This explanation DOES ring the bell to me. The phenomenon looked like crossover distortion to me, and the third tone "bias" just "iron out those wrinkles".
Dither is also used at very low levers, comparable with the LSB in high end audio processors to reduce the distortion. In a 16 or 24 bit processor it can reduce measurably the remanent distortion without being noticeable itself at levels 90 dB or more below the maximum ADC output. Nevertheless, I also agree that adding dither, while academically interesting, may not be necesary for receiving, in practice, at least on the HF bands. The received signal carries already some noise, If the distortion products of the linear amplifiers during transmission are higher than the ADC distortion, most likely there will be no overall gain. 73, Jose, CO2JA --- Nick Whyborn wrote: > The distortion introduced by ADC's is intrinsically very different from > that introduced by typical analog circuitry. ADC's have a transfer > function which is highly non-linear at the small-scale (partly because > of the quantisation steps intrinsic to the ADC process, but mainly > because of the dynamic non-linearity and the structure in the integral > non-linearity). On the other hand normal analog circuitry has a very > smooth transfer function on the small scale and a smooth transition into > saturation (an exception here would be cross-over distortion class AB or > B amplifiers). Put another way, if we tried to describe the transfer > function by a polynomial then an ADC would require a polynomial with an > order up to about the number of distinct levels in the ADC (i.e. for a > 16bit ADC we would need a polynomial of order 65000), whereas a good > analog circuit used below saturation would only need a polynomial of > order less than ten. > > This fundamental difference is the reason why distortion products in an > ADC are roughly constant in absolute power level (i.e. dB relative to > full scale or dBFS) as the signal level is increased as you noted in > your 2-tone tests. This can often be seen in the data sheets, for > example p. 20 of the TI AFE8406 dual digital receiver, or p. 20 of the > Analog Devices AD9446 ADC. So agreed, TOIP has little use for ADC's > where the transfer function is dominated by higher order distortion. > > In contrast, for an analog circuit the absolute level of the major > distortion products increases *faster* than the signal level as the > level is increased, and hence the concept of TOIP does make sense for > this type of circuit. > > One way of effectively "smoothing out" the transfer function and to > reduce the level of the distortion products is to add a dither signal. > The effect here is to randomise the errors that occur for a given input > signal level. Without dither and for a certain input voltage the ADC > output will be in error from the ideal infinite resolution case and this > error will always be the same. However, if we add a changing noise-like > signal then successive suamples of the ADC output will have different > uncorrelated errors. The fact that they are uncorrelated implies that > they will not appear in a spectrum as a distint spur anymore but as > noise smeared out over the whole bandwidth, i.e. it will add a small > amount to the noise floor (if the dither signal is itself a wideband > noise signal then it to will raise the noise floor by an amount > dependent on its relative level). I think the 16 MHz weak tone you added > in your investigation is acting as a dither signal. In this case, since > the dither is not a random noise signal, the power from the distortion > products will not be spread evenly over the whole bandwidth but will be > in a number of weak spurs. Still the effect will be to reduce the third > order IMD level. > > There is a good article by Walt Kester on the Analog Devices website > which explains why dither can improve the SFDR of an ADC: > http://www.analog.com/library/analogdialogue/archives/40-02/adc_noise.html > > If the application we are interested in is SDR here for shortwave > reception using an ADC followed by digital down converter (i.e. not the > IQ mixer plus audio ADC approach), then my guess it that dither is > completely unnecessary since the input signal will intrinsically contain > a lot of noise and uncorrelated weak signals spread all over the input > frequency range and these will effectively provide the desirable dither > function. In fact, it even may be desirable from this point of view not > to restrict the ADC input bandwidth by filtering too much to ensure > there is enough noise to dither out the ADC small-scale nonlinearities! > For the direct conversion approach the performance of a (24bit) audio > ADC may be good enough to go without dither at all. > > Nick, G4JNX. > __________________________________________ Participe en Universidad 2008. 11 al 15 de febrero del 2008. Palacio de las Convenciones, Ciudad de la Habana, Cuba http://www.universidad2008.cu
