The HP 53310A Modulation Domain Analyzer

Technical Backgrounder

The HP 53310A Modulation Domain Analyzer has been designed with the goals of providing new levels of speed, ease of use, and affordability in an instrument capable of performing continuous frequency and time-interval measurements and displaying them graphically with respect to time. The instrument's real-time, interactive display, intuitive, menu-driven operation, numerous automated functions, and high degree of integration all contribute to achieving these goals.

Many of these attributes have been made possible through the development of an HP designed, custom integrated circuit that makes the HP 53310A the first instrument of its kind to combine measurement functions and data processing capabilities on a single chip. A custom, HP-designed universal counter integrated circuit -- the UCIC -- keeps track of data events as well as time through the use of ``zero-dead-time'' counters, and computes results on the fly, enabling them to be updated and displayed graphically 24 times per second. The UCIC's instantaneous computing capability also provides the HP 53310A with the unique ability to trigger on user-specified frequency or time-interval values.

The UCIC works in conjunction with the HP 53310A's host 68000 microprocessor and a hardware histogram coprocessor to provide all the instrument's measurement and statistical functions. These components also are responsible for the HP 53310A's significant improvements in ease of use by enabling a number of one-button operations, providing oscilloscope-like menu-driven operation, and permitting the real-time graphical display. The high degree of integration provided by this design has dramatically reduced the number of components, resulting in a moderately priced instrument that is also more compact and lighter.

The Modulation Domain

The resulting ``counter,'' generically called a frequency and time-interval analyzer, is able to provide frequency versus time (or time-interval versus time) analysis, called the modulation domain. It thus does for frequency measurement what the digital oscilloscope does for voltage measurement: continuously sampling the input and displaying the results graphically.

The HP 53310A can perform up to 64,000 continuous measurements (samples) on input signals up to 200 MHz (2 GHz, optional) at a rate of up to 2 million measurements per second using HP's continuous measurement technology. An input signal is sampled at positive-going zero crossings, and each sample has associated with it the total cycle count, and the time stamp at that point. The cycle count comes from reading, on the fly, a counter totalling the number of zero crossings. Every cycle is counted, not just those on which a sample occurs. When sampling occurs, that particular cycle's upcrossing is gated by a synchronizer for time stamping, which is accomplished by reading (also on the fly) another counter counting the time base clock. Then, by dividing the number of cycles by the time interval of the sample, the average frequency over the length of the sample can be calculated.

The modulation domain is the missing view in the three dimensional view of any signal, a view that has been difficult or impossible to achieve until recently. The view of an input signal provided by the ubiquitous oscilloscope is in the time domain, displaying variations in amplitude (voltage) as a function of time. The frequency domain is the view of an input signal in terms of its amplitude as a function of frequency, showing how the power in the signal is distributed across its bandwidth. A spectrum analyzer is an example of an instrument operating in the frequency domain.

The view of an input signal in the modulation domain reveals the variations in frequency as a function of time. The resulting display represents the frequency modulation on the signal. This modulation may be desired, such as the chirps on an agile-carrier, staggered PRI radar signal, or undesired, such as the jitter on a high-speed digital communications channel.

UCIC -- A Frequency and Time-Interval Analyzer On a Chip

With the introduction of the HP 53310A, HP has integrated all the functionality of its continuous measurement technology onto a single integrated circuit. Designed in conjunction with HP's Colorado Integrated Circuit Division, the Universal Counter Integrated Circuit (UCIC) contains all the circuitry necessary to provide arming functions, trigger synchronization, generation of time interpolation pulses, measurement sequencing, and zero-dead-time counters necessary for implementing HP's continuous measurement technology. In addition, the chip contains extensive RISC-based data processing capabilities. It is this on-chip data processing that provides the increased information content in a form usable for quick graphical display and other automated functions by greatly reducing the load placed on the host microprocessor. The 9.4 mm by 9.0 mm CMOS chip contains over 100,000 field-effect transistors (FETs) and is packaged in a 132-pin PCPGA.

The UCIC contains two major functional blocks: a measurement section that controls the capturing of information from an input signal, and a data processing section that pre-processes the data extracted from the input signal. The UCIC is under control of the host microprocessor only prior to a measurement by the setting of internal control bits. In addition, the host microprocessor downloads 16-bit instructions into the UCIC's 32-word instruction register. Once the UCIC is programmed for a particular measurement scheme, enabling an external RUN signal begins the measurement with the UCIC controlling all aspects of the measurement independently of the host microprocessor. The HP 53310A's host processor is then able to obtain high-speed access of the results via the direct memory access (DMA) port on the UCIC for real-time graphical display.

Improved Continuous Measurement Technology

Unlike conventional frequency and time-interval measuring instruments that lose valuable data while computing results and preparing for the next measurement, the HP 53310A Modulation Domain Analyzer uses HP's patented continuous measurement technology to provide up to 64,000 measurements at a rate of up to 2 million measurements per second. The HP 53310A's time sampling technique is called ``continuous measurement'' because the count hardware is not reset between measurements. Eliminating this reset time dramatically increases measurement throughput. By sampling fast enough, the time variation of frequency or time-interval data can be displayed.

The UCIC's measurement section provides to the data processing section ``time stamps,'' representing when an input signal occurred with respect to an internal time base, and ``event stamps,'' indicating which input pulse is associated with the time stamp. The measurement section, consisting of 35,000 FETs operating off four asynchronous 100-MHz signals, contains the arming, trigger synchronizer, sequencer, and second-generation zero-dead-time counter functional blocks.

