RITEC RAM-5 Versaile Compuer Conrolled Ulrasonic Sysem: Modular Approach allows Cusomizaion o Specific Experimenal Requiremens. High Power RF Burs Oupus as high as 5 kilowas for frequencies o 7 MHz. Three Sandard Frequency Ranges covering frequencies from 5 khz o 4 MHz. Cusom Frequency Ranges also available. Signal Processing Allows Accurae Deerminaion of boh Signal Ampliude and Phase. A phase-sensiive superheerodyne receiver provides he abiliy o exrac oherwise undeecable signals from he noise.
Powerful Ulrasonic Research Tool The RITEC RAM-5 is a complee ulrasonic measuremen sysem designed for ulrasonic research and applicaions of he nondesrucive evaluaion of maerials properies. Some special capabiliies no available in oher commercial insrumens include: The abiliy o make reproducible measuremens using shor (down o single cycle) RF burs exciaions in composies and oher difficul maerials, High power RF one burs exciaions up o 5 kilowas, providing abiliy o drive inefficien ransducers, A modular approach, (This permis a sysem o be cusomized by he user for specific experimenal requiremens. Sandard configuraions include a high power one burs pulser, a broadband pulser/receiver and he complee superheerodyne measuremen sysem.) The RAM-5 power frame was designed o accep up o five gaed amplifier modules; his allows a wider frequency range using wo or more gaed amplifiers. Superior signal processing for ampliude and phase measuremens of pulsed RF signals. Measuremens of he phase angle are reproducible o wihin.3 degrees and ampliudes are reproducible o wihin.1 db. For example, in a es sample wih a ransi ime of 5 microseconds wih 1 MHz ransducers, a resoluion of 4 picoseconds is possible. This abiliy o measure signals auomaically and accuraely coupled wih sofware o process hese readings ino acousic ime of fligh and aenuaion informaion combine o make he RAM-5 a very powerful ulrasonic research ool. The modular RAM-5 sysem consiss of he following elemens in conjuncion wih he RAM- 5 power frame: In our mos popular sysems, a high qualiy, fas swiching, synhesized frequency source is used o produce he ransmiing signal and o process he received signal. This approach enables simulaneous measuremens of he phase and he ampliude as well as making he lineariy of he sysem essenially independen of he signal level. A Modular Approach The insrumenaion for his sophisicaed, complee ulrasonic sysem has been divided ino funcional modules. Each module performs a specific ask and can be undersood, esed, and modified individually. This approach offers a number of advanages. 1. Each module can be developed individually and modified for special requiremens. 2. No all applicaions require he same funcional arrangemen. For example, some applicaions require only he broadband receiver, and a pulser/receiver sysem can purchased wihou he addiional modules for he superheerodyne phase sensiive receiver. 3. New modules may be added o mee he needs of a paricular cusom applicaion a a fracion of he developmen cos of a complee sysem. 4. Modules may be replaced as needed o keep he sysem curren wih new developmens in measuremen echnology. This modular approach maximizes RITEC's abiliy o make sae-of-he-ar insrumenaion available and gives he user he bes possibiliy of keeping his sysem curren. These advanages resul in grealy increased capabiliy, more flexibiliy, and a lower price as compared wih non-modularized sand-alone sysems or hose creaed from componens purchased from several manufacurers. I. One or more broadband, high power gaed amplifiers for producing RF burss, derived
