Abstract:
A portable network analyzer and method having multiple channel transmit and receive capability for real-time monitoring of processes which maintains phase integrity, requires low power, is adapted to provide full vector analysis, provides output frequencies of up to 62.5 MHz and provides fine sensitivity frequency resolution. The present invention includes a multi-channel means for transmitting and a multi-channel means for receiving, both in electrical communication with a software means for controlling. The means for controlling is programmed to provide a signal to a system under investigation which steps consecutively over a range of predetermined frequencies. The resulting received signal from the system provides complete time domain response information by executing a frequency transform of the magnitude and phase information acquired at each frequency step.

Description:
This invention was made with Government support Contract No. W-7405-ENG36 awarded by the United States Department of Energy to The Regents of the University of California. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to network analyzers wherein at least one transmit signal is transmitted to a system being analyzed, and at least one receive signal is received from the system being analyzed, the transmitted signal being mixed with the receive signal to obtain the phase and amplitude data of the receive signal and performing calculations to obtain time domain response data. 
     BACKGROUND OF THE INVENTION 
     Several commercial products exist in the market today which are used to obtain similar information from a measured system (such as those manufactured by Hagtronics, Venable Industries and DKD Instruments). Such analyzers, however, only provide scalar information (and hence, are incapable of providing phase information), have a limited maximum frequency, are limited to a low number of transmit and receive channels and are costly. What is required is a small, multi-channel, wide-band network analyzer which maintains phase information, allows for inverse fast Fourier transforms to product time responses and is able to generate system information not discernable from the system&#39;s magnitude response alone. 
     Accordingly, it is an object of the present invention to provide a method for real-time network analysis of a system to determine the health or operational state of individual components forming the system and which allows prompt action to be taken in the event of a detected problem. 
     It is also an object of the present invention to provide a method for real-time analysis of a system which measures system responses using any arrangement of transmitters and receivers, measures system network phase and magnitude transfer functions, measures the transfer function of passive or active filters, measures closed-loop control system phase and magnitude, measures the system&#39;s DC gain, open loop bandwidth and phase margin of amplifiers, measures impedance versus frequency of selected system components, measures system resonances, characterizes phase lock loops, measures performance of nonlinear system components (such as signal mixers), determines the phase and noise characteristics of a system and measures other non-linear system characteristics. 
     It is a further object of the present invention to provide a multiple channel phase coherent, swept-frequency network analyzer which accurately maintains phase integrity of system data, provides enhanced measurement capabilities, is low cost, is small, requires low power for operation and is programmable in frequency, step size, bandwidth and gain. 
     It is a further object of the present invention to provide a network analyzer having a plurality of pure sinusoidal transmit and measurement channels, the network analyzer maintaining phase integrity between the transmitted signal and the measured signal so that an operational analysis can be executed on the measured signal. 
     It is also an object of the present invention to provide a network analyzer having a plurality of pure sinusoidal transmit and measurement channels, the network analyzer minimizing direct current drive while enhancing dynamic range of measurements. 
     It is a further object of the present invention to provide a network analyzer having a plurality of pure sinusoidal transmit and measurement channels, the network analyzer measuring predefined signals by mixing the received signal with a reference, phase-shifted transmit signal and the in-phase transmitted signal. 
     It is also an object of the present invention to provide a portable system analyzer which measures system responses using any arrangement of transmitters and receivers, measures system network phase and magnitude transfer functions, measures the transfer function of passive or active filters, measures closed-loop control system phase and magnitude, measures the system&#39;s DC gain, open loop bandwidth and phase margin of amplifiers, measures impedance versus frequency of selected system components, measures system resonances, characterizes phase lock loops, measures performance of nonlinear system components (such as signal mixers), determines the phase and noise characteristics of a system and measures non-linear system characteristics. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those of skill in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     The present invention is a small, portable, system or network analyzer and method having multiple channel transmit and receive capability for real-time monitoring of processes which maintains phase integrity, requires low power, is adapted to provide full vector analysis, provides output frequencies of up to 62.5 MHz and provides fine sensitivity frequency resolution. The present invention also provides a highly sensitive dynamic range response, allows for programmable phase and magnitude spectrum drive signal shaping, variable signal sweep speeds and low signal power for each of the transmit and receive channels employed. The present invention also maintains phase integrity between any signals transmitted to the system being analyzed and any response signals from the system so that time domain analysis can be executed on the recovered response signals. 
