Which of the following techniques are used to generate frequency modulated signal : 1. Armstrong Which one of the following is true ?
2. Foster-Sealy Discriminator
3. Balanced Modulator
4. Reactance Modulator
1 and 4
Sort the four by what they actually do — only two of them generate FM.
| Technique | Function | Generates FM? |
|---|---|---|
| Armstrong method | Indirect FM: phase-modulate a crystal carrier, then multiply | ✓ |
| Foster-Seeley discriminator | Demodulates FM | ✗ |
| Balanced modulator | Produces DSB-SC for amplitude modulation | ✗ |
| Reactance modulator | Direct FM: varies an oscillator's tank reactance | ✓ |
So 1 and 4 — option 4.
The two generation methods represent the two families.
The reactance modulator is direct FM. A transistor or FET is connected across the oscillator tank so that it presents an effective reactance proportional to the modulating voltage, pulling the oscillation frequency directly:
\(f_{i}(t)=f_{c}+k_{f}\,m(t)\)
It gives large deviation easily, but since the oscillator must be free-running to be pullable, its centre frequency drifts — so an AFC loop is usually added.
The Armstrong method is indirect FM. The message is first integrated and then used to phase-modulate a highly stable crystal oscillator; integrating beforehand converts phase modulation into frequency modulation. Frequency stability is excellent, but the phase deviation obtainable linearly is tiny, so chains of multipliers and mixers are needed to build it up to broadcast deviation.
Foster-Seeley is the deliberate trap — it is an FM circuit, but at the receiving end. It uses a tuned transformer to convert frequency shifts into phase shifts and then into amplitude, and it must be preceded by a limiter because it responds to amplitude as well. Its close relative, the ratio detector, is inherently amplitude-insensitive and needs no limiter.
The balanced modulator belongs to AM entirely: by cancelling the carrier it produces double-sideband suppressed-carrier output, the first stage of an SSB transmitter.
Hence, the FM generation techniques are 1 and 4.
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 ?
In case of wideband FM, the modulation index value is :
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 ?
A carrier is frequency modulated with a sinusoidal signal of ____ KHz resulting in a maximum frequency deviation of 5 KHz. Modulation index and bandwidth of the modulating signal is respectively is given as
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 ?
Which of the following is true ?
Important Questions from Frequency Modulation
Given an angle-modulated signal
$x_c(t) = 10\cos[(10^8) \pi t+5\sin 2\pi (10^3)t]$
The maximum phase deviation and maximum frequency deviation is given byIn 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 ?
In case of wideband FM, the modulation index value is :
A high frequency signal is frequency modulated by n number of modulating signals. The ideal number of sidebands in the modulated signal will be :
