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. The NATS teleport standard is 23 dB, which turns a -7 dBm modem IF into 16 dBm of RF. Be careful not to drive the output up to the 1 dB compression point (P1dB), where compression and distortion set in.
• 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) - case temperature. The BUC heats with the power it dissipates (DC power in minus RF power out): it settles at 25 °C + 0.30 °C per watt dissipated, approaching that value with a 10-minute time constant, heating and cooling alike. At the NATS operating point it runs about 47 °C. Alarm threshold is 70 °C.
• CURRENT (A) - current draw from the 24 V supply. About 2.6 A of it is idle draw (synthesiser, mixers, IF stages, fan); the output stage adds up to 2.4 A more as the output rises toward saturation. The NATS operating point draws about 3.1 A; driven into saturation, about 5.0 A. High current draw (>4.5 A) means the BUC is overdriven or its output stage is failing.
• FREQ ERR (kHz) - frequency error of the Local Oscillator in kHz. When locked to the external 10 MHz reference (from GPSDO), the LO carries only the reference’s own error, effectively zero (displayed in green). When unlocked, the BUC runs on its internal crystal and shows its offset (displayed in red): up to about ±10 ppm, which is 70 kHz at 7 GHz.
• OUT PWR (dBm) - the BUC’s RF output power in dBm: input power plus gain, with the soft compression of the amplifier applied as it nears saturation. Displayed in green normally, yellow when approaching saturation (within 2 dB of P1dB).
• P1dB Margin (dB) (TX Chain tab) - how far the output sits below P1dB (P1dB minus output). At the NATS operating point (16 dBm out, P1dB 28 dBm) it reads 12 dB.

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, so it carries the reference’s own error and nothing more. When the GPSDO is locked to GPS that error is a few parts in 10^12, essentially zero. In holdover it is about 5 parts in 10^10 (3.5 Hz at 7 GHz), and during the GPSDO’s warm-up it is much larger. A GNSS spoof that drags the GPSDO’s time also drags every LO locked to it. Locked operation is required for professional satellite communications.
• Unlocked mode (LOCK LED red): With no usable reference (none present, or the GPSDO not warmed up) the BUC runs on its internal crystal oscillator (TCXO). It starts up to about ±10 ppm off and wanders slowly from there (a couple of ppm per square-root hour, never beyond ±30 ppm). At 7000 MHz, 10 ppm is 70 kHz. That is too far off 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’s output amplifier compresses gradually (a soft “Rapp” curve). P1dB, the 1 dB compression point, is set per station; the NATS teleport BUCs have a P1dB of 28 dBm and saturate (Psat) at about 30 dBm, 2.16 dB higher. Well below P1dB the BUC behaves linearly: increase input by X dB, output increases by X dB. Approaching P1dB, the gain starts to drop and distortion increases; at Psat the output cannot rise any further.

Example: NATS BUC, P1dB = 28 dBm, IF input = -7 dBm.

• Gain 23 dB: output 16 dBm, 12 dB below P1dB (linear, P1dB margin 12 dB, about 3.1 A)
• Gain 33 dB: linear output would be 26 dBm, the BUC delivers 25.7 dBm (0.3 dB of compression, margin 2.3 dB, about 4.0 A)
• Gain 50 dB: linear output would be 43 dBm, but the BUC is saturated at about 30 dBm (over 10 dB of compression, about 5.0 A, saturation and high-current alarms)

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): -95 dBc/Hz out to 10 kHz offset. The synthesiser is clean on the external reference.
• Unlocked (poor): -75 dBc/Hz out to 10 kHz offset on the internal reference, degrading modulation quality and effective SNR.

The phase noise rides on your carrier all the way to the far-end demodulator, where it costs a small amount of Es/N0 (more when the BUC is unlocked, and far more from a big phase-noise plateau like a free-running LNB’s). High-order modulation schemes (16-APSK, 32-APSK, etc.) are the most sensitive to it. 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). Any variation causes phase distortion - different parts of your signal arrive at slightly different times.

In SignalRange the BUC’s group delay is 3 ns at 25 °C and rises 0.1 ns for every °C above that (about 5 ns at the NATS operating temperature of 47 °C, 7.5 ns at the 70 °C alarm). For wideband signals, excessive group delay degrades modulation quality, so a hot BUC is worth 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): -35 dBc
• 2×LO + IF (second harmonic mixing): -40 dBc
• 3×LO - IF (third harmonic): -47 dBc

SignalRange lists these spurs at these fixed levels for every carrier.