     The present invention includes a means for transmitting, software means for controlling and a means for receiving in electrical communication with the software means for controlling. The means for transmitting and the means for receiving include multi-channel multiplexer circuitry, thereby allowing analysis of multiple systems or networks. The means for controlling is programmed to provide a signal to a system under investigation which steps consecutively over a range of predetermined frequencies. The resulting received signal from the system provides a frequency response over the range of predetermined frequencies. Time response information is provided by taking a frequency transform of the magnitude and phase information acquired at each frequency step. The conversion back to the time domain would not be possible without using a device such as the present invention which maintains phase integrity. 
     The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical illustration of the preferred embodiment of the present invention; 
     FIG. 2 is a graphical illustration of the means for transmitting as disclosed and claimed in the present invention; and 
     FIG. 3 is a graphical illustration of the means for receiving as disclosed and claimed in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in FIGS. 1-3, the present invention  10  is a method and apparatus for transmitting sinusoidal signals on one of a plurality of channels to a system and receiving corresponding response signals on one of a plurality of receive channels corresponding to various system operational characteristics. As seen in FIG. 2 a  and  2   b,  the present invention  10  includes a means for transmitting  20 , a means for receiving  40  and a means for controlling  60 . 
     As seen in FIG. 2, the means for transmitting  20  transmits at least one sinusoidal signal  27   n  to the system S, or network, being analyzed. The means for transmitting  20  includes a first means for digital synthesizing  21 , a second means for digital synthesizing  23 , optional electronic means for signal conditioning  25 , a multi-channel means for multiplexing  27 , an optional output buffer  29 , all in communication with the software means for controlling  60 . 
     First means for digital synthesizing  21  produces a pure sinusoidal signal with a spurious free dynamic range of over 50 db, which prevents intermodulation from corrupting the result of extremely sensitive measurements of signal  41   n  (shown in FIGS. 1 and 3) received from the system S. Preferably, first means for digital synthesizing  21  is capable of generating sinusoidal signals over a wide frequency range with low frequency tuning resolution, producing a square wave reference sinusoidal signal corresponding to the sinusoidal signal being generated, and generating an analog signal corresponding to the sine wave reference signal. A preferred first means for digital synthesizing  21 , for example, is part number AD9851 sold by Analog Devices, which generates signals from a few hertz to over 60 megahertz, has a frequency tuning resolution of 0.04 Hz and outputs an analog reference signal  21   a  at a predetermined frequency. This resolution allows the measurement of received signals  41   n  to a wide range of transmitted sinusoidal signals  27   n . 
     Second means for digital synthesizing  23  also produces a pure sinusoidal signal with a spurious free dynamic range of over 50 db, generates sinusoidal signals over a wide frequency range with high frequency tuning resolution and produces an analog signal. The preferred second means for digital synthesizing  23  is the same synthesizer as the first means for digital synthesizing  21 , which generates sinusoidal signals from a few hertz to over 60 megahertz and has a frequency tuning resolution of 0.04 Hz. However, the analog output signal  23   a  of second digital synthesizer  23  is phase shifted with regard to the reference analog signal  21   a  generated by the first means for digital synthesizing  21 , thereby resulting in a phase-shifted frequency signal  23   a.  Preferably, frequency signal  23   a  is phase-shifted by 90 degrees resulting in a cosine signal. The phase-shifted frequency signal  23   a  is then transmitted to the means for receiving  40  (as seen in FIGS. 1 and 3) to allow the determination of phase information from the system S being analyzed as further described below. 
