MATLAB: How to preserve base band signal’s bandwidth

band pass signalbase band signalCommunications ToolboxDSP System Toolboxlocal oscillator

I am using a baseband signal with bandwidth 100 kHz, which I then multiply with a local oscillator of frequency 1.5 MHz. Now, when I see the output after the up-conversion, the base band signal is up-converted to 1.5 MHz frequency, but, the original bandwidth of 100 kHz is not preserved. Rather, the bandwidth is also stretched to 1.5 MHz (the same as the local oscillator's frequency).

How can I up-convert my signal while keeping the original bandwidth of 100 kHz?

I have been trying to look into a lot of things, but couldn't find any solution.

Any help or guidance would be valuable.

Best Answer

For up-conversion you typically need to first increase the sample rate of your input through (multistage) interpolation and then multiply by local oscillator. See the example: http://www.mathworks.com/help/dsp/examples/digital-up-and-down-conversion-for-family-radio-service.html

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MATLAB: Changing the LO frequency of ad9361_RX block in ad9361_QPSK testbench model causes unexpected behavior

If you change both the LO and the RF center frequency in the ad9361_RX block, you will notice very minor differences. The RF center frequency of the RX should the same as the TX LO center frequency. The input signal to the RX is a complex equivalent baseband signal, and the RF center frequency of the RX defines the RF center frequency of the incoming signal.

The LO RX center frequency should be close (within the simulation bandwidth) to the RF center frequency. This is described in the support package documentation. To access this, please type the following command at the Command Window -

open(ad93xx_getdoc)

If you set the TX LO frequency at 2 GHz, the RX RF frequency at 2GHz, and the LO center frequency at 1GHz, results will be incorrect.

MATLAB: How to use AD9361 TX RF block set in Simulink

The “External LO” block is use to add noise on the LO signal of 2GHz specified in the “ad9361_tx” block mask, while the “Ideal Source” complex sine signal of 2MHz (look under the mask) is the modified input to the modular found in the subsystem “ad9361_tx/ad9361_tx/RF_TX”. The noise skirt is specified in the “External LO” block, while the bandwidth of the signal is related to the “Step size”, Tstep, found in the “ad9361_tx/ad9361_tx/RF_TX/Configuration” block. The Configuration block is also use to specify the simulation frequencies.

The “ad9361_tx/ad9361_tx/RF_TX” has three Inport blocks. The blocks “Inport_I”(real part of complex baseband) and “Inport_Q”(imaginary part of complex baseband) specify that the complex sine signal resides on a DC carrier (Carrier frequency = 0). If the carrier frequency is not DC, then the complex baseband signal is handled as I+jQ. The “ad9361_tx/ad9361_tx/RF_TX/LO Signal” block specifies the LOFreq and the “ad9361_tx/ad9361_tx/RF_TX/Outport” block specifies that the desired output carrier frequency is ‘RF’. The output carrier frequency of interest should be DC+LO=LO=RF=2GHz. The original baseband signal, after system processing, will reside on this RF signal. However, a baseband signal is I=jQ where the carrier is implied but not shown.

You can follow the documentation which is provided in Add-on Manager.

Best Answer

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MATLAB: Changing the LO frequency of ad9361_RX block in ad9361_QPSK testbench model causes unexpected behavior

MATLAB: How to use AD9361 TX RF block set in Simulink

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