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 ?
Consider an FM signal
\(s(t) = 10\sin\left(4\pi \times 10^6 t + 9\cos\left(2\pi \times 10^3 t\right)\right)\)
the frequency deviation and bandwidth of FM wave are
In case of wideband FM, the modulation index value is :
A fm kc/s modulating frequency provides Mf = 2 (significant Bessel functions i.e. n = 4) in F.M. wave. What bandwidth is required for passing this wave keeping ∆f constant, if frequency of modulating signal is doubled what will be its effect on Mf ?
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 ?
Important Questions from Frequency Modulation
Which of the following is NOT the advantage of frequency modulation ?
Which of the following statements is true for FM?
The modulation technique in which frequency of the carrier wave is changed with respect to the modulating wave is called:
A phase locked loop can be used to demodulate
What is the modulation index in a frequency modulated signal with a modulating frequency of 500 Hz and frequency deviation of 10 kHz?
