What transmitter architectures are used?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Transmitters convert baseband data to RF at the required power and spectral purity. Main architectures are direct up-conversion (quadrature modulator at the carrier), two-step heterodyne (modulate at an IF, then up-convert), polar (separate amplitude and phase paths, enabling switching PAs), outphasing, and direct RF DAC synthesis. Each must meet output power, EVM, ACLR and spurious emission limits.^[1]^[2]

What issues does direct up-conversion have?

LO pulling by the PA output when the VCO runs at or near the carrier; mitigated by running the VCO at 2× or 4/3× and dividing.
LO leakage and I/Q imbalance (sideband) requiring calibration.
Noise in the receive band in FDD systems.

^[1]

What are polar and envelope-tracking transmitters?

Polar transmitters modulate phase on a constant-envelope signal driving a saturated PA, and impose amplitude through the supply; bandwidth expansion of amplitude and phase signals limits them for wide channels. Envelope tracking keeps a linear PA but varies its supply with the envelope to improve efficiency.^[2]

What spectral purity must transmitters meet?

Spurious emission limits such as those in ITU-R SM.329 and national rules, harmonic limits, ACLR and spectral emission masks from the relevant standard, and EVM. Harmonic filtering after the PA is usually required.^[3]^[4]

Frequently asked questions

Why run the VCO at twice the carrier?

To prevent the PA output from pulling the oscillator.

What is an RF DAC?

A data converter fast enough to synthesize RF directly or in a higher Nyquist zone.

Do transmitters need filters?

Yes, for harmonics and out-of-band noise.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
“RF Power Amplifiers for Wireless Communications, 2nd ed.”. S. C. Cripps, Artech House. 2006.
“Recommendation ITU-R SM.329: Unwanted emissions in the spurious domain”. ITU-R. current version.
“47 CFR Part 15: Radio Frequency Devices”. U.S. Federal Communications Commission, via eCFR. current.

Categories: Receivers and systems

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