Contents |  General |  MB86060 16-bit 100MSa/s Interpolating DAC |  MB86061 12-bit 400MSa/s (ECL i/p) DAC |  Application Questions |  Glossary |

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1.1: Why use a High Performance Digital to Analog Converter (DAC)?

Synthesised 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.

1.2: In what application area 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 transself 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.

Key communication system architectures:

AMPS - old analog system. Wide-band solutions provide a cost and power reduction

DAMPS (IS-136) - TDMA based AMPS, big in USA, possible cost reduction programmes

CDMA-2000 - development of Qualcomm's CDMA technology [Lucent]

W-CMDA - the European/Japanese version of CDMA-2000 [Ericsson, NTT Docomo.]

GSM - an obvious target but DAC performance not yet adequate enough for all situations

1.3: 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.

1.4: Are the products a proprietary Fujitsu design?

Yes, and both products have patents applied for in the US and elsewhere, including GB2333191A, EP0935345A, JP11-274934A, GB2333171A, EP0930717A, JP11-274935A,GB2333190A, EP0929158A, JP11-243339A, GB2335097A, EP0940923A, JP11-317667A, GB2335076A, EP0940852A, JP11-251530A.

1.5: 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.

1.6: 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 pairs separated by an unused 'channel' which usually gives a worse result.

1.7: 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.

1.8: 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.

The decision whether to use segment shuffling will depend on the target system and whether noise or spurious performance is more critical.

1.9: 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.

1.10: 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.

1.11: 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 maximise capacity.

1.12: 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.

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