from a coninuous wave (CW) RF source, o drive various ypes of ransducers, including piezoelecric, air-coupled and elecro-magneic acousic ransducers (EMATs). II. Timing o produce he gaes required o urn he amplifier on and off coherenly wih he CW source. III. An accurae direc digial synhesizer (DDS) for high-resoluion frequency conrol and he abiliy for rapid frequency sweeps. IV. A broadband receiver sage o amplify ulrasonic signals eiher from he ransmiing ransducer, using a diplexer, or from a separae receiving ransducer. V. The broadband receiver is ypically followed by superheerodyne phasesensiive receiver sages o improve he deecion of received signals in noisy environmens. VI. A pair of inegraors ha are gaed on and off o process he informaion from he phase sensiive receiver oupus for precise measuremens of he phase and ampliude. All conrol and measuremen funcions are under compuer conrol and sofware is provided for many of he more sandard measuremen echniques used in ulrasonics. Each of he elemens lised above can be added o a signal channel sysem o add increased measuremen capabiliies o a RAM-5 sysem. Mark I. A Mark I configuraion consiss of he gaed amplifier, elemen I, and he power frame. For operaion, an exernal RF source and iming gaes are required. Mark II. The Mark II configuraion adds he iming funcions in elemen II o he Mark I configuraion and can be used wih an exernal CW RF synhesizer. Mark III. The Mark III, wih he addiion of he synhesizer, is a complee sandalone compuer conrolled one burs pulser. Mark IV. The Mark IV configuraion, wih he addiion of he broadband receiver module, discussed above as elemen IV, is a compuer conrolled broadband pulser and receiver. Mark V. The Mark V configuraion adds he superheerodyne mixer and phase sensiive deecors o he broadband Mark IV sysem for addiional processing by digiizing he phase deecor oupus. Mark VI. The complee RAM-5 superheerodyne phase sensiive measuremen sysem, designaed as he Mark VI configuraion, uilizes all six elemens. Cusom Configuraions The addiion of a second high power gaed amplifier can be used o exend he frequency range of he sysem. The addiion of a second gaed amplifier and a second receiver can be used o drive wo ransducers in hroughransmission or pulse-echo mode. The addiion of a special riple synhesizer module allows independen conrol of he frequency and iming up o hree synhesizers and gaed amplifiers, resuling in a versaile hree channel pulser sysem. A block diagram of he single channel Mark VI RAM-5 measuremen sysem is shown in Figure 1.
SAMPLE HIGH AND LOW PASS FILTERS & GAIN CONTROL OUTPUT AMPLITUDE CONTROL HIGH POWER RF GATED AMPLIFIER DIGITAL FREQUENCY CONTROL DDS FREQUENCY SYNTHESIZER RF SIGNAL BROADBAND RF RECEIVER RAM-5 BLOCK DIAGRAM F+IF IF BANDWIDTH CONTROL MIXER & IF AMPLIFIER DIGITAL CONTROL OF ALL MODULES & ANALOG-TO-DIGITAL CONVERSION OF INTEGRATOR OUTPUTS DATA ACQUISITION CARD AMPLIFIER GATE F IF OSCILLATOR o o /18 o o 9 /27 REFERENCE PHASE SELECTION QUADRATURE PHASE SENSITIVE DETECTION CONTROL OF RF BURST WIDTH AND INTEGRATOR GATE POSITION AND WIDTH NO. 1 NO. 2 INTEGRATE RATE CONTROL TIMING INTEGRATOR GATE GATED INTEGRATORS TO DATA ACQUISITION CARD CW in from Synhesizer Coheren Amplifier Gae (Deermines RF Burs Widh) Amp + Inpu Amp - Inpu Block Diagram of Riec Gaed Amplifier Buffer Buffer CW Rejec Pedesal Adjusmen Adjusmen 1 Gaed Muliplier Amplifiers Power Amplifiers Differenial Amplifier RF Level Conrol Coheren Oupu Amp 1 2 Bias 2 Amplifier Transformer Buffer Conrol Pre-Gae DC Drive Adjus Pre-Gae Time ( 1 - ) equals 1or 4 Cycles of he CW Signal 22 144 V peak BNC Oupu Figure 1 Block Diagram of he complee RAM-5 Mark VI Sysem Each of he various sysem funcions will be briefly described below. High Power Gaed RF Amplifier The RITEC gaed RF amplifier module is designed o derive from an exernal frequency source he very high power RF burss needed for modern research sudies, wih he addiion of iming gaes produced exernally or from he iming module. These RF burss may hen be used o drive various ypes of ulrasonic ransducers, such as piezoelecrics or EMATs. When highly aenuaive maerials are encounered, long burss conaining numerous cycles may be used o generae very large sound ampliudes. These long one burss can also be used o drive he sample ino acousic resonance, which allows he use of inefficien ransducers and improves he signal o noise. When he greaes ime resoluion