Synthesizers Tune Under USB Control

May 17, 2012
These compact, low-cost test signal generators can sweep frequencies from 250 to 4000 MHz and power levels across 70-dB dynamic ranges with high frequency and power accuracy.
Signal generators are vital to testing high-frequency components and systems, essentially emulating the types of signals that those components and systems will handle in different applications. The types of test signals have grown in complexity over the years, from simple sine waves with amplitude modulation (AM) and frequency modulation (FM) to frequency-agile signals with advanced digital modulation.

In many cases, however, while a signal generator's performance is still important, the many functions and features may not be needed. To fill a need for more "no frills" test-signal generators, Mini-Circuits has introduced a pair of high-performance (but low-cost and easy-to-use) signal generators. Dubbed models SSG-4000LH and SSG-4000HP, they are intended for applications from 250 to 4000 MHz. The first is designed for low output harmonics while the second is optimized for high output-power levels. Both signal generators feature Universal Serial Bus (USB) 2.0 ports and are supplied with personal computer (PC) software, allowing for ease of control.

The compact frequency-synthesized model SSG-4000HP test source (see photo) measures just 8.37 x 8.50 x 2.15 in. (212.60 x 215.9 x 54.6 mm)the SSG-4000LH is several inches longermaking these signal generators ideal for integration into larger or portable test solutions. The small size and low cost of these signal generators invites their use in test systems requiring multitone signals.

Both signal generators are USB Human Interface Devices (HIDs) that require no drivers. They are supplied with an installation compact disc (CD) containing installation and graphical-user-interface (GUI) control software for a Windows-based personal computer (PC). The intuitive software provides clear displays of signal-generator frequency and output power, and allows operators to select from several operating modes. Among these are sweep mode (for sweeping frequency or power) and hop mode (for jumping between frequencies or power levels). In addition to the installation CD, they are packed with a 24-V AC/DC power adapter, an AC power cord, and a 2.7-ft USB cable.

The two models offer tradeoffs between lower harmonics (typically -66 dBc in the SSG-4000LH) with reduced output power and slightly higher harmonics with considerably more output power. Model SSG-4000LH has an output-power range of -60 to +10 dBm, while model SSG-4000HP boosts those levels by 10 dB, to -50 to +20 dBm. The output-power levels for both signal generators is adjustable in 0.25-dB steps, and both generators boast 0.25-dB output-power accuracy. With the RF power shut off, the RF power level at the output port typically reads -95 dBm. Both signal generators can tune from 250 to 4000 MHz in steps as small as 5 kHz or as large as a full-band frequency hop, with 5-ms typical frequency settling time to a new frequency. The typical VSWR is 1.15:1 from 250 to 2000 MHz and 1.70:1 from 2000 to 4000 MHz.

In addition to sweep power and frequency control, the software also enables pulse modulation for both USB signal generators. This capability allows minimum pulse widths of 1 s and minimum pulse periods of 2 s. The pulse repetition frequency (PRF) can be set to a maximum rate of 500 kHz.

The outstanding harmonic performance of the model SSG-4000LH signal generator comes at a slight premium in price ($2395) compared to the $1995 cost of the model SSG-4000HP signal generator. Otherwise, the performance specifications between the two signal generators are very similar. Both generators promise frequency accuracy of 1 ppm/year using the internal reference with an aging rate of 1 ppm/year. Both sources provide access to the 10-MHz signals from the internal reference oscillator at a female BNC connector, at a typical output level of +7 dBm. Both signal generators also provide provisions for the use of an external 10-MHz reference oscillator, via a female BNC input connector.

As an example of the value in spectral purity provided by these signal generators, low-noise model SSG-4000LH signal generator achieves low harmonics, spurious noise, and phase-noise levels belying its price tag. Harmonic and subharmonic levels are a function of the signal generator's output-power level, with harmonic/subharmonic levels of typically -57 dBc at an output power level of -50 dBm, -77 dBc at output power levels of -30 and -10 dBm, -74 dBc at an output power level of 0 dBm, and -47 dBc at the maximum rated output-power level of +10 dBm.

The SSG-4000LH's spurious levels are a function of the frequency step size, whether as a frequency-hopped output signal or a frequency sweep, with larger step sizes yielding lower spurious levels. For example, the SSG-4000LH's spurious levels are typically -50 dBc for a frequency step size of 5 kHz, -66 dBc for a frequency step size of 100 kHz, -80 dBc for a frequency step size of 1 MHz, and -95 dBc for a frequency step size of 10 MHz.

