Upconverter
Understanding Upconverters
Upconversion is the essential step in any transmitter that generates the signal at a convenient IF for modulation and processing, then translates it to the final RF frequency for transmission. The upconverter determines the output frequency accuracy, spectral purity, and sideband rejection of the transmitted signal.
Upconverter Components
- IF input: Modulated signal at IF, typically 70 MHz, 140 MHz, or L-band.
- Mixer: Multiplies IF with LO to produce RF output.
- LO source: Synthesizer providing frequency reference.
- Output filter: Bandpass filter selecting the desired sideband and rejecting LO leakage, image, and spurious products.
- IF driver / RF output amplifier: Provides gain as needed.
f_RF = f_LO + f_IF (upper sideband)
f_RF = f_LO - f_IF (lower sideband)
Unwanted outputs:
f_LO (LO leakage): must be filtered
f_LO +/- f_IF (unwanted sideband): must be filtered
f_LO +/- n x f_IF (harmonics): must be filtered
Frequently Asked Questions
What is an upconverter?
An upconverter translates a lower-frequency signal (IF) to a higher RF frequency using a mixer and LO. It is the transmit complement of a downconverter. The output includes the desired sideband plus unwanted products that must be filtered.
What is sideband selection?
The mixer produces both upper (LO+IF) and lower (LO-IF) sidebands. Only one is desired. A bandpass filter on the output selects the desired sideband and rejects the other plus the LO leakage. The filter rejection determines sideband suppression.
What is a BUC?
A BUC (Block Up Converter) is an outdoor unit that upconverts the L-band IF signal from the modem to the satellite transmit frequency (Ku-band or Ka-band) and amplifies it for transmission. It is mounted at the antenna feed to minimize cable loss.