Test signal generators were once much simpler, reflective of a time when engineers simply wanted to send a signal through a circuit to check a design's functionality. But as high-frequency circuit designs have grown in complexity, test signal generators have been forced to follow, so as to accurately emulate the signals that will be found under actual operating conditions. The new MXG and EXG X-Series signal generators from Agilent Technologies reflect this obsession with accuracy, delivering new standards for signal purity for analog and vector test signals through 6 GHz.

The X-Series (Fig. 1) features four 6-GHz signal generators, two of which offer traditional analog output signals (models N5171B and N5181B) and two capable of providing vector in-phase/quadrature (I/Q) output signals (models N5172B and N5182B). The EXG models N5171B and N5172B deliver output levels to +21 dBm, while the higher-power MXG models N5181B and N5182B reach output levels to +24 dBm. Among other performance enhancements, the MXG models boast improved spectral purity over their EXG counterparts, with considerably better phase noise at all offsets and carrier frequencies (see table).

However, even the "economy" models in this signal-generator family offer noteworthy performance over a bandwidth that is a match for many commercial communications applicationsas well as a good number of military and industrial test applications through 6 GHz. The "low-end" model EXG N5171B (Fig. 2), for example, is actually available per three different options (501, 503, and 506) in frequency ranges of 9 kHz to 1 GHz, 9 kHz to 3 GHz, and 9 kHz to 6 GHz, each with 0.01-Hz frequency resolution. The frequency synthesis architecture of the N5171B and the other signal generators enables a great deal of flexibility in terms of performance options, allowing users to select the limits that best suit their applications.

The N5171B signal generator features analog modulation (amplitude, frequency, phase, and narrowband pulse), while the model N5172B provides as much as 120-MHz internal I/Q digital modulation bandwidth (200 MHz with external inputs. Either can switch frequencies quickly in list/step sweep mode or under Standard Commands for Programmable Instruments (SCPI) program control. For standard models, the switching speed is typically less than 5 ms. Option UNZ can reduce the SCPI switching speed to less than 1.15 ms and the list/step sweep-mode switching speed to under 900 s. Typical switching speed is better than 800 s for both modes with this option. Switching is slightly slower in the model N5172B when digital modulation is switched on. For both models, the amplitude switching speed is similar: about 5 ms for standard units and under 1 ms for signal generators with fast-switching options, using list/step sweep mode or SCPI control.

The two EXG signal generators are stabilized by an internal 10-MHz crystal reference oscillator with nominal aging rate of better than 5 ppm/10 years and temperature stability of 1 ppm from 0 to +55C. The generators also provide access to the internal reference via a coaxial connector, at a nominal level of +4 dBm into a 50-Ω load. The EXG signal generators can also be used with an external frequency reference source, operating at 10 MHz (and -3 to +20 dBm), with another option allowing the use of external frequency reference sources from 1 to 50 MHz.

Signal output levels for the N5171B and N5172B signal generators can be adjusted from -144 to +30 dBm with 0.01-dB resolution. Amplitude adjustments are made with the aid of an internal electronic step attenuator with an attenuation range of 0 to 130 dB, adjustable in 5-dB steps. The two EXG signal generators achieve maximum output-power levels in standard units of +13 dBm through 10 MHz, better than +18 dBm from 10 MHz to 3 GHz, and better than +16 dBm from 3 to 6 GHz. An option provides for increased output levels of +17 dBm through 10 MHz, +21 dBm from 10 MHz to 3 GHz, and +18 dBm from 3 to 6 GHz in both EXG signal generators. The absolute level accuracy for these generators in continuous-wave (CW) output mode and with the automatic level control (ALC) on is 0.6 dB or better for output levels from -127 to -110 dBm from 5 MHz to 6 GHz, 0.9 dB or better for output levels from -110 to -60 dBm from 9 kHz to 3 GHz, 1.1 dB for those same output levels from 3 to 6 GHz, and 0.8 dB or better for all frequencies at output levels from -60 to +21 dBm. For the N5172B, the absolute level accuracy in digital I/Q mode adds 0.25 dB to the level accuracy numbers across the full frequency range.

