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A microwave toolbox: simultaneous pulse power and frequency measurement.

Introduction

For many years, microwave measurements of power and frequency on pulsed signals have been relatively difficult or complex to implement. The evolution of pulsed frequency measurements started with manual measurements using wavemeters and has evolved into instruments that can automatically profile the frequency with a pulse. These advanced measurement instruments have a price range of $10,000 to $15,000.

Peak power meters have made similar progress from the diode/oscilloscope combinations to system with internal displays of detected waveforms that offer sophisticated measurement enhancements, including automatic measurements of pulse width, pulse repetition interval and pulse repetition frequency (PRF). These relatively recent instruments put the price of this measurement sophistication in the range of $7000 to $15,000, including sensors. For the first time, two new instruments, the models 595A (to 20 GHz) and 598A (to 26.5 GHz, with extensions available to 170 GHz), combine a single microwave peak power meter with a pulsed/CW frequency counter at a price of only 15 percent more than the pulsed frequency counting capability alone.

Description

These models offer automatic and self-contained frequency profiling that is ideally suited to such applications as chirped radar profiling, carrier frequency measurement, pulsed radar analysis, VCO measurements and frequency agile system analysis over a frequency range up to 170 GHz (depending upon the model selected). Automatic measurements are possible to pulse widths as narrow as 50 ns. Synchronous outputs on the rear panel show the actual measurement window for viewing on an oscilloscope. In addition, automatic measurements of pulse width, pulse period and PRF simplify the measurement task. Frequency resolutions are user selectable from 1 GHz to 1 Hz. All functions are fully programmable using the IEEE 488 GPIB.

Careful design consideration was given to accurately and automatically measuring rapidly varying pulse bursts typical of frequency hopping and wideband chirp signals. The 595A and 598A pulse/CW microwave frequency counters actually reacquire the microwave signal for each measurement window, allowing, essentially, unlimited frequency changes from window to window as opposed to requiring external gating if the frequency changes more than 10 MHz within the detected pulse burst.

These pulse/CW microwave frequency counters utilize a YIG preselected heterodyne downconversion technique. This spectrum analyzer type preselector prevents harmonics and other spurious signals from interfering with the measurement of the desired signal. Furthermore, this frequency selectivity allows any desired signal to be selected for measurement in a multisignal environment over the entire +7 to -25 dBm dynamic range, which is typical of the instrument. The YIG preselector also acts as a power limiter, allowing the instrument to tolerate up to +53 dBm (200 W) of pulsed input power and up to +45 dBm (30 W) of CW power without damage.

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In addition, these instruments excel in the practical measurement of CW and peak power. The power within a pulse burst can be profiled automatically and simultaneously with frequency over the entire frequency range of the instrument, as shown in Figure 2. Dynamic range for the power measurement is the same as that of the frequency measurement. The YIG preselector provides performance advantages for power measurement as well. This multistage YIG filter effectively eliminates odd and even harmonics as well as spurious signals outside its 50 MHz passband, making the measurement more accurate than conventional schemes. Conventional peak power measurement schemes using balanced diode detection are sensitive to odd harmonics, or to both even and odd harmonics using single diode detection. In addition, preselection allows the power of these interfering signals to be actually measured if desired. YIG preselection also lowers the broadband noise floor of the power measurement and allows tolerance of the same high input power levels (+53 dBm/200 W) as the frequency measurement.

By measuring the frequency of the incoming signal, the instrument automatically corrects the power reading for the calibration factor of the internal power sensor. Manual entering of calibration factors or measurement frequency is unnecessary. Careful design and internal calibration tables result in an accuracy better than [+ or -]1 dB (typ) and repeatability better than [+ or -]0.3 dB (typ) over the entire frequency range of the instrument.

Conclusion

Simultaneous frequency and power measurements are now possible on CW and pulsed signals using a single convenient compact instrument without sacrificing on performance or price. The instruments are 3.5" high in a 19" rack-mount housing. The 595A and 598A pulse/CW microwave frequency counters are available for delivery in 12 weeks at $13,900 and $16,500, respectively.
COPYRIGHT 1994 Horizon House Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Product Feature
Publication:Microwave Journal
Date:Sep 1, 1994
Words:750
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