RF Coverage and Limitations
The SkyWalker-1 was designed as a DVB-S satellite receiver, but the BCM4500’s signal monitoring registers work whether or not a signal is locked. This makes the hardware useful as a crude spectrum analyzer and RF power detector across any band an LNB can translate into the 950—2150 MHz IF window. Understanding what the hardware actually measures — and where the measurement breaks down — is essential to interpreting sweep results correctly.
How the Power Detector Works
Section titled “How the Power Detector Works”The BCM4500 demodulator contains an AGC (Automatic Gain Control) loop that adjusts the tuner gain to maintain a constant signal level at the ADC. The AGC register values (indirect registers 0x02—0x05) reflect how much gain the system is applying. This mechanism operates continuously, regardless of whether the demodulator achieves signal lock.
The key insight: higher AGC values mean weaker signals (the system needs more gain to reach the target level). The AGC responds to the total received power within the tuner’s passband at the tuned frequency. It does not discriminate between modulated carriers, noise floors, or interference — it measures raw RF energy.
The SIGNAL_MONITOR command (0xB7) reads both the SNR and AGC registers in a single USB transfer, avoiding the round-trip overhead of separate indirect register reads. The TUNE_MONITOR command (0xB8) combines tune + settle + read into one operation, forming the building block for spectrum sweeps. See Signal Monitoring for the register-level details.
LNB Frequency Shifting
Section titled “LNB Frequency Shifting”A Low-Noise Block downconverter (LNB) mounted at the dish feed performs frequency translation: it mixes the incoming RF signal with its internal local oscillator (LO), producing an intermediate frequency (IF) output that travels down the coaxial cable to the receiver.
actual_rf = if_frequency + lnb_loif_frequency = actual_rf - lnb_loThe SkyWalker-1 tunes within a fixed IF range: 950—2150 MHz. The actual RF band covered depends entirely on which LNB is connected and what its LO frequency is. The receiver has no knowledge of the LNB’s LO — the host software must account for the frequency offset when computing tune parameters and interpreting results.
RF Coverage Map
Section titled “RF Coverage Map”| Configuration | LNB LO (MHz) | IF Range (MHz) | Actual RF Covered (MHz) | Typical Use |
|---|---|---|---|---|
| Ku-band low | 9,750 | 950—2,150 | 10,700—11,900 | Satellite TV low band |
| Ku-band high | 10,600 | 950—2,150 | 11,550—12,750 | Satellite TV high band |
| C-band | 5,150 | 950—2,150 | 6,100—7,300 | C-band satellite |
| No LNB (direct) | 0 | 950—2,150 | 950—2,150 | L-band direct input |
| Custom (9.0 GHz) | 9,000 | 950—2,150 | 9,950—11,150 | QO-100 DATV |
L-Band Direct Input
Section titled “L-Band Direct Input”With no LNB — cable connected directly to an antenna or feed — the hardware covers the raw 950—2,150 MHz IF range. This encompasses several interesting allocations:
| Range (MHz) | Allocation | Detectable? | Demodulatable? |
|---|---|---|---|
| 1,240—1,300 | Amateur 23 cm band | Yes (energy) | No (SSB/CW/FM) |
| 1,525—1,559 | Inmarsat downlink | Yes (energy) | No (proprietary) |
| 1,559—1,610 | GNSS (GPS L1, Galileo E1) | Yes (spread spectrum) | No (CDMA/BOC) |
| 1,610—1,626 | Iridium downlink | Yes (energy) | No (TDMA/FDMA) |
| 1,670—1,710 | MetSat (GOES LRIT, NOAA HRPT) | Yes (carrier) | No (non-DVB framing) |
| 1,710—1,785 | LTE/AWS uplink | Yes (energy) | No (OFDM) |
| 1,920—2,025 | UMTS uplink | Yes (energy) | No (WCDMA) |
“Detectable” means the AGC registers respond to RF energy at that frequency — the hardware sees something. “Demodulatable” means the BCM4500 can lock onto the signal and produce a decoded transport stream. Only DVB-S, Turbo-coded, DCII, and DSS signals can be demodulated. Everything else shows up as an energy level without any ability to extract data.
