Verification of complex signal timing requires the sophistication and preciseness that only a dedicated counter can offer. A dedicated counter provides the 1-ns resolution needed to accurately measure the high frequencies found in communications systems as well as radar, television and satellite telecommunications products.
The exponential growth in wireless communications has pushed the need for higher counter resolution to ensure precise tuning of RF transmitters and receivers, said Bill Griffith, Product Marketing Engineer for Hewlett-Packard. The best instrument for performing these tests is the stand-alone counter. It offers time-interval measurements, signal conditioning and triggering.
Multipurpose instruments, such as oscilloscopes, typically cannot make accurate time interval measurements. This is especially true if the signal to be measured is delayed a significant amount of time from the trigger pulse. Precise time interval measurements require a consistent trigger point. The input filters in counters are designed to provide the necessary signal conditioning to obtain precise measurements.
Both counters and oscilloscopes check jitter, the cycle-to-cycle deviations of a periodic event. Examples of jitter are period variations of a computer clock oscillator or clock-to-data jitter in a communications system.
Jitter plagues high-speed clock distribution networks found in microprocessors, PCs, computer mainframes and instruments. As clock speeds increase, jitter can consume a significant portion of the timing budget and cause errors. By reducing jitter you can reduce errors or increase the clock speed and improve system performance.2
To measure jitter, you must perform many single pulse-width measurements and statistically process the samples to get the maximum, minimum and standard deviation values from the samples. For example, the specification for a computer clock distribution system might be £ 1 error in 1 billion clock cycles. With a 50-MHz clock, this means the system can have only 1 error every 20 seconds.
To measure cycle-to-cycle jitter, you must capture every edge. The peak-to-peak jitter is important but typically the rms jitter, which is the standard deviation value, is more important because it can help you understand the nature of the jitter.
But only the counter measures the rms jitter and displays the distribution of the measurements in a histogram. The histogram helps to reveal the nature of the jitter. For example, jitter caused by sine modulation presents a histogram that looks like a bathtub curve. Jitter caused by a square wave modulation has two distinct bars at the maximum or minimum values.1
A frequency-modulated signal also is difficult to characterize with a typical oscilloscope because as the frequency varies, the period also changes. These frequency/period fluctuations prevent the scope from getting a stable trigger and reduce your job of figuring out the nature of the signal to just guesswork. A counter with time and frequency analysis capabilities is what you need. It displays the needed frequency-varying-over-time information.
Applications that need high-end counters include high-speed testing of components, subassemblies and systems. Their testing requires the speeds supported by counters, including 8,000 measurements/s with single-shot resolution of 50 ps, said Charles Holtom, Product Marketing Manager for Fluke.
A typical application of a high-end counter is the field installation and maintenance of digital base stations, such as global systems for mobile communication (GSM), said Mr. Holtom. The frequency accuracy for a GSM base station must be <5 ´ 10-8, which requires an instrument with a frequency accuracy of at least 5 ´ 10-9.
Several GSM network providers use a central synchronization system to meet this calibration requirement and to avoid regular preventive calibrations, said Mr. Holtom. A high-accuracy tool, however, is still required during installation and when the system loses frequency synchronization.
Stand-alone counters used in communications testing also have a frequency capability higher than most multipurpose instruments, said Tom Hayden of Keithley Instruments. For example, the Keithley Model 776 measures 225 MHz on channel A or B and 2.4 GHz on channel C. It also provides variable gate lengths, which makes it suitable for measuring short RF bursts.
Dedicated counters are the best bet for microwave applications too, especially when a quick reading with 1-Hz resolution is needed, said Steve Ashby, Applications Engineer for EIP Microwave. A microwave spectrum analyzer can perform a reading on a high-frequency signal but often will require many range/resolution bandwidth changes to complete the test. The breadth of the signal also makes it difficult to determine the exact frequency.
Microwave counters also are needed for high-resolution measurements of pulsed signals. Spectrum analyzers often give an ambiguous or difficult-to-interpret display of a pulsed signal while a pulsed counter quickly and accurately provides the measurement.
Other features on a dedicated microwave counter are measurement averaging of modulated or moving signals and source locking, said Mr. Ashby. Source locking holds unstable sources to an in-phase 10-MHz standard up to 110 GHz. This capability is important where unstable sources are common and synthesizers are unavailable, he said.
Reference
1. “ABCs of Time and Frequency Analysis PM 6681 Timer/Counter/Analyzer With TimeView™ Software,” Fluke, Application Note, 1996.
2. “Characterize and Reduce Clock Jitter to Improve System Speed and Reliability,” Hewlett-Packard, 1994.
