Harvey of Cambridge 193-R
Tech's Filing Cabinet Miscellaneous General Electric RCA Tube Tech Semi Tech Audio Radio Components

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.