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Volume 10 Number 8 May 1, 2006
 

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This Issue's Feature Articles

Selecting and Purchasing Frequency Synthesizer Products
By:

Mike Harris, Director of Business Development, Meret Optical Communications

Product/Service Focus
This issue's focus is Frequency Synthesizers. You can view and add to our existing list of Test Products/Services, Test Vendors, Test Literature, Test Definitions
 
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Selecting and Purchasing Frequency Synthesizer Products

Mike Harris, Director of Business Development, Meret Optical Communications

Selecting and purchasing a frequency synthesis product starts with an understanding of frequency synthesis techniques, for there are several methodologies, each with significant advantages and disadvantages. Cost, power consumption, size, switching speed and frequency resolution are but a few factors that will influence the purchasing decision. Each synthesis technique has at least one measurable advantage over the others, so your decision will be dependent on the parameters that are most important to you.

What is a frequency synthesizer? The following description applies to all frequency synthesis techniques.

To qualify as a "frequency synthesizer", a device must accept a reference frequency, perhaps from a crystal or another synthesizer, and generate a new frequency that has the same accuracy as the reference.  Typically, the spectral purity (noise) will correlate with that of the reference as well.  The frequency synthesizer normally accepts as inputs a frequency control word, the necessary power supplies to operate the synthesizer, and a reference signal. The desired output frequency is derived from this combination of inputs. This generalization is common to all methods of frequency synthesis.

Let’s look at the basic frequency synthesis methods, and examine their performance differences, and evaluate how the differences will affect your selection.

Frequency Synthesis Methods

There are four common frequency syntheses methods, and below we will examine the advantages and limitations of each.

Direct Analog
This method uses multiplication, division, addition and subtraction of the reference frequency in order to create the desired output frequency. A set of reference frequencies is derived from a single crystal oscillator. These signals are fed through a mix-filter signal processing chain, which provides clean and fast switching frequencies.
Advantages:
Very good phase noise, very fast switching speed, (0.1-20 uS is common) and very good spurious performance. Very high upper frequency ranges are possible.
Limitations:
Not phase continuous, switching transients exist, difficult to modulate, generally complex and therefore, expensive. Fine steps require significant additional circuitry.

Indirect Synthesis, often called Phase-Locked Loop (PLL)
This method utilizes a voltage controlled oscillator (VCO) and a phase detector, which will generate a correction signal when there is any change in output frequency due to drift. The correction signal changes the VCO tuning voltage to bring the drifting signal back on track. Thus, the output signal is never truly locked to the reference signal. The amount that the signal deviates from the programmed output frequency before correction occurs defines the phase noise performance of the synthesizer.
Advantages:
Good spurious performance and good phase noise performance when step size is large. It is very easy to add modulation.
Limitations:
Resolution is limited, and affects phase noise, spurious and switching speed performance. This design is not phase continuous, and is limited to 1-octave bandwidths for a single loop synthesizer.

ALL (Meret Optical Communications proprietary PLL plus DSP circuitry) 
Improvements over standard PLL:
Lower power consumption, reduction in complexity, substantially better phase noise and spurious performance, and a 20 dB improvement in phase noise within 10 kHz are the main improvements allowed by this Meret Optical proprietary architecture.

DDS (Direct Digital Synthesis)
This method is often called the only true synthesizer. The digital output signal is constructed from a frequency accumulator and a lookup table embedded in a ROM. Then, the output of the ROM is converted by a D-A converter to create a sinusoidal analog output frequency. There are numerous variants, one of which is a Chirp DDS. The Chirp DDS utilizes a pipelined Phase-Frequency accumulator, which is really two accumulators, and can synthesize a new frequency each clock cycle.
Advantages:
The DDS is phase continuous, and it has excellent switching speed. (2nS for Meret’s DCP-1A Chirp Synthesizer). Phase noise is superior to any other synthesis method, and so is frequency resolution. It is simple to add phase control to a DDS.
Limitations:
Spurious performance and the upper bandwidth limitations created by the maximum allowable clock frequency are the primary limitations. It is also difficult to modulate a DDS.

Note that direct-analog frequency synthesis, the oldest method to date, exhibits good marks in several critical areas. Excellent switching speed, phase noise and spurious performance are significant and often essential parameters. Be advised that this synthesis method is also typically significantly more expensive than all other methods, with some synthesizers in this category priced at $250,000 and more. These complex machines, in addition to being quite pricey, often are energy hogs and fairly large in size, when compared with other synthesizers. Most direct-analog synthesizers are not custom or easily configurable.

Meret Optical Communications, San Diego, CA,focuses on the newer methods of frequency synthesis, utilizing DDS and DDS+PLL techniques to manufacture custom synthesizers designed to meet the customer’s exact requirements.

Having a feel for the differences in frequency synthesizer methods is not enough information to make a decision. In order to achieve the proper balance between price and performance, the first step that you, as the synthesizer procurement candidate, must accomplish is to create a critical specification list. Now, let’s review the specifications that must be considered. These parameters will be the most important  criteria that you will rely on when making your frequency synthesizer selection.

The Big Three: Range, Resolution, Control
Every synthesizer decision starts with these three. Frequency range (the lowest and highest frequency), step size (frequency resolution), and the frequency control scheme (BCD, Binary, TTL, ECL, Serial, Parallel are several options) will direct you to the general category.  Top your specification list with these parameters. Your information source will be the end user, whether the end user is an internal customer, such as your engineering staff, or an external customer.

Next on your selection criteria list should be the critical performance specifications. Top on this list should be switching speed (the time between when you tell the synthesizer to change frequency, and when the frequency change is complete), phase noise, harmonic and spurious performance. Phase noise, harmonics and spurious are the three primary types of measurable noise in the synthesizer. These parameters begin to refine your selection.

Less critical parameters that must be considered are, output power, output flatness, output impedance (typically 50 ohms) and frequency reference frequency and amplitude. These parameters are often provided as options, and are easily configurable in custom units.

It should be evident by now that numerous factors determine which synthesizer will fit your needs, and that often a balance must be considered when making your decision.

In order to allow our customers maximum input in the decision-making process, Meret Optical Communications utilizes a synthesizer configuration form. This tool is essential in allowing a custom frequency synthesizer manufacturer to provide the exact product to fit their customer’s requirements. This approach allows the buyer significant latitude. In addition, it places the decision of which synthesizer is best for you on the manufacturer, where it belongs.

Here’s a representative example:

 

 

In this article, we’ve discussed the basic frequency synthesis methods, their advantages and disadvantages. We’ve taken a look at specifications, both critical and optional.

With this information in mind, we have developed a specification list that the purchaser can utilize as a guide to obtain the frequency synthesizer that best fits his/her application. Utilizing this important tool, frequency synthesizer selection becomes a significantly less complex process.

 
Next Issue's Product/Service Focus
In our next issue of Product/Service Focus we will cover Telecommunications Test/Bus Analyzers. You can add or upgrade a listing before the next issue comes out.

If you would like to include an exclusive article on how to best select Telecommunications Test/Bus Analyzers, please contact LouisUngar@ieee.org.
 

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