United States Patent       5,978,488     Nov. 2, 1999
SIMULATED AM RADIO       Margolin

Abstract
A realistic illusion of listening to an actual AM radio is produced by providing a number of audio program signals and by simulating the process of tuning across the radio dial where each station is playing a different program. In a superheterodyne receiver using envelope detection, tuning a station off-channel results in a changing mixture of distortion and noise. The distortion function is caused by a variety of processes, among them being the filtering action as parts of the signal spectrum are cut off by the bandpass filter in the IF amplifier. Another is caused by the reduction in the carrier which causes the foldover of negative peaks in the envelope detector.
 
 

U.S. Patent 1,342,885  METHOD OF RECEIVING HIGH-FREQUENCY OSCILLATIONS   issued June 8, 1920 to Armstrong.

The superheterodyne was invented by Edwin H. Armstrong.

The architecture of Armstrong's receiver is shown in FIG. 13a. The broadcast signal is received by Antenna 1301 and multiplied in Mixer 1302 by the signal from Local Oscillator (LO) 1303. The resulting difference signal is amplified in Intermediate Amplifier (IF) 1304 and envelope detected in Detector 1305. The listener hears the demodulated signal in Headphone 1306 (referred to by Armstrong as a telephone).

Note that Armstrong did not use an RF amplifier. The reason for this is that the triode vacuum tubes of the day were not capable of operating as amplifiers at the high frequencies (above 1 MHz.) which were Armstrong's goal. The parasitic capacitance between the plate and the grid caused amplifiers to oscillate when operated at high gain. (The higher the frequency, the greater the positive feedback through the parasitic capacitance.)

By converting high incoming frequencies to a low frequency, Armstrong's method allowed IF Amplifier 1304 to operate at a frequency where triodes could be used at a much higher gain without oscillating. In addition, by tuning Local Oscillator 1303, all frequencies were converted to the same IF frequency so that IF Amplifier 1304 could be optimized to operate at a single frequency. This made it possible to tailor the gain and filter characteristics. Armstrong's '885 patent even teaches a double conversion superheterodyne receiver operating at 10 MHz.
 

The main object of this patent is to provide a variable capacitor that can be accurately and precisely adjusted.

What I find most interesting is that FIG. 4 shows the axis of rotation of Rotor 9 and Stator 8 offset from the center of the midline.  This provides a capacitance curve that linearizes the dial so that the stations are not bunched up at one end.

This became the standard method used in the construction of AM radios and is discussed in my patent (5,978,488).
 

The main object of this patent is to provide a method of construction that is efficient and reliable.

Another object, which I find more interesting, is to provide a capacitance curve to linearize the dial so that the stations are not bunched up at one end.

FIGS. 4 and 5 show Stator 17 with a curve along the midline. As Rotor 31 is rotated, the capacitance will be modified by the shape of the curve.

Dial linearization is discussed in my patent (5,978,488).