Scanned articles covering the technology of FM radio transmission and reception. Unless otherwise noted, these scans were done by John Atwood. Articles marked "*" were extracted from worldradiohistory.com (formerly americanradiohistory.com) scans. scans. Listed chronologically.
Early FM
Roder 1936 Noise in Frequency Modulation * - A theoretical explanation of how FM improves
the signal-to-noise ratio. 
Landon 1941 Impulse Noise in FM Reception * - Experiments are done to evaluate the impact of
impulse noise in an FM receiver. Takeaways: perfect symmetry is important, the signal must be tuned to the center
of the channel, limiters help when symmetry or center tuning are not perfect.
Goldman 1941 FM Noise and Interference * - A theoretical treatment of noise, both random
and impulsive in an FM receiver.
Wheeler 1942 Common-Channel Interference Between Two Frequency-Modulated Signals - A
theoretical analysis of what happens when two different FM signals occupy the same channel. Although this term is
not used, Wheeler essentially explains "capture ratio".
Hund 1942 Reactance Tubes in FM Applications * - Details on how reactance tube circuits are
used as modulators in FM transmitters.
Rodgers 1943 Tuning Indicators and Circuits for Frequency-Modulation Receivers - Various
ways of using a conventional tuning eye tube to accurately tune an FM receiver. After the war, GE came out with
the 6AL7GT designed specifically to indicate FM tuning.
Martin 1944 FM Reciever Design Considerations * - Stromberg-Carlson weighs-in on how to
optimize consumer FM radio design. Low-band, of course.
Parker 1944 Design of Intermediate-Frequency System for Frequency-Modulated Recievers -
The design of an FM IF system using 8.25 mc instead of the then-common 4.3 mc. After the war, when the FM band
was moved to 88-108 mc, the FM IF frequency was standardized on 10.7 mc.
Arguimbau 1945 Discriminator Linearity * - How to design the most linear Foster-Seely
discriminator.
Johnson 1945 Interference in FM Receivers * - More theory and experiments on interference
reduction in FM broadcasting.
GE 1946 2H21 Phasitron Ad * - General Electric's ad for the first phasitron. For more
details, see Adler 1947. GE also
worked with Adler on the 6BN6 beam switching tube.
Meyers 1946 Nonlinearity in Frequency-Modulation Radio Systems Due to Multipath Propagation
- A theoretical study of FM multipath distortion. It is worst when the path delay difference is large and the signal
strength is similar.
Smith & Bradley 1946 The Theory of Impulse Noise in Ideal FM Receivers * - A theoretical
analysis of impulse noise on FM reception.
Adler 1947 A New System of Frequency Modulation * - A description of the development of the
"Phasitron" - a special tube that implements phase modulation, used in the generation of FM. This tube was
realized as the General Electric 2H21 and 5593 tubes, which were used in FM transmitters made by GE, Raytheon, and
others.
Adams 1947 IF Amplifiers for FM Receivers * - Issues involved with the design of FM IF
amplifiers. The main problem is the reduction of capacitive feedback, which, if not causing oscillation, causes
unsymetrical frequency response.
Plusc 1947 Investigation of FM Signal Interference * - A theoretical evaluation of on-channel
and adjacent-channel interference in FM broadcasting. One finding: discriminator bandwidths well in excess of the
usual 200 kc helps reduce this problem.
Gladwin 1947 The Distortion of Frequency-Modulated Waves by Transmission Networks * - The
effects of limited bandwidth on the distortion of FM signals.
Electronics McKenzie 1948 Fremodyne FM Receivers * - The Fremodyne FM detector is a clever
circuit developed by the Hazeltine Labs to be an inexpensive way to add FM to a regular receiver. It uses a
single 12AT7 dual triode as both a local oscillator, converter, and superregenerative detector, using an IF frequency
of 21.75 mc. This article describes extensive tests of various FM receivers, including two with the Fremodydne circuit.
The Freodyne uses slope detection, so tuning for minimum noise maximizes distortion. The superregenerative
detection has no "capture" to reject interfering signals. The radiated interference is bad enough that two
Fremodyne set 100 feet apart interfere with each other. The only advantage over AM was that ignition and other
local noise sources are less. So: simple - yes, decent performance - no.
Labin 1948 Theory of Counting and Its Application to the Detection of FM Waves * - The
theoretical basis of the "counting" type of FM detectors.
Smith & Fouty 1948 Circular Polarization in FM Broadcasting * - By using a
circularly-polarized transmitting antenna, a higher received signal level for a randomly-placed receiving antenna can
be obtained. The FCC allows twice the transmitted power level with circular polarization. Circular polarization
became important when FM car radios with vertical polarization came into use. Circular polarization is standard
today.
Day 1948 Serrasoid FM Modulator * - The phasing method of FM generation, used by Armstrong,
Raytheon, GE, and others requires no troublesome phase-locked loops, but the early phase modulators could only
generate limited phase shifts without distortion. This required many frequency multiplier stages. The Serrasoid
phase modulator used a sawtooth wave and threshold detector to give much higher phase shifts with low distortion.
This was developed by R.E.L., but was also used by Gates. A nice endorsment by Armstrong is included.
Arguimbau & Granlund 1949 Sky-Wave F-M Receiver * - The idea of sky-wave FM transmission is
investigated. A very high performance receiver for this purpose is described.
Dishington 1949 Diode Phase-Discriminators * - How to make the best phase detectors -
probably applicable to FM detectors.
Loughlin 1952 The Theory of AM Rejection in the Ratio Detector * - Detailed theory of how
ratio detectors reject AM noise. Interestingly, imperfect diodes have better AM rejection than perfect diodes.
Wilmotte 1952 Reduction of Interference in FM Receiver by Feedback Across the Limiter * -
When frequency-limited feedback is used around a limiter, the amount of interference from a signal on the same
channel is reduced. This improves the "capture ratio" (although this term is not used).
Johnson 1956 FM Receiver Design * - Points out that conventional FM receivers, based on
Armstrong's original design, have poor capture ratio due to narrow-band discriminators. By using wide-band
discriminators or other techniques, a capture ratio of 0.5dB or better can be achieved.
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 1 *
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 2 *
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 3a *
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 3b *
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 4 *
Johnstone 1957 Limiters & Discriminators for FM Receivers Part 5 * - A five-part series
describing in detail the various types of FM detectors and limiter circuits. Note letter from Arguimbau at the
end of Part 1.