In spite of their low price tags, these USB signal generators offer surprisingly low single-sideband (SSB) phase-noise floors, with quite respectable overall phase-noise performance at all offset frequencies. For example, for the SSG-4000LH, when the software has set the signal generator for the lowest spurious levels, the close-in phase noise for a 250-MHz signal is -96 dBc/Hz offset 100 Hz from the carrier. When the software is set for the lowest noise levels, the resulting phase noise is -99 dBc/Hz offset 100 Hz from the 250-MHz carrier. With these software setting of lowest-spurious/lowest-noise, the corresponding typical phase-noise levels for the SSG-4000LH signal generator are -104/-114 dBc/Hz offset 1 kHz from the carrier, -103/-107 dBc/Hz offset 10 kHz from the carrier, -121/-123 dBc/Hz offset 100 kHz from the carrier, and -147/-146 dBc/Hz offset 1 MHz from the carrier.

For a higher-frequency carrier of 1060 MHz, the SSG-4000LH exhibits typical phase-noise levels at the lowest-spurious and lowest-noise software settings: -83 and -85 dBc/Hz offset 100 Hz from the carrier, -92 and -100 dBc/Hz offset 1 kHz from the carrier, -91 and -94 dBc/Hz offset 10 kHz from the carrier, -111 and -112 dBc/Hz offset 100 kHz from the carrier, and -139 dBc/Hz offset 1 MHz from the carrier at both software noise settings. At a carrier of 2600 MHz, the phase-noise levels are -75 and -60 dBc/Hz offset 100 Hz from the carrier, -84 and -93 dBc/Hz offset 1 kHz from the carrier, -84 and -89 dBc/Hz offset 10 kHz from the carrier, -103 and -106 dBc/Hz offset 100 kHz from the carrier, and -132 dBc/Hz offset 1 MHz from the carrier at both software noise settings.

At the SGG-4000LH's upper-frequency limit of 4000 MHz, the phase noise levels at the two software settings are -70 and -75 dBc/Hz offset 100 Hz from the carrier, -80 and -90 dBc/Hz offset 1 kHz from the carrier, -82 and -88 dBc/Hz offset 10 kHz from the carrier, -102 and -106 dBc/Hz offset 100 kHz from the carrier, and -131 dBc/Hz at both software settings offset 1 MHz from the carrier.

Except for the increase in output power and the slightly higher harmonic levels, the SSG-4000HP signal generator offers performance quite similar to that of the model SSG-4000LH synthesizer. The higher-power signal generator exhibits harmonic and subharmonic levels of -50 dBc at an output signal level of -50 dBm, -67 dBc at -30 dBm, -46 dBc at 0 dBm, -45 dBc at +10 dBm, and -38 dBc at +15 dBm. Spurious levels are dependent on frequency step size, with typical levels of -53 dBc for a 5-kHz step, -63 dBc for a 100-kHz step, -80 dBc for a 1-MHz step, and -95 dBc for a 10-MHz step. The SSB phase noise is typically -93 dBc/Hz or better at a 100-Hz offset from a 250-MHz carrier, -103 dBc/Hz or better at a 10-kHz offset from the same carrier, and -146 dBc/Hz or better at a 1-MHz offset from the same carrier. At the highest carrier frequency of 4000 MHz, the SSG-4000HP has phase noise of better than -72 dBc/Hz offset 100 Hz from the carrier, better than -82 dBc/Hz offset 10 kHz from the carrier, and better than -131 dBc/Hz offset 1 MHz from the carrier.

Both synthesized signal generators are supplied in shielded metal housings, typically drawing 400 to 424 mA current from the +24-VDC AC/DC adapter. Both operate with low trigger input levels from 0 to 0.8 V and high trigger input levels 2.4 to 3.3 V, while providing output low trigger levels of 0 to 0.4 V and high trigger levels of 2.4 to 3.3 V. BNC connectors are provided for trigger input and output voltages. Both signal generators provide their specified performance levels across operating temperatures from 0 to +50C.

The included installation and control software is written for a PC with a Pentium II or better processor. The software is compatible with 32-b and 64-b operating systems. A mounting bracket for rack-mount installations is available for both signal generators as an option, so as to ease their installation into automatic-test-equipment (ATE) systems.

Mini-Circuits, Inc.
P.O. Box 350166
Brooklyn, NY 11235-0003
(718) 934-4500
FAX: (718) 332-4661

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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