DSB and SSB Modulation Questions and Answers

Answer: A

Both begin with a balanced modulator to produce a suppressed-carrier double-sideband signal; they differ only in how the unwanted sideband is then removed. The sharp filter is peculiar to the first method and the quadrature network to the second. A frequency multiplier appears in neither, since multiplying a single sideband would distort it rather than translate it, which is why mixing is used instead. It is the same reasoning that decides whether a service keeps its carrier or discards it.

Answer: B

Multiplication acts on the whole signal, so it multiplies the sideband's frequency offsets as well as the carrier and destroys the message; mixing translates without scaling and is therefore the correct operation. Multipliers work to very high frequencies. They do not selectively suppress a sideband. And while a constant envelope is indeed needed for efficient multiplication, that is a consequence of the same underlying point rather than the primary reason.

Answer: C

The ninety-degree network must hold its accuracy across the whole message band, and any error leaves an unwanted sideband residue, which is why forty decibels of suppression is respectable for the method while a good crystal filter reaches sixty. Carrier suppression comes from the balanced modulators and is unaffected. It can work at the operating frequency directly, which is one of its advantages. And it does cancel one sideband, which is the whole point. Both economies are independent, which is why the three schemes form a ladder rather than a pair.

Answer: D

The colour subcarrier of analogue television is quadrature-modulated DSB-SC, with the carrier suppressed and a burst transmitted separately to give the receiver its phase reference. Medium wave broadcasting is full-carrier AM for receiver simplicity. Aeronautical voice is full-carrier AM for its non-capture property. And marine high-frequency voice is single sideband, chosen for power and bandwidth efficiency. The three schemes differ on exactly two axes, power and bandwidth, and every question on the topic reduces to one of them.

Answer: A

The burst is a short sample of the suppressed subcarrier sent during the line blanking interval, and it gives the receiver the phase reference its quadrature detectors need. Without it the hues would be wrong, since colour is encoded in the subcarrier's phase. Brightness is carried by the luminance signal. Sound has its own carrier and its own gain control. And field synchronisation is provided by separate pulses. Keeping the carrier is a decision about the receiver, and keeping the second sideband a decision about the channel.

Answer: B

With no carrier, the total is simply the two sidebands:

\[ P_t = 200 + 200 = 400 \ \mathrm{W} \]

so 400 watts, all of it carrying information, which is the whole point of suppressing the carrier. 200 W counts one sideband only. 600 W would include a 200 W carrier that this scheme does not transmit. And 100 W is half a sideband. Comparing this with full-carrier AM is instructive: to put 200 W into each sideband, an AM transmitter at full modulation would need an 800 W carrier and would radiate 1200 W in total.

Answer: C

At unity index the carrier is two thirds of the total and each sideband a sixth:

\[ \begin{aligned} P_c &= 1000 \ \mathrm{W} \\ P_{sb} &= \frac{1000}{4} = 250 \ \mathrm{W} \end{aligned} \]

so 250 watts in the single sideband SSB transmits, a saving of eighty-three per cent. 500 W is both sidebands. 750 W and 1000 W correspond to keeping more of the redundant power. The arithmetic is worth doing once carefully, because the ratio is the standard argument for the scheme.

Answer: D

By definition one, either the upper or the lower, the other having been removed by filtering or cancellation. Two is the double-sideband case. None would leave nothing to transmit, since the sidebands carry all the information. And four would arise from two modulating tones in a double-sideband scheme. The single sideband is what halves the bandwidth and, with the carrier gone as well, puts every watt into the message.

Answer: A

Only the carrier is removed, both sidebands being retained, which is why the bandwidth is unchanged from full-carrier AM while the power efficiency is transformed. Removing one sideband as well gives SSB. Removing a sideband alone is not a standard scheme. And the message's low-frequency content is preserved, though it is exactly that content which makes the SSB filter difficult and forces vestigial sideband on television.

Answer: B

It saves bandwidth, halving the occupied channel, and that is the second and independent economy the suppressed-carrier family offers. Linearity is a requirement rather than a saving, and SSB needs more of it. Frequency stability requirements become tighter, not looser. And receiver complexity increases at every step away from full-carrier AM. Keeping the two savings separate - power from the carrier, bandwidth from the sideband - makes the whole comparison straightforward.