Signal Generator Melds Speed With Low Phase Noise

Test-instrument manufacturers are faced with trying to provide performance that is one step ahead of emerging technologies. As difficult as this task may appear, many measurement equipment suppliers succeed by working closely with their device and component suppliers and their customers, keeping in tune with the latest test requirements. For frequency synthesizers, one of these requirements is fast frequency switching speed; another is low phase noise. Typically, it is difficult to find both in the same instrument. But the new Panther 2500 series of synthesized signal generators manages to stay ahead of the pack with outstanding phase-noise performance and lightning-like switching speed. Four models are available in the Panther 2500 series, covering 100 kHz to 8 GHz, 100 kHz to 20 GHz, 100 kHz to 26.5 GHz, and 100 kHz to 40 GHz, each with microscopic frequency resolution of 0.0001 Hz across its full band. These signal sources are offered in two different configurations, for bench-top (with full front-panel controls) and automatic-test-equipment (ATE) applications (with rear-panel output).

The new test signal generators (Fig. 1) employ the company's patent-pending "Accumulative High Frequency Feedback" (AHFF) technology to achieve low single-sideband (SSB) offset phase-noise performance while maintaining fast frequency switching speed. Loaded with standard performance features such as high stability time base, frequency modulation (FM), high-speed pulse/square wave modulation (PM), amplitude modulation (AM), and high leveled output power, these new generators provide excellent test solutions for a CW, modulated, swept-frequency, and fast-frequency-switching applications on both research and development (R&D) and manufacturing environments.

Frequency synthesizers can be categorized as having either direct or indirect synthesis architectures. Direct synthesizers can use either analog methods (mixing, multiplying, dividing, and filtering), direct-digital synthesis (DDS), or a combination of the two. Direct frequency synthesizers offer outstanding performance, but tend to be bulky and costly with high power consumption and poor reliability.

Synthesizers in the same class as the 2500 signal generator tend to use indirect synthesis architectures. Indirect synthesis architecture makes use of signal-switching-path phase-locked loops (PLLs) and are relatively low in cost and simple compared to direct synthesis designs, resulting in smaller size and lower power consumption. Unfortunately, to meet the high level of performance demanded in military and high-volume test applications, multiple PLLs, sometimes as many as 8 to 10, are required. As a result, the PLL implementations used in this class leads to slow switching speed and design complexity.

Single-sideband (SSB) phase noise is defined as the ratio of the power in a 1-Hz bandwidth at a given frequency offset from the carrier to the power of the carrier itself.

In a well-designed PLL, the phase-noise profile as a function of offset from the carrier consists of two distinct sections. The first is a pedestal extending from the carrier to the loop bandwidth followed by the noise of the free running oscillator. The latter usually drops monotonically at a rate of 20-dB per decade until it reaches a noise floor. The foregoing is a simplified depiction of the phase noise in a PLL that is useful for the purpose of this discussion.

Over the years, many engineers have expended much effort in making the pedestal be no more than the theoretical limit determined by the noise of the reference times the ratio of the output and reference frequency. It is very important to keep this ratio as low as possible, often by using multiple PLLs with frequency dividers between them. This approach provides good phase noise, but with added complexity and cost.

The phase noise in PLL-based synthesizer architectures is largely proportional to the ratio of the output frequency of the PLL to the input (or reference) frequency. By using the highest possible reference source with the lowest phase noise, good output phase noise can be achieved since the multiplication factor (N) for a given output frequency is minimized. Unfortunately, low N numbers make it difficult to achieve fine frequency resolution. To resolve this difficulty while still achieving low phase noise, fractional N and sigma-delta systems are used.

However, the PLLs for such systems must have relatively narrow loop filter bandwidths to prevent the passing of spurious signal sidebands resulting in slow switching speed.

AHFF^TM TECHNOLOGY
AHFF technology was developed by engineers at Giga-tronics to overcome the limitations in fractional N and sigma-delta systems. This technology employed in the Panther 2500 series synthesizers, achieves low N numbers and very fine resolution in single loop (Fig. 2) . The approach makes use of a high-frequency reference source with a variable component to drive the PLL. This is achieved by the judicious combination of several low-noise techniques. The PLL uses a novel technique of high-frequency fractional-frequency prescaling. This allows the ratio of the output frequency to the reference frequency to be quite low for a given phase-noise level compared to traditional PLL synthesis methods.

The partitioning of the PLL frequency steps and reference tuning in the Panther 2500 series is very carefully calculated. The object is to attempt to maintain a wide PLL bandwidth while providing sufficient suppression of spurious sidebands as required in a high-performance signal generator. It must be noted that frequency switching speed and phase settling time benefit from a wider loop bandwidth. Carefully crafted twin tuning algorithms control these parameters in such a way that signal purity and wide loop bandwidth are achieved simultaneously.

