Saturday, August 31, 2019

Direct conversion receivers

One of the simplest type of receivers for practical amateur band listening is the direct conversion type. There's only a few stages: A local oscillator, a product detector and an audio amplifier. Better receivers have extras like RF amplification and audio filtering. 

The simple type of direct conversion receiver has only one main disadvantage compared to the superhet. Its selectivity. It responds evenly to signals both above and below where the set it tuned to. This gives it a selectivity double that of a superhet. It can mean that on a crowded band you hear more signals than are actually there because you are hearing everything twice, at two spots on the dial. 

You can overcome that either by using the direct conversion receiver as the back end of a superhet and using some RF filtering before it (eg a crystal filter) or make the direct conversion set more complex by incorporating phasing techniques to remove reception of the undesired sideband. 

Even if you don't do that the direct conversion receiver is still very useful, especially for homebrew QRP gear. A major advantage is you can use the local oscillator to form a CW transmitter. That's easy with the only thing required being to shift its frequency down by about 800 Hz on transmit and provide adequate buffering, amplification and keying for the transmit section. 

Here are some demonstrations of direct conversion receivers. 

Portable 40/20m DC receiver

A more complex direct conversion receiver - built quite a few years back

CW receiving with an unfinished DC receiver

Have you built a direct conversion receiver? If so please leave your comments below. 

PS: Heard about my new book? It's the Australian Ham Radio Handbook. It's now available both as an ebook and paperback. Find out more here!

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