How do I allocate gain and noise figure budgets across the stages of a receiver chain?
Receiver Chain Budget Engineering
A receiver chain budget is a stage-by-stage spreadsheet that tracks gain, noise figure, P1dB, IP3, and signal levels through every component from the antenna to the ADC. This is the most important analysis document in receiver design.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
Margin Allocation
Each row represents a component (LNA, filter, mixer, IF amplifier, etc.). Columns track: (1) Component NF (dB) and gain (dB). (2) Cascaded NF through this stage. (3) Cascaded gain through this stage. (4) Component P1dB and IP3 (input-referred and output-referred). (5) Cascaded IP3 (limited by the weakest stage). (6) Signal levels: minimum (sensitivity), typical, and maximum (strongest expected signal). (7) Component output power at maximum input: must be below the component P1dB. (8) Spurious-free dynamic range (SFDR) and instantaneous dynamic range at each stage. Example receiver chain: Stage 1 (preselector filter): gain = -1.5 dB, NF = 1.5 dB. Stage 2 (LNA): gain = 22 dB, NF = 0.7 dB. Stage 3 (image reject filter): gain = -2.0 dB, NF = 2.0 dB. Stage 4 (mixer): gain = -7 dB, NF = 7 dB, IIP3 = +15 dBm. Stage 5 (IF filter): gain = -3 dB, NF = 3 dB. Stage 6 (IF amplifier): gain = 30 dB, NF = 3 dB. Stage 7 (ADC driver): gain = 10 dB, NF = 5 dB. Cascaded NF = 1.5 + (10^(0.7/10)-1)/10^(-1.5/10) ≈ 2.4 dB (preselector loss + LNA NF dominates). Cascaded gain = -1.5 + 22 - 2 - 7 - 3 + 30 + 10 = 48.5 dB.
Propagation Modeling
The system dynamic range is limited by: (1) Sensitivity floor: MDS = -174 + 10×log10(BW) + NF_sys. For 10 MHz BW, NF = 2.4 dB: MDS = -174 + 70 + 2.4 = -101.6 dBm. (2) Compression ceiling: the maximum input signal that does not compress any stage. With 48.5 dB gain and IF amplifier P1dB_out = +15 dBm: maximum input = 15 - 48.5 = -33.5 dBm (without AGC). Dynamic range = -33.5 - (-101.6) = 68.1 dB. (3) Spurious-free DR (SFDR): limited by intermodulation in the mixer and IF amplifier. SFDR = 2/3 × (IIP3 - MDS) for third-order products. If cascaded IIP3 = -5 dBm: SFDR = 2/3 × (-5 - (-101.6)) = 64.4 dB. To improve dynamic range: add AGC (variable attenuator before the mixer): 30-60 dB variable range extends the dynamic range to 100+ dB. Use a higher-linearity mixer (IIP3 > +20 dBm). Split the gain between pre-mixer and post-mixer stages.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Fade Mitigation
(1) LNA gain: 15-25 dB is optimal. Less than 15 dB: second stage noise contributes significantly. More than 25 dB: the mixer may be overdriven by strong signals. (2) Preselector insertion loss: keep below 1.5 dB (each dB directly increases system NF). Use low-loss filters (waveguide, ceramic, or high-Q lumped element). (3) Mixer IIP3: should be at least 10 dB above the maximum expected signal at the mixer input. Rule of thumb: IIP3_mixer > P_max_input + cascaded_gain_to_mixer + 10 dB. (4) IF amplifier: provide bulk gain (20-40 dB) with moderate NF (3-5 dB is fine since it is divided by the pre-IF gain). (5) AGC placement: before the mixer for maximum dynamic range, or between LNA and mixer if the LNA has sufficient linearity. (6) Filter loss budget: total filter loss before the LNA should be minimized (< 2 dB). Filters after the LNA protect downstream stages from overload but their loss does not significantly affect NF (divided by LNA gain in Friis equation).
Frequently Asked Questions
Should I optimize for noise figure or dynamic range?
It depends on the application: for satellite communications and radio astronomy (weak signals, no strong interferers): optimize NF. Use the lowest-NF LNA with high gain to suppress downstream noise. Dynamic range is secondary. For cellular base stations (many signals at varying levels): balance NF and DR. Use a moderate-NF LNA (1-2 dB) with moderate gain (15-20 dB) plus a high-linearity mixer. For electronic warfare and spectrum monitoring (unpredictable signal environment): prioritize dynamic range and SFDR. Use high-linearity front ends (IIP3 > +30 dBm) even at the cost of 3-5 dB higher NF.
How do I handle AGC in the budget spreadsheet?
Create two (or more) columns: one for minimum attenuation (maximum gain, used for sensitivity analysis) and one for maximum attenuation (minimum gain, used for maximum signal handling). At minimum attenuation: verify that NF meets the requirement and that the minimum signal reaches the ADC above its noise floor. At maximum attenuation: verify that the maximum expected signal does not compress any stage. The AGC range should be at least: AGC_range = P_max_input - P_min_input - SFDR_without_AGC. Typical AGC range: 30-60 dB for military receivers, 20-40 dB for commercial receivers.
What cascaded IP3 is needed for my application?
The required cascaded IIP3 depends on the signal environment: for a commercial cellular receiver (single-carrier, controlled environment): IIP3 ≈ -15 to -5 dBm is sufficient. For a multi-carrier base station receiver: IIP3 ≈ +5 to +15 dBm (many carriers create intermodulation products). For a military ESM receiver: IIP3 ≈ +15 to +30 dBm (must handle strong uncontrolled signals without generating false targets). Calculate from SFDR requirement: IIP3 = MDS + 3/2 × SFDR. For MDS = -100 dBm and SFDR = 70 dB: IIP3 = -100 + 105 = +5 dBm.