NCO in simulink - 90 degree phase shift?

Hello folks,

I'm trying to simulate a quadrature Digital Down Converter (DDC). For now, I'm using a sine wave and a cosine wave for the NCO from the DSP toolbox. If I want to use a single clock source to yield the I/Q signals, I need to shift the I signal (cosine) by 90 degrees. What is the best way to implement this for sample-based systems?

Also, any guidance on how to make this quadrature DDC model more realistic would be appreciated. Thanks!

Answer 1

[Ryan G]

You can utilize a sine wave function block. This block has a dialog to input the phase shift (in radians) and you can use 2 of them hooked to the same digital input signal.

[Paul]

I'm actually already using the sine wave function block, but from the signal processing block set.

I guess I'll be more specific about my appliation. I'm modeling a system that has a DDC. Eventually this design will be implemented using the Xilinx System Blockset and burned to a FPGA. So, I'd like to model the DDC as accurately as possible (as proof of concept) using the appropriate blocks, so that they can eventually be replaced with the Xilinx blocks.

I see there is a NCO block. I'm guessing this would be a better solution to accurately model a digital local oscillator, correct?

[Ryan G]

There are actually 2 sine wave blocks in base simulink, the one I linked has an input attached so you can hook up the clock. I did not see an equivalent one to that in DSP.

You could use the trigonometric function block as well since it also has sine and cosine. I was under the impression that the Xilinx blocks (if used in Simulink) should be friendly with the Simulink blocks which should be friendly with HDL coder. That's not really my field but I didn't think this would be an issue.