📡 RF Circuit Analysis Suite — Qucs-S

RF and mixed-signal circuit designs simulated in Qucs-S with Ngspice backend. Covers low-noise amplifiers, bandpass filters, mixers, and impedance matching networks with S-parameter characterization and noise figure analysis.

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📊 Simulation Results

433 MHz Low-Noise Amplifier

LNA Circuit Schematic:

S-Parameter Response (S11, S21, S22, S12):

Smith Chart (Input & Output Match):

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📋 Circuit Index

Design	Frequency	Key Metrics	Application
Low-Noise Amplifier	433 MHz	NF=1.2dB, Gain=15dB, IIP3=-5dBm	ISM band receiver front-end
Chebyshev BPF	2.4 GHz	BW=80MHz, IL=1.8dB, rejection=40dB	Wi-Fi pre-select filter
Gilbert Cell Mixer	915 MHz	CG=8dB, NF=12dB, IIP3=+2dBm	LoRa downconverter
L-Network Match	50Ω ↔ (12-j18)Ω		S11
Pi-Network Attenuator	DC–6 GHz	10dB ±0.3dB, VSWR < 1.15	Test and measurement

🏗️ Repository Structure

├── simulations/
│   ├── lna/
│   │   ├── ism-433-lna.sch                # Qucs-S schematic
│   │   ├── ism-433-lna.spice              # Exported SPICE netlist
│   │   └── bfp740-model.lib               # Transistor SPICE model
│   ├── bandpass-filter/
│   │   ├── wifi-2g4-bpf.sch               # 2.4GHz Chebyshev BPF
│   │   ├── filter-synthesis.py            # Automated coefficient calculator
│   │   └── substrate-parameters.txt       # FR4 dielectric properties
│   ├── mixer/
│   │   ├── gilbert-cell-915.sch           # Gilbert cell downconverter
│   │   └── mixer-spurs.py                 # Spurious response calculator
│   └── impedance-matching/
│       ├── l-network.sch                  # L-match design
│       ├── smith-chart-overlay.py         # Smith chart visualization
│       └── matching-sweep.sch             # Frequency-swept match quality
├── docs/
│   ├── rf-design-methodology.md
│   └── s-parameter-primer.md
├── results/
│   └── (generated S-parameter plots, Smith charts)
└── README.md

🔬 Simulation Methodology

S-Parameter Analysis

All RF circuits characterized using 2-port S-parameter simulation:

• S11 (Input Return Loss): Target < -10dB across operating band
• S21 (Forward Gain/Loss): Primary performance metric
• S12 (Reverse Isolation): Critical for amplifier stability
• S22 (Output Return Loss): Load matching quality

Stability Analysis

Amplifier designs include Rollett stability factor (K) and auxiliary stability factor (Δ):

K = (1 - |S11|² - |S22|² + |Δ|²) / (2|S12||S21|)
Δ = S11·S22 - S12·S21
Unconditionally stable when: K > 1 AND |Δ| < 1

Noise Analysis

LNA designs include full noise characterization:

• Minimum noise figure (NF_min)
• Optimum source impedance (Γ_opt)
• Noise resistance (R_n)
• Noise circles plotted on Smith chart

📊 Key Results

433 MHz LNA Performance

Parameter	Simulated	Target	Unit
Gain (S21)	15.3	>14	dB
Noise Figure	1.18	<1.5	dB
Input RL (S11)	-18.5	<-10	dB
Output RL (S22)	-14.2	<-10	dB
Reverse Isolation (S12)	-28	<-20	dB
IIP3	-4.8	>-8	dBm
DC Current	12.3	<15	mA
K (stability)	3.7	>1	—

2.4 GHz Bandpass Filter

Parameter	Simulated	Target	Unit
Center Frequency	2.442	2.440	GHz
3dB Bandwidth	82	80	MHz
Insertion Loss	1.76	<2.0	dB
Return Loss	-16.8	<-12	dB
Stopband (2.0 GHz)	-38	<-35	dB
Stopband (2.8 GHz)	-41	<-35	dB

🛠️ Qucs-S Configuration

These simulations use Qucs-S with the Ngspice backend:

1. Install Qucs-S ≥ 24.3.0 and Ngspice ≥ 43
2. Set backend: Edit → Application Settings → Simulation → Ngspice
3. Load .sch files from simulations/ directory
4. Exported SPICE netlists provided for direct Ngspice use

📄 License

MIT — schematics, scripts, and documentation freely available.