Sample Rate Change

This tutorial describes how to implement sample rate change within GNU Radio.

The previous tutorial, Designing Filter Taps, demonstrates how to design filter taps and use them in signal processing blocks. Please complete the Designing Filter Taps tutorial before completing this one.

Interpolation

Interpolation is the process of increasing the sampling rate and thus the available bandwidth. This example will demonstrate how to increase the sampling rate using the Interpolating FIR Filter block.

Start by adding the following blocks to the flowgraph and connect them:

1. Two Variable blocks
2. Low-Pass Filter Taps
3. QT GUI Range
4. Signal Source
5. Interpolating FIR Filter
6. Throttle
7. QT Frequency GUI Sink

Edit the first of the two new variable blocks:

• Id: interpolation_rate
• Value: 4

Edit the second of the two new variable blocks:

• Id: samp_rate_interpolated
• Value: samp_rate*interpolation_rate"

Edit the properties of the Low-Pass Filter Taps block:

• Id: lowPassTaps
• Sample Rate (Hz): samp_rate_interpolated
• Cutoff Freq (Hz): samp_rate_interpolated/(interpolation_rate*2)
• Transition Width (Hz): samp_rate_interpolated/(interpolation_rate*4)

Edit the properties of the QT GUI Range block:

• Id: frequency
• Default Value: 0
• Start: -samp_rate/2
• Stop: samp_rate/2

Edit the property of the Signal Source:

• Frequency: frequency

Edit the properties of the Interpolating FIR Filter block:

• Interpolation: interpolation_rate
• Taps: lowPassTaps

The Interpolating FIR Filter will increase the sampling rate from 32 kHz to 128 kHz, a factor of 4 due to the interpolation_rate variable. Make a note of this by editing the Comment field under the Advanced tab:

The comment is then displayed as a visual reminder in GRC:

Edit the Throttle property:

• Sample Rate: samp_rate_interpolated

Edit the QT GUI Frequency Sink property:

• Bandwidth (Hz): samp_rate_interpolated

The flowgraph should now look like the following:

Running the flowgraph will show the following QT GUI Frequency Sink:

Arbitrary Rate Resampling

• updating sample rate variable