The arming block provides control signals for the trigger synchronizer to determine how many measurements to make, the number of cycles or time delay to wait before sampling, and which input signal to sample. By doing so, it creates the various measurement modes available in the HP 53310A, primarily frequency and time-interval.

Besides its interaction with the arming block, the trigger synchronizer block is critical to the overall performance of the HP 53310A. It sends an interpolation pulse to an external charge/flash interpolator where it is converted to a time measurement that is supplied back to the UCIC. Interpolation increases the effective resolution of the system. The low-noise interpolation pulses require such extremely tight jitter specifications to insure the accuracy of the timing function that Hewlett-Packard derived a new synchronization theory to handle non-symmetrical, cascaded latches for ratioed and ratioless CMOS synchronizers.

The zero-dead-time counters (ZDTs) perform the continuous count function for the HP 53310A by eliminating the need to reset the counters between measurements. Consisting of two 100-MHz, 28-bit synchronous counters and eight 28-bit registers, the zero-dead-time counter outputs can be read on-the-fly without introducing any errors. Previous designs used a combination of synchronous and ripple counters, delaying reading of the counter until all bits were settled. Because these designs were dependent on the clock state for reading the counters, they resulted in a slower relative throughput. The HP 53310A's ZDTs, unlike these designs, ignore the counter clock altogether, and allow the count to be read on command. This read-on-command capability is made possible through the use of Gray coding, a counting method in which only one bit at a time changes as the counting progresses.

On-Chip Data Processing

The UCIC's data processing section, containing 62,000 FETs and operating off a two-phase, non-overlapping, 25-MHz, on-chip clock, is essentially a 56-bit, 25-MIPS reduced instruction set processor specifically designed to operate on the ZDTs' continuous count data. It accounts for a significant part of the HP 53310A's real-time graphical display capability, as well as other speed and ease-of-use features.

The processing section reads time and event stamps from the measurement section ZDTs. Then, depending on the program instructions contained in the instruction sequencer, it performs operations on the data that result in various measurements, calculating either instantaneous frequency or time-interval. The instruction set comprises nine 16-bit instructions tailored to performing the types of calculations necessary for frequency and time-interval measurement. The results are stored in external memory, available for further processing and graphical display by the host processor.

As adjustments are made to the instrument while it is displaying a signal, such as changing the time base, the host microprocessor loads new values into the on-chip processor's program store, changing the necessary parameters on the fly. The change is immediately reflected on the display. Selection of a new measurement function loads an entirely new program into the program store.

Real-Time Display

One of the primary design goals of the HP 53310A was to have a display update rate sufficient to provide a real-time picture of the input signal, allowing the user to immediately see, among other things, the results of adjustments made to the circuit under test. While previous designs of the modulation domain analyzer provide a more robust set of measurement and analysis functions, as well as greater accuracy, the architecture of these instruments limits their ability to provide a real-time, interactive display of the input signal. In these earlier instruments, input events and time stamps are collected and stored in memory, and the raw data is post-processed by the host microprocessor for display, resulting in only about 2 display updates per second.

Offloading the burden of calculating measurements from the host processor dramatically improves the update rate. By adding on-chip data processing to the measurement functions, measurement data can be pre-processed before being sent to the host processor, enabling the HP 53310A to perform 24 display updates per second. Further benefits of this pre-processing include the ability to trigger the measurement of the input signal based on a specified value of frequency or time-interval, and the ability to automatically scale and center the display.

Autoscale

Autoscale is an innovative feature introduced with the HP 53310A that makes acquisition as easy as the touch of a button. It examines the input signal and attempts to set the instrument to a state which will produce a stable, centered, properly scaled display of the signal, automatically. It is a major contributor to the HP 53310A's ease of use, automatically performing a number of functions previously done manually. The autoscale function is made possible by the immediate availability of pre-processed measurement information from the UCIC and that chip's programmability.

Throughout the autoscale process, the system automatically changes only those parameters necessary to present a meaningful display to the user, and leaves unchanged other parameters the user may have intentionally set. Autoscaling of both the frequency function and the time-interval function are available.

The autoscale algorithm for the frequency function is a multi-step process for determining values for the vertical frequency scale and the horizontal time scale. After finding the 50 percent voltage threshold on whichever input channel is being used, the autoscale algorithm then seeks to determine the best center and span values for the vertical display. First it performs an iterative process to determine the minimum and maximum input frequency. It then checks the ratio of these two frequencies against instrument limits, and adjusts the display values, if necessary to show only the higher frequencies. It then seeks the best resolution. The autoscale function attempts to display the signal in the center half of the display, though sometimes this is not possible due to instrument limits, and tries to use ``nice'' values (numbers with no more than two significant digits) for the center and span. Lastly, the horizontal time per division is set to display approximately four cycles of the modulation. The autoscale function for time-interval measurements follows a similar process.

The HP 53310A features a number of auto parameters, relieving the user from setting them manually. For example, the frequency trigger can be set to automatically trigger in the middle of the vertical range selected. As a result, ranges and instrument states can be changed without resetting the value of the trigger.

The Next Standard in Instrumentation

With the introduction of the HP 53310A Modulation Domain Analyzer, Hewlett-Packard Company has brought time-dependent frequency and time-interval measurements within reach of thousands of design engineers. Its breakthroughs in speed, cost, and ease of use have made this second-generation instrument ideal for gaining quick, qualitative insight into the operation of complex circuits used in a broad range of modern electronic systems. Its unique ability to directly display complex signals in real time, affordability, and ease-of-use features make the HP 53310A Modulation Domain Analyzer ready to take its place beside the circuit designer's other established test instruments.

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