Radio Receiver Design - Direct Conversion

Direct Conversion

In the Direct conversion receiver, the signals from the antenna are only tuned by a single tuned circuit before entering a mixer where they are mixed with a signal from a local oscillator which is tuned to the carrier wave frequency of the transmitted signal (unlike the superheterodyne design, where the local oscillator is at an offset frequency). The output of this mixer is thus audio frequency, which is passed through a low pass filter into an audio amplifier which may drive a speaker.

For receiving CW (morse code) the local oscillator is tuned to a frequency slightly different from that of the transmitter in order to turn the received signal into an audible "beep."

• Advantages
  • Simpler than a superheterodyne receiver

• Disadvantages
  • Poor rejection of strong signals at adjacent frequencies compared to a superheterodyne receiver.
  • Increased noise or interference when receiving a SSB signal since there is no selectivity against the undesired sideband.