WorksheetsBalanced Modulator (DSBSC)
Total questions: 20
Worksheet time: 10mins
In a balanced modulator for DSB‑SC generation, what is the primary purpose of the tuned load circuit at the output?
To select the sum and difference sidebands only
To convert the signal from analog to digital form
To amplify the carrier frequency component strongly
To suppress the message and pass only the carrier
Two identical nonlinear devices in a balanced modulator are driven by vc±vm. If i1=a+bv1+cv1^2 and i2=a+bv2+cv2^2, which operation produces suppression of the carrier term at the output?
Taking the difference current io=i1−i2
Adding currents so that io=i1+i2
Differentiating current with respect to time
Filtering with a low‑pass RC network
Given vc=Vc sin ωc t and vm=Vm sin ωm t, the difference current contains terms k1 sin ωm t + k2 cos(ωc−ωm)t + k2 cos(ωc+ωm)t. After proper tuning, which expression represents the DSB‑SC output current?
k2 cos(ωc−ωm)t + k2 cos(ωc+ωm)t
k1 sin ωm t + k2 cos ωc t
k2 cos ωc t + k2 sin ωm t
k1 sin ωc t + k1 sin ωm t
In the ring modulator shown, what is applied to the center taps of both transformers to drive the diode switching?
A triangular ramp waveform
A constant DC bias voltage
A sinusoidal message signal
A square wave carrier signal
During a large positive value of the square-wave drive w(t), which diode pair conducts and how are nodes connected?
D1 and D4 conduct; A to D and B to C
D1 and D2 conduct; A to C and B to D
D3 and D4 conduct; A to C and B to D
All diodes conduct; A to B and C to D
Which statement best explains how the ring modulator generates a DSB-SC signal at the output of T2?
It multiplies the message by a ±1 square wave
It adds the carrier to the message directly
It filters the message with a low-pass only
It rectifies the message to remove negatives
Which statement best defines demodulation in amplitude modulation?
Filtering noise from a baseband audio signal
Amplifying a carrier to increase transmission distance
Combining two baseband signals into a carrier wave
Recovering the original message from a modulated wave
In a square law detector for AM, what is the primary role of the low-pass filter after the nonlinear device?
Shift the message spectrum to higher frequencies
Remove the high-frequency carrier components
Increase the envelope amplitude for transmission
Linearize the diode for ideal detection
For a square law detector using a diode with characteristic i(t)=av+bv2 , which term after low-pass filtering is primarily responsible for recovering the message m(t) when the AM input is v=(VDC+m(t))cos(ωct) ?
the squared term b m(t)^2 high-frequency component
the linear term a V_DC cos(ω_c t) component
the squared term b V_DC m(t) baseband component
the linear term a m(t) cos(ω_c t) component
the squared term b V_DC^2 cos(2ω_c t) component
When expanding v^2 = [(VDC+m(t))cos(ωct)]2 in a square law detector, which trigonometric identity allows separation into baseband and high-frequency terms?
cos2(x)=21+cos(2x)
tan2(x)=sec2(x)−1
cos(2x) = 2 sin(x) cos(x)
sin(2x)=2sin2(x)
sin2(x)=1−cos(2x)
In an AM envelope detector, what happens during the positive half-cycle of the input when the diode conducts?
The capacitor charges toward the input peak
The capacitor discharges through the load resistor
The diode remains reverse-biased and open
The RC network blocks all carrier components
Which statement best describes the role of the RC time constant in an envelope detector?
RC must be smaller than the carrier period only
RC must be larger than the message period
RC must equal the carrier period exactly
RC must be between carrier and message periods
If RC is chosen too large in a peak detector, which outcome is most likely?
Carrier components are fully removed always
Output fails to follow fast envelope drops
Output exhibits excessive ripple between peaks
Diode remains forward-biased continuously
To prevent diagonal clipping while minimizing ripple, which qualitative choice is appropriate for RC?
Slightly less than 1/fc but greater than 1/fm
Equal to the geometric mean of fc and fm
Independent of both carrier and message rates
Much greater than 1/fc yet much less than 1/fm
In coherent AM demodulation, what condition must the local oscillator satisfy relative to the incoming carrier for correct detection?
Same frequency but arbitrary phase offset
Random phase with automatic gain control
Slight frequency offset for envelope tracking
Same frequency and in phase alignment
Twice the carrier frequency with phase lock
A receiver lacks a strong carrier component in its AM signal but still needs a coherent LO. Which block is best suited to regenerate a phase-synchronized LO from the input?
Band-pass filter centered at carrier
Phase-locked loop locked to carrier
Low-pass filter for baseband extraction
Automatic gain control amplifier
Quadrature mixer with fixed clock
In a synchronous AM demodulator with zero path delay, the local oscillator (LO) is best modeled as which signal?
cos(ωc t)
sin(ωc t + π/2)
m(t) cos(ωc t)
cos(2ωc t)
Given an AM input of (V_DC + m(t)) cos(ωc t) multiplied by a synchronous LO cos(ωc t), what is Vx before filtering?
(V_DC/2) cos(ωc t) + (m(t)/2) sin(2ωc t)
(V_DC + m(t)) cos(2ωc t)
V_DC + m(t) + cos(2ωc t)
V_DC/2 + (V_DC/2) cos(2ωc t) + m(t)/2 + (m(t)/2) cos(2ωc t)
If the LPF cut-off is approximately fc and Vx contains baseband and 2ωc terms, which output is expected at Vout?
m(t) only with zero DC component
V_DC/2 + m(t)/2
V_DC + m(t)
(V_DC/2) cos(2ωc t) + m(t)
If there is a path delay τ so the LO becomes cos(ωc(t−τ)), which practical effect is most critical for coherent demodulation?
It doubles the baseband bandwidth
It removes the DC component entirely
It introduces a phase offset relative to carrier
It slightly boosts carrier amplitude