     The electronic means for signal conditioning  25  is optionally necessary depending on the type of digital synthesizer ( 21  or  23 ) employed. In the preferred embodiment of the present invention, device part number AD9851 outputs analog signals  21   a,    23   a.  However, those of skill in the art will realize that output signals  21   a,    23   a  employed in the preferred embodiment are current output signals, and further, that these signals are differential signals. Thus, signal  21   a  requires further refinement to reduce or eliminate DC offset and account for the proper amplification and voltage level for the proper transmission to the system S being analyzed. In one preferred embodiment of the present invention, a means for filtering electronics (not shown) is first employed to filter signal  21   a  to eliminate any remaining higher order harmonics and to produce a differential voltage signal. Then, a dual amplifier (not shown) is configured to amplify the signal, which is then transmitted to a high pass filter circuit resulting in a conditioned signal  25   a  to thereby eliminate any DC offset and to further provide a proper signal drive voltage level. Those skilled in the art will realize that numerous methods may exist in which to properly condition signal  21   a  by a means for filtering electronics which remain within the spirit and scope of this invention. 
     The conditioned signal  25   a  (or, signal  21   a  if the signal did not require conditioning) is then transmitted to a multi-channel means for multiplexing  27  which multiplexes signal  25   a  to the required channel or channels  27   n  by the software means for controlling  60 . In the preferred embodiment, means for multiplexing  27  is a MAX308 eight channel multiplexor sold by Maxim Industries. Each multiplexed signal  27   n  is then controlled by the means for controlling  60  to be transmitted to an output buffer (not shown) such as an LM6182 manufactured by National Semiconductor, which can drive up to ½ watt of power and remain stable under capacitive loads. Each output buffer is optional, but is necessary in one preferred embodiment due to the internal series resistance found in the means for multiplexing  27  employed in the preferred embodiment. Such resistance needs to be isolated from the system S being analyzed to ensure accurate response measurements. 
     As shown in FIGS. 1-3, the software means for controlling  60  is stored on a computer means, is in communication with a communications bus B and is software employed to control (and therefore, is in communication with) the means for digital synthesizing  21 ,  23 , the electronic means for signal conditioning  25 , the multi-channel means for multiplexing  27 , a multi-channel means for demultiplexing  41 , a low noise means for amplifying  43 , a means for mixing electronic signals  45 , optional low pass filtering means  47   a,    47   b  and a multi-channel, simultaneous sampling, means for converting signals  49 . While those of skill in the art will come to realize that numerous software applications could be used to control these devices to achieve the same or similar results of those disclosed in this invention, the preferred embodiment of the present invention employs Labview control software (developed by National Instruments). In particular, the means for controlling  60  is programmed for stepped frequency modulation, continuous wave processing so that frequencies are discretely stepped over a predetermined frequency range and over a predefined frequency increment by programming tasks known in the art. Optimally, the means for controlling is programmed so that at each step, the means for controlling is programmed to dwell for a short time interval to allow for filter settling. At each interval, preselected measurements can be taken [such as, for example, the magnitide of the in-phase signal (I) and the quadrature signal (Q)) over a particular frequency sweep, which can then be stored, analyzed and displayed by the means for controlling  60  as needed. When the stepped programming is completed over the preselected frequencies, the frequency magnitude of each response signal  41   n  can be calculated as {square root over (I 2 +L +Q 2 +L )}, the phase data of each response signal  41   n  is computed as the arctan (Q/I), while the time domain impulse response of each response signal  41   n  is computed by performing a complex inverse frequency transform, all techniques known to those of skill in the art. By executing such a frequency sweep at every frequency of interest, the spectral magnitude and phase information about system S is obtainable by employing the means for controlling  60 . The inverse frequency transform can then be performed to yield an equivalent time domain response from system S. 
     As seen in FIG. 3, the means for receiving  40  includes a multi-channel means for demultiplexing  41 , a low noise means for amplifying  43 , a means for mixing electronic signals  45 , optional low pass filtering means  47   a,    47   b  and a multi-channel, simultaneous sampling, means for converting signals  49 , all in communication with the means for controlling  60 . 
     The means for demultiplexing  41 , controlled by the software means for controlling  60 , demultiplexes each receive signal  41   n  resulting in a demultiplexed receive signal  41   a.  The ability to transform multiplexed signals to a demultiplexed signal by a conventional multiplexor circuit is well known in the art. As such, in this configuration, a plurality of transmit signals  27   n  can be transmitted to a system S for analyzing, and the corresponding plurality of receive signals  41   n  can be used to receive the signals from the system S, thereby allowing greater system flexibility. In the preferred embodiment, means for demultiplexing  41  is a MAX308 eight channel multiplexor circuitry manufactured by Maxim Industries. 
