|‘Technical Shorts’ is a series of (fairly) short articles written by Gerry O'Hara, a CVRS Director and vintage radio enthusiast, each focussing on a technical issue of relevance in repairing, restoring or using Eddystone valve radios. However, much of the content is also applicable to non-Eddystone valve receivers. The articles are the author’s personal opinion, based on his experience and are meant to be of interest or help to the novice or hobbyist – they are not meant to be a definitive or exhaustive treatise on the topic under discussion….
References are provided for those wishing to explore the subjects discussed in more depth. The author encourages feedback and discussion on any topic covered through the CVRS forum.
The intent of this ‘Short’ is not to deliver a treatise on detection or demodulation, but rather briefly outline the various forms of detector used in valve receivers, when and why they are used, discuss typical detector circuits used in Eddystone valve receivers of the post-WWII era, and provide some commentary on their performance and faults that may develop in them. Detector circuits are closely integrated with automatic gain control (AGC) circuits in many receivers (though not Eddystone) and the ‘Short’ on AGC could usefully be read in conjunction with this article. This article also describes frequency modulation (FM) detector circuits and their adjustment in some detail, which will be of interest to folks using sets with FM capability, such as the S.770/R and S.770/U series.
What is a ‘Detector’?
In essence, ‘detection’, or more correctly, ‘demodulation’ in the radio world is the process of obtaining intelligence from a received signal - though whether or not the original modulation was actually intelligent may, in many cases, be debatable: in this process, the desired signal is first selected and amplified by the radio frequency (RF) and intermediate frequency (IF) stages of a receiver, the next step is to derive the waveform that the RF signal was modulated with prior to transmission.
The method of demodulation depends on the original method of modulation of the signal, though the actual demodulation technique used may differ for any particular modulation type. The device used for demodulation in a receiver is commonly referred to as a ‘detector’. In superheterodyne (superhet) receivers, the signal detector is often referred to as the ‘second detector’, the term ‘first detector’ being used for the mixer stage.
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