Sherwood's list focusses on a few (important) performance metrics, mostly for receivers. But these metrics by themselves do not indicate the practical capabiity, utility and quality of use. Case in point - see user reviews for the Yaesu FTDX-3000 and of course the Icom IC-7300. They're not at the top of Rob's list, but they are at or near the top of user's favorites.
Make no mistake, Rob's list is VERY useful in understanding how a receiver may behave at some of it's corner conditions, and his discoveries, widely published, have driven manufacturers to focus on phase noise and selectivity, among other things, leading to improved products. But in some areas, this has gone to an extreme that's just a little silly. Case in point, the Yaesu FTDX-101D, where isolation and tuning of the analog front end seem to be so critical that Yaesu forbids the user from opening it up, even to install optional filters.
Regarding DSP radios - quality is quality and quality costs. Do not ever expect a $200 "SDR" to outperform a $1000 radio (analog, digital or hybrid), because some of the most critical components that set performance are in the front end, and good ones are expensive.
In a DSP (SDR) radio, it's the bandpass/preselection filters and ADC. In an analog radio its the bandpass/preselection filters and 1st mixer. In both cases, the quality of the frequency reference also directly drives performance - and good ones are expensive.
$200 DSP radios are usually based on commercial chipsets developed for other applications, like digital video broadcasting (DVB). These chipsets are designed to have ADEQUATE performance in their INTENDED APPLICATION at the LOWEST POSSIBLE COST. They usually incorporate cheap and relatively low performance front end filters and I/Q mixers to allow cheaper, lower dynamic range ADCs to be built into the devices, with all the resulting I/Q imbalance that implies. The ADCs are followed by a digital decimator to reduce sample bandwidth to something an attached computer (that you provide) can handle) - audio frequencies.
Actual software post-processing happens at AUDIO bandwidths inside the attached computer, and the lower the processed digital bandwidth the less performance the digital portion of the system will have.
This performance is VERY low in a $100 or $200 dongle.
Yaesu has improved performance of its radios by moving back end digital processing bandwidth from 10's of KHz in the FTDX-5000, 3000, 1200 and FT991 to multiple MHz in the FTDX-101D, making their digital performance much better. And in the FTDX-101D, they've mated a top-notch and very expensive analog front end to a "high IF sampling" back end, to hit the top of Sherwoods list in performance.
When the cost/performance of the best front end analog mixers is equivalant to the cost/performance of the best available ADCs, then use of an analog front end (mixer) becomes redundant, for equal performance. At HF frequencies (very low), we are almost there, as shown by the relative performance of the Flex-6600, Icom IC-7610 and Yaesu FTDX-101D.
But further up the frequency bands, that battle is still being fought.
The big attractions of an all digital processing chain is:1. Flexibility/agility, and 2. Mechanical/elecrical simplicity. For the latter, simplicity is gained by moving all of the incredible complexity of DSP inside a digital signal processing IC (ASIC, FPGA or HA CPU), making electical/mechanical design and production easier and cheaper.
Have you ever looked inside a direct digital sampling Flex-6600? Surprise! It's almost empty, has few shielded compartments and is made mostly of bent metal. It's pretty inexpensive to make, once development cost has been retired.
In contrast, the IF sampling, hybrid analog/digital FTDX-101D is packed to the brim with RF-tight cavities, custom castings, hundreds of components and needs a lot of "touch labor" to complete. It's an expensive radio to build, no doubt about it. And its very critical internal tuning WILL drift over time, degrading performance - That's a future problem that owners have not realized yet.
So - should new hams play with cheap "SDR radios"? Yep. It's like a teaser. It may take them off into development of things other than radios (like hobbyist NVAs and SAs) and will likely lead them to more serious radios later on. Lot's of good MAY come from this (some already has). But if and only if they are technically capable AND have some good training and mentoring.
Otherwise they'll hit the same dead end that uninformed amateur SDR enthusiasts have hit when assuming a DSP system with 3 KHz of processed bandwidth can become a "perfect radio". It can't, and they still don't know why.
Hopefully, young people will explore this field and go into it professionally - because there is just as much (and at present much more) money to be made in digital signal processing (for communications, control systems and remote sensing) as there is in robotics. In fact, advanced robotics relies on DSP to perform.
Best Regards (and sorry for the very long essay!)
Brian - K6BRN