The modem generates a modulated L-band IF signal (950-1450 MHz) which travels via coax to the BUC mounted at the antenna feed. The BUC mixes the IF with a local oscillator (e.g., 13.05 GHz for Ku-band) to produce the RF uplink frequency (14.0-14.5 GHz). An integrated SSPA or GaN PA amplifies the signal to the required transmit power (1-25 W typical for VSAT).

VSAT terminals: 1-4 W for small dishes (0.75-1.2 m). Medium terminals: 8-16 W for 1.8-2.4 m dishes. Large earth stations: 25-100 W, often using separate HPAs instead of integrated BUCs. Power is rated at P1dB (saturated output is 1-2 dB higher). GaN technology has improved efficiency to 25-35% from 15-20% for older GaAs.

A BUC integrates upconversion and power amplification in one unit, typically for VSAT/small terminals (1-25 W). An HPA (High Power Amplifier) is a separate unit providing higher power (50 W to several kW), used with standalone upconverters in large earth stations. BUCs are more compact and cost-effective for small installations.

Active Components

Block Upconverter

A Block Upconverter (BUC) converts an L-band IF signal (950-1450 MHz) from a satellite modem to the RF uplink frequency (Ku-band 14.0-14.5 GHz or Ka-band 29.5-30.0 GHz) with integrated power amplification. Mounted at the antenna feed, the BUC minimizes cable losses between the modem and antenna. Output power ranges from 1 W for small VSAT terminals to 25 W for medium earth stations. Modern BUCs use GaN SSPA technology for 25-35% DC-to-RF efficiency.

Category: Active Components

Power: 1-25 W

Understanding Block Upconverters

The BUC is the transmit counterpart to the LNB (receive). Together, they form the outdoor unit (ODU) of a VSAT terminal. The modem generates a modulated L-band signal that travels via coaxial cable to the BUC. The BUC's local oscillator and mixer upconvert the entire L-band block to the satellite uplink frequency, then the integrated PA amplifies it to the required EIRP.

Phase noise of the BUC's LO is critical for wideband modulations (16APSK, 32APSK in DVB-S2X). Modern BUCs achieve −85 dBc/Hz at 100 Hz offset, sufficient for 32APSK operation.

Upconversion

f_RF = f_IF + f_LO

Ku-band: 1000 MHz + 13.05 GHz = 14.05 GHz
Ka-band: 1000 MHz + 28.55 GHz = 29.55 GHz

EIRP = P_BUC + G_antenna − L_feed

BUC Specifications by Class

Class	Power	Band	Dish	Technology
Micro VSAT	1-2 W	Ku	0.75-1.0 m	GaAs SSPA
Standard VSAT	4-8 W	Ku/Ka	1.2-1.8 m	GaN SSPA
Medium	16-25 W	Ku/Ka	2.4-3.8 m	GaN SSPA
Large (HPA)	50-400 W	Ku/C	5-13 m	TWTA/SSPA

Common Questions

Frequently Asked Questions

How does a BUC work?

L-band IF from modem → LO mixer upconverts to Ku/Ka → SSPA amplifies to 1-25 W. Mounted at antenna feed to minimize cable loss.

Power ratings?

VSAT: 1-4 W (small dish). Medium: 8-16 W (1.8-2.4 m). Large: 25-100 W (separate HPA). GaN: 25-35% efficiency.

BUC vs HPA?

BUC: integrated upconverter+PA, 1-25 W, compact, VSAT. HPA: separate high-power amp, 50 W-kW, large earth stations.

Related Terms

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