There are several subtopics or sub-sections under each main topic in the GATE ECE exam syllabus. These subtopics provide a detailed breakdown of the main topics and help candidates in their preparation.

Here are the main topics and their corresponding subtopics in the GATE ECE exam syllabus:

1. Networks and Signals:
- Network graphs
- Network theorems
- Two-port networks
- Fourier series
- Fourier transform
- Laplace transform
- Network functions

2. Electronic Devices and Analog Circuits:
- Semiconductor physics
- Diodes and its applications
- Bipolar junction transistors (BJTs)
- Field-effect transistors (FETs)
- Amplifiers
- Oscillators
- Operational amplifiers (Op-Amps)
- Filters

3. Digital Circuits:
- Number systems
- Combinatorial circuits
- Sequential circuits
- Boolean algebra
- Minimization techniques
- Arithmetic circuits
- Memory and programmable logic

4. Control Systems:
- Mathematical modeling of systems
- Transfer functions
- Block diagrams and signal flow graphs
- Time-domain analysis
- Stability analysis
- Frequency-domain analysis
- Controllers
- Compensators

5. Communications:
- Analog communications
- Random variables and processes
- Digital communications
- Information theory and coding
- Modulation and demodulation techniques
- Multiple access techniques
- Spread spectrum techniques
- Error control coding

6. Electromagnetics:
- Electrostatics and magnetostatics
- Maxwell's equations
- Plane waves and wave propagation
- Transmission lines
- Waveguides
- Antennas
- Microwave engineering
- Optical fiber communication

7. Signals and Systems:
- Continuous-time signals and systems
- Discrete-time signals and systems
- Fourier series and Fourier transform
- Laplace transform
- Z-transform
- Linear time-invariant systems
- Filter design
- Sampling and reconstruction of signals

These subtopics cover the breadth and depth of the GATE ECE exam syllabus, ensuring that candidates have a thorough understanding of each main topic. It is essential to study and prepare for all these subtopics to excel in the GATE ECE exam.