[Elecraft] I/Q

[Edward R Cole]

not going to repeat what everyone has already posted:

One begins with an RF signal which has modulation (easiest to 
consider the modulation SSB and even easier if you take the example 
of a two-tone test signal for SSB).

The first step in producing IQ output is to spit the RF into two 
parallel mixers that use the same frequency LO, but one is shifted 
90-degrees in phase.  The LO is the same frequency as the RF so one 
gets audio as an output (same thing one does in an SSB 
demodulator).  The two resulting audio outputs are 90-degree out of 
phase with each other.  In the typical SDR the next step it convert 
to digital using a analog to digital converter (ADC)(hams are using 
their soundcard as a ADC).  The resulting two digital signals are out 
of phase with each other and can be processed by a computer (called 
an DSP = digital signal processor).

Due to the unique mathematical properties of the two digital signals 
one can recover CW/DSB/SSB/AM/FM/PSK modulation types (and I may have 
left out others).

This is a very rudimentary (non-mathematical) description.  The true 
beauty of using IQ is its versatility.  No longer does the receiver 
have to have a product detector, discriminator, envelope detector, 
etc.  The DSP does it all in digital form and re-converts the 
demodulated signal back to audio (or displays the signal on a screen).
http://www.kl7uw.com/MAP65.htm

the diagram may help understanding.

73, Ed - KL7UW
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