FM mode now has the same three-layer architecture as PM mode:
- fm_mod/fm_demod for carrier-level FM modulation
- fm_signal_source/fm_downlink_receiver convenience wrappers
- fm_loopback_demo.py verifying round-trip SCO voltage recovery
Includes GRC YAML for all 4 blocks and doc updates across
blocks reference, SCO guide, and signal architecture pages.
- sco_mod: 9-channel FM subcarrier oscillator modulator (inverse of sco_demod),
with round-trip tests proving voltage recovery across all channels
- fm_voice_subcarrier_mod: add audio_input parameter to accept external float
streams (e.g., Apollo mission voice recordings) instead of internal test tone
- loopback_demo.py: streaming TX->RX round-trip with CLI for noise/voice/frames
- agc_loopback_demo.py: full Virtual AGC integration via TCP bridge
Implement the transmit/generate side as streaming GNU Radio blocks,
complementing the existing receive chain. Each block maps to a physical
instrument on CuriousMarc's Keysight bench:
pcm_frame_source - PCM bit stream generator (sync_block + FrameSourceEngine)
nrz_encoder - bits to NRZ waveform (+1/-1) with upsampling
bpsk_subcarrier_mod - NRZ x cos(1.024 MHz) BPSK modulator
fm_voice_subcarrier_mod - 1.25 MHz FM test tone source
pm_mod - phase modulator: exp(j * deviation * input)
usb_signal_source - convenience wrapper wiring all blocks together
Includes GRC YAML definitions for all blocks under [Apollo USB] category,
49 new tests (271 total, all passing), and a loopback test that validates
the full TX->RX round trip including frame recovery with 30 dB AWGN.
