Explain with reference to $AM RX^r$,

1] Fidelity.

2] Selectivity

3] Sensitivity.

4] Image frequency and its rejection.

5] Double spotting.

1] Fidelity : Fidelity is the ability of the $RX^r$ to reproduce all modulating frequencies equally. The frequency response of AF amplifier is shown above that dictates the Fidelity of the $RX^r$. High Fidelity is obtained when the frequency response is flat in the desired range of frequencies.

2] Selectivity : Selectivity of a receiver is its ability to reject all unwanted s/gs and have high gain for the desired frequencies. It is usually plotted as attenuation vs frequency. For the desired frequency, the attenuation should be as less as possible.

Selectivity decides the adjacent channel rejection of a $RX^r$

3] Sensitivity : Sensitivity of a receiver is its ability to amplify weak s/g, it is measured in $\mu$ v or dB.

It also determines how much the o/p will differ by minute changes in any of the parameter of the receiver.

4] Image frequency and its rejection : AM receiver broadcast system, the local OSC, frequency is higher than incoming slg (fs)

Frequency by Intermediate Frequency (If)

fo = fs + If

If = fo - fs

If two frequency components F1 and F2 are mixed in a mixer, then mixer o/p consists of sum and differences frequencies like (F1 > F2)

o/p of mixer: F1, F2, ( F1 - F2), (F1 + F2)

Out of these, the difference component is selected using tuned ckt after the mixer.

Now assume that local oscillator frequency is set to 'fo' and unwanted lg frequency f = fsi + If

then the mixer o/p contains 4