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Official Documentation

Block Upconverter

The BUC (Block Upconverter) is a critical component in the transmit chain that converts low-frequency IF (Intermediate Frequency) signals to high-frequency RF (Radio Frequency) signals suitable for satellite transmission. Think of it as the transmit counterpart to the LNB: while the LNB downconverts received RF to IF, the BUC upconverts transmitted IF to RF. The BUC sits between your modems (which generate IF signals) and the HPA (High Power Amplifier), performing frequency translation and providing gain to bring signals up to the power level needed for final amplification.

Key controls

  • POWER - enables or disables the BUC. When powered, the BUC upconverts and amplifies signals. When off, no signal processing occurs. The BUC is a prerequisite for the HPA - the HPA cannot operate without the BUC being powered first.
  • MUTE - a safety feature that stops RF output without powering down the entire BUC. When muted, the BUC remains powered and locked but produces no output signal. Use this to temporarily silence transmission without losing lock or thermal stability.
  • LO (MHz) - sets the Local Oscillator frequency (typically 3700–4200 MHz for C-band). This determines the frequency translation: RF_out = IF_in + LO. For example, if your modem generates a 70 MHz IF signal and LO is 4200 MHz, the BUC outputs 4270 MHz RF.
  • GAIN (dB) - controls amplification (0–70 dB). Higher gain means stronger output signals. Typical BUC gain is 55–65 dB. Be careful not to overdrive into saturation (P1dB), which causes compression and distortion.
  • LOOPBACK TO LNB - a test mode that routes the BUC’s output back to the LNB input instead of to the antenna. This creates a local RF loop for testing without transmitting to the satellite. Useful for validating the signal path without radiating.

Readouts you will see

  • LO (MHz) - displays the current Local Oscillator frequency. This directly determines what RF frequencies you’ll transmit.
  • TEMP (°C) - operating temperature. The BUC warms up during operation due to power dissipation. Starts at ambient (~25°C) and rises based on output power and gain. Alarm threshold is 70°C.
  • CURRENT (A) - current draw from the power supply. Increases with gain and output power. Typical range is 0.5–4.5 A. High current draw (>4.5 A) may indicate a fault or over-temperature condition.
  • FREQ ERR (kHz) - frequency error of the Local Oscillator in kHz. When locked to the external 10 MHz reference (from GPSDO), this should be near zero (displayed in green). When unlocked, it shows drift (displayed in red), typically 10–100 ppm of the LO frequency.
  • OUT PWR (dBm) - the BUC’s RF output power in dBm. This is calculated from input power plus gain, with compression applied when approaching saturation (P1dB). Displayed in green normally, yellow when approaching saturation (within 2 dB of P1dB).

LED indicators

  • LOCK - shows whether the BUC’s Local Oscillator is locked to the external 10 MHz reference from the GPSDO.
    • Green - locked to reference. Frequency is stable and accurate.
    • Amber - acquiring lock or degraded reference.
    • Red - not locked. BUC is free-running with frequency drift.
  • LOOPBACK - indicates loopback mode status.
    • Blue - loopback enabled. Output routed to LNB, not antenna.
    • Off - normal mode. Output goes to antenna via HPA.

How frequency translation works

The BUC performs upconversion by mixing (multiplying) the IF input signal with the Local Oscillator. The mathematical result is sum and difference frequencies: RF = IF + LO and RF = IF - LO. The BUC uses filtering to select the sum product (IF + LO) and reject the difference product.

Example: Your modem transmits at 70 MHz IF. You set the BUC LO to 4200 MHz. The mixer produces:

  • Sum: 70 MHz + 4200 MHz = 4270 MHz (desired RF output)
  • Difference: 4200 MHz - 70 MHz = 4130 MHz (filtered out)

The BUC’s output filter passes 4270 MHz and blocks 4130 MHz. This is how you can transmit multiple carriers at different IF frequencies and have them appear at the correct RF frequencies for the satellite.