is required, he unique gaing circuiry on he final power amplifier sage allows he amplifier o produce a clean single cycle of RF (up o 2 MHz). Single cycle RF burss give comparable resoluion o spike pulsers in many siuaions. A block diagram of he gaed amplifier is shown in Figure 2. Figure 2 A block diagram of he gaed amplifier module. The high power RF burs is creaed by selecing a number of RF cycles from a synhesizer running in a coninuous mode or in a gaed mode. This is accomplished by urning on he amplifier wih a gae ha is coheren wih he RF signal. This low level signal is hen spli ino wo signals 18 degrees apar o drive wo final amplifier sages. The high power oupus from hese final amplifier sages are combined in an oupu ransformer o produce he high power bipolar RF burs. This configuraion is known as a push-pull configuraion. Depending on he ransisors used in he final amplifier sages and he frequency range and bandwidh of he oupu power ransformer, he gaed amplifier can be cusomized for specific ranges of frequency. Two versions of he gaed amplifier wih differen oupu power levels are available over mos frequency ranges. The high power version has a maximum available oupu pulse power of 5 kilowas (KW) roo-mean-squared (RMS) ino a 5 Ohm load. The low power version has a maximum available oupu pulse power of 1.5 KW RMS ino a 5 Ohm load. Please noe ha hese power measuremens are RMS measuremens, peak pulse power measuremens would be double of hose RMS power measuremens, (1 KW peak and 3 KW peak). Some oher amplifier manufacurers quoe peak pulse powers.
Wih he high power version he frequency range where he 5 KW power is available is resriced o 1 decade of frequency up o a maximum frequency of 7 MHz. Typical frequency ranges are 5 khz o 5 khz, 25 khz o 2.5 MHz, and 5 khz o 5 MHz. Wih he sandard power version he frequency range where he 1.5 KW power is available is resriced o greaer han 1 decade of frequency up o a maximum frequency of 1 MHz. 1KW is ypically available o 15 MHz. Typical frequency ranges are 5 khz o.5 MHz, and 5 khz o 5 MHz. One of he mos common frequency ranges is 25 khz o 1 MHz wih a specified oupu of 1KW a he endpoins. RMS Pulse Power (was) ino a 5Ω Load Typical RMS Pulse Power of High Power RAM-.5-5 Specified RMS Pulse Power of High Power RAM-.5-5 Typical RMS Pulse Power of Sandard RAM-.25-17.5 Specified RMS Pulse Power of Sandard RAM-.25-17.5 8 7 6 5 4 3 2 1 1-1 1 1 1 Frequency (MHz) Ampliude (Vols) 1 5-5 -1 3 8 13 18 Time (microseconds) Figure 4 Available powers as a funcion of frequency for he 1.5 KW amplifier and he 5 KW amplifiers. The push-pull arrangemen for he oupu circuiry of he amplifier resuls in a lower harmonic conen for he even harmonics, even a he maximum oupu power of 5 KW. The harmonic conens of a 5KW oupu burs are shown in Figure 5 as a funcion of frequency. Harmonic Disorion in 5KW oupu burs Figure 3 A high power RF burs of five cycles a 35 khz A ypical burs, ino a high power 5 Ohm load, is shown in Figure 3. A plo of he available powers as a funcion of frequency for he wo versions of he amplifiers is shown in Figure 4. -3-4 -5 Relaive Ampliude (db)-2 3rd 5h 2nd 4h 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 1 Oupu Frequency (MHz) Figure 5 Harmonic Conen of Oupu Pulse as a funcion of Frequency. The noise level of he gaed amplifier during he receive ime is also very low because he
amplifier has been urned off. The raio of he pulse ampliude o he CW leakage hrough he amplifier (On-Off raio) is greaer han 14 db. This high On-Off raio is imporan because a very small amoun of leakage can overwhelm he receiver inpu a high gains and cause disorions in he oupus of he quadraure phase sensiive deecors. This reducion in leakage of he CW signal is a major advanage of using an amplifier ha is gaed as opposed o an amplifier ha operaes coninuously amplifying exernally produced lowlevel RF burss. Anoher advanage is ha he amplifier has been urned off and is no longer amplifying any noise ha may presen on he