The FM transmitters have the following blocks as per the following correct sequence : (A) Crystal Oscillator Choose the most appropriate answer from the options given below :
(B) Antenna
(C) Frequency multiplier
(D) Phase modulate / Audio source
(E) Power amplifier
(A), (D), (C), (E), (B)
Follow the signal from where it originates to where it leaves, and the order is forced: crystal oscillator, phase modulator, frequency multiplier, power amplifier, antenna — option 4.
Two blocks fix the ends immediately. A signal has to start somewhere, and that is the oscillator; it has to leave somewhere, and that is the antenna. Any sequence beginning with the antenna, as option 2 does, or ending with the modulator, as option 3 does, can be rejected without further thought.
| Stage | What it does |
|---|---|
| (A) Crystal oscillator | Generates a very stable low-frequency carrier |
| (D) Phase modulator | Impresses the audio, giving a small deviation |
| (C) Frequency multiplier | Raises carrier and deviation to the wanted values |
| (E) Power amplifier | Raises the level to the transmitted power |
| (B) Antenna | Radiates |
Why this is the indirect or Armstrong method, and why the order must be this way. A crystal oscillator cannot itself be frequency modulated to any useful extent without destroying the stability that is its whole reason for being there. So the carrier is generated at a low frequency and phase modulated instead, which produces only a very small deviation — a few tens of hertz.
The frequency multiplier then solves that problem, and it must come after the modulator because it multiplies the deviation by the same factor as the carrier:
\(f_{c}\rightarrow nf_{c},\qquad \Delta f\rightarrow n\Delta f\)
A chain of doublers and triplers giving \(n=\) several hundred turns a 25 Hz deviation into the 75 kHz that broadcast FM requires, while carrying the crystal's stability up with it. Placing the multiplier before the modulator, as options 1 and 3 do in effect, would multiply nothing useful.
The power amplifier comes last before the antenna, and in FM it can be a highly efficient non-linear class C stage, because the signal has a constant envelope and no amplitude information to preserve — one of FM's practical advantages over AM.
Hence, the sequence is (A), (D), (C), (E), (B).
Similar Questions
In FM
Consider the following :
ST1 : F.M. signal produces more side bands than A.M.
ST2 : The carrier in a F.M. signal can never be dropped to zero amplitude.Which of the following is valid ?
Which of the following is true ?
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\(s(t) = 10\sin\left(4\pi \times 10^6 t + 9\cos\left(2\pi \times 10^3 t\right)\right)\)
the frequency deviation and bandwidth of FM wave are
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A high frequency signal is frequency modulated by n number of modulating signals. The ideal number of sidebands in the modulated signal will be :
The approximate rule for transmission of an FM signal generated by a single-tone modulating signal of frequency fm, modulation index β and maximum frequency deviation Δf, is defined as :
Assertion (A) : The FM radio broadcast of analog signals provides higher fidelity.
Reason (R) : FM uses significantly larger channel bandwidth for signal transmission.
Select your answer using the codes given below :
What bandwidth is needed for an FM signal that has a peak deviation of ± 3 KHz and handles audio signals from 200 Hz to 5 KHz ?
Which of the following techniques are used to generate frequency modulated signal :
1. Armstrong
2. Foster-Sealy Discriminator
3. Balanced Modulator
4. Reactance ModulatorWhich one of the following is true ?
Important Questions from Frequency Modulation
Which of the following statements is true for FM?
What is the modulation index in a frequency modulated signal with a modulating frequency of 500 Hz and frequency deviation of 10 kHz?
Which of the following rules states that the bandwidth required to transmit an angle modulated wave is twice the sum of the peak frequency deviation and highest modulating signal frequency?
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In FM, "M" stands for
