Checking How RF Gain Order Changes Cascaded Noise Performance

A receiver’s noise figure is not the sum of the noise figures printed on its component data sheets. Earlier gain changes the significance of noise added later, while loss before the first amplifier can be especially costly. Build the cascade in its actual order before comparing alternative lineups.

Define the reference conditions

The Analog Devices AN2168 RF system development note describes cascaded noise calculations alongside gain and linearity budgeting. Use compatible definitions for gain, impedance, bandwidth, and noise figure rather than mixing unrelated measurement conditions.

Draw the input reference plane and every stage, including connectors, filters, attenuators, and switches. Passive loss that is omitted from the diagram is also omitted from the calculation.

Calculate in linear units

For a simple matched two-stage cascade, the noise factor is the first stage factor plus the second stage factor minus one, divided by first stage power gain. Convert decibel quantities to linear values before applying that relationship.

For illustration, a first stage with noise factor two and power gain ten followed by noise factor four gives a total factor of 2.3, or about 3.62 dB. Increasing the first stage gain reduces the second stage’s contribution in this model, but it does not remove the first stage’s own noise.

Include passive loss and frequency conversion carefully

Evaluate a front-end filter or attenuator at its actual loss and temperature conditions. Moving it before or after gain can change the cascade result even if overall gain stays the same.

For mixers and image responses, use a model appropriate to the receiver architecture and the specified single or double sideband convention. Do not insert a mixer number into a simple two-port calculation without checking how image noise is handled.

Balance noise with strong signal behavior

Higher early gain can improve sensitivity while reducing headroom for blockers and large wanted signals. Review compression and intermodulation at each stage before accepting the lowest calculated noise figure.

Measure the assembled chain at defined reference planes and compare gain and noise over frequency. Include the intended gain states rather than qualifying only the maximum gain configuration.

Save stage order, linear calculations, reference conditions, passive losses, and measured results. A useful receiver budget shows where noise performance is established and what tradeoff accompanies a change. It does not claim that adding gain anywhere in the chain provides the same sensitivity improvement or that the lowest standalone amplifier noise figure guarantees the best complete receiver.