High Power Amplifier

The HPA (High Power Amplifier) is the final amplification stage before signals are transmitted to the satellite. It takes the BUC’s output (a few milliwatts to a watt) and boosts it to the power the uplink needs: a few watts for a small Ku-band SSPA, hundreds of watts to a kilowatt for a C-band teleport TWTA. The HPA sits after the BUC (Block Upconverter) in the transmit chain and drives the feed through the OMT.

Key controls

• POWER - enables or disables the HPA. The HPA can only be powered on if the BUC is already powered (power sequencing requirement).
• HPA ENABLE - enables or disables the RF output. Even when powered, the HPA won’t transmit until this switch is on.
• ALC - Automatic Level Control. With ALC on, the amplifier adjusts its own gain so the output stays at the back-off setpoint whatever the drive from the BUC. With ALC off the gain is fixed, and the output follows the drive dB for dB.
• OUTPUT BACK-OFF FROM P1dB (dB) - how far below the amplifier’s 1 dB compression point (P1dB) you want to run (0-30 dB), applied with Apply Changes. With ALC on: output = P1dB - back-off. With ALC off: the back-off sets the gain so that the amplifier’s rated drive comes out at P1dB - back-off.

Readouts you will see

• Input - the composite drive from the BUC, dBm.
• Output - the composite RF output, dBm, with a power meter and the output in watts.
• P1dB - the amplifier’s 1 dB compression point, dBm.
• Back-off - the actual output back-off: P1dB minus the output. With ALC on it equals your setpoint; with ALC off it moves with the drive.
• Gain - output minus input. The readouts always add up: input + gain = output.
• IM3 (2-tone) - the third-order intermodulation level at the present drive, dBc: how far below each carrier the products of two equal carriers would be. More negative is cleaner.
• Temp - case temperature. It follows the dissipated power (DC input minus RF output) with a lag of a couple of minutes, heating and cooling alike.

What is P1dB and why does it matter?

P1dB is the output power at which the amplifier’s gain has dropped 1 dB below its small-signal value. Below P1dB the amplifier is nearly linear; above it, the gain compresses quickly toward saturation (Psat). A traveling-wave tube amplifier (TWTA) saturates about 4 dB above its P1dB; a solid-state amplifier (SSPA) about 2 dB above.

Compression is nonlinear, and a nonlinear amplifier creates new signals: intermodulation products between carriers, and spectral regrowth (shoulders) beside a single wideband carrier. That is why HPAs run backed off from P1dB.

Understanding back-off

For a 1 kW TWTA (P1dB 60 dBm) in ALC:

Back-off	Output	IM3 (2-tone)
10 dB	50 dBm (100 W)	about -43 dBc
6 dB	54 dBm (250 W)	about -35 dBc
3 dB	57 dBm (500 W)	about -29 dBc
1 dB	59 dBm (800 W)	about -25 dBc

Each dB of back-off you give up buys 1 dB of output and costs about 2 dB of IM3: the third-order products rise three times as fast as the carriers. Below 3 dB of output back-off the amplifier raises the overdrive alarm.

Seeing the intermodulation

Tap the coupler at TX RF Post-HPA (or Post-OMT) and look at the spectrum analyzer. With one carrier you will see low shoulders either side of it (spectral regrowth); with two or more carriers, IM3 products at 2f1 - f2 and 2f2 - f1. Lower the back-off and watch them rise.

Note

In SignalRange the intermodulation products are drawn on the TX analyzer; they are not yet radiated into the satellite’s transponder.

What the satellite does with your power

Your EIRP crosses about 200 dB of free-space loss to a GEO satellite and arrives as a flux density. The transponder compares it with its saturation flux density (SFD): the difference is its input back-off. A Campaign 1 station at its normal 100 W runs a TIDEMARK transponder about 3 dB short of saturation, which leaves it about half a dB under its maximum output.

• More power near saturation buys almost nothing. Six dB more HPA output moves the downlink by a fraction of a dB, and past saturation the tube folds back: the downlink falls again.
• Less power (or a mispointed dish) costs downlink in fixed-gain transponders, roughly dB for dB once you are well backed off, and it always costs uplink C/N. A transponder in ALC holds its output while the uplink fade stays inside its range; only the uplink C/N suffers.
• The C/N you measure on the downlink includes the uplink: (C/N)⁻¹ = (C/N_up)⁻¹ + (C/N_down)⁻¹. With two or more carriers in one transponder, its own intermodulation adds a third term.

ALC on or off?

• ALC on: change the BUC gain and the HPA output does not move - its gain changes instead. This keeps the uplink steady, and it hides a BUC that is driving harder than it needs to. If the drive falls too low for the setpoint, the ALC runs out of range and raises an alarm.
• ALC off (fixed gain): the back-off is calibrated on the rated drive. If the BUC delivers less than that (a gain left low after maintenance, say), the HPA delivers that much less, with every indicator green. Read the chain from the modem outward before trusting the number at the end of it.

Thermal management

The HPA turns DC power into RF and heat. Its DC draw rises with output (a class-AB stage’s current follows the output amplitude), so the case runs warmer at low back-off. A 1 kW TWTA runs about 44 °C at 10 dB back-off and about 60 °C at 2 dB. The over-temperature alarm is at 85 °C. Disabling the output does not drop the temperature instantly: the heatsink cools over minutes.

Power sequencing

The HPA can only be powered if the BUC is powered. An HPA enabled while the BUC is muted or off amplifies nothing but noise and fails the mission: disable the HPA before muting the BUC, and enable it only after the BUC is unmuted.

Alarms and warning conditions

• HPA overdrive - IMD degradation - output back-off under 3 dB. Increase the back-off.
• HPA ALC at maximum gain - the drive is too low for the setpoint; check the BUC and the modem.
• HPA ALC at minimum gain - the drive is too high for the ALC to bring down.
• HPA over-temperature - case above 85 °C. Reduce output, check cooling.
• HPA enabled without BUC power - power sequencing violation.

Simple troubleshooting steps

1. No RF output - check Power and HPA Enable, and that the BUC is powered, unmuted and carrying the modem’s carrier (Input should read a level, not “–”).
2. Overdrive alarm - output back-off is under 3 dB. Raise the back-off.
3. Output lower than the setpoint - with ALC on, look for the ALC alarm (weak drive). With ALC off, compare Input with the rated drive.
4. More output needed - lower the back-off, and watch IM3 and the temperature as you do.