LO — Complex Phasor Generator

LO chains a 32-bit NCO with a 2¹⁶-entry sin/cos LUT to produce CF32 IQ phasors. freq is a normalised frequency in cycles/sample (0.25 = fs/4).

Free-running tone

from doppler.source import LO
import numpy as np

lo = LO(0.25)          # quarter-rate tone
iq = lo.steps(8)
print(iq)

[ 1.+0.j  0.+1.j -1.+0.j  0.-1.j  1.+0.j  0.+1.j -1.+0.j  0.-1.j]

CF32 precision

The LUT is 16-bit, so values like 0+1j appear as -8.7e-08+1.000000e+00j in repr(). The absolute error is < 1.6 × 10⁻⁵ radians, well below −96 dBc.

FM via control port

Per-sample frequency deviation added on top of the base frequency:

ctrl = (0.002 * np.sin(2 * np.pi * 0.01 * np.arange(1024))).astype(np.float32)
lo2 = LO(0.1)
iq = lo2.steps_ctrl(ctrl)

Phase continuity

Successive calls resume exactly where the last left off:

lo3 = LO(0.25)
a = lo3.steps(4)   # samples 0–3
b = lo3.steps(4)   # samples 4–7, phase continuous

print(np.concatenate([a, b]))

[ 1.+0.j  0.+1.j -1.+0.j  0.-1.j  1.+0.j  0.+1.j -1.+0.j  0.-1.j]