Wireless ASSPs
Frequently Asked Questions
DAC
Q1. Why Use a High Performance Digital to Analog Converter (DAC)?
Q2. In What Application Areas is Analog Synthesis most often used?
Q3. What Aspects of the DAC are Crucial to its Success?
Q4. When is SFDR not SFDR?
Q5. Single Tone, Two Tone, or Multi Tone?
Q6. Adjacent Channel Power Ratio
Q7. What Advantage is Segment Shuffling?
Q8. What is the Highest Frequency that can be Generated?
Q9. Bandwidth - what is it worth?
Q10. Programmability
Q11. Wide-Band DAC - all the Problems Solved?
Q1. Why Use a High Performance Digital to Analog Converter (DAC)?
Synthesized analog signals are used in many applications, and in particular for communication systems - both analog and digital.
By generating analog signals digitally all the usual advantages of 'digital’ can be gained - accuracy, stability, repeatability,
and not forgetting fast to change. Digital processing is used to create a specific frequency spectrum mathematically which
when output through a DAC produces the desired analog signal. Not surprisingly with advances in DSP it is relatively easy
to ensure the desired accuracy of the digital signal and therefore the DAC tends to limit absolute performance. Because of
this potential limitation, that could ultimately determine overall system performance, designers require high performance
devices such as the new Fujitsu DAC ASSPs.
Q2. In What Application Areas is Analog Synthesis most often used?
One major application for analog signal synthesis where these new DACs are ideally suited is wireless communications systems
– both analog and digital since any modulation scheme applied to the signal is transmitted to the converter itself. Another
area is test & instrumentation where a precise yet programmable test signal is required. The area of test & instrumentation
often overlaps that of communications systems in the form of dedicated test equipment. AMPS, DAMPS, CDMA-2000
Q3. What Aspects of the DAC are Crucial to its Success?
For the target applications it is critical that the converter be capable of accurately reproducing Sine wave signals (tones)
across a wide range (band) of frequencies. Producing an inaccurate Sine wave will generate other, albeit attenuated, tones
at multiples of the original frequency (harmonics). A measure of the converter's ability here is defined by the Spurious Free
Dynamic Range (SFDR), measured as the highest spurious product (including harmonic and non-harmonically related). Since SFDR
will vary with signal amplitude this should either be quoted in units of dBFS (dBs down from full scale) with the test signal
amplitude or dBc (dBs down from the signal). These unwanted tones can become a major problem by interfering with other Sine
waves being generated simultaneously
Q4. When is SFDR not SFDR?
As always there are ways to make an SFDR figure look better. Firstly it is important to ensure that the figure quoted is at
a relevant frequency as maintaining SFDR at higher frequencies is difficult. Secondly establish the bandwidth over which the
measurement has been quoted. Traditionally this would be DC to Nyquist, but the requirement may be for a SFDR within a defined
band e.g. 10 to 20 MHz allowing spurious products to be classified as either in or out of band, and obviously disregarding
those that are out-of-band.
Some competitive products may be specified over a narrow-band as a means to present a best case performance. Unfortunately
rarely does this quoted band correspond to the systems wanted band and so can be virtually useless. MB86060/1 are, in general,
quoted to Nyquist - if this meets the required specification then everything is OK, if it is just outside then considering
only the system band then sufficient performance may be achieved.
Q5. Single Tone, Two Tone, or Multi Tone?
In the same way that SFDR can be measured at various test signal amplitude and frequency it can also be measured with more
than one tone. Most important to note is that it is virtually impossible to predict one measurement result from the other.
While a single tone test might be performed at full-scale (typically -1dBFS) multi tone tests require each tone to be backed
off even further (typically 3dBs per tone) from full scale so to avoid overloading the converter. A multi tone test is often
used to simulate using the converter under typical conditions, e.g. a four channel communication system which has four tones
separated by 200kHz. These may be arranged as four adjacent tones or as two separated by an unused 'channel' which usually
gives a worse result.
Q6. Adjacent Channel Power Ratio
The Adjacent Channel Power Ratio (ACPR) is a useful measure, in addition to SFDR, that has been adopted to more closely represent
performance under more realistic operating conditions compared to one or more discrete tones. ACPR is a ratio of power in
a desired channel compared to that in another adjacent channel, thus giving an indication of spread. Typically the desired
channel will include a wide-band signal and to complete the definition both channel bandwidth and channel spacing should be
quoted.
Q7. What Advantage is Segment Shuffling
The concept of segment shuffling is unique to both MB86060 & MB86061. Simplistically the main DAC core, used to generate the
analog output current, consists of four identical quadrants each nominally associated with a part of the converter's transfer
function. These quadrants will differ slightly through manufacturing tolerances and this manifests itself as spurious products.
By enabling segment shuffling an internal controller randomly swaps the allocation of quadrants. This has the effect of averaging
errors thus improving spurious performance, but being impossible to get something for nothing the spurious energy is redistributed
and manifests itself as a rise in the noise floor. Segment Shuffling is most effective on large-scale signals, -10dBFS and
above, differing from Dither which is also incorporated into MB86060, see section 2.5.
Q8. What is the Highest Frequency that can be Generated?
In theory, signals up to the Nyquist frequency, defined by the DAC rate divided by 2, can be generated. In reality converter
performance degrades significantly when approaching the Nyquist frequency as well as introducing impossible requirements on
the output reconstruction filter. The situation is slightly different for the interpolating modes of MB86060 where the internal
digital filter band limits the output spectrum. Either way through the combination of high speed and performance the MB86060/1
DACs enable significantly higher frequencies to be generated compared to competitive products. In many communications systems
this allows designers to adopt a higher IF and benefit from simplified RF up-conversion stages.
Q9. Bandwidth - what is it worth?
Both DACs are aimed at wide-band or high-IF applications. It is important that a target application makes maximum use of one
or both of these so as to ensure a good systems advantage is achieved. A wide-band system will typically be one where several
communication channels are required to be generated through a single DAC, rather than using individual converters [at base-band]
and dedicated IF up-converters per channel. The resultant saving in hardware and ultimately programmability [signals can be
positioned in the frequency spectrum under software control] provides designers with significant cost trade-offs.
Q10. Programmability
The ability to create a complete wide-band transmit signal in software and convert through a single DAC provides an enormous
systems advantage of programmability. The IF/RF chain can be fixed and signal frequencies determined by software permitting
virtually instantaneous adjustment (frequency hopping) without the need for conventional PLLs to settle. Another advantage
is being able to achieve closer channel spacing, and thus maximize capacity.
Q11 . Wide-Band DAC - all the Problems Solved?
Implementing a wide-band architecture, using a high performance DAC, doesn't solve all the problems. The system also requires
sufficient wide-band performance from all subsequent parts of the IF/RF chain including up-conversion and the power amplifier
(PA). The latter probably being the most difficult to achieve, so much so that often an alternative approach is sought. Assuming
that the linearity of the DAC and IF/RF chain can be accurately designed, then non-linearity of the PA can be compensated
for by digital pre-processing using an inverse transfer function.
PLL
Q1. What advantages do Fujitsu's "SL" Phase-locked loops (PLLs) have over the competition?
Q1. What advantages do Fujitsu's "SL" Phase-locked loops (PLLs) have over the competition?
Fujitsu’s new "SL" series lineup of single (E) and dual (F) PLLs offers your RF/Wireless development customer very low
current consumption, excellent
