1. FIGURE 1 shows the block diagram of a superhetrodyne radio receiver. In a test a 20 ?V signal was fed from the aerial into the first stage of the receiver, a radio frequency amplifier. This signal is then passed through several stages of the receiver to eventually appear at the input to the AM detector. For the AM detector to work satisfactory it requires a minimum signal level of -3dB(mW). Calculate the required minimum gain of the fifth intermediate amplifier given the data in TABLE 1.
|
Stage
|
Gain
|
|
RFamplifier
|
+10dB
|
|
Mixer
|
-7 dB
|
|
BPF
|
-1dB
|
|
IF1
|
+15dB
|
|
IF2
|
+15dB
|
|
IF3
|
+20dB
|
|
IF4
|
+10dB
|
TABLE 1
2. As a rule of thumb, the perceived noise from a source falls by 6 dBA with every doubling of the distance from thesource.
figure 2
If a 1 MW wind turbine (FIGURE 2) produces 103 dBA of noise within one metre of its base, estimate the distance from the turbine at which the noise can be reckoned to have fallen below the background noise level of 40dBA.
3. A simplified model of ADC noise refers the noise to a noisy input source resistanceRnwhileassumingtherestofthesignalpathtobenoiseless.
FIGURE3representsaparticular18-bitADCthathasa10V input voltage range. The ADC has a bandwidth of 1MHz.
Calculate the maximum value of Rn if the resolution of the ADC is not to be adversely affected by thermal noise. Assume the ADC operates at 25°C.
[N.b. The voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete values possible on itsoutput.]
4. The specification for a 4-way TV antenna amplifier [FIGURE 4] is given in TABLE 2,below.
|
Bandwidth
|
40-862MHz
|
|
Gain
|
20 dB
|
|
NoiseFigure
|
6 dBB
|
|
Max.Output
|
85dBmV
|
|
InputImpedance
|
75W
|
|
OutputImpedance
|
75W
|
Table 2
(a) Determine the voltage required on the input of the amplifier to give the maximum output of 85dBμV.
(b) If the signal level from the aerial is 5 dBmV and the input noise level is 20 dBμV, calculate the signal-to-noise ratio on the output of the amplifier.