Understanding lock and frequency stability

The BUC’s Local Oscillator must be extremely stable - even small frequency errors translate directly to the RF output and can cause signals to miss the satellite transponder’s bandwidth or drift off the receiver’s tuning. There are two operating modes:

  • Locked mode (LOCK LED green): The LO is phase-locked to the 10 MHz reference from the GPSDO. The GPSDO itself is locked to GPS satellites, providing accuracy better than 1 part per billion. Frequency error is essentially zero. This is required for professional satellite communications.
  • Unlocked mode (LOCK LED red): The LO is free-running with only its internal crystal oscillator for stability. Drift is 10–100 ppm (parts per million), meaning at 4200 MHz, frequency error could be ±42–420 kHz. This drift is too large for most satellite operations and will cause signal degradation or loss.

Always verify the LOCK LED is green before transmission. If unlocked, check that the GPSDO is powered, warmed up (typically 10–30 minutes), and locked to GPS satellites.

Saturation and compression (P1dB)

The BUC has a maximum linear output power called P1dB (the 1 dB compression point), which in this module is 15 dBm. Below P1dB, the BUC behaves linearly: increase input by X dB, output increases by X dB. At and above P1dB, the amplifier compresses - gain drops and distortion increases.

Example: BUC gain = 60 dB, P1dB = 15 dBm, IF input = -50 dBm.

  • Linear output would be: -50 dBm + 60 dB = 10 dBm (below P1dB, no compression)
  • If input increases to -45 dBm: linear output = 15 dBm (at P1dB, minimal compression)
  • If input increases to -40 dBm: linear output = 20 dBm, but BUC compresses to ~18.5 dBm

The OUT PWR readout turns yellow when within 2 dB of P1dB to warn you of approaching saturation. Operating in compression creates unwanted distortion (intermodulation products) and wastes power. If you see yellow, reduce gain or input power.

Phase noise and signal quality

Phase noise is random fluctuations in the phase of the Local Oscillator. These fluctuations are translated to the RF output and appear as noise sidebands around your carrier, degrading signal quality and potentially interfering with adjacent channels. Phase noise is measured in dBc/Hz (decibels relative to carrier per Hz of bandwidth) at a specific frequency offset from the carrier.

  • Locked (good): -100 to -105 dBc/Hz @ 10 kHz offset. The external reference provides excellent stability.
  • Unlocked (poor): -70 to -80 dBc/Hz @ 10 kHz offset. Free-running oscillator has much worse phase noise, degrading modulation quality and effective SNR.

High-order modulation schemes (16-APSK, 32-APSK, etc.) are very sensitive to phase noise. If your link quality degrades despite good power levels, check that the BUC is locked and phase noise is nominal.

Group delay and bandwidth

Group delay is the time it takes for different frequency components of your signal to pass through the BUC. Ideally, all frequencies experience the same delay (flat group delay). In reality, there’s variation (typically 2–10 nanoseconds across the bandwidth), which causes phase distortion - different parts of your signal arrive at slightly different times.

Group delay increases with temperature and at the edges of the BUC’s bandwidth. For wideband signals, excessive group delay variation degrades modulation quality. If you see group delay >10 ns with high temperature, consider improving cooling.

Spurious outputs

Mixing is inherently nonlinear and produces unwanted outputs at harmonic frequencies: N×LO ± M×IF. The primary desired output is 1×LO + 1×IF, but you also get:

  • 2×LO - IF (second harmonic mixing): typically -30 to -40 dBc
  • 2×LO + IF (second harmonic mixing): typically -35 to -45 dBc
  • 3×LO - IF (third harmonic): typically -40 to -55 dBc

These spurious outputs are unwanted signals at frequencies you didn’t intend to transmit. The BUC’s output filter suppresses them, but some residual spurious energy may remain. In a well-designed BUC, spurious outputs are <-30 dBc and meet regulatory limits. If spurious outputs are excessive, they can cause interference to other services or violate licensing terms.

