Receiver Architecture
(1 stage downconverting, low IF or Zero IF) structure, what's the progress of
design for this ? I mean why it can not just downconvert the freq from RF to
Zero-IF in the past instead of using 2 mixers for downconversion.
Is it because limition of RF component/design(capability of mixer ?) or
immature DSP(DC offset problem) then?
Thanks for your instruction.
Edwin Armstrong discovered heterodyne during WW1. It was much later when a group of British scientists discovered homodyne, however heterodyne has already become a popular radio architecture used in the States for domestic and military FM radios.
Homodyne then was very difficult to tune and it is not flexible. It suffers from baseband saturation and DC offset due to LO leakage. It was easily jammed by the German Nazis. It was not until Spread Spectrum was invented by the same group of British scientists to resolve this issue. However, they realised that is actually much easier to implement Spread Spectrum on heterodyne.
With the proliferation of Spread Spectrum used in Satellite, CDMA, WLAN etc, heterodyne is preferred. Also, early 1st Generation mobile cellular techonology based on FDM such as the early AMPS (USA) and TAC (UK) and other cellular networks in the 60s to 70s are using FM radios, based on heterodyne.
Heterodyne architecture continued to be used because there is demand to switch to different GSM bands used in different countries, for example, 1800 in the USA and 900 in Europe.
It is also used in WLAN to switch among the 11 WLAN subbands.
It is used in Bluetooth to switch among 79 subbands.
Such cannot be achieved in homodyne.
Homodyne can only be used if you have a dedicated band. With the widespread of RFID, this is used. For AM radio, this is used. For point-to-point single channel radio, this is used.[/code]