bccount

for fiequency dependent cable and filter ( for the VNA measurement) losses[4]. The results from these two sets of measurements are shown in Figure 4. Note that conversion gain is plotted, which is the negative of conversion loss. The residual discrepancies in these measurements are due to inaccuracies and non-linearities in the amplitude measurements made by SA and the VNA in broadband detector mode. Neither of these instruments are, by design, highly accurate power meters. As it is unclear which approach is more accurate, the average of the SA and VNA measurements of 5.8dB, at 915h4Hz, was used in the simulations below.

The 1dB compression level of the mixer was also measured. Using methods similar to that described above to characterize PldB for the ampWiers, the broadband detector mode of the VNA was used in conjunction with a power sweep to measure PldB for the mixer. In this case, the PldB corresponds to the power level at which

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the conversion loss increases by 1dB over its small signal value, Using an RF frequency of 915MHz, and an LO frequency of 985MHz (at +4dJ3m), a swept power measurement yielded an input PldB RF power of -1.7dBm, corresponding to an output PldB lF power of -8SdBm.

0.00

6 s -2.00 z 8 4.00 z 0

-6.00 > z 8 -8.00

Figure 4. Mixer conversion loss measurements made using a VNA and a spectrum analyzer. The

785 91 5 965 1005 1055 11 85 1485 LO signal is constant at 985- +4dBm. The RF power level was approximately -20dBm.

-1 0.00

RF FREQUENCY (MHZ)

C. Filter Characterization

VNA measurements of passive components, such as filters, is much more straight forward than the previously described amplifier and mixer measurements. One consideration for filter measurement is that both the broadband as well as narrow-band Characteristics are usually of interest Also, measurement averaging and isolation calibration may be necessary for accurate measurements of high out-of-band attenuation levels. Figure 5 shows the measured small signal response for the 915MHz filter, custom made by Piezo Technologies Inc. 0. The results show a 4dB insertion loss, a 3 lMHz bandwidth and out-of-band rejection levels on the order of 9OdB. As a rule, due to the in-band insertion loss is more accurately taken fiom a narrow-band measurement, as the frequency resolution is improved.

(a) Broadband 0.3-3000METz @) N-w-band: 800-1000Mt~.

Figure 5. Measured response of the 915MHz pre-select filter. The broadband response (a) shows out-of-band rejection on the order of 90dB. The narrow-band return loss measurement is also shown @).

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The measured response of the 70MHz IF filter is shown in Figure 6. It is seen that for this filter the out-of-band rejection increasingly degrades to 16dB at high RF fiequencies. This problem can be overcome by cascading the filter with a low pass filter to improve the signal purity of the IF output. The narrow-band response shows an insertion loss of about I&, and a bandwidth of 25.5MHZ.

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1

S t a r t 1.300 MHr stop 3 mm.naa HH=

(a) Broadband 0.3-3OOOMHz. @)Narrowband 0.3-100MH~.

Figure 6. Broadband (a) and narrow-band @I) responses for the Mini-Circuits 70MHz IF bandpass filter. broadband response shows that the out-of-band rejection of this filter degrades at high RF frequencies. The narrow- band return loss measurement is also shown @).

The

III. Receiver Subsvstem Simulation and Measurement

A. System Simulation Model and Power Level Comparisons

The CDS model schematic for the receiver is shown in Figure 7. The schematic represents one of several approaches to modeling the receiver, and consists of functional block models for the various components. The amplifier models are simple (constant) gain blocks, and the two filters are idealized band-pass filters specified by type and order. A i3lh order Tschebychev is used for the RF filter, whereas a 5& order eliptic filter is used for the IF filter. These were chosen in accordance with the authors’ best understanding of the type and order of the actual filters used. The mixer model shown is also the simplest. It treats the mixer and LO together as a block, and will yield the desired IF product, but it will not produce mixer RF and LO leakage, harmonics nor intermodulation terms. The primary input parameters for the functional models are the IS211 and output referenced PldB values, corresponding to the nominal 91JMHz RF frequency and 70MHz IF frequency, along with filter 3dB fiequencies. Initially these values were assigned based on manufacturers specifications, where available, and reasonable estimates for other parameter values (Figure 7). The model was then “calibrated” using measured component level characterization data, such as described in Section II. Table 1 shows input parameters for the system model so calibrated against component data for Sys 1 and Sys 2. Resistive losses are not modeled in the filter models used. Instead expected, or measured, filter losses in excess of the simulated IS211 for the filter models used are included into the IS211 of adjacent pads.

