I disagree with the idea that AM radios are overall worse than they were 50 years ago. That's not necessarily true.
I'll agree that AM car radios were good back then.
But when it came to your average consumer AM radio used in the 1960's or 1970's -- like the typical tinny sounding clock radio of that time period (some with with just four or five transistors), or little ubiquitous transistor radios used during that era (the ones with the tinny sounding 2.5 inch speaker, no RF amp and maybe one stage of IF amplification), AM radios today are in many ways superior, because they at least have enough performance to where you can actually hear the station if you live anywhere that isn't in the main lobe of the station's pattern. This is probably because a lot of IF chips have a built in RF amp stage.
And none of the older clock, table, or handheld transistor radios I've heard sound as good as my Superadio, or even my boombox.
Modern AM car radios do seem to sound like crap. I don't know why that is.
Car radios are a different issue. Delco radios in the late 60's were amazing - both sensitivity and selectivity. Sensitivity came from a tuned RF stage, something not standard in portable radios, and of course a 60 inch whip antenna - still electrically short for AM, but Delco increased the electrical length with an internal inductor. Selectivity came from halving the IF frequency to 262.5 kHz. Add to that noise filtering that actually WORKED - and you had some really great radios. But you had to be careful - there was another version that lacked the sensitivity. My dad had one of those cars - I hated when we took the red car out places because reception was limited to local only no matter if the antenna was extended or not.
AM radios into the early 70's were based on a 6 transistor reference design from Japan. For those that aren't familiar with the concept of reference designs - they are created by a savvy person and suggested by the manufacturer of a part, or parts. You have complete freedom to use the design to manufacture your product. I don't know who presented the all Japanese six design, but my money is on the transformer people. You look inside 90% of the transistor radios of the time, you have the same four transformers: red oscillator, yellow, white, and black IF transformers. The part number of the transistors changes, but not the architecture. Ferrite loopstick antenna and tuning capacitor for stage one, no transistor. Red slug Oscillator / Mixer = converter stage (combined oscillator and mixer), second section of tuning capacitor, one transistor. Three IF transformers, two transistors. Pre-audio amp - one transistor. Push / pull class B output audio amp, output audio transformer - two transistors.
There were variations - germanium PNP transistors were replaced with silicon NPN eventually, add a seventh transistor and maybe an interstage audio transformer to make it louder in a bigger radio with bigger speaker. Of course the tuned RF versions with a green slug RF coil and another transistor. Those required a three section tuning capacitor. But the variations required some extra design skills on the part of the manufacturer - something that was not a priority. Companies tended to spend the time on cool looking boxes and the circuitry was copied over and over again. New transistor came out to replace one going obsolete? No problem, tinker around with the bias some and it worked. Probably the biggest change was that changeover from PNP germanium to NPN silicon, which probably had a lot more to do with getting rid of the thermistor that was required so the things would work outside on hot summer days than it did the desire for better performance.
4 transistor radios like the earliest TI Regencies were comparatively low volume and poor performance. You would only want one for the curioscity, display, or investment value - they won't be your primary radio for listening. There were countless "boys radios" with just one or two transistors - a lot of which were headphone only. That was to avoid some legal restriction of some sort. Tuned RF architecture and enough amplification to make them work - little better than a crystal radio.
Then came IC's - and the death of the 6 transistor reference design in the mid to late 70's. The IF cans held on for a while, I have some radios with IC's that still have three or four IF cans - but the other development - the ceramic filter spelled doom for the transformer manufacturers. They fought hard to keep their niche, and they still manage to hold onto the red converter can for AM, but it is a rare radio these days that has IF cans. Hence the one ceramic filter abominations. Take out the low pass filter on audio, you have an instant broadband radio that sounds GREAT with music! But it comes at a cost - selectivity. My daughter had one with +/- 40 kHz bandwidth. Not so bad for a 7 year old, except that a local 660 swamped out her favorite station on 620. She cried and - knowing my skill - demanded that I fix it. A decent ceramic filter and her station came through just fine.
AM performance is what you want from it. I've documented the problems with tuning voltage steps on digitally tuned radios - the AM IF bandwidth has to be wide or you miss stations completely. And cheap mechanical tuning mechanisms on analog radios - half inch diameter direct drive, flexible plastic linear scale with no mechancial lash preventing hysteresis. If those radios don't have wide bandwidth, you can't tune them at all.
As for Silabs - I have yet to see a radio actually using them for AM. Silabs does a decent job with the IC, I have their demo USB radio. But plugging it into a computer and you automatically have such a noisy RF environment I can't characterize it as good for anything but local. Their application note for AM specifies a single trace on a PC board wound around the perimeter, that is so small there is no chance of anything but local. Take the trace out and use a decent loop, who knows? But I don't know what the IF bandwidth would be. These are digitally tuned chips, so they need to get the tuning steps right on frequency to avoid missing stations, or make the IF bandwidth broad. I won't be a believer until CCrane or somebody makes a decent radio using one.
They have fairly good selectivity in the "IF" section, from what I've heard and personally observed. I have several Tecsuns - a PL-380, two PL-606s, a PL-398mp and a PL-398BT.
