QT GUI Frequency Sink

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A graphical sink to display multiple signals in frequency.

This is a QT-based graphical sink that takes a set of floating point streams and plots the PSD. Each signal is plotted with a different color, and functions can be used to change the label and color for a given input number.

The sink supports plotting streaming float data or messages. The message port is named "in". The two modes cannot be used simultaneously, and should be set to 0 when using the message mode. GRC handles this issue by providing the "Float Message" type that removes the streaming port(s).


(R): Run-time adjustable

options: [Complex, Float, Complex Message, Float Message]
title for the plot
FFT Size
size of the FFT to compute and display. If using the PDU message port to plot samples, the length of each PDU must be a multiple of the FFT size.
options: [32,64,128,256,512,1024,2048,4096,8192,16384,32768]
default: 1024
Spectrum Width
options (if Float input): [Full, Half]
Window Type
options: [Blackman-harris, Hamming, Hann, Blackman, Rectangular, Kaiser, Flat-top]
default: window.WIN_BLACKMAN_hARRIS
Normalize Window Power
options: [Yes, No]
Center Frequency (Hz) (R)
center frequency of signal (only used for x-axis labels)
Bandwidth (Hz)
bandwidth of signal (used to set x-axis labels)
default: samp_rate
options: [Yes, No]
options: [Yes, No]
options: [None, Low, Medium, High]
Y min
default: -140
Y max
default: 10
Y label
default: "Relative Gain"
Y units
default: "dB"
Number of Inputs
default: 1
Update Period (R)
default: 0.10
Show Msg Ports
options: [Yes, No]
GUI Hint
See GUI Hint



set the center frequency
set the bandwidth


the frequency where the output plot was double-clicked

Example Flowgraph


Source Files

C++ files
for Complex input
for Float input
Header files
for Complex input
for Float input
Public header files
Block definition