Why this topic is important for ECET?
Receivers are the heart of any communication system. Without receivers, transmitted information cannot be understood.
In ECET exams, questions come on superheterodyne receiver, sensitivity, selectivity, image frequency, noise figure, etc.
Mastering this topic ensures quick scoring because formulas are direct + conceptual.
📘 Concept Notes
1. What is a Receiver?
A receiver is a device that accepts a modulated signal from the channel (air/wire), extracts the information, and delivers it to the user.
2. Types of Receivers
- TRF (Tuned Radio Frequency Receiver):
- Old design.
- Selects the frequency by tuning RF amplifiers directly.
- Problems: Poor selectivity, instability, and no constant bandwidth.
- Superheterodyne Receiver:
- Standard receiver used today.
- Converts incoming RF signal to a fixed intermediate frequency (IF) using a local oscillator and mixer.
- Advantages: High selectivity, sensitivity, stable gain, easy filtering.
3. Superheterodyne Receiver – Block Diagram
- Antenna – Captures signal.
- RF amplifier – Boosts weak signals.
- Mixer + Local Oscillator (LO) – Converts RF to IF.
- IF = |f_RF – f_LO|.
- IF amplifier – High gain, filters noise.
- Detector (Demodulator) – Extracts audio (baseband).
- AF amplifier + Speaker – Produces output sound.
4. Receiver Characteristics
- Sensitivity: Ability to detect weak signals.
- Selectivity: Ability to reject adjacent channel interference.
- Fidelity: Faithfulness of output signal to original input.
- Image Frequency Rejection Ratio (IFRR): Ability to reject unwanted image frequencies.
- Noise Figure (NF): Measures degradation of SNR due to receiver.
5. Example
Suppose a receiver works at RF = 1000 kHz, IF = 455 kHz.
- Then Local Oscillator = 1455 kHz.
- Image frequency = f_RF + 2 × IF = 1910 kHz.
So, any signal near 1910 kHz may interfere unless proper filtering is used.
⚙️ Formulas
- Intermediate Frequency:
Image Frequency:
Selectivity:
Sensitivity (minimum input power required):
Noise Figure:
Gain of cascaded stages (Friis formula for noise):
🔟 10 MCQs
Q1. Which of the following is NOT an advantage of superheterodyne receivers?
a) Better selectivity
b) Constant bandwidth
c) Simple design
d) High sensitivity
Q2. If RF = 1000 kHz and IF = 455 kHz, what is the LO frequency?
a) 545 kHz
b) 1455 kHz
c) 455 kHz
d) 1000 kHz
Q3. For the same RF and IF (1000 kHz, 455 kHz), find the image frequency.
a) 1455 kHz
b) 1910 kHz
c) 910 kHz
d) 455 kHz
Q4. Selectivity of a receiver is improved by:
a) Wideband filters
b) Narrowband filters at IF
c) Increasing bandwidth
d) Reducing gain
Q5. Sensitivity is defined as:
a) Ability to tune different stations
b) Ability to reject noise
c) Ability to amplify weak signals
d) Ability to reduce distortion
Q6. If SNR_in = 30 dB and SNR_out = 20 dB, find NF.
a) 1.5
b) 2
c) 10
d) 0.5
Q7. The TRF receiver suffers mainly due to:
a) High fidelity
b) Poor selectivity
c) High gain
d) Easy tuning
Q8. In a superheterodyne receiver, the IF is usually kept constant because:
a) It simplifies design and improves selectivity
b) It increases noise
c) It reduces fidelity
d) It complicates tuning
Q9. If IF = 455 kHz, RF = 1200 kHz, find image frequency.
a) 1655 kHz
b) 2110 kHz
c) 910 kHz
d) 745 kHz
Q10. Noise figure of an ideal receiver is:
a) Zero
b) Infinity
c) One
d) Two
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | c |
| Q2 | b |
| Q3 | b |
| Q4 | b |
| Q5 | c |
| Q6 | b |
| Q7 | b |
| Q8 | a |
| Q9 | b |
| Q10 | c |
🧠 Explanations
- Q1: Superheterodyne has many advantages, but design is complex, not simple → (c).
- Q2: LO = RF + IF = 1000 + 455 = 1455 kHz → (b).
- Q3: Image = RF + 2IF = 1000 + 910 = 1910 kHz → (b).
- Q4: Selectivity is achieved using narrowband IF filters → (b).
- Q5: Sensitivity is the ability to detect weak signals → (c).
- Q6: NF = SNR_in / SNR_out (linear).
- Convert: 30 dB → 1000, 20 dB → 100.
- NF = 1000/100 = 10 → (b).
- Q7: TRF problem is poor selectivity → (b).
- Q8: Constant IF simplifies tuning, improves filtering → (a).
- Q9: Image = RF + 2IF = 1200 + 910 = 2110 kHz → (b).
- Q10: Ideal noise figure = 1 (no degradation) → (c).
🎯 Motivation / Why Practice Matters
In ECET, receivers give direct scoring questions like IF frequency, image frequency, noise figure.
- Most formulas are one-step calculations, perfect for speed.
- Practicing ensures you don’t confuse LO frequency vs Image frequency.
- Strong grip here saves time → extra edge for tougher problems in Communication & Electronics.
📲 CTA
Join our WHATSAPP group for ECET 2026 updates and discussions →
👉 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9
