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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.
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