Replay IQ Signals with rpitx on Raspberry Pi

This guide explains how to record and replay IQ signals using rtl_sdr and sendiq with the rpitx software on a Raspberry Pi. It provides step-by-step instructions for capturing radio signals and replaying them via your Pi.

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📦 Hardware Requirements

• Raspberry Pi (compatible with rpitx — check the rpitx repo for details)
• RTL-SDR USB dongle for signal capture
• A wire antenna connected to GPIO 4 (physical pin 7) for transmission

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✅ Features

• Capture IQ samples from a target frequency using rtl_sdr
• Replay recorded IQ data as RF signals with sendiq
• Simple commands tailored for 433 MHz ISM band operation
• Instructions to help ensure correct hardware setup and parameters

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🔧 How to Record IQ Samples

Run the following command to capture raw IQ data on 433.813 MHz:

rtl_sdr -f 433813000 -g 35 -s 250000 rec.iq

Explanation

• -f 433813000 sets the frequency to 433.813 MHz
• -g 35 sets the gain to 35 dB (adjust as needed)
• -s 250000 sets sample rate to 250 kS/s
• rec.iq is the output filename storing the IQ samples Press Ctrl+C to stop recording when you have enough data.

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▶️ How to Replay Recorded IQ Data

Replay your recorded IQ file with:

./sendiq -i rec.iq -s 250000 -f 433828000 -t u8

• -i rec.iq specifies the IQ input file
• -s 250000 matches the sample rate used during recording
• -f 433828000 sets the transmit frequency to 433.828 MHz
• -t u8 specifies the IQ data type (unsigned 8-bit)