IQ-DATA
Table of Contents
INTRO
Any sinusoidal signal 
can be composed of two components named I (inphase) and Q (quadrature, meaning a phase difference of 
between I- and Q- components) which are multiplied with an appropriate oscillator with a frequency of 
.
can be composed of two components named I (inphase) and Q (quadrature, meaning a phase difference of between I- and Q- components) which are multiplied with an appropriate oscillator with a frequency of . 
, with oscillator 
, with oscillator 
It can be seen that these equations are one and the same as the defintions for magitude and phase of a complex value. This is the reason why i(t) and q(t) data are usually interpreted as complex value pairs. The concept of transmission and reception with the help of direct conversion is the latest concept of SDR and also used by the transceiver systems with an AD936x IC onboard. Therefore it will be described more detailed in the next section.
TRANSMISSION
The baseband signal is sinusoidal with a momentary amplitude 
and phase 
. It is represented in the form of complex value pairs of 
and 
.
and phase . It is represented in the form of complex value pairs of and . When IQ-data is sent to an SDR - transmitter it is first upsampled (by interpolating and lowpassfiltering) to meet the ADCs' sampling frequencies. Lowpassfilter provide additional bandshaping of the analog baseband signals before they are fed to the RF-mixer stage where the output signal is synthesized combining the IQ-data: 
. The gain is adjusted by a following attenuation stage before the signal is led to an output amplifier.
. The gain is adjusted by a following attenuation stage before the signal is led to an output amplifier. RECEPTION
The receiver contains a low noise amplifier followed by an IQ-mixer stage where the signal 
is mixed down to baseband using two synchronized and 
phase shifted oscillators 
and 
.
is mixed down to baseband using two synchronized and phase shifted oscillators and .
It should be noted, that 
can be obtained exactly, even if there is a phase difference between the TX- and RX- oscillators. 
represents the phase difference between the oscillators of transmitter and receiver.
can be obtained exactly, even if there is a phase difference between the TX- and RX- oscillators. represents the phase difference between the oscillators of transmitter and receiver.After the signal has been direct converted to baseband it has to be digitized by highspeed ADCs. The next stage is filtering and decimating the signal to obtain the desired sampling frequency used by the target system. The SDR-receiver gets the downsampled IQ-data at a rate named 
here. The IQ-data of the baseband signal contains information which has to be typically demodulated at the next stage. This is done by signal processing on a host system, for example with the help of Matlab.
here. The IQ-data of the baseband signal contains information which has to be typically demodulated at the next stage. This is done by signal processing on a host system, for example with the help of Matlab.