low level CW inpu, hus improving he signal-onoise. Oher advanages are he economies realized in size, hea dissipaion, and power supply requiremens. Because he amplifier is gaed on and off, an imporan facor o consider when deermining he burs widh and burs repeiion rae or rigger frequency is he duy cycle. This is defined as he ime he amplifier is urned on, specifically he burs widh muliplied by he repeiion rae, expressed as a percenage. For example, as shown above in Figure 3, a an operaing frequency of 35 khz a burs widh of five cycles resuls in a burs 14 microseconds long. A a repeiion rae of 1 Hz, he duy cycle would be.14 or.14%. A a repeiion rae of 1kHz, he duy cycle would be 1.4%. No appreciable sag in oupu ampliude will occur as he duy cycle limis are approached a full power. If he limis are exceeded, however, an auomaic shudown circui will be acivaed and he power sage will be urned off wihou damage. (Full power oupu can be resored by swiching he high volage off, correcing he excessive duy cycle, and urning he high volage on again.) The high power available makes i possible o drive very low efficiency ransducers such as EMATs and sill have reasonable sysem performance. In addiion, when maerials exhibiing high ulrasonic losses are o be examined, he use of high power in conjuncion wih efficien ransducers can mean he difference beween meaningful daa and no observable signals. Single Channel Frequency Synhesizer The frequency source is provided hrough a direc digial synhesizer ha provides insananeous changes in frequency as conras o he seling imes required in phase locked looped synhesizers. The synhesizer operaes up o 4 MHz in he broadband Mark III and Mark IV configuraions and up o 64 MHz in he superheerodyne Mark VI configuraions. The synhesizer acceps an exernal clock signal a frequencies up o 25 MHz and processes inernally resuling in a sysem clock as high as 3 MHz. Frequency conrol is provided hrough 48-bi conrol, resuling in a ypical resoluion of up o.5 x 1-6 herz. In he broadband Mark III and Mark IV configuraions, he synhesizer oupu is sen o he iming circuiry and o he gaed amplifier for gaing and amplificaion. Wih he superheerodyne receiver in he Mark VI sysems, he oupu of he frequency synhesizer is used as a "local oscillaor" in he superheerodyne receiver. I also is mixed wih he quarz inermediae frequency oscillaor o produce he CW operaing frequency ha is sen o he gaed amplifier and he iming circuiry. Signal Channel Timing All he digial iming funcions of he RAM have been made coheren wih respec o he CW operaing frequency. When a rigger is received from he compuer, an exernal source, or he inernal rep-rae generaor, he iming circuiry wais for a posiive zero crossing of he CW signal before beginning he gaing process. The RF cycles are hen couned o he desired number o produce a burs wih he correc widh. A he same ime he gaing funcion begins, a rigger pulse is sen o he rigger oupu connecor and a 1 MHz gaed clock oscillaor is sared. This clock is hus made coheren wih respec o he CW and is used o generae he signal processing gae delays and widhs. The coherency feaure avoids he gae posiion jier
ha would be eviden if a coninuous clock were used. Muliple Gaed Amplifier Sysems In some applicaions, he unique capabiliies of he RAM-5 sysem o accep muliple gaed amplifiers are paricularly valuable. The abiliy o widen he frequency range of a sysem wih wo gaed amplifiers has already been menioned. This permis one RAM-5 sysem o cover he frequency range of 5 khz o 7 MHz, mainaining he 5 KW oupu, beween wo gaed amplifiers. Anoher configuraion, wih up o four gaed amplifiers, allows he sysem o fire several ransducers simulaneously or in a designaed sequence. Muliple Channel Sysems Wih he addiion of a special riple synhesizer/iming module, i is possible o configure a hree channel Mark IV sysem, wih hree gaed amplifiers driving hree ransducers and hree independenly conrolled receivers. Such a sysem has been paricularly useful in online indusrial