How to use the "bandwidth" parameter to baseband a complex signal from an intermediate frequency to DC with "dsp.Digit​alDownConv​erter" in R2024b?

I am looking for advice to baseband a complex signal from an intermediate frequency to DC. I would like to use "dsp.DigitalDownConverter".

The "visualize" function draws the filter response with some dashed lines is red. One line follows the "Bandwidth" (BW) while the other seems to be:

Fs/2-BW

where Fs being the low sample rate.

My workflow is: 

1. Create sin wave signal
2. Modulate Signal with "Pmmod"
3. Add noise to the signal
4. Demodulate signal with "pmdemod"
5. Create the down converter with "dsp.DigitalDownConverter"
6. Plot the results 

What are these red lines in the graph from "visualizing" the Digital Down Converter? What does the “bandwidth” parameter of "dsp.DigitalDownConverter" do? It looks like it does not set 3dB as the down point on the filter cascade. 

Answer 1

[MathWorks Support Team]

To get the demodulated message from the Digital Down Converter (DDC) output, it still needs to be demodulated (e.g., using "pmdemod" or other PM demodulation techniques) at the new, lower sample rate.

The key point is that DDC and phase demodulation are fundamentally different operations. The DDC object is designed to perform "downconversion" (mixing to baseband) and decimation; it does not perform phase demodulation.

As an aside, "dsp.ComplexBandpassDecimator" might also be worth considering, depending on the application. That said, phase demodulation, as you noted, is handled by functions such as "pmmod" and "pmdemod", so it needs to be added manually after the down conversion.

The "Bandwidth" property determines the frequency range of the input signal and, in turn, sets the passband for the filter stages. The red dashed vertical lines indicate the passband (left) and stopband (right) frequencies of the overall equivalent filter for the current design configuration. The passband frequency is always equal to half of the Bandwidth .

Inputs with smaller bandwidths allow for wider transition bands in the filter stages, which reduces their complexity.

For example, compare the two designs below. Notice that the design with the lower input bandwidth results in lower-order filters.

G20 = dsp.DigitalDownConverter(Bandwidth=200000)
G5 = dsp.DigitalDownConverter(Bandwidth=50000)


G20.getFilterOrders


ans = 


  struct with fields:


       NumCICSections: 4
    SecondFilterOrder: 12
     ThirdFilterOrder: 24


G5.getFilterOrders


ans = 


  struct with fields:


       NumCICSections: 3
    SecondFilterOrder: 12
     ThirdFilterOrder: 10

Note: The "Bandwidth" property reflects prior knowledge that the input signal contains no frequency components beyond the specified bandwidth. In other words, the assumption is not merely that any out-of-band input content is unimportant or can be filtered out; rather, such content is assumed to be entirely absent.