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.