The Hazeltine FreModyne FM Receiver

The Hazeltine FreModyne FM Receiver (Part 1).

Introduction

Readers of Electronics Australia during the 1960's will remember their "Fremodyne Four" project. It was a simple VHF communications receiver which was very popular. The author had mentioned that the design was based on the "FreModyne"; a simple super-regenerative superhet receiver which the editor had seen reference to in a late 1940's electronics magazine from the U.S.(ref.6). The Electronics Australia article appeared twice during the 1960s, and then in May 1970, a solid state version appeared.

EA's updated Fremodyne Four from March 1967. See further down page for more on this set

However, the Fremodyne has its origins as a low cost FM receiver, developed by the Hazeltine Corporation in the U.S, back in 1947, well before Electronics Australia presented their constructional articles. EA's design was essentially a copy of the Hazeltine circuit, but with plug in coils to allow 30-250Mc/s reception.

Where does the name come from? FREquency MODulation, and DYNE being a generic sort of suffix added to the names of various radio circuits since broadcasting began. In fact, Hazeltine spelled their circuit's name as "FreModyne" to emphasise it was for FM reception.

My own interest in the Fremodyne came about in the late 1980's as a result of the AM stations dropping their music formats or transferring to FM. Being the type of person who likes constructing regenerative receivers in preferences to superhets, I looked for a similarly simple way to receive FM. The first course of action was to look at the articles in Electronics Australia. From here on I started my fascinating research into, and construction of, super-regenerative receivers.

Simple FM Receivers.

Contrary to what many people believe, it is not necessary to use a complicated multi valve superhet with numerous tuned circuits to receive FM. It is in fact possible to use crystal set techniques for VHF FM reception and there exist a number of designs. However, they require a strong signal. The fact that slope detection is used means that sensitivity is further reduced (the receiver cannot be tuned to the peak of its response curve, where maximum gain occurs, in order to detect FM). The next step up, a regenerative detector, can also be used. The limitation is that as the regeneration control is advanced to increase gain, bandwidth decreases and thus distortion increases. So, this type of receiver cannot be used at full gain with wideband FM. Nevertheless, experiments have shown it is capable of excellent audio quality. Furthermore, at VHF, the operation of the regeneration control is very critical. While these designs can be used by technically minded people who accept their limitations, they are totally unsuitable for the general public. For these reasons, the super-regenerative detector is preferred where a simple circuit is required. It provides good sensitivity and high audio output along with wide bandwidth, and can be used by non technical persons.