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Wireless Workshop - Direct Conversion Receivers

Tropper Technologies' Wireless Workshop is designed to explore various aspects of emerging wireless technologies...

Direct conversion receivers are not a new concept.  The original designs, which date back to the early 1900s, were called Homodyne receivers. The efficient implementation of Direct conversion receivers has however only recently become achievable.

 
 
  • Background

A conventional Superheterodyne receiver converts the RF signal to an IF frequency signal and then converts the IF signal to baseband for further processing.  One of the more costly and space consuming components in the superhet receiver chain is the IF filter.  This filter is typically realized as a Surface Acoustic Wave (SAW) filter and must exhibit very steep skirts in order to achieve a high degree of selectivity.

 

Radio2.jpg (12901 bytes)

 

 

The Direct conversion receiver does not require an IF filter as the RF signal is converted directly to baseband.

 

Note that in the superheterdyne design, the creation of the I/Q data streams is performed at IF frequencies.  In the direct conversion design above, the creation of the I/Q data streams must be performed in the RF stage (at RF frequencies).  This is more difficult and expensive to accomplish as it necessitates the use of two mixers at RF rather than two mixers at IF.  Recent advances in IC process technologies have however made this achievable.

 

 

  • Benefits of Direct Conversion Receivers
  1. No IF SAW filter.
  2. As the IF frequency is zero, there is no need to mitigate the image frequency. This means that there is no need to employ the more complex image reject mixers in the design.
  3. Direct conversion of the signal at RF is a considerable step toward multi-standard/multi-mode radios as more and more of the waveform processing is handled not in hardware but in software (DSP etc...) at baseband. This makes the radio more flexible.
  • Tradeoffs
  1. Above implementation requires two mixers at RF frequencies. his opens up the possibility that the local oscillators might radiate back in the antenna.
  2. Direct conversion receivers are very susceptible to DC errors due to leakage of the LO into the two mixer RF ports since they operate at roughly the same frequency.
  3. The DC offset can be removed via filtering prior to A/D and demodulation.  This would require extra filtering.
  4. Or the DC offset can be passed through the A/Ds and removed via additional processing. This would require A/Ds that have very high dynamic range.

 

The next app note in this series will focus on RF amplifiers and the need for linearization.