applicaions using elecromagneic acousic ransducers (EMATS). These ransducers are very versaile bu have a low efficiency in convering he RF elecrical energy ino acousic energy. Wih he high power, high curren oupu of he RAM gaed amplifier and wih proper impedance maching in he ransmi line, i is possible o produce ens of amperes of curren in he EMAT. Wih he special riple synhesizer/iming module, i is possible o adjus he relaive delay beween he oupus of each gaed amplifier in 12.5 ns seps, wih furher conrol over he relaive phases by adjusing he phase of he synhesizer oupus. Unique iming circuiry mainains coherency beween each of he synhesizer oupus and herefore beween he high power RF burss. Superheerodyne Receiver Superheerodyne circuiry has been used in nearly every radio-frequency receiver manufacured since he 193s. The concep allows he use of fixed uning elemens a he inermediae frequency (IF), a consan bandwidh independen of he operaion frequency, and rejecion of ou-of-band spurious signals. In RITEC's implemenaion, he Broadband RF Receiver, Mixer and IF Amplifier, Direc Digial Synhesizer, and IF Oscillaor and Quadraure Phase-Sensiive Deecors modules are used in combinaion o complee he phase sensiive superheerodyne receiver. Selecion of he ransmier frequency and receiver uning are accomplished simulaneously by seing he synhesizer. This feaure grealy simplifies he compuer conrol of he receiver, especially in applicaions requiring he frequency o be swep. The adjusmen of he phase sensiive deecion circuiry is also simplified because he muliplicaion process occurs a he fixed IF frequency. Specifically, he reference oupus from he IF oscillaor are adjused as closely as possible o quadraure (9º) before being applied o he phase sensiive deecion muliplier circuis. Broadband Receiver The digial conrol of gain in wo db seps makes possible accurae, repeaable measuremens of signal srengh, and low noise, high gain exernal pre-amplifiers are available for use when weak signals are encounered. The addiion of adjusable IF band-pass, high-pass, and low-pass filers make his one of he mos versaile receiving sysems available for ulrasonic research. Quadraure Phase Sensiive Deecion and Analog Inegraion One of he significan innovaions of he RAM is a signal processing echnique ha involves he use of boh quadraure phase sensiive deecion and gaed analog inegraors. Quadraure deecion allows signals o be processed in much he same way as is done wih a vecor volmeer. The wo deecor (muliplier) circuis produce wo orhogonal vecor componens (real and imaginary) of he
signal from which he ampliude and phase angle can be calculaed. To gain more insigh ino he operaion of he phase-sensiive receiver circuiry, i is insrucive o consider he received ulrasonic informaion as a monochromaic signal a frequency (f r ) modulaed by a erm, which defines he widh, ampliude, and shape of he received signal, A r (). f() = Ar() sin (2π f + φ ) (1) r r Therefore, signal-averaging echniques may be employed o recover signals from noise. In Figure 7, he signal o noise raio of he broadband receiver oupu was purposely reduced o approximaely one, bu i is relaively o deec he wo echoes in he phase deeced signals wih a reasonable signal o noise raio. Advanages of Quadraure Phase Deecion Phase Deeced Signal (5 khz Low Pass Video Filer) Afer conversion o he inermediae frequency, he received signal can be processed in he phase deecors. The oupu of he Phase Deecor No. 1 is given by: D ( ) = g () sin 1 3 Ar φ r (2) where he oal gain and conversion efficiencies are all included in he erm g 3. The oupu of Phase Deecor No. 2 is given by: volage (millivols) 4 3 2 1 Phase Deeced Signal (25 khz Low Pass Video Filer) Phase Deeced Signal (2 MHz Low Pass Video Filer) RF Signal from Receiver RF Monior 1 12 14 16 18 2 ime (microseconds) D ( ) = g () cos 2 3 Ar φ r (3) The wo phase deecor oupus are shown below in Figure 6, along wih he broadband receiver echo. Phase Deecor Oupus (mv) 2 1-1 -2 Phase Deecor 1 Phase Deecor 2 