In GNU Radio how can I make positive complex number negative?

Nice, this is a very GNU Radio–ish question 🙂

In GNU Radio, a complex number doesn’t have a “positive” or “negative” sign the way a real number does. If you mean invert the complex signal (i.e. multiply it by −1), that’s easy and very common.

The simple way (recommended)

Use a Multiply Const block:

• Block: Multiply Const
• Type: complex
• Constant: -1

This turns

z = I + jQ \quad \rightarrow \quad -z = -I - jQ

That’s usually what people mean by “make it negative”.

Alternative: using Add Const

You could also do this with an Add Const block, but it’s less clean:

• Add Const (complex)
• Constant: (-2*I, -2*Q) after some offset

So… don’t 😄 Multiply Const is the right tool.

If you meant something else

Just to cover the common interpretations:

1️⃣ Flip only the real or imaginary part

Use Complex to Float, negate one stream, then Float to Complex.

Example:

I + jQ \rightarrow -I + jQ

2️⃣ Force magnitude to be negative (not mathematically valid)

Magnitudes are always ≥ 0. If you’re working with magnitude only, you can negate the float magnitude, but it’s no longer a complex number.

3️⃣ Conjugate instead of negate

If you meant:

I + jQ \rightarrow I - jQ

TL;DR

To negate a complex signal in GNU Radio:

Multiply Const (complex) = −1