How can IQ samples be obtained by shifting a signal down to DC?

[Djaferbey]

Hello,

Im trying to figure out how different receiver architectures obtain IQ samples. (Direct sampling architecture and direct conversion architectures). If a direct conversion method is used, how can the IQ samples be obtained by shifting the desired channel down to DC. ?

 

[D.A.(Tony)Stewart]

Frequency conversion requires stable LOs with dual IQ phase PLL and a multiplier to act as a frequency converter and shift f by the difference f =0 then filter out 2f.

 

[Djaferbey]

Frequency conversion requires stable LOs with dual IQ phase PLL and a multiplier to act as a frequency converter and shift f by the difference f =0 then filter out 2f.

In a direct sampling architecture where the first Nyquist zone is sampled, how can IQ samples be obtained by downconverting a recorded QPSK-modulated signal to DC? What is the relationship between this down conversion process and the IQ components?

 

[D.A.(Tony)Stewart]

The DSP performs downconversion using a numerically controlled oscillator (NCO), which is a digital equivalent of the linear local oscillator, LO. The NCO must be locked to the carrier and the sampling rate must avoid aliasing after the LPF's so often 2.5x fc

 

[Djaferbey]

The DSP performs downconversion using a numerically controlled oscillator (NCO), which is a digital equivalent of the linear local oscillator, LO. The NCO must be locked to the carrier and the sampling rate must avoid aliasing after the LPF's so often 2.5x fc

Thanks.

I'm new to this, so I might be a little slow. I'm processing ADC data with MATLAB that contains a DQPSK-modulated signal. I read that to obtain the IQ samples, I need to shift the desired channel down to DC. So as far as i understand the NCO performs a similar process?. Also, what is the relationship between downconversion to DC and obtaining the IQ components? I really don't understand how we can obtain IQ samples by shifting the signal to DC.

The signal is real, by multiplying any complex exponential signal will result in a complex signal.

This is the function I have for performing the frequency shift:

 

[D.A.(Tony)Stewart]

I dont use Matlab but openAI does.

 

[kaz1]

Thanks.

what is the relationship between downconversion to DC and obtaining the IQ components? I really don't understand how we can obtain IQ samples by shifting the signal to DC.

You need to visualise the spectrum from transmitter to receiver. An I/Q pair of channels is translated to one channel by shifting its frequency centre away from dc.
This will produce asymmetric spectrum containing information from both I & Q channels.
At receiver. You reverse that back. onto I/Q channels by shifting frequency centre back to dc.