Digiized RF Echo 55 6 65 7 75 Time (microseconds) Figure 6 Broadband receiver monior and he wo phase deecor oupus. This ype of deecion also has he advanage of mainaining excellen lineariy even when he received signal is small; his ype of deecion is also no affeced by he presence of noise. Figure 7 In order o obain accurae signal ampliude and phase informaion, he phase-deeced signals are processed wih analog inegraion circuis. This mehod has he value of making he gae posiion non-criical, removing he RF componens, and improving he signal-o-noise raio. The ime limis of he inegraion are conrolled by he inegraor gae, and he inegrae rae (r I ) is under compuer conrol. The gae is posiioned so ha i begins before and ends afer he signal. The inegraor oupus are given by: and I 1 = r I D1 ( ) d = A( )cosφ r d (4) 2 1 2 1 I = r D ( 2 I 2 ) d = A( )sinφ r d (5) 2 1 2 1 where I 1 and I 2 are he oupus of inegraors No. 1 and No. 2, 1 and 2 are he sar and sop imes defined by he inegraor gae, and D 1 () and D 2 () are he phase sensiive deecor oupus. The inegraor oupu volages are read
by inernal 16-bi analog-o-digial converers inernal o he RAM and hen ransmied back o he daa acquisiion compuer. The phase angle of he received signal can be calculaed from he wo inegraor oupus by: -1 I 2 φ an r = (6) I1 and he signal ampliude may be obained from: been achieved. Unforunaely, seady sae condiions in he oupus of he phase deecors are someimes difficul o achieve as shown in he ypical siuaion shown in Figure 9. In his example, he RF signal (echo) was produced by ransmiing a single cycle RF pulse o he ransducer. I is clear ha filering ou he RF componens in he deeced oupus will resul in significan degradaion of he rise and fall imes. A I 1 + I 2 2 2 =. (7) Furher improvemen in he signal o noise can be achieved using he gaed inegraors. The inegraor oupus for he signals shown in Figure 6 are shown below in Figure 8. Volage Deecion & Inegraion Phase Angle = deg RF Signal Unfilered 9 deg Phase Deecor (M2) 9 deg Inegraor (I2) 6 Unfilered deg Phase Deecor (M1) deg Inegraor (I1) Inegraor Oupus (mv) 4 2-2 Inegraor Oupu No. 1 Inegraor Oupu No. 2 5 6 7 8 9 1 Time (microseconds) Figure 8 Oupus of he Gaed Inegraors for he RF Echo and phase deeced signals shown in Figure 6. However, if he signals are a very small number of RF periods wide, measuring he magniude of he vecor componens has some experimenal difficulies. Typical signals are no fla opped and he peak value may no be he bes choice for an accurae measuremen of he echo ampliude. In an ideal siuaion, he RF erms can be compleely filered ou, and he ampliude (A) and phase (φ r ) may be calculaed afer measuring he insananeous value of he phase deeced oupus a he cener of he signal or where seady sae condiions have 2 4 6 8 1 12 14 16 18 2 Time Figure 9 Oupus of he RF Receiver, he Quadraure Phase Sensiive Deecors, and he gaed Inegraors for a Typical Signal wih φ r = o The oupus from he wo inegraors are also shown in Figure 9. Even hough i is difficul o deermine he seady-sae ampliude in he phase-deeced signals, i is easy o deermine he seady-sae ampliude for he oupus of he wo phase deecors. In his example, he oupu of he degree phase deecor inegraes o a maximum value and he oupu of he 9 degree phase deecor inegraes o zero, indicaing a phase angle, φ r, of degrees. In order o illusrae he effeciveness of his signal-processing scheme and o deermine if aenuaion measuremens would be affeced by burs widh, a simple es was performed wih a Plexiglas sample. The logarihmic raio of he
