Simulator Serves 85-MHz Channels

June 18, 2010
This modular simulator can reproduce the signal conditions found in communications channels where transmitters and/or receivers are moving, controlling fading, attenuation, Doppler, and other effects.

Communications links often involve mobile transmitters or receivers, as in the case of satellite communications (satcom) systems and unmanned aerial vehicles (UAVs). Testing such communications links under realistic conditions involves a signal simulator rather than a conventional signal generator. One such signal simulator, the Telemetrix 400 Channel Simulator (T400CS) from RT Logic, was recently enhanced from an available channel bandwidth of 40 MHz to a real-time bandwidth of 85 MHz. The modular system can now handle the widest communications channels, emulating the mix of natural effects between transmitters and receivers, such as propagation losses, delays, and multipath effects, in the test conditions. The T400CS Channel Simulator is a modular card-based system that allows the selection of specific function and performance levels, operating bands from UHF through Ku-band frequencies, spectrum/interference analysis and test/interference signal generation, with as many as eight communications channels supported per card.

When transmitters and receivers are in motion relative to one another, their signals are affected by carrier and signal Doppler shifts, time delays, attenuation, noise, and interference. Weather, terrain effects, and multipath effects can also hinder the operation of a communications system. The T400CS is designed to accurately emulate these signal effects to model and test a communications link under near-actual operating signal conditions.

The basic Channel Simulator supports instantaneous channel bandwidths of 40 or 85 MHz, and can "condition" signals for emulating the effects of Doppler shifts, range delays, and signal-path losses. Signal Doppler can be adjusted phase continuously over a range of 500 kHz with 1-Hz resolution; carrier Doppler can be adjusted by 0 kHz 20 MHz in the 40-MHz version and 0 kHz 42.5 MHz in the 85-MHz version, phase continuously and with 1-Hz resolution. Delays due to the range or distance between receivers and transmitters can be adjusted phase continuously from 3 s to 2.5 s with 1-ns resolution. Range attenuation can be set from 0 to 60 dB, with as much as 60 dB phase-continuous fading effects also available with 0.5-dB resolution. For adding noise to a signal, additive Gaussian white noise (AWGN) is available -168 dBm/Hz to -102 dBm/ Hz while the same levels of noise are also available for checking a receiver's signal sensitivity in the presence of noise. The T400CS system features overall amplitude response of 0.5 dB with an instantaneous dynamic range of 40 dB. The system can be used with internal or external triggers.

The T400CS Channel Simulator can be equipped with cards for test signal generation and analysis. The signal generator cards can produce as many as eight signals per card with independently adjustable frequency offset, modulation type, data rate, pseudorandom-noise (PRN) code, amplitude, and filtering. Test and/or interference signals can be generated with a wide range of modulation formats, including binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), minimum shift keying (MSK), frequency shift keying (FSK), phase shift keying (PSK), amplitude modulation (AM), and frequency modulation (FM). Signals can be processed by means of standard filters, including rectangular, raised cosine, and root raised cosine filters, with standard PRN codes of 127, 511, 2047, 215 1, 220 1, and 223 1. Signals can be controlled with independent trigger delays and individually set signal characteristics.

When equipped with frequency downconverters, the system can accept RF input signals at UHF (225 to 400 MHz), L-band (950 to 1750 MHz), S-band (1650 to 3000 MHz), C-band (3.4 to 4.2 GHz), X-band (7.25 to 7.75 GHz), and Ku-band (10.70 to 12.75 GHz) frequencies.

The flexible T400CS Channel Simulator architecture is based on the use of compact PCI (cPCI) function modules controlled by a cPCI CPU running Microsoft Windows XP Professional. The approach allows for multiple channel simulator cards, signal generator cards, spectrum analyzers, frequency converters, and other measurement functions.

The T400CS system can be configured with an IF of 70, 160, or 266 MHz. The system works with a number of different front-end software programs, including STK software from Analytical Graphics to produces RF and intermediatefrequency (IF) signals with required signal behavior. The Channel Simulator can be seamlessly controlled by means of the STK software. The Channel Simulator can also be commanded by user programs via TCP/IP socket communications over Ethernet connections, from supplied user interface software, and from usercreated spreadsheets.

RT Logic, a subsidiary of Integral Systems, Colorado Springs, CO, (719) 598-2801, FAX: (719) 598-2655; Internet: www.rtlogic.com.

About the Author

Jack Browne | Technical Contributor

Jack Browne, Technical Contributor, has worked in technical publishing for over 30 years. He managed the content and production of three technical journals while at the American Institute of Physics, including Medical Physics and the Journal of Vacuum Science & Technology. He has been a Publisher and Editor for Penton Media, started the firm’s Wireless Symposium & Exhibition trade show in 1993, and currently serves as Technical Contributor for that company's Microwaves & RF magazine. Browne, who holds a BS in Mathematics from City College of New York and BA degrees in English and Philosophy from Fordham University, is a member of the IEEE.

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