The N5171B and N5172B EXG signal generators offer outstanding spectral purity, with standard units showing harmonic levels of typically -35 dBc from 9 kHz to 4 GHz and -53 dBc from 4 to 6 GHz when measured at an output signal level of +4 dBm. Nonharmonic spurious levels for standard units are typically -65 dBc from 9 kHz to 5 MHz, -75 dBc from 5 to 750 MHz, -72 dBc from 750 to 1500 MHz, -66 dBc from 1.5 to 3.0 GHz, and -60 dBc from 3.0 to 6.0 GHz. The absolute phase noise measured at a 20-kHz offset is typically -133 dBc/Hz at a carrier of 250.1 MHz, -122 dBc/Hz at 1 GHz, -110 dBc/Hz at 3 GHz, and -103 dBc/Hz at 6 GHz.

Of course, phase noise is a frequency-domain measure of a signal generator's stability. For high-speed data communications systems, the time-domain parameter jitter is a more familiar means of evaluating a signal generator's performance, and the N5171B and N5172B signal generators also shine in the time domain. The time jitter for a carrier of 155 MHz (an equivalent SONET or SDH data rate of 155 Mb/s) is typically 0.9 ps; for 622 MHz (an equivalent data rate of 622 Mb/s) is typically 0.11 ps; and for 2.488 GHz (a data rate of 2.488 Gb/s) is typically 0.11 ps.

The higher-performance models N5181B and N5182B MXG signal generators (Fig. 3) are available with two frequency rangesfrom 9 kHz to 3 GHz or from 9 kHz to 6 GHzalso with 0.01-Hz resolution. Their frequency switching speeds in SCPI or list/step sweep modes are similar to those of the EXG generators, at typically 5 ms for standard units and 1.05 ms or better with a high-speed option. The amplitude switching speed for the MXG signal generators is also typically 5 ms for standard units in SCPI or list/step sweep modes, with amplitude switching speeds of 400 s or better in list/step sweep mode and 950 s or better in SCPI mode with the high-speed option.

Both MXG signal generators can be set at power levels from -144 to +30 dBm with 0.01-dB resolution. As with the EXG signal generators, the MXG instruments leverage the control of an internal electronic step attenuator with attenuation range of 0 to 130 dB, adjustable in 5-dB steps. The maximum output-power levels for standard MXG models is +13 dBm from 9 kHz to 10 MHz, +18 dBm from 10 MHz to 3 GHz, and +16 dBm from 3 to 6 GHz. With a high-power option (Option 1EA), the following maximum levels are available: +17 dBm from 9 kHz to 10 MHz, +24 dBm from 10 MHz to 3 GHz, +19 dBm from 3 to 5 GHz, and +18 dBm from 5 to 6 GHz.

The absolute level accuracy for the MXG signal generators in CW mode and with the ALC on is 0.6 dB from 9 to 100 kHz at power levels from -60 to +24 dBm; it is 0.9 dB across the same frequency range at power levels from -110 to -60 dBm. The absolute level accuracy is 0.8 dB from 100 kHz to 5 MHz at power levels from -60 to +24 dBm; it is 0.9 dB across the same frequency range at power levels from -110 to -60 dBm. The level accuracy is 0.6 dB from 5 MHz to 3 GHz at power levels from -60 to +24 dBm; it is 0.8 dB across the same frequency range at power levels from -110 to -60 dBm, and 0.6 dB from 3 to 6 GHz at power levels from -60 to +24 dBm. Also, it is 1.1 dB across the same frequency range at power levels from -110 to -60 dBm. For the model N5182B, the absolute level accuracy in digital I/Q mode adds 0.25 dB to the level accuracy numbers across the full frequency range.

