Which of following four alternatives is not correct? We need modulation:

Options

A. to reduce the time lag between transmission andreception of the information signal
B. to reduce the size of antenna
C. to reduce the e fractional band width, that is the ratio of the signal band width to the centre frequency
D. to increase the selectivity.

Answer

To solve the question regarding which alternative is not correct about the need for modulation, we can follow these steps:

### Step 1: Understand Modulation
Modulation is the process of varying one or more properties of a periodic waveform (the carrier signal) in accordance with a modulating signal that contains the information to be transmitted. This is essential for effective communication.

### Step 2: Analyze the Options
We need to evaluate each of the given alternatives to determine which one is incorrect regarding the purpose of modulation.

### Step 3: Evaluate Option 1
- **Option 1:** "To reduce the time lag between transmission and reception of the information signal."
- This statement is **not correct**. Modulation does not influence the time lag between transmission and reception. The time lag is more related to the propagation delay and the processing time in the system rather than the modulation process itself.

### Step 4: Evaluate Option 2
- **Option 2:** "To reduce the size of antenna."
- This statement is **correct**. Modulation allows the transmission of signals at higher frequencies, which enables the use of smaller antennas.

### Step 5: Evaluate Option 3
- **Option 3:** "To reduce the fractional bandwidth that is the ratio of the signal bandwidth to the center frequency."
- This statement is **correct**. Modulation can help in reducing the fractional bandwidth, which enhances selectivity in communication systems.

### Step 6: Evaluate Option 4
- **Option 4:** "To increase selectivity."
- This statement is **correct**. Modulation allows for better selectivity between different signals, enabling the receiver to distinguish between them more effectively.

### Conclusion
After evaluating all the options, we find that:
- Option 1 is the only statement that is not correct about the need for modulation. Therefore, the answer to the question is **Option 1**.

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