ampliude of wo echoes, which is a measure of he aenuaion, was measured as a funcion of frequency for a number of differen ransmier burs widhs. The inegraor gae widh and posiion remained consan during he invesigaion. The resuls are shown in wo differen formas in Figure 1. Ampliude (db) 34 32 3 28 26 24 1 cycle burs 2 cycle burs 3 cycle burs 4 cycle burs 5 cycle burs 6 cycle burs 7 cycle burs 8 cycle burs 9 cycle burs.82.92 1.2 1.12 1.22 Frequency (MHz) burs widh of a single cycle. However, he daa aken wih a single cycle burs is more sensiive o noise and has a poorer signal-o-noise raio (SNR) as expeced. The case for phase sensiive deecion and inegraion is made even sronger in his example because he specrum of he received signal produced from a shor burs is dominaed by low frequency componens. This is rue because here is always some low frequency componen in he driving pulse and he aenuaion is relaively low a his end of he specrum. The phase sensiive deecion and inegraion process is very effecive in rejecing hese componens. In fac, i can be shown mahemaically ha he quadraure phase sensiive deecion and inegraion process produces an ampliude equal o he value of he Fourier Transform a he ransmi frequency. The conclusion hen is ha his signal processing echnique is he bes mehod known o us for obaining eiher ampliude or phase informaion from pulsed acousic signals. Evaluaion Tess In order o illusrae he capabiliies of he RITEC RAM sysem he resuls of a series of simple ess are presened. Measuremens of he Absolue Transi Time Figure 1 Raio of he Ampliude of Echo 1 o Echo 2 in decibels as a Funcion of Frequency and Widh The figure shows ha for each frequency a consan value for he aenuaion is obained which is independen of he pulse widh. These resuls could be repeaed wih a sample-andhold echnique if he gae were placed wihin he fla-opped porion of a long echo and sufficien filering were used o remove he RF componens from he deeced signal. Quie good agreemen can be found beween he daa aken wih he maximum burs widh of eigh cycles and he daa aken wih he minimum Afer deermining he slope of he phase versus frequency curve for a signal, he oal ime of arrival including delays hrough he elecronics and acousic elemens as well as he acousic ime of fligh in he sample associaed wih he group velociy can be deermined from he relaion: φ r T = 2π F (5) where φ r is he change in phase and F is he frequency change. When wo echoes are measured, an accurae measure of he acousic ransi ime can be obained by aking he difference beween he resuls and dividing by he appropriae number
of ransis for he echoes chosen. In he example shown in Figure 11, he firs wo echoes were measured. may occur in he elecronics or he acousic bond. However, he acousic bond mus remain sable over he course of he readings. In order o documen he sysem's abiliy o measure small changes in signal arrival ime, a es was devised using an adjusable lengh air dielecric coaxial line. Because he dielecric is air, he increase in ime may be easily calculaed using he speed of ligh and he change in line lengh. The block diagram of he es is shown in Figure 12. Figure 11 Acousic Phase Measuremens in a Fused Silica Rod. A leas squares daa fi produced values for he phase versus frequency slopes of 61.533 and 121.489 radians per MHz, and he acousic ransi ime was han calculaed as 4.771 microseconds. I is no difficul o obain beer han four-place reproducibiliy in daa of his ype. However, he researcher mus be careful o include all relevan acousic effecs, such as diffracion and phase shifs a he bond-ransducer-sample inerface, if he wishes o claim his level of absolue accuracy. Changes in Acousic Transi Time Many invesigaions are more concerned wih changes in acousic velociy or ime as a funcion of some oher parameer such as emperaure or pressure han hey are wih absolue imes. These changes can also be deermined wih more accuracy and precision han absolue measuremens. The calculaions are made from he relaion: RITEC RAM-5 Rec. Inpu RF Burs Ou High Power Diplexer for Pulse/echo RF Burs operaion Bishop Precision Adjusable Lengh Coaxial Line Figure 12 6 db 5 Ohm aenuaor Fused Silica Sample Sysem for Delaying he Arrival of an Acousic Echo by a Known Amoun. In order o have confidence in delay produced by he adjusable line, i is necessary o insure ha he delay line is erminaed in 5 Ω in boh direcions. To erminae he inpu o he adjusable delay line, he 6 db aenuaor was added beween he diplexer and he delay line; his aenuaor would no be required in an acual acousic applicaion. The resuls of a es performed a 18.312 MHz are shown in Figure 13. φr Change in Time = (6) 2π F Noe ha he frequency remains consan during hese invesigaions. This eliminaes problems wih frequency dependen phase shifs, which