Loopback mode

The LOOPBACK TO LNB switch routes the BUC’s RF output back to the LNB input, creating a local test loop. This allows you to:

  • Test the entire signal path (modem → BUC → LNB → modem) without radiating
  • Verify frequency translation, gain, and signal quality
  • Calibrate or align equipment in the lab before deployment
  • Troubleshoot issues without interfering with live satellite operations

When loopback is enabled, the LOOPBACK LED turns blue. Your transmitted signals will appear on the receive side if the LNB and receiver are configured correctly. This is a powerful diagnostic tool but should not be used during live transmission - it bypasses the HPA and antenna entirely.

Alarms and warning conditions

The system will raise alarms for several conditions:

  • BUC not locked to reference - The LO cannot lock to the 10 MHz external reference. Check that the GPSDO is powered, warmed up, and GPS-locked. Frequency stability will be poor (10–100 ppm drift).
  • BUC frequency error: X kHz - When unlocked, frequency drift exceeds 50 kHz. Transmitted signals may be off-frequency and miss the satellite transponder. Lock to external reference immediately.
  • BUC approaching saturation (X dBm) - Output power is within 2 dB of P1dB (15 dBm). Operating in compression creates distortion. Reduce gain or input power to bring output below 13 dBm for linear operation.
  • BUC over-temperature (X °C) - Temperature exceeds 70°C. This can occur at high gain/power levels or in hot ambient conditions. Reduce power, improve cooling, or allow cool-down time. Prolonged over-temperature operation may damage components.
  • BUC high current draw (X A) - Current exceeds 4.5 A, indicating possible fault, saturation, or thermal runaway. Check output power, temperature, and gain settings.
  • BUC phase noise degraded (unlocked) - Phase noise is worse than -85 dBc/Hz due to unlocked operation. This degrades modulation quality, especially for high-order modulations. Lock to external reference.

Simple troubleshooting steps

  1. No output signal - Check power switch is on. Verify MUTE is off (switch down). Check that modem is transmitting IF signals. Verify gain is not zero.
  2. LOCK LED is red - The BUC cannot lock to external reference. Verify GPSDO is powered and GPS-locked. Check reference cable connection. Wait for GPSDO warm-up (typically 10–30 minutes from cold start).
  3. Frequency error is large (>50 kHz) - BUC is unlocked. Follow steps above to achieve lock. Do not transmit with large frequency errors - signals may be off-frequency and unusable.
  4. Output power too low - Increase gain. Check that modem is transmitting at proper IF level (typically -10 to 0 dBm). Verify MUTE is off.
  5. Output power display is yellow (approaching saturation) - Reduce gain by 3–6 dB to operate in linear region. Saturation causes distortion and wastes power.
  6. High temperature or current draw - Reduce gain or output power. Improve cooling or reduce ambient temperature. Allow BUC to cool before resuming high-power operation.
  7. Poor modulation quality despite good power - Check LOCK status (should be green). Verify phase noise is nominal (<-95 dBc/Hz when locked). Check for saturation (output power should be <13 dBm). Verify group delay is <10 ns.
  8. Testing signal path without transmitting - Enable LOOPBACK TO LNB switch. The LOOPBACK LED turns blue. Configure LNB and receiver to receive the looped-back signal. This validates the chain without radiating.