Counters
Multifunction Microwave Counter
Frequency Extends to 2.4 GHz
The Model 1856A is a multifunction microwave counter with a frequency bandwidth from 5 Hz to 2.4 GHz. The temperature-compensated crystal oscillator time base has 0.5-ppm stability from 18° C to 28° C and 1 ppm from 0° C to 50° C. Sensitivity at 2.4 GHz is 50 mV. Applications include checking radio transmitter frequencies to the microwave range, military aeronautical systems, nuclear test facilities communications and cellular telephones. $499. B+K Precision, (312) 889-1448.
Power Readings for Calibration
Are Automatically Corrected
The IP 595A/598A Pulse/CW Microwave Counters offer automatic and self-contained frequency and power profiling for chirped radar, carrier-frequency measurements, pulsed radar analysis, VCO measurements and frequency-agile system analysis to 170 GHz. The instruments detect and measure pulse widths as narrow as 50 ns for CW, frequency-modulated and pulsed RF signals. Power readings are automatically corrected for the internal power-sensing calibration factor. The time-base aging rate is 2 ´ 10-7/yr, and calibration is required once every two years. 595A: $13,900; 598A: $16,500. EIP Microwave, (800) 232-3471.
Frequency Reference Instrument
Has Rubidium Reference
The PM 6681R Rubidium Frequency Reference/Calibrator provides a primary frequency and time reference for the lab and on-site. It offers a 10-MHz frequency reference with six buffered outputs. Basic instrument performance is 11 digits/s frequency resolution and 50-ps single-shot time resolution. The PM 6681R and an external receiver can monitor the GPS frequency and compare it with the unit’s atomic reference. Frequency drift is <2 ´ 10-10/yr and traceability is 1 ´ 10-9. The instrument is available in versions that calibrate frequencies to 4.5 GHz for RF and microwave applications. The TimeView software handles time and frequency analysis and advanced statistical processing in the modulation domain. $12,950. Fluke, (800) 44-FLUKE.
Programmable Unit Offers
External Gate and Arming
The HM8122 Universal Counter provides three inputs and a signal measurement capability from DC to 1.6 GHz. The instrument has a 100-MHz reference oscillator and 10-ns resolution. Time-interval averaging allows 1-ps resolution. Low-frequency measurements have 8-digit resolution at a 1-s gate time. Nine measurement functions include a preselectable number of pulses per rotation, offset, display-hold and trigger-view. Rear input connections allow measurements of channel A gated by channel B. The HM8122 performs a self-test and calibration routine during power-up. $1,120. Hameg, (619) 630-4080.
Universal Counter Offers
10 Digits/s Resolution
The HP 53131A Universal Counter offers 10 digits/s-resolution at up to 225 MHz on two channels. Measurements include frequency, time interval, pulse parameters, phase angles, period, rise/fall time, positive/negative pulse width, duty cycle and peak voltage. The instrument uses real-time digital signal processing to analyze data and simultaneously take readings. It performs automated limit tests, scale and offset, and statistical calculations. $1,725. Hewlett-Packard, (800) 452-4844.
Universal Counter Measures
Frequencies to 2.7 GHz
The GUC-2270 Universal Counter has a low range to measure frequencies from 5 Hz to 10 MHz; a high range to measure from 5 MHz to 200 MHz; and channel C to measure from 100 MHz to 2.7 GHz. Gate times extend from 0.01 s to 20 s. Measurement functions include frequency, period, frequency ratio, time interval, unit counter and check mode. Accuracy is ± 1 ppm/month ± 1 count. $700. Instek, (818) 336-6537.
Programmable Counter Has
13 Measurement Functions
The Model 776 Counter/Timer provides two independent channels for frequency measurements to 225 MHz. Thirteen measurement functions are available including peak voltage, time interval, ratio and phase measurement. Ten front-panel setup procedures can be stored. The unit is programmable on the IEEE 488 bus and supplies up to 100 ASCII-formatted readings/s to the bus. A 10-MHz time base with 5- ppm temperature stability is standard. $1,995. Keithley Instruments, (800) 552-1115.
Instrument Offers
25-ps Resolution
The SR620 Time Interval Counter makes single-shot time measurements with 25-ps resolution and 1-s measurements with 11-digit resolution. The instrument measures period, counts, pulse-width, and rise and fall time, and reports the statistics of 1 million samples. Interfaces include RS-232, GPIB/IEEE 488 and a printer port. $4,500. Stanford Research, (408) 744-9040.
Copyright 1996 Nelson Publishing Inc.
June 1996