The QO-100 Exception
Section titled “The QO-100 Exception”The Es’hail-2/QO-100 amateur satellite at 25.9 degrees East carries DVB-S DATV signals in the 10,491—10,499 MHz range. These are actual DVB-S QPSK signals at low symbol rates (333—2,000 ksps) — the one case where amateur satellite signals use a modulation the SkyWalker-1 can natively demodulate.
The problem is IF range. With a standard 9,750 MHz universal LNB, 10,494 MHz maps to 744 MHz IF — below the SkyWalker-1’s 950 MHz minimum. A custom LNB with a ~9.0 GHz local oscillator puts the same signal at ~1,494 MHz IF, comfortably in range.
Hardware Limitations
Section titled “Hardware Limitations”The SkyWalker-1 is a satellite receiver repurposed as a measurement tool. The results are useful but carry inherent constraints worth understanding before interpreting sweep data.
Resolution Bandwidth
Section titled “Resolution Bandwidth”The minimum symbol rate is 256 ksps, giving a minimum resolution bandwidth of approximately 346 kHz (symbol_rate * 1.35 roll-off factor). This is far coarser than a dedicated spectrum analyzer, which might offer 1 kHz or even 10 Hz RBW. Narrowband signals — SSB at 3 kHz, CW at 500 Hz, FM repeater outputs at 25 kHz — cannot be individually resolved. They appear as a single energy bump within the ~346 kHz measurement window, indistinguishable from each other or from broadband noise at similar levels.
Dynamic Range
Section titled “Dynamic Range”The AGC provides approximately 30—40 dB of usable dynamic range. A professional spectrum analyzer offers 70+ dB. The practical consequence: weak signals near strong ones get masked. A satellite transponder 25 dB above the noise floor is clearly visible; a signal 5 dB above the noise floor next to a strong adjacent carrier may not be.
Sweep Speed
Section titled “Sweep Speed”Each tune-measure step takes approximately 12 ms (tune settling + dwell + USB transfer). A full 950—2,150 MHz sweep at 5 MHz steps requires about 240 steps at 12 ms each, totaling approximately 2.9 seconds. This is adequate for mapping satellite transponders or identifying persistent carriers, but too slow for capturing transient signals or frequency-hopping transmissions.
What It Does Well
Section titled “What It Does Well”| Strength | Detail |
|---|---|
| Built-in LNB power | 13V/18V, 22 kHz tone, DiSEqC 1.0/1.2 — complete satellite receiver chain with no external hardware |
| Native DVB-S demodulation | Can lock, decode, and stream DVB-S, Turbo, DCII, and DSS signals as MPEG-2 transport streams |
| Power measurement | Detects RF energy across the full 950—2,150 MHz IF range regardless of modulation |
| Transport stream capture | GPIF streaming provides real MPEG-2 TS data for locked signals |
| Low cost | Repurposes existing satellite receiver hardware for spectrum awareness |
What It Cannot Do
Section titled “What It Cannot Do”| Limitation | Detail |
|---|---|
| Not an SDR | Cannot capture raw IQ samples or demodulate arbitrary waveforms |
| Fixed demod pipeline | Only DVB-S / Turbo / DCII / DSS modulations — no FM, SSB, CW, OFDM, ATSC |
| Coarse RBW | Minimum ~346 kHz resolution bandwidth; narrowband signals are unresolvable |
| Limited dynamic range | ~30—40 dB usable vs 70+ dB for dedicated spectrum analyzers |
| No DVB-S2 | Incompatible FEC (LDPC vs Reed-Solomon) — see DVB-S2 Incompatibility |
See Also
Section titled “See Also”- Tuning Tool — the primary user-facing tool for tuning, monitoring, and capture
- RF Specifications — electrical parameters, signal path, and LNB current limits
- Signal Monitoring — AGC and SNR register details used by the power detector
- LNB Control — voltage, tone, and DiSEqC command interface