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. The BUC runs on its internal crystal (up to about ±10 ppm off, wandering slowly).
• BUC frequency error: X kHz - When unlocked, the frequency error 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 (26 dBm or more on a NATS BUC with its 28 dBm P1dB). Operating in compression creates distortion. Reduce gain or input power until the P1dB margin is comfortably above 2 dB.
• BUC over-temperature (X °C) - Temperature exceeds 70°C. This can occur when the BUC is driven hard, when its fan or heatsink is degraded, or when a failing output stage draws extra current. Reduce gain (or mute the BUC), and allow cool-down time: the case cools with the same 10-minute time constant it heats with. Prolonged over-temperature operation may damage components.
• BUC cooling fault - fan/heatsink degraded - the fan or heatsink is not shedding heat, so the BUC runs hotter at any output. Run it at lower gain until it is repaired.
• BUC high current draw (X A) - Current exceeds 4.5 A: the output is driven into saturation, or a failing output stage is drawing extra current. Reduce gain; muting the BUC removes the output stage’s bias and brings the current back to its 2.6 A idle draw.
• BUC phase noise degraded (unlocked) - Phase noise is -75 dBc/Hz instead of -95 dBc/Hz because the BUC is running on its internal reference. 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 until the P1dB margin is well above 2 dB (the NATS standard of 23 dB gives 12 dB of margin). Saturation causes distortion and wastes power.
6. High temperature or current draw - Reduce gain or output power. If the current stays high at normal gain, suspect the output stage; mute the BUC to drop it to idle. Allow the BUC to cool before resuming high-power operation: the temperature falls over minutes, not seconds.
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 (the P1dB margin should be more than 2 dB). Check the temperature, since group delay grows as the BUC heats.
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 = 7000 MHz, Gain = 23 dB, IF input -7 dBm, Loopback OFF, LOCK LED = Green, LOOPBACK LED = Off, Temp = 47°C, Current = 3.1 A, Freq Err = 0.0 kHz (green), Out Pwr = 16 dBm (green), P1dB Margin = 12 dB. BUC is locked, operating in linear region, all parameters nominal. Ready for transmission.
• Example B - Unlocked (degraded): Power ON, Mute OFF, LO = 7000 MHz, Gain = 23 dB, LOCK LED = Red, Freq Err = 58.8 kHz (red), Phase Noise = -75 dBc/Hz. GPSDO is not providing reference or not warmed up, so the BUC is on its internal crystal, about 8.4 ppm off (58.8 kHz / 7000 MHz). Signal quality will be poor. Check GPSDO status and wait for lock before transmitting.
• Example C - Close to saturation: Power ON, Mute OFF, IF input -7 dBm, Gain = 33 dB, Out Pwr = 25.7 dBm (still green), P1dB Margin = 2.3 dB, Current = 4.0 A, LOCK LED = Green. Ten dB of gain too many: the output is already compressing slightly (0.3 dB) and a little more drive would turn the readout yellow. Return the gain to 23 dB to restore the 12 dB margin and avoid distortion.
• Example D - Muted (safe): Power ON, Mute ON, LOCK LED = Green, Out Pwr = -∞ dBm, Current = 2.6 A. BUC is powered and locked but producing no output; only the idle draw remains, and the case slowly cools toward about 44°C. 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 = 16 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 = 23 dB, Out Pwr = 16 dBm, Temp = 72°C (alarm), Current = 5.4 A (alarm), cooling fault alarm. At normal gain the BUC should draw 3.1 A, so the extra 2.3 A points to a failing output stage, and a degraded fan keeps the heat in. Mute the BUC: the current drops to 2.6 A at once and the temperature passes back under 70°C about a minute later. Report the fault before unmuting.

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 the net gain you set on the GAIN control (23 dB at the NATS teleport) 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), the LO is an exact multiple of the reference, so it inherits the reference’s fractional error: whatever the GPSDO is off by, in parts per billion, the LO is off by the same parts per billion. 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, and a reference that is locked but wrong (a GPSDO in holdover, or one being steered by a GNSS spoof) moves every LO that follows it.

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.