Frequency Shifting

This tutorial describes how to perform frequency shifting, causing the frequency of a signal to change.

Frequency Shifting

Frequency shifting is the process of changing the position of a signal within the frequency domain. Equivalently, it can be stated that frequency shifting is the process of changing the center frequency of a signal. Frequency shifted can be implemented many different ways, although this tutorial will focus on the simple method of multiplication by a complex sinusoid.

Multiplying a signal by a complex sinusoid with frequency f Hz will translate or shift the center frequency of the signal by f Hz. For example, to frequency shift a signal by 1 MHz a complex signal must be generated with frequency 1 MHz, and then multiplied against the desired signal in order to frequency shift it.

Build Example Signal

First an example signal needs to be built. A simple signal of filtered noise is created. Drag in the following blocks and connect them:

• Noise Source
• Throttle
• Low Pass Filter
• QT GUI Frequency Sink

The flowgraph is an offline simulation to the choice of sampling rate is somewhat arbitrary, but a sample rate of 10 MHz is chosen to realistic number.

The low-pass filter properties are updated to define the cutoff frequency and transition width:

• Cutoff Freq: samp_rate/8
• Transition Width: samp_rate/16