A more complete modeling wuld be done, for example, by including full 2-port S-parameters for measured components. This would take into account fiequency dependent characteristics of the components as well as include the effects of reflections in the system, something not addressed in the present model. It can be argued, however, that a model should be no more complicated than it needs to be. Starting with a fairly simple modeling approach and comparing to measurements allows for determining whether or not additional modeling complexity is needed. Further, in designing a new system, measured data likely does not exist and what degree of

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confidence to place in various built-in functional models used by a system simulator is very important to establish.

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Figure 7. CDS simulation model with initial component values chosen from manufacturers spec's, where available, and other reasonable estimates for other parameters. Note that the first pad is used only to set the proper power level for the small signal budget analysis. TP1, "2, "P3 and TP4 correspond to the test points indicated in Figure 1.

Table 1. Summary of input parameter values for receiver subsystem simulation model as "calibrated" against measured component data for Sys 1 and Sys 2 hardware (Only differences from Figure 7 are listed)

Table 2 shows power levels at the test points denoted Tpl, TP2, TP3, and Tp4 in Figure 1 and Figure 7. In obtaining the simulated results using CDS, the system schematic was used along with a "Small Signal Budget Test Bench" at an input CW fkquency of 915MH~. For this simulation, the simulator assumes a OdBm input, hence the first attenuator (Component 1) in Figure 7 is used to set the receiver input power level to match the measured condition for the simulation. For other simulation results discussed below, the value of the f is t attenuator is set to OdB. The results show good agreement between measured and simulated powers at various test points, with expected improvements to the agreement for the simulations which have been "calibrated" against measured component data.

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Table 2 Comparison of power levels at various receiver test points for initial simulation model, measured values, and final simulation model for Sys 1 and Sys 2. AU powers listed are in dBm, measured using SA at either the 915MHz RF frequency or the 70MHz IF frequency.

I I 26.3 I -30.4 I SA Measuranent

B. Small Signal Response of Receiver Subsystem

Figure 8a and Figure 8b show the broadband measured and simulated small signal response of the entire receiver subsystem. For this measurement, the broadband detector mode of the VNA is used (since the stimulus and response hquencies are different), the input power is held at approximately -30dBm and the RF kquency is swept from 0.3MHz to 3000MHz. The simulation was performed using a ‘‘Small Signal Sweep Test Bench” within CDS. As desired, the subsystem only responds to RF fkquencies in the vicinity of 915MHz, which is predicted properly by the simulated response. The measured and simulated responses differ in the shape of the out-of-band characteristics. Note that the scale is significantly different on the two plots. The measurement dynamic range is limited by the sensitivity of the detector (the noise floor of the measurement is only on the order of 50dBc versus 130dBc for the simulation). The measurements do not include a Comtion related to the difference between the output cable loss at the RF and IF frequencies [4]. The correction varies with frequency, but is generally less than 1dB. Consequently, exact agreement of the in-band conversion gain between measurement and simulation is not to be expected here. Also, a narrower band sweep should be used if such agreement were to be sought.

Figure 8c shows a dramatically different small signal response for the receiver with the 915MHz pre-select filter removed. Without this filter, strong responses are seen at the image frequency of 1055MHz as well as at 2885MHZ, corresponding to 3fm - fm. Note that the VCO output is unfiltered and has reasonably strong (-25dBc) harmonic content at 2 f~0 and 3fL0. As a result, without the RF preselect filter responses might be expected at fLo+/- fm,, 2f~O+/- f”, and 3fL0 +/- fm. The corresponding frequencies are 915,1055,1900,2040, 2885, and 3025MHz. The response at 3025MHz is also present, but outside the frequency range of the HP8714 VNA. The mixer’s double-balanced design suppresses the second harmonic response.