The MXG models N5181B and N5182B signal generators derive their outstanding spectral purity from a new triple loop synthesizer architecture and an internal frequency reference with 1 x 10-7/year nominal aging rate. After 30 days, this reference source exhibits a nominal aging rate of 5 x 10-10/day. The signal generators can also operate with an external 10-MHz reference oscillator (in standard units) or external reference oscillators operating from 1 to 50 MHz with square or sine-wave outputs (in signal generators with the proper option, Option 1ER).

Their spectral-purity performance levels include harmonics of typically -35 dBc measured at an output setting of +4 dBm from 9 kHz to 4 GHz and typically -53 dBc from 4 to 6 GHz. Non-harmonic spurious levels for signal generators with low phase noise options are -65 dBc from 9 kHz to 5 MHz, -75 dBc from 5 to 250 MHz, -96 dBc from 250 to 750 MHz,-92 dBc from 750 to 1.5 GHz, -86 dBc from 1.5 to 3.0 GHz, and -80 dBc from 3.0 to 6.0 GHz.

The absolute phase noise, which is noticeably lower than that of the EXG signal generators, measured at a 10-kHz offset with low noise option (UNY) is -144 dBc/Hz at a carrier of 250.1 MHz, -141 dBc/Hz at 1 GHz, -132 dBc/Hz at 3 GHz, and -126 dBc/Hz at 6 GHz. With an option (UNX), the phase noise can be diminished even furtheras much a 10 dB or more at some carriers. The time-domain jitter for low phase noise optioned (UNX/UNY) MXG models for a carrier of 155 MHz (an equivalent SONET or SDH data rate of 155 Mb/s) is typically 0.25 ps; for 622 MHz (an equivalent data rate of 622 Mb/s) is typically 33 fs; and for 2.488 GHz (a data rate of 2.488 Gb/s) is typically 29 fs.

Both the N5172B and N5182B signal generators with wideband digital (I/Q) modulation can operate with either an internal or external I/Q modulation source. As much as 160 MHz I/Q bandwidth is available with the internal source and as much as 200 MHz I/Q modulation bandwidth is available with an external source. The two signal generators provide great flexibility in modulation formats, with capabilities for phase-shift-keying (PSK), quadrature-amplitude-modulation (QAM), frequency-shift-keying (FSK), minimum-shift-keying (MSK), and amplitude-shift-keying (ASK) formats.

In addition, these digitally modulated signal generators can be equipped with an optional (Option 403) internal additive white Gaussian noise (AWGN) generator with bandwidth of 1 to 60 MHz or 1 to 160 MHz, depending upon other options. Capable of a carrier-to-noise ratio (CNR) of better than 100 dB, the AWGN source is ideal for carrier-to-noise (C/N) measurements and evaluation of a device under test (DUT) under impaired performance conditions.

The MXG achieve their outstanding phase-noise performance (Fig. 4) thanks to a new triple-loop frequency synthesizer architecture and frequency plan that works with that synthesis approach. The result is improved phase noise close to the carrier and at wide offsets from the carrier. The combination of carefully selected reference frequency and frequency conversion, through mixers and multipliers, provides adequate spacing to allow filtering of signal artifacts (such as spurious signal products). The MXG vector signal generators even incorporate a real-time application-specific integrated circuit (ASIC) for some control over phase-noise levels in both CW and modulated signals.

Given the many features and capabilities of these signal generators, operating them may seem intimidating. To help gain the greatest benefits from these test sources, each instrument is supplied with a trial version of Agilent's powerful Signal Studio control software for creating a wide range of test waveforms and controlling the signal generators.

For example, the software can greatly simplify the creation of digitally modulated waveforms as well as standardized test signals for wireless communications, such as for wireless local area networks (WLANs), Bluetooth, Global Positioning System (GPS) & GLONASS navigation systems, and cellular GSM, EDGE, and Long-Term-Evolution (LTE) testing. In fact, those not yet committed to the purchase of an EXG or MXG signal generator can try a free 14-day trial license of Signal Studio. P&A: from $6,900 (N5171B, from $16,970 N(5172B), from $15,500 (N5181B), and from $19,320 (N5182B); 30 days.

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