Transmitter
Understanding RF Transmitters
The transmitter converts baseband information into a modulated RF signal at the required power level for radiation through the antenna. Transmitter design must balance output power, efficiency, spectral purity, and modulation quality.
Transmitter Architecture
- Direct modulation: Modulate the carrier directly. Simple but limited to simple modulations.
- IF-based: Modulate at IF, then upconvert to RF. Better spectral purity and flexibility.
- Direct digital: DAC generates the modulated signal at IF or RF. Maximum flexibility.
Key Specifications
- Output power: Typically specified as average power (dBm or watts).
- Frequency accuracy: Often within 0.1 ppm for communications.
- EVM: Modulation quality metric. Must meet standard requirements.
- ACLR: Adjacent Channel Leakage Ratio. Spectral regrowth into adjacent channels.
- Harmonics: Must be below regulatory limits.
Frequently Asked Questions
What does a transmitter do?
A transmitter generates a modulated RF signal at the required power and frequency for transmission through an antenna. It includes a signal source, modulator, amplifier chain, filtering, and antenna interface.
What determines transmitter power?
Required transmitter power is determined by the link budget: enough EIRP (transmit power + antenna gain) to close the link with adequate margin at the intended range. Typical values range from milliwatts (Bluetooth) to megawatts (broadcast TV).
What is ACLR?
ACLR (Adjacent Channel Leakage Ratio) measures how much transmit signal power leaks into adjacent frequency channels. It is caused by amplifier nonlinearity spreading the signal spectrum (spectral regrowth). ACLR requirements are typically -30 to -45 dBc.