Short examples

  • Example A - Normal operation: Power ON, Mute OFF, LO = 4200 MHz, Gain = 60 dB, Loopback OFF, LOCK LED = Green, LOOPBACK LED = Off, Temp = 45°C, Current = 2.5 A, Freq Err = 0.0 kHz (green), Out Pwr = 10 dBm (green). BUC is locked, operating in linear region, all parameters nominal. Ready for transmission.
  • Example B - Unlocked (degraded): Power ON, Mute OFF, LO = 4200 MHz, Gain = 60 dB, LOCK LED = Red, Freq Err = 84.3 kHz (red), Phase Noise = -78 dBc/Hz. GPSDO is not providing reference or not warmed up. Frequency drift is ~20 ppm (84 kHz / 4200 MHz). Signal quality will be poor. Check GPSDO status and wait for lock before transmitting.
  • Example C - Approaching saturation: Power ON, Mute OFF, Gain = 65 dB, Out Pwr = 13.5 dBm (yellow), LOCK LED = Green. Output is within 1.5 dB of P1dB (15 dBm). Operating near compression - reduce gain to 60–62 dB to maintain linearity and avoid distortion.
  • Example D - Muted (safe): Power ON, Mute ON, LOCK LED = Green, Out Pwr = -∞ dBm, Temp = 30°C, Current = 0.8 A. BUC is powered and locked but producing no output. Useful for maintaining lock and thermal stability during transmission pauses without powering down.
  • Example E - Loopback test mode: Power ON, Mute OFF, Loopback ON, LOOPBACK LED = Blue, LOCK LED = Green, Out Pwr = 10 dBm. BUC output is routed to LNB input for local testing. No RF transmitted to antenna/HPA. Check that receive side sees the looped-back signal at expected frequency (IF_modem + LO_BUC).
  • Example F - Over-temperature alarm: Power ON, Gain = 70 dB, Out Pwr = 14 dBm, Temp = 75°C (alarm), Current = 4.8 A (alarm). Operating at maximum gain near saturation. High power dissipation caused over-temperature. Immediately reduce gain to 55–60 dB and allow cooling. Check ambient temperature and airflow.

Advanced: Understanding mixer theory

The BUC’s core function is frequency translation, performed by a mixer. Mathematically, mixing is multiplication in the time domain, which corresponds to convolution (sum and difference) in the frequency domain. An ideal mixer with inputs at f_IF and f_LO produces outputs at f_IF + f_LO and |f_IF - f_LO|.

Real mixers are nonlinear and produce additional products at N×f_LO ± M×f_IF for various integer values of N and M. These are spurious outputs. The BUC uses filtering to select only the desired f_IF + f_LO product and suppress all others by 30+ dB.

The mixer’s conversion loss (typically 5–8 dB) is compensated by the BUC’s built-in amplification. Overall, the BUC provides net gain (typically 55–65 dB) from IF input to RF output.

Advanced: Phase-locked loop (PLL) operation

The BUC’s Local Oscillator uses a Phase-Locked Loop (PLL) to achieve lock to the external 10 MHz reference. The PLL compares the phase of the LO (divided down by N) with the reference, and generates an error signal that adjusts the LO frequency until phase alignment is achieved.

Lock acquisition takes 2–5 seconds as the PLL’s feedback loop converges. During this time, the LOCK LED shows amber. Once locked (LOCK LED green), frequency error drops to near zero and the LO tracks the reference within the tracking range (typically ±10 kHz). If frequency disturbances exceed the tracking range, the PLL loses lock and must re-acquire.

The external reference must be clean, stable, and within the PLL’s capture range. A noisy or drifting reference can prevent lock or cause frequent lock/unlock cycling.

Final notes

The BUC is the “translator” in your transmit chain, converting low-frequency IF from your modems to high-frequency RF for satellite uplink. Its performance directly affects signal quality, frequency accuracy, and transmission reliability.

Best practices include: always verify lock before transmission (LOCK LED green, Freq Err near zero), operate in the linear region (output power at least 2–3 dB below P1dB), monitor temperature and current draw, use MUTE instead of power cycling to pause transmission, and use loopback mode for testing without radiating. Remember that the BUC must be powered before the HPA can operate - this is a safety interlock to ensure proper signal flow.

Frequency stability is paramount in satellite communications. Even small errors (a few kHz) can cause signals to fall outside the transponder bandwidth or miss the receiver’s tuning. The external reference from the GPSDO is essential - do not attempt to operate unlocked for production traffic. During initial setup or troubleshooting, unlocked operation can help verify signal flow, but always achieve lock before live transmission.