The simulated response, displayed in Figure 84 shows only the peaks at the 915MHz frequency of interest and the image frequency of 1055MHz. This is due to two simplifications in the simulation setup. First, is the simple mixer model used, as mentioned earlier, and second is the fact that the significant harmonic content of the LO signal is not modeled.

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(a) Measured @) Simulated

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(c) Measured w/o RF filter (d) Simulated wlo RF filter

Figure 8. Measured and simulated small signal response of the receiver subsystem with and without the RF pre-select filter. . In these plots, the x-axis corresponds to RF frequency and the y-axis corresponds to conversion gain (dB). The conversion gain measurements were made using the HP8714VNA using the broadband detection mode. The receiver input level was -3OdBm,and the LO frequency was set to 985MHz (at +4dBm).

C. Power Saturation of the Receiver Subsystem

The measured and simulated compression characteristics of the Sys 1 receiver are shown in Figure 9. The measured response was obtained using the VNA in the same contiguration as used for a mixer PldB measurement. This measurement was corrected for the difference between the output cable loss at IF and RF frequencies [4]. The input power plotted on the x-axis is the measured (or corrected) input power corresponding

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to the power sweep used. The nominal small signal conversion gain is seen to be 27.0dB with a system input PldB power of -23.2dBm. The corresponding output PldB power is: -23.2+26.0 = +2.8dBm. A summary of the relevant PldB powers for the system and each of the pertinent components thereof is given in Table 3. It might be noted that the 27.0d.B small signal conversion gain is not in exact agreement with the 26.4dB obtained using SA measurements for Sys 1, as recorded in Table 2. This residual discrepancy is a result of the same amplitude measurement accuracy issues affecting the differences observed in Figure 4.

Recelver Subsystem RF AmpWer 915MHz input ROMHz 915MH4 Inpri1l915MBe

The Sys 1 component PldB powers were used to calibrate the corresponding values in the simulation model of Figure 7flable 1 to obtain the simulated response shown in Figure 9. The small signal conversion gains between simulation and this VNA measurement differ by about 0.4dB, with the simulation showing better consistency with the SA measurement of conversion gain (see Table 2). The simulated input PldB value is in closer agreement with the VNA measurement. The simulation shows the PldB to be at -23. ldBm, referenced to the input, or 2.4 dBm referenced to the output.

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2.8dBm (ref. output) -23.2dBm (re€ input)

Figure 9. Measured and simulated 1dB conversion gain compression results for entire (Sys 1) receiver subsystem. The input power plotted was measured using the VNA Dower measurement mode. The RF, LO and

output hqnency output flpgoency output kqnency

+ 13.ldBm (ref. output) -8.5” (ref output) +4.8dBm (ref. output) -17.3dBm (ref input) -1.7” (d. input) -16.3dBm (d itlput)

32.47 30.24 -27.8 -25.69 -23.54 -21.32 iF frequencies are 915- 985MzIz, and 70MHz, respectively. Input Power (dBm)

D. Receiver Performance with Antenna Received Signals

Performance with an antenna received signal is the “bottom line” for a wireless receiver. As implied in Figure 1, a test was pedormed with a lab-generated transmit tone at 915MHz. This was set up using an HP8648C signal generator connected to a log-periodic antenna. Some distance (-1 8 feet) away a monopole antenna COMected to the input of the receiver subsystem picked up this signal along with whatever other signals were present in the local electromagnetic environment. An example input s p e d ” received by the antenna in the vicinity of 915MHz is shown in Figure loa. This was measured by connecting the antenna directly to the SA. Notice

51

that in addition to the lab-generated signal, strong peaks are seen in the range of 865 to 890MHz, corresponding to local cellular telephone tr&ic, as well as even stronger peaks fiom 930 to 935, fiom local paging signals.

3 5 - 4''' ,931

I Frequency (MHz)

"I

-70

51 58 64 70 76 83 89 95

Frequency (MHz)

(a) RF input spectrum from receive antenna

Figure 10. Spectrum analyzer measured RF input (a) and IF output @) signal amplitude and frequency content, with antenna transmit signal at 915- (Figure 1). For input spectrum (a) antenna is connected directly to SA. For output spectrum @), antenna is connected to receiver input and IF output of receiver is connected to SA.

