MMillimeter-wave test signals are essential for evaluating a wide range of systems and their components, including aerospace, defense, and satellite communications systems. Those test signals should feature low phase noise, minimal harmonics and spurious content and, ideally, fast switching speed to cut the time needed for production testing of components. To fill those requirements, Giga-tronics (www.gigatronics.com) has introduced a new line of six frequency-agile synthesized signal generators with frequency ranges extending as high as 50 GHz. The six new 2500B series of microwave signal generators offer frequency coverage of 100 kHz to 2.5 GHz, 100 kHz to 8 GHz, 100 kHz to 20 GHz, 100 kHz to 26.5 GHz, 100 kHz to 40 GHz, and 100 kHz to 50 GHz.

The Giga-tronics 2500B series signal generators (Fig. 1) are optimized to achieve the best compromise between switching speed and high signal purity. The firm's patented Accumulator High Frequency Feedback (AHFF) technology provides switching speed of typically better than 500 s for a 1-GHz step and as fast as 100 s for smaller frequency steps. The six models (see table) represent a choice in frequency coverage that suits the needs of many different applications. Lower-frequency models, for example, are well suited to commercial wireless testing, while the wide bandwidths of the 40- and 50-GHz models can provide the test signals needed for military satellite communications, electronic warfare (EW), and radar component and system measurements.

The frequency accuracy of these new signal generators derives from a high-performance internal reference oscillator, an oven-controlled crystal oscillator (OCXO). The generators provide frequency accuracy that is the same as this OCXO, which is better than 5 x 10-10/day aging rate with temperature stability of better than 2.5 x 10-8. The 2500B signal generators can also work with an external frequency reference at 10 or 100 MHz. By connecting an external reference oscillator, the 2500B signal generators automatically disconnect the internal OCXO and phase-lock the signal generator to the external reference source. The 10-MHz external reference should provide at least -5 dBm input power while the 100-MHz external reference should deliver at least +5 dBm input power to the signal generator external source port. For a pair of 2500B generators, the most stable phase tracking over time is by locking the sources to an external 100-MHz reference oscillator.

The low-noise OCXO internal reference source contributes to outstanding spectral purity for the 2500B series signal generators. For standard models, the single-sideband (SSB) phase noise (Fig. 2) measured for a 10-GHz carrier is -77 dBc/Hz offset 100 Hz from the carrier, -96 dBc/Hz offset 1 kHz, -109 dBc/Hz offset 10 kHz, -108 dBc/Hz offset 100 kHz, and -138 dBc/Hz offset 1 MHz. For a 30-GHz carrier, the SSB phase noise is -67 dBc/Hz offset 100 Hz from the carrier, -79 dBc/Hz offset 1 kHz, -99 dBc/Hz offset 10 kHz, -99 dBc/Hz offset 100 kHz, and -127 dBc/ Hz offset 1 MHz. The signal generators can also be equipped with an option that enhances the phase-noise performance close to the carrier. With this option (Option 28), the SSB phase noise for a 10-GHz carrier is -70 dBc/Hz offset 10 Hz from the carrier, -90 dBc/Hz offset 100 Hz, -102 dBc/Hz offset 1 kHz, -109 dBc/Hz offset 10 kHz, -108 dBc/Hz offset 100 kHz, and -138 dBc/Hz offset 1 MHz. The SSB phase noise with this option for a 30-GHz carrier is -56 dBc/ Hz offset 10 Hz from the carrier, -78 dBc/Hz offset 100 Hz, -90 dBc/Hz offset 1 kHz, -99 dBc/Hz offset 10 kHz, -99 dBc/Hz offset 100 kHz, and -127 dBc/ Hz offset 1 MHz from the carrier.

Harmonic levels are typically -50 dBc from 100 MHz to 39.6 GHz and -30 dBc from 39.6 to 50 GHz when measured at maximum leveled output power or +10 dBm, whichever is lower. Subharmonics are typically -80 dBc from 100 kHz to 2 GHz, -60 dBc from 2 to 20.2 GHz, and -40 dBc from 20.2 to 50 GHz. Spurious content is typically -65 dBc to 10.1 GHz, -58 dBc from 10.1 to 20.2 GHz, -50 dBc from 20.2 to 39.6 GHz, and -40 dBc from 39.6 to 50 GHz.

Although designed for high spectral purity at millimeter-wave frequencies, the new signal generators do not trade off output power. At 15 GHz, the sources provide +13 dBm, and +20 dBm output power at 20 GHz in standard models. Even at 50 GHz, the model 2550B delivers +13 dBm output power. The maximum leveled output power for the 50-GHz model 2550B is +10 dBm to 10 MHz, +14 dBm to 2 GHz, +12 dBm to 8 GHz, +15 dBm from 8 to 40 GHz, and +13 dBm from 40 to 50 GHz. The output power can be adjusted with 0.05-dB resolution.

For more power, the company offers a line of broadband, solid-state microwave power amplifiers that generate +40 dBm output power to 10 GHz, +35 dBm output power to 20 GHz, +23 dBm output power to 40 GHz, and +20 dBm output power to 50 GHz. To control outputpower levels on the signal generators, users can select from optional mechanical step attenuators in all models, as well as an electronic step attenuator with 110-dB attenuation range in the 8-GHz 2508B frequency synthesizer model. The electronic attenuator option is ideal for radio-frequency integratedcircuit (RFIC) test applications where fast amplitude switching is required over a wide dynamic range. Inclusion of either type of step attenuator does reduce final output power somewhat, with the specified maximum output power for the 2550B at +10 dBm when equipped with a mechanical step attenuator. The step attenuators work with high precision frequency-compensated automatic level control (ALC) capability within the signal generators to maintain precise amplitude control over wide amplitude dynamic ranges and across wide frequency ranges.

