QT GUI Vector Sink in GNU Radio – does it show offset of the signal from centre?

Short answer: no — not by itself 🙂

The QT GUI Vector Sink does not inherently show frequency offset from centre. It just plots whatever numbers you feed into it, index vs value.

Here’s how to think about it:

What the QT GUI Vector Sink actually is

• It displays a vector of values (floats or complex magnitudes/phases).
• X-axis = index (0, 1, 2, …), unless you explicitly reinterpret it
• Y-axis = value

There is no built-in concept of “centre frequency” or “offset” in the sink.

When it can look like frequency offset

If you feed it:

• an FFT output, and
• that FFT is arranged as DC in the middle (e.g. via fftshift),

then:

• the centre bin corresponds to 0 Hz
• bins left/right correspond to negative/positive frequency offsets

But that interpretation comes from your signal processing, not the Vector Sink.

So in that case:

You are inferring offset from centre, not the sink showing it explicitly.

Common gotchas

• No FFT shift → DC is at index 0, not the middle
• Vector Sink won’t label frequency (unlike FFT Sink)
• Complex vectors are usually shown as:
  • real
  • imaginary
  • magnitude
  • phase
    (depending on settings)

If you want to see frequency offset clearly

Use instead:

• QT GUI FFT Sink → explicitly shows frequency axis centred on tuned frequency
• or:
  • FFT → fftshift → Vector Sink, and you mentally map bins to Hz

Bin-to-Hz mapping:

Δf = sample_rate / FFT_size

offset = (bin_index − FFT_size/2) × Δf

TL;DR

• QT GUI Vector Sink: shows values only, no inherent frequency meaning
• Offset from centre: only visible if you arrange the data that way
• Want obvious frequency offset? → use QT GUI FFT Sink