     The demultiplexed receive signal  41   a  is then transmitted to the means for amplifying  43  to increase the magnitude of signal  41   a.  The gain of means for amplifying  43  is optionally adjusted by the software means for controlling  60  to allow analysis of high or low level return signals. Means for amplifying  43  is low noise adjustable because, in certain cases, the received signal  41   a  will be near the system noise floor. 
     After receive signal  41   a  is amplified, it is transmitted to a means for electronically mixing signals  45  (such as, for example, a solid state mixer integrated circuit such as part number AD831) which executes a phase coherent mixing operation to obtain the rectangular format of the in-phase (I) and quadrature (Q) components of each signal, and extracts information pertaining to the phase and magnitude of the return signal  41   a  with respect to the transmitted signals  21   a  and  23   a.  To obtain the I and Q components, the receive signal  41   a  is divided into multiple signals by the means for electronically mixing signals  45 , and preferably, two signals  41   a   I  and  41   a   Q . Means for electronically mixing signals  45  electronically mixes in-phase signal  41   a   I  with sine wave transmit signal  21   a,  and further, electronically mixes quadrature signal  41   a   Q . with cosine wave transmit signal  23   a.  Software means for controlling  60  obtains receive signal  41   a  from the system S which is either an inverted or non-inverted signal, depending on the state of the transmitted reference signals  21   a  or  23   a.  Thus, for example, when the reference square wave signal  21   a  is low, the receive signal  41   a  is non-inverted. When the reference square wave signal  21   a  is high, the receive signal  41   a  is inverted. The mixing of receive signal  41   a  with the reference sine wave signal  21   a  and with the reference cosine wave signal  23   a  is accomplished by hardware configuring the means for electronically mixing signals  45  to select the reference sine wave signal  21   a  or the reference cosine wave signal  23   a.  This mixing process is analogous to a radio frequency (RF) mixer where an RF signal is sequentially inverted or not inverted by transmitting saturating local oscillator current through a set of ring connected switching diodes. Thus, according to the present invention, the mixing produces a frequency translation which produces the difference of the transmit signal and the received signal. 
     Preferably, as seen in FIG. 1, both in-phase signal  41   a   I  and quadrature signal  41   a   Q  are next processed through low pass filtering means  47   a,    47   b  to condition both signals so that each signal has a frequency content which represents the difference of the two mixer signals. Thus, the transmitted reference signal is mixed with the received signal to produce an output signal which is the difference between the transmitted reference signal and the received signal. While other methods may exist to obtain the same type of signals, such methods may be deficient. For example, analog multipliers typically suffer from generating high DC drift signals while offering low dynamic range. The present invention solves such deficiencies. In any case, the component I and Q can thereafter easily be mathematically transformed into polar phase and magnitude format by means well known in the art for further computational processing by the means for controlling  60 . 
     Next, in-phase signal  41   I  and quadrature signal  41   a   Q  are both transmitted to a multi-channel, simultaneous sampling, means for converting signals  49  to produce a signal  49   a  for processing by the means for controlling  60 . Preferably, means for converting signals  49  is capable of converting an analog signal to a high resolution digital signal for processing by the means for controlling  60 . A representative means for converting signals  49  includes a MAX125 (sold by Maxim Industries), which has the capability to accurately and simultaneously sample up to four input signals, and rapidly convert each of these signals to a fourteen bit resolution digital representation. While the present invention only requires use of two input signals to the means for converting signals  49 , the other inputs could be used to sample a higher bandwidth signal if desired (since using a higher bandwidth allows for a faster scan rate, but less signal range above the noise floor—thus, if more time is selected, lower voltage signals can be recovered). 
     Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The particular values and configurations discussed above can be varied, are cited to illustrate particular embodiments of the present invention and are not intended to limit the scope of the invention. It is contemplated that the use of the present invention can involve components having different characteristics as long as the principle, the presentation of a multi-channel network analyzer, is followed.