Figure 13 Experimenal Measuremens of Echo Delays and Line Showing he Theoreical Delay Calculaed Using he Speed of Ligh. The fac ha he daa fall on he heoreical line clearly suppor he abiliy of he apparaus o measure changes in ime; he small deviaions are ascribed o he difficuly of seing he line lengh exacly. Everyhing else being equal, his resoluion is direcly proporional o he operaing frequency. However, he accuracy of acousic ampliude and phase measuremens depends on many experimenal facors oher han he qualiy of he elecronic insrumenaion. Some measure of he sabiliy of he insrumenaion and he value of using wo echoes for he measuremen of ime change is illusraed when he changes in arrival ime for he firs wo echoes in he Silica rod are ploed over a longer period of ime. For his es, he sample was placed on he able wihou any effor o mainain emperaure sabiliy in any par of he apparaus. The resuls are shown in Figure 14. Figure 14 Changes in he Arrival Times for he Firs Two Echoes in a Silica Rod and Changes in he Round Trip Time Calculaed from This Daa. The upward drifs of he arrival ime of he wo echoes follow each oher closely. This general drif canno be explained by a change in eiher he sample dimensions or sound velociy because ha effec would cause he wo lines o diverge. The major porion of he effec mus come from changes in he insrumenaion. Therefore, he difference calculaions of he changes in round rip ime are clearly superior because he upward drif is canceled and he remaining effecs are eiher noise or real changes in he sample. Noe: hese exremely small changes in ransi ime were observed using signal-averaging echniques. Changes in Acousic Aenuaion In order o precisely deermine he acousic aenuaion or relaive changes in aenuaion, accurae measuremens of he ampliude mus be performed. Some discussion of his requiremen has previously been made and resuls shown in Figure 4. The following ess were designed o show he lineariy of he insrumenaion and check measured values of ampliude change agains he precision aenuaors responsible for his change. The block diagram of he seup is shown in Figure 15.
RITEC RAM-5 inserion loss. These resuls are shown in Figure 17. Rec. Inpu RF Burs Ou High Power Diplexer for Pulse/echo RF Burs operaion Precision Adjusable Aenuaor 6 db 5 Ohm aenuaor Fused Silica Sample Figure 15 Seup for Producing Calibraed Changes in Echo Ampliude In he firs es, he aenuaor was a precision adjusable wave-guide operaing beyond cu-off wih an inserion loss of 34 db and a resoluion of.2 db. The daa shown in Figure 16 fall close o he heoreical line and agree wihin he resoluion of he aenuaor. Figure 16 Measured Signal Change and Line Showing Expeced Resul over a 1 db Range. Figure 17 Measured Signal Change and Line Showing Expeced Resul over an 8 db Range. In his case, he daa deviaes from wha is heoreically expeced when a large aenuaion is swiched ino he sysem. The reason for his discrepancy is mos likely improper compensaion for CW leakage. However, he signal size wih 8 db insered is more han an order of magniude smaller han he bi size of he A/D converer and even wih averaging some error is expeced. Every effor was made o anicipae he quesions of poenial users in his descripive shee. However, i is recognized ha i is no possible o predic all he possible uses of he RITEC RAM-5 sysem or even he concerns of researchers wishing o use i for sandard applicaions. If here are any quesions abou he applicabiliy of his insrumenaion for a specific or general requiremen, please conac Bruce Chick, Gary Peersen, Michael Ragosa or Mark McKenna and hey will endeavor o be of assisance. The second es was performed using swiched aenuaors wih 1 db seps and a db