Vai's Uni Notes

Low-Noise Amplifiers

4 min read

The LNA (low-noise amplifier) is the first active stage of the receiver; by the Friis noise equation it largely sets the receiver noise.

Reading an LNA Datasheet

  • The catalog gives NF and gain at specified frequencies and bias
  • Example, Mini-Circuits PGA-103+ (catalog-reading example, not the 8.4 GHz link LNA):
    • NF: 0.6 dB at 1 GHz
    • NF: 0.9 dB at 2 GHz
    • Gain: 11 dB at 2 GHz
  • Also check power, matching, linearity, and operating band

Where Should the LNA Go?

  • Source: 30 K. Cable: 290 K physical, 3.0 dB loss (exactly half power). LNA: NF 0.69 dB, 20 dB gain
    • A: cable → LNA
    • B: LNA → cable
  • Both give 20−3.0=17.0dB of signal gain. Will they give the same output noise?

Both Component Orders

  • Friis: 𝐹total=𝐹1+(𝐹2−1)/𝐺1; NFtotal=10log10𝐹total
  • Matched 290 K cable: 𝐹=2, 𝐺=0.5. LNA: 𝐹≈1.17, 𝐺=100
  • A (cable first):
    • 𝐹total=2+𝐹LNA−10.5⇒NFtotal≈3.7dB
  • B (LNA first):
    • 𝐹total=𝐹LNA+2−1100⇒NFtotal≈0.73dB
  • NF improvement ≈ 3.0 dB, using the 290 K reference source
  • Convert to added temperature and add the actual source:
    • 𝑇𝑒,total=290(𝐹total−1),𝑇sys=𝑇source+𝑇𝑒,total
  • Cable first adds 390 K; LNA first adds 52.9 K
  • NF compares both arrangements with a 290 K reference source:
    • 10log10(290+390290+52.9)≈3.0dB
  • Our antenna is only 30 K, so the actual [𝑆/𝑁0] improves by:
    • 10log10(30+39030+52.9)≈7.0dB

NOTE

What It Means for Receiver Layout

  • Antenna feed → (shortest practical low-loss connection) → LNA → long cable and later receiver
  • Low LNA noise figure sets a low receiver-noise floor
  • Enough early gain reduces later input-referred noise
  • Check overload, stability, bandwidth, and input filtering
  • Design guidance: JPL DESCANSO Vol. 10, Ch. 2; Analog Devices AN-2622, Selecting an RF Low Noise Amplifier

Why DSN Uses Cryogenic LNAs

  • A refrigerator cools the LNA: physical cooling can lower its NF and receiver-added noise
  • A lower LNA NF reduces the receiver contribution
  • But the remaining antenna and feed noise limit the improvement in total system sensitivity
  • Source: JPL DESCANSO Ch. 5, HEMT Low-Noise Amplifiers

Lunar Link: Cryogenic Cooling

  • Receiver NF: 0.69 → 0.15 dB (illustrative specs), with 𝑇𝑒=290(10NF/10−1)
  • 𝑇𝑒,rx: about 50 K → 10 K
  • 𝑇sys,𝐴=𝑇𝐴+(𝐿𝑓−1)𝑇phys+𝐿𝑓𝑇𝑒,rx
    • 168.0 K (22.3 dB-K) becomes 117.7 K (20.7 dB-K)
  • Δ[𝐺/𝑇]=Δ[𝑆/𝑁0]=10log10(168.0117.7)≈1.5dB
  • Only 1.5 dB: the 30 K antenna and 75.1 K feed emission are untouched by cooling the LNA

Lesson Takeaways

  • LNA placement: minimize loss before the first amplifier
  • LNA parameters: low NF and enough early gain reduce receiver noise
    • Gain must also satisfy overload and stability limits; antenna and feed noise remain
  • Receiving system: G/T combines antenna gain with all system noise
  • Link trades: more EIRP or G/T improves S/N₀; propagation, pointing, and polarization losses reduce it

Lesson review questions

  • Why should the LNA be close to the antenna?
  • Which LNA parameters reduce total receiver noise?
  • What does G/T tell us that receiver NF alone cannot?
  • Which S/N₀ terms can the spacecraft or ground provider change?
  • How would range, weather, or pointing affect the budget?

Graph View

Backlinks