That's all about the ADC, not about little Rafael's (mis)behaviour under tough conditions.
The Rafael R820T (yes this is just as much a mainstream DVB-T chip as the Miricstakes all the beating from your antenna.
There seems to be a general perception that the MSi001 is simply a DVB-T tuner that has been applied to SDR applications and as a result cannot deliver performance as good as it might, had it been designed with SDR in mind from the outset. This perception is quite wrong. The MSi001 was designed specifically as a tuner front end for a wide range of broadcast receiver applications using a SDR approach. DVB-T was simply one of several applications to which it has been successfully applied. The MSi001 was developed as part of a silicon and software solution for broadcast applications. The silicon acts as a ‘hardware accelerator’ for software demodulation. By using hardware to pre-select the wanted signal, the burden of software processing is reduced significantly. It is not necessary for example to process adjacent channels in software – unless you actually want to, because they can first be removed in hardware. The complete chipset is the MSi3101 which comprises the MSi001 and MSi2500. The MSI3101 was not designed to interface to any hardware DVB-T demodulators and has never been used as such.
To get the best performance from the MSi001, it is necessary to understand how the device works and crucially, how it differs from other silicon tuners. The first point to note is that the MSi001 has no autonomous AGC, either at RF or at IF. It simply has digitally programmable gain for every stage within the signal chain. The reason for this approach is that the algorithms for AGC need to be tailored to the specific needs of the system and as such there is no ‘one size fits all’ AGC solution that works well for all systems. When developing the MSi001 it was decided that AGC algorithms should be integrated into the demodulation software and tailored to the specific needs of the system. This is precisely what Mirics did with their own software for applications such as DAB/DAB+, ISDB-T, FM, and DVB-T. The AGC algorithms for FM are completely different for those used in DAB. To get the best out of the MSi001, it is necessary to implement an appropriate AGC, or at least ensure that the gain is set to an appropriate level. Setting the gain to maximum will in most cases simply overload the ADCs, particularly if an external LNA is also added. General purpose SDR application software such as SDR# or SDR Console do not natively include appropriate AGC feedback for the MSi3101 and so it is essential to ensure that the gain is set correctly to get the best possible performance. Where people have used the original Mirics dongles for general purpose SDR applications, it is most likely that any shortfall in performance is as a result of the lack of AGC or an inappropriate gain setting as these dongles were only designed with Mirics’ own software in mind.
The second point to note is that unlike conventional silicon TV tuners, the MSi001 has very high performance 5th order Chebyshev filters which can be programmed to set the bandwidth as low as 200 KHz or as high as 8 MHz. These filters use a self calibrated active R-C approach and as a consequence have exceptional signal handling. The correct use of these filters substantially eases the ADC performance requirements as if there are strong blockers, it is almost always possible to filter them out or substantially attenuate them prior to them reaching the ADCs. The ADCs in the MSi2500 are native 12 bit converters which deliver 10.5 ENOB at around 10 MS/s. As there are 2 converters (one each for I and Q), this is equivalent to 10.5 ENOB at 20 MS/s for a single ADC in a conventional low IF DVB-T front end. It should be noted that the MSi001 can support both zero IF and a Low IF of up to 2.048 MHz. In low IF mode, the filters operate as bandpass filters with the same selectivity as in low pass mode. As a consequence, the ‘Q’ of these filters is extremely high in band pass mode. Conventional DVB-T tuners simply cannot deliver this level of selectivity because the minimum bandwidth is typically only 6 MHz. I/Q correction is always necessary in ZIF receivers (albeit simple to implement) but unnecessary in Low IF receivers.
The gain/noise/linearity/ADC ENOB of the MSi3101 was specifically designed such that no one factor dominates performance.
Whilst the MSi001 was not designed for general purpose SDR applications, it certainly was designed as part of an SDR solution. The most obvious short-coming of the device for general purpose SDR applications is the oft commented ‘gap’ between the upper end of broadcast band III and the lower end of band IV. This exists, simply because there are no broadcast TV or radio transmissions at these frequencies and it was therefore not considered to be relevant when the part was developed. That aside and granted that the part will not perform to the same level as a very high quality (and very expensive) ‘professional’ receiver, the MSI001 can deliver good performance for a wide range of analogue and digital radio applications across a very wide range of frequencies and in many cases will significantly outperform the tuners used in the RTL-SDR dongles if used to its full potential