Output-power levels are precisely controlled in the 2500B series signal generators, with metrology-grade accuracy. The accuracy is a function of the output power level and frequency of operation, with accuracy of 0.85 dB for power levels greater than +5 dBm from 100 kHz to 20 GHz, 1.05 dB for power levels greater than +5 dBm from 20.0 to 39.6 GHz, and 0.85 dB for power levels greater than +5 dBm from 39.6 to 50.0 GHz. At reduced power levels, the output-power accuracy is 0.70 dB for power levels from -10 to +5 dBm from 100 kHz to 20 GHz, 0.90 dB for power levels from -10 to +5 dBm from 20.0 to 39.6 GHz, and 0.90 dB for power levels from -10 to +5 dBm from 39.6 to 50.0 GHz. At the lowest output-power levels, the accuracy is 1.20 dB for power levels from -10 to -90 dBm from 100 kHz to 20 GHz, 1.50 dB for power levels from -10 to -90 dBm from 20.0 to 39.6 GHz, and 2.50 dB for power levels from -10 to -90 dBm from 39.6 to 50.0 GHz.

The 2550B series frequency synthesizers support their fast switching speed (Fig. 3) with a wide range of tuning functions and high-frequency resolution. For example, the frequency resolution for all models is a fine 0.001 Hz. For flexible testing, the signal generators offer analog and digital sweep capabilities for frequency and power. The power can be swept at different rates over the full frequency range (or parts thereof) for each signal generator. Power ramps can be adjusted over a range of 0 to 25 dB and the slope of a power sweep can be adjusted at rates from 0 to 0.5 dB/ GHz. Sweep times can be set from 10 ms to 200 s. Also, the 2500B series signal generators offer high-rate digital sweep modes that allow the output frequency to sweep linearly between start and stop frequencies. By coupling a 2500B series signal generator with a Giga-tronics model 8003 scalar network analyzer, a wide range of swept stimulus/response measurements, such as gain, isolation, and return loss, can be performed on high-frequency components automatically. In addition to the standard sweep modes, the 2500B signal generators can operate in list mode with as many as 4000 discrete frequency and/or amplitude points. The step times can be set from 150 s to 1 s in standard mode (with an optional range of 2 ms to 1 s). The frequency settling time, which is defined as the time for the frequency to settle within 50 kHz of the final value after a change in frequency, is better than 550 s. The amplitude settling time, which is defined as the time for the amplitude to settle within 0.1 dB of a final value after a change in amplitude, is less than 500 s.

Because modern systems operate with a variety of modulation formats, the 2500B signal generators are equipped with extensive internal modulation capabilities as well as the capability to work with external modulation sources. An internal function generator produces amplitude modulation (AM), frequency modulation (FM), phase modulation, and pulse modulation. It can generate a wide range of AM and FM waveforms, including sine waves, square waves, triangle waveforms, ramps, and Gaussian noise. The AM waveforms are available at rates of 0.01 Hz to 100 kHz with 0.01-Hz resolution, while the FM and phase-modulation waveforms can be programmed with rates from 0.01 Hz to 1 MHz with 0.01-Hz resolution.

For radar testing, the 2500B signal generators also feature internal pulse modulation in continuous, gated, triggered, and pulse burst modes. The signal generators offer internal pulse widths from 10 ns to 10 ms at pulse repetition rates of 0.2 s to 1 s and resolution of 10 ns. Pulse modulation is available at on/off ratios of greater than 80 dB with rise/fall times of less than 10 ns from 0.5 to 20 GHz and less than 25 ns from 20 to 50 GHz. The level accuracy is 0.5 dB for pulse widths of greater than 350 ns, +1.5/-0.5 dB for pulse widths from 100 to 350 ns, and +2.5/-0.5 dB for pulse widths from 25 to 100 ns. In addition, the 2500B signal generators offer a new pulse burst mode in which as many as 1 to 300 pulses can be programmed as part of a pulse burst signal. This mode allows operators to set the pulse width, repetition rate, and burst period.

The 2500B series of frequency synthesizers include hardware triggering and synchronization signals with programmable delays to allow coordination with other measurement instruments in an automatic-test-equipment (ATE) system. To simplify connections to personal computers and networks, each 2500B signal generator is equipped with Universal Serial Bus (USB) and Ethernet local-area-network (LAN) interfaces. Every 2500B series generator is supplied with Giga-tronics Automation Xpress, a personal computer (PC) software package designed for enhanced user interface and automatic test systems. Automation Xpress leverages industry leading software applications, familiar Windows drop-down menus, and other functions to perform tasks. Using Windows-based applications, such as Microsoft Excel or Notepad, engineers can create, manage, and download complex lists in seconds. The 2500B signal generators feature full command compatibility with the firm's earlier 2500C series of signal generators as well as previous-generation signal generators from Giga-tronics. The company offers optional command control sets to simplify upgrades of legacy signal generators in ATE installations. The 2500B series signal generators feature a two-year calibration cycle, occupy 3U rack spaces (5.25 in.), and weigh under 35 lbs. Giga-tronics, Inc., 4650 Norris Canyon Road, San Ramon, CA 94583; (925) 328-4650, FAX: (925) 328-4700, E-mail: info@gigatronics.com, Internet: www.gigatronics.com.