@) IF output spednun h m receiver

Connecting the input antenna to the receiver input and the receiver output to the SA yields an IF spectrum similar to that shown in Figure lob. With the exception of the band close (e.g. +/- 10MHz) to the lab-generated signal, the spectrum is dynamic; both plots shown in Figure 10 are snapshots of time varying spectral pictures. A strong peak is seen at the desired IF hquency of 7OMHz. There are also peaks in the neighborhood of 55MHZ and 93MHz. These correspond to paging and cell phone trafEc, respectively. These signals are still observed in the IF spectrum as both the RF and IF filter bandwidths are not sufficiently narrow to block these potentially interfering signals h m view. A narrower band IF filter is being fabricated which will result in a cleaner IF spectrum.

IV. Summarv and Conclusions

A versatile test bench has been shown to be a usem tool for measurement characterization at the component as well as subsystem level. In particular, the broadband detection mode and absolute power measurement features of the Hp8714 VNA are valuable supplements to the conventional VNA tuned receiver mode. Accuracy limitations of the test bench do exist in the areas of mixer and subsystem conversion loss or conversion gain measurements. While an uncertainty analysis is outside the swpe, experience suggest that these measurements probably m o t be specified any better than within +/- 0.5dB of the true values using the equipment and techniques applied. The addition of a calibrated power meter to the bench would allow more accurate conversion loss and absolute power measurements to be made at spot fiequencies.

System modeling, as implemented, was found to be very useful for predicting the in-bhd characteristics of a fiequency converting subsystem, such as that studied. Additional modeling complexity can be implemented, however, there are clear advantages to the use of simple built-in functional component models. The presented model was shown to be easily calibrated with measured data and demonstrated to produce useful simulations of swept response characteristics, power at various points in the system, and system 1dB compression characteristics. This work forms a firm foundation which can be built upon. For example, adding accurate

52

treatment of noise figure would allow receiver sensitivity and dynamic range to be quantified. Also, to the receiver hardware and/or associated simulation model, can be added IF processing and demodulation circuitry. A similar approach can be followed to assemble a transmitter subsystem and model which could be used in symphony with the receiver for complete wireless communication system measurement/simuIation analyses.

The case study presented has excellent educational merits. A system designer could use such a case study to gain experience with system hardware, software and instrumentation that would lead to more efficient measurement and simulation strategies for hture designs. It can also be used for short course and in-house training, in addition to university instruction where it already has proven to be extremely usem for training new RF engineers in the fundamentals of wireless/RF systems.

References

[1]* P.G. Flikkema, L.P. Dunleavy, H.C. Gordon, RE. Henning, T. M. Weller, "Wireless Circuit and System Design: ANew Undergraduate Laboratory", Proc. Frontiers in Education '97, Nov. 5-8, Pittsburgh, PA. [2]* L. Dunleavy, "A Versatile Yet Economical Test Bench for Characterization of Wireless lW/Microwave Circuits and Systems ",Wireless andMicrowave TechnoIogy '97, October 6-10,1997, Chantilly, VA [3] HP8712B and HP8714B RFNetwork Analyzers Users Guide, Hewlett Packard PM 08712-90003. [4]* L. Dunleavy, T. Weller, E. Grimes and J. Culver,"Mixer Measurements using Network and Spectrum Analysis, I' 48'h ARFTG Conference, Dec. 5-6,1996 Clearwater FL,.

* These references are currently available fiom web site http:/lwww.eng.usf.edu/wAMI.

Acknowledgments

The equipment utilized in preparing this tutorial was made possible by combined grants fiom the National Science Foundation (Grant # DUE-9650529) and Hewlett Packard Company with matchmg funds fi-om the University of South Florida. General support and assistance in the area of parts and supplies h m the following companies is greatly appreciated: Mini-Circuits, Cushcraft, Miteq, and Piezo Technology Inc. . We wish to acknowledge Rudy Henning, Tom Weller, and Horace Gordon of the University of South Florida who co- developed the WAMI lab with the first two authors.

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