Very good presentation about how fantastic radio is! Beat frequency for the brain.
AM radios must meet Part 15 specifications because the internal oscillator produced by beating the IF with a spectrum of AM station frequencies has an innate signal strength which must meet 15.209. To sample this use a second AM radio tuned to an empty channel and tune the first radio until you hear the swish of the small "carrier" coming from the first radio. In past years I tinkered with modulating this IF by-product oscillator using a small modulation transformer wired into the B+ power supply to the oscillator tube. I wasn't quite smart enough to re-wire another tube to perform as an RF amplifier to put out a decent signal, but if you're ever illegally imprisoned you might convert an AM radio to a transmitter and use the loudspeaker as a microphone to put out a distress call.
Another way to make an AM radio would be to have a crystal tuned to each AM channel, very many crystals. To reduce cost you could only use crystals for the few stations you actually wanted to hear.
Makes you wonder about FM frequencies. Do FM radios employ the superheterodyne method? If so, what is their IF frequency?
Thinking about IFs.
Indeed the FM band has its own IF, mentioned along with many other IFs for other bands including shortwave, longwave, and more on Wikipedia under "Intermediate Frequency".
And I imagine that one of us using a spectrum analyzer might come across an IF signal from a nearby radio which might be mistaken for an actual empty carrier from somebody's transmitting tower.
Yes FM is superheterodyne also at 10.7 mHz
All radio receivers AM FM Shortwave use this system.
But the now DSP method of reception does away with the conventional system and takes an incoming signal and with a potentiometer varies a voltage to varactor diodes to select a specific incoming frequency and a chip converts the analog to digital and gets processed and then converted back to analog for an audio amp to get it to the speaker. No tuning capacitor, RF, IF converter, detector or anything like that.