College

Design (only the block diagram) an Armstrong indirect FM modulator to generate an FM carrier with a carrier frequency of 95 MHz and \(\Delta f=20\) kHz.

- A narrowband FM generator is available at a carrier frequency of 200 kHz and a frequency deviation \(\Delta f=9\) Hz.
- An oscillator with an adjustable frequency in the range of 9 to 10 MHz is also available.
- There is a bandpass filter with any center frequency, and only frequency doublers are available.

Answer :

An Armstrong indirect FM modulator can be designed using a narrowband FM generator, an adjustable frequency oscillator, frequency doublers, and a bandpass filter.

The modulator generates an FM carrier with a frequency of 95 MHz and a frequency deviation of 20 kHz.

The block diagram of the Armstrong indirect FM modulator consists of several components. The narrowband FM generator with a carrier frequency of 200 kHz and a frequency deviation of 9 Hz is used as the message signal source. An adjustable frequency oscillator in the range of 9 to 10 MHz provides the carrier signal.

Frequency doublers are employed to double the frequency of the carrier signal successively. This ensures that the carrier frequency reaches 95 MHz. The modulating signal from the narrowband FM generator is combined with the carrier signal using a mixer or multiplier. A bandpass filter is utilized to eliminate any unwanted frequencies and ensure that the output consists of the desired FM carrier frequency with a frequency deviation of 20 kHz.

By carefully selecting and connecting these components, the Armstrong indirect FM modulator generates the desired FM carrier signal with a frequency of 95 MHz and a frequency deviation of 20 kHz. The modulator design takes advantage of the available narrowband FM generator, adjustable frequency oscillator, frequency doublers, and bandpass filter to achieve the desired modulation characteristics.

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