AM Modulator and Superheterodyne AM Receiver

Overview

This project implements an Amplitude Modulation (AM) transmitter and a corresponding Superheterodyne AM receiver for the medium-wave (MW) band (535 kHz – 1605 kHz). The entire design and testing were performed in Proteus 8, using discrete electronic components (resistors, capacitors, inductors, diodes, MOSFETs) and frequency down-conversion to 455 kHz IF, and signal recovery via envelope detection.

Objective

• Design a MOSFET‑based AM transmitter that modulates a 120 kHz message onto a 1.2 MHz carrier.
• Build a Superheterodyne receiver that down‑converts the incoming 1.2 MHz AM signal to a 455 kHz IF, then recovers the original audio via an envelope detector.
• Verify and visualize each stage—message, carrier, modulated RF, IF, and demodulated output—using Proteus’s oscilloscope.

Key Components & Circuit Design

AM Transmitter

• Carrier Frequency: 1.2 MHz (displayed at 12 MHz in Proteus for clearer oscilloscope traces)
• Message Signal: 120 kHz sine wave from VSM Signal Generator
• Modulator Topology: Common‑source MOSFET (2N7000) with passive bias network
  • Gate bias network: 10 kΩ & 100 kΩ resistors
  • RF coupling capacitor: 0.01 µF (gate to carrier source)
  • Drain resistor: 4.7 kΩ
• RF Amplifier Stage:
  • LC tank tuned to 1.2 MHz (L = 10 µH, C = 100 pF)
  • Common‑source MOSFET amplifier for post‑modulation gain

Superheterodyne Receiver

1. RF Front End
   • Band‑pass LC filter centered at 1.2 MHz (L = 10 µH, C = 120 pF)
   • Low-noise MOSFET amplifier for initial RF gain
2. Mixer / Frequency Converter
   • Local Oscillator: 1.655 MHz (to mix 1.2 MHz → 455 kHz IF)
   • Mixer implemented with a diode ring (or single‑balanced MOSFET mixer)
   • Output filtered to isolate 455 kHz IF
3. IF Amplifier
   • Tuned LC amplifier at 455 kHz (L = 100 µH, C = 1000 pF) for selectivity and gain
4. Envelope Detector
   • Diode (1N4148) + RC network (R = 10 kΩ, C = 0.1 µF)
   • Recovers the 120 kHz message envelope → baseband audio
5. Complete Circuit
   

Simulation Environment

• Tool: Proteus 8 (Version 8.x or higher)
• Key Instruments: Virtual Signal Generator (VSM), Virtual Oscilloscope
• Carrier Display Workaround: Proteus oscilloscope struggled to show a clean 1.2 MHz trace, so all “carrier” screenshots were taken at 12 MHz, though component values are calculated for 1.2 MHz.

Results (Oscilloscope Captures)

Stage	Frequency Shown	Observations
Message Input	120 kHz	Clean sine wave from VSM generator
Carrier (displayed)	12 MHz	Stable sinusoid (actual tuned to 1.2 MHz)
AM Modulated Output	12 MHz	Envelope follows 120 kHz message
IF Signal After Mixer	455 kHz	Clean IF sine wave, ready for IF amplifier
Demodulated Audio	120 kHz (envelope)	Recovered baseband envelope after RC filter

• Modulation Depth: Approximately 80 – 90%
• IF Selectivity: Good rejection of unwanted mixer products
• Audio Recovery: Envelope detector output matches original 120 kHz message envelope

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