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The phase detector in the Panther 2500 series generators operates at a high frequency compared to traditional designs. The tunable oscillator in the generators is a YIG-tuned oscillator operating from 4 to10 GHz. A portion of its signal is processed by the fractional prescaler so that the signal at the phase detector is close in frequency to that of the reference. The phase detector feeds the difference in phase between the two signals to the loop circuitry and the YIG oscillator in order to stabilize the YIG oscillator. The high-frequency reference and the fractional prescaler are tuned by means of algorithms that provide the right conditions for spurious suppression with a sufficiently wide band loop to ensure fast phase settling.

What are the results of this unique architecture? The Panther 2500 series signal generator delivers excellent close-in phase-noise performance (Fig. 3) of better than -84 dBc/Hz offset 100 Hz from a 10-GHz carrier, -104 dBc/Hz offset 1 kHz, and better than -111 dBc/Hz offset 10 kHz and 100 kHz from the same carrier. Harmonics are better than -55 dBc from 100 MHz to 20 GHz and better than -30 dBc above 20 GHz. The harmonic specification from 10 MHz to 100 MHz is better -44 dBc and better than -30 dBc below 10 MHz. Nonharmonic spurious content is better than -64 dBc though 10 GHz, better than -60 dBc from 10 to 20 GHz, and better than -54 dBc above 20 GHz.

In the Panther 2500 series generators, the YIG oscillator's outputs are monitored by the reference loop. The reference loop has the OCXO at 100 MHz that is locked to 10 MHz (internal or external). The PLL controls the frequency of the YIG oscillator so the output frequency has the same accuracy and stability as that of the reference oscillator. If the reference crystal has a stability specification of 3 × 10^-8/day, the worst-case-frequency error at 1 GHz will be 900 Hz measured 30 days after the last calibration. The Panthers are supplied with a high-stability oven-controlled crystal oscillator (OCXO) reference with aging rate of better than 5 × 10^-10/day and temperature stability of better than 2.5 × 10^-8/°C. The accuracy can be improved by using a better external 10- or 100-MHz reference source.

The Panther 2500 signal generators deliver amplitude and frequency switching speed of better than 550 µs for frequency steps to 1 GHz. Especially for high-throughput testing, such as antenna testing and radar-cross-section (RCS) measurements, this switching speed can dramatically reduce test time, assuming analysis equipment that can take advantage. By using the company's Automation Xpress personal-computer (PC) software, which is supplied with every generator, 1-ms CW frequency and power switching is possible for faster testing and more device throughput.

The generators provide as much as +20 dBm leveled output power from 8 to 20 GHz (Fig. 4). Output power control is augmented by the inclusion of a 90-dB programmable step attenuator. The generators can tune power levels down to -110 dBm through 20 GHz and down to -100 dBm through 40 GHz for precise control of output power. In addition, using the built-in automatic-level-control (ALC) circuitry, the level accuracy at 0 dBm is better than ±0.5 dB from 100 kHz to 20 GHz and better than ±1.0 dB from 20 GHz to 40 GHz (Fig. 5).

In addition, the 2500A series includes external ALC functionality, DC FM, and phase-adjust capability. External ALC allows the 2500A to automatically compensate its own output to a given level based on load and test conditions. The external ALC feature is commonly used when the output of the 2500 is connected to a device through a cable or other devices that cause significant standing waves or signal attenuation. One such application is in traveling-wave-tube (TWT) manufacturing and test. Since the VSWR of these vacuum devices can vary greatly, the external ALC can be used to compensate for input variations to the TWTs that might otherwise be several decibels. Using DC FM, operators can fine-tune the output frequency of the Panther 2500 generators by means of external control. The Phase Adjust function provides adjustment over a signal's output phase, particularly useful in applications where the phase (or frequency) of the test signals must be fine-tuned.

Finally, for ease of use, the front-panel menu of the Panther 2500 series generators has been streamlined to enhance operator productivity. The number of soft screens and menu layers has been minimized, therefore simplifying the content and improving operational performance. All Panther 2500 series generators are shipped with Automation Xpress^TM PC software, written for enhanced manual user interaction and for higher ATE programming productivity. It uses the familiar Microsoft Windows environment to simplify complex ATE tasks. Using industry-leading software applications, such as Microsoft^TM Excel or Notepad, engineers can create, manage, and download complex lists in seconds. With its high-speed programming interface, Automation Xpress software dramatically speeds execution time. The Panther 2500 series' Auto Programmer feature ensures that error-free ATE sequences can be generated in the time that it takes to configure the signal generator manually.

Gigatronics, Inc.,
4650 Norris Canyon Rd.,
San Ramon, CA 94583;
(925) 328-4650, FAX: (925) 328-4700,
Internet: www.gigatronics.com