Direct Modulation and Subcarriers
- Direct BPSK: the data spectrum is centered on the RF carrier
- BPSK on a subcarrier (here 70 kHz): the data first modulates a subcarrier, so the data sidebands move to ±70 kHz around the RF carrier
- Calculated average spectra: uncoded 10 kbit/s NRZ, no pulse shaping; PSD relative to unit complex-envelope power per Hz; no receiver noise; display stops at −80 dB/Hz
Residual Carrier With a BPSK Subcarrier
- Illustrative RF spectrum: 32 kbit/s BPSK on a 1.024 MHz subcarrier
- Lower sideband at −1.024 MHz, residual carrier at 0, upper sideband at +1.024 MHz
- Unshaped NRZ data: each sideband has a 64 kHz first-null span ( kHz)
- The residual carrier is power left in the carrier itself — useful for acquisition and tracking (modulation trade), but it doesn’t carry data
Data Power Is Part of Total Signal Power
- Illustrative allocation of total received RF power: ranging 20%, residual carrier 30%, data 50%
Data Power Fraction
q = 0.5
- : the data-channel S/N₀ is 3.0 dB lower than the total S/N₀
- Assumed power fractions sum to 100%; modulation indices and waveform design determine the actual split (the diagram shows allocation, not signal flow)
- appears explicitly in the Eb/N₀ link equation and feeds energy per bit
Subcarrier Examples and Ranging Tones
- Direct: 100 kbit/s telemetry modulates the RF carrier without an added subcarrier
- Subcarrier: 100 kbit/s data on a 1.024 MHz subcarrier — account for both RF sidebands
- Combined: 20 kbit/s telemetry on a 70 kHz subcarrier, plus ranging tones at 4, 20, 100, and 500 kHz
- These are illustrative plans: modulation indices set the power split; receiver compatibility sets usable combinations
- PN ranging uses a known pseudo-random pattern and correlation; it need not use a telemetry subcarrier
- Reference: JPL DSN 810-005, Module 206 (Telemetry General Information)