Build Receiver

Build Receiver

Turn a target signal into a working receiver. The method below is general; FM voice and an unmodulated carrier are shown as two worked examples so the shape transfers to modulations the demo doesn't cover.

General method (any modulation)

1. Know the target. Get its center frequency and occupied bandwidth from survey-spectrum / measure. Compute the offset from the capture's center: center_offset = target_freq − capture_center_freq.
2. Discover the blocks. Call list_blocks. Compose only from these types; check each block's input/output dtype ('c' complex, 'f' float) so connections match.
3. Channelize. Use freq_xlating_lowpass to shift the target to baseband and decimate. Choose decimation so the post-decimation rate is ~2–3× the signal bandwidth; set cutoff ≈ signal_bandwidth and transition ≈ cutoff/2.
4. Demodulate to match the modulation (see examples below).
5. Validate. validate_pipeline(pipeline) and fix every issue (each names the block and field) before running.
6. Run. run_pipeline(pipeline); check status == "ok".
7. Verify before you claim success (see Verify). Never report a working receiver without evidence.
8. Persist. save_pipeline(pipeline) and export_grc(pipeline) so the user keeps a reusable receiver they can open in GNU Radio Companion.

Worked example A — NBFM voice

Channelize so the post-decimation rate equals the FM quad rate (a multiple of the audio rate), then demodulate to a WAV:

• freq_xlating_lowpass (decimation chosen so e.g. 2 MHz / 20 = 100 kHz = quad_rate; cutoff ≈ 8 kHz, transition ≈ 4 kHz)
• nbfm_rx with audio_rate=25000, quad_rate=100000 (quad_rate must be an integer multiple of audio_rate)
• wav_sink (sample_rate=25000)

Connections: src → chan → demod → audio.

Worked example B — unmodulated carrier / CW (non-FM)

There is no demodulator here — receiving a carrier means channelizing it to baseband and confirming it:

• freq_xlating_lowpass (narrow cutoff, e.g. a few kHz) → file_sink (give the sink a path ending in .cf32).
• run_pipeline, then bridge the raw output into an analyzable capture: take the sink path from the run's artifacts[0] and call load_capture(path, sample_rate=<post-decimation rate>, center_freq=<target_freq>) (raw .cf32 needs the sample rate supplied).
• Then measure(capture_path, center_freq=<target_freq>) (or render a spectrogram) to confirm the carrier sits where expected with the predicted SNR.

This proves the channelizer/offset logic independently of any demodulator — the same skeleton as example A with the demod stage removed. The run → load_capture → analyze bridge is general: that's how you inspect any pipeline's file output.

Other modulations

AM, SSB, and digital (OOK/PPM/FSK) demodulators are not in the v1.0 vocabulary. If the target needs one, report the gap and switch to the escape-hatch skill — do not force an FM demodulator onto a non-FM signal.

Verify (mandatory)

• FM voice: read the recovered WAV and confirm energy in the audio band (e.g. a 1 kHz modulating tone shows up at 1 kHz).
• CW/other: measure or spectrogram the channelized output and confirm the carrier/structure is where you expect.
• If verification fails → switch to the debug-no-signal skill. Do not retry blindly.