What parameters are tracked in an RF budget analysis?

A complete RF budget analysis tracks at least six parameters at every stage: signal power level (dBm), cumulative gain/loss (dB), noise figure (dB) or noise temperature, cascaded noise figure (Friis formula), input-referred IP3 (dBm), and 1 dB compression point (dBm). From these, the system-level metrics are derived: sensitivity (minimum detectable signal), spurious-free dynamic range (SFDR), blocking dynamic range, and signal-to-noise ratio at the ADC input or demodulator. Some analyses also track image rejection, LO leakage, and DC offset.

How does the Friis noise figure cascade formula work?

The Friis formula calculates the total noise factor of cascaded stages: F_total = F1 + (F2-1)/G1 + (F3-1)/(G1×G2) + ... where Fn is the noise factor (linear, not dB) and Gn is the gain (linear) of each stage. This shows that the first stage dominates overall noise figure because subsequent stage contributions are divided by the preceding gain. For example, an LNA with NF=1 dB and gain=20 dB followed by a mixer with NF=8 dB gives F_total = 1.26 + (6.31-1)/100 = 1.31, or 1.18 dB, only 0.18 dB worse than the LNA alone.

What is the difference between a link budget and an RF budget analysis?

A link budget calculates the end-to-end power balance from transmitter to receiver including path loss, antenna gains, and atmospheric effects to determine whether the received signal exceeds the receiver sensitivity with adequate margin. An RF budget analysis is a more detailed stage-by-stage calculation within the transmitter or receiver hardware, tracking signal level, noise, and linearity through each component (filter, amplifier, mixer, attenuator). The link budget uses the receiver sensitivity as a single number; the RF budget analysis is how that sensitivity number is derived from the cascade of component specifications.

What is RF Budget Analysis? | Link Budget Design

Definition & Purpose

RF budget analysis is the systematic, stage-by-stage calculation of signal power, noise figure, and linearity through every component of an RF signal chain, from antenna to ADC (receiver) or DAC to antenna (transmitter). The analysis produces a complete picture of the system's dynamic range: the minimum signal it can detect (sensitivity) and the maximum signal it can handle without distortion (compression or IP3 limit). It is the single most important design document in any RF system, performed before selecting components and iterated throughout the design cycle.

The analysis is built as a spreadsheet or software model with one column per component (filter, LNA, mixer, IF amplifier, attenuator, ADC) and one row per tracked parameter. At each stage, the cumulative gain updates the signal level, the Friis formula updates the cascaded noise figure, and the reverse cascade (from output to input) calculates the referred linearity metrics. The goal is to ensure that at every point in the chain, the signal is above the noise floor (with margin) and below the compression point (with margin), across all operating conditions including temperature, aging, and component tolerances.

Key Formulas

Friis Cascade Noise Factor:

F_total = F₁ + (F₂−1)/G₁ + (F₃−1)/(G₁G₂) + ...

Cascade IIP3 (reverse cascade):

1/IIP3_total = 1/IIP3₁ + G₁/IIP3₂ + G₁G₂/IIP3₃ + ...

(all values in linear power, not dBm)

Sensitivity:

MDS = −174 + 10log₁₀(BW) + NF + SNR_min [dBm]

Spurious-Free Dynamic Range:

SFDR = 2/3 × (IIP3 − MDS) [dB]

Example Receiver Budget (X-band, 9.4 GHz)

Stage	Gain (dB)	NF (dB)	IIP3 (dBm)	Cumul. Gain	Cumul. NF	Signal (dBm)
Antenna	0	0	—	0	0	−80
Preselector Filter	−1.5	1.5	+60	−1.5	1.5	−81.5
LNA	+25	1.2	+10	+23.5	2.7	−56.5
Image Filter	−2.0	2.0	+60	+21.5	2.7	−58.5
Mixer	−7.0	7.0	+15	+14.5	2.8	−65.5
IF Amplifier	+30	3.0	+25	+44.5	2.8	−35.5
IF Filter	−3.0	3.0	+60	+41.5	2.8	−38.5
ADC Driver	+10	5.0	+30	+51.5	2.8	−28.5

System NF = 2.8 dB | Sensitivity (10 MHz BW, 10 dB SNR) = −174 + 70 + 2.8 + 10 = −91.2 dBm

Practical Application

An X-band weather radar receiver requires −90 dBm sensitivity in a 10 MHz bandwidth to detect precipitation at 200 km range. The RF budget analysis reveals that a system noise figure of 2.8 dB (driven primarily by the 1.2 dB LNA after a 1.5 dB preselector filter) yields an MDS of −91.2 dBm, meeting the requirement with 1.2 dB margin. The cascaded IIP3 of +5 dBm (dominated by the mixer at +15 dBm after 23.5 dB of preceding gain) provides an SFDR of 64 dB. If the SFDR target is 70 dB, the designer can improve the mixer IIP3 to +22 dBm or add 3 dB of attenuation before the mixer (accepting 0.3 dB degradation in noise figure).

Frequently Asked Questions

What parameters are tracked?

At minimum: signal level (dBm), cumulative gain (dB), noise figure (cascaded via Friis), IIP3, and P1dB at every stage. Derived: sensitivity (MDS), SFDR, blocking dynamic range, and SNR at the ADC input.

How does the Friis cascade work?

F_total = F₁ + (F₂−1)/G₁ + ... The first stage dominates because subsequent contributions are divided by preceding gain. An LNA with 1 dB NF and 20 dB gain reduces a following 8 dB mixer NF contribution to just 0.18 dB system impact.

Link budget vs RF budget analysis?

Link budget: end-to-end power balance (Tx to Rx) including path loss and antenna gains. RF budget analysis: detailed stage-by-stage cascade within the hardware tracking noise and linearity. The link budget uses sensitivity as one number; the RF budget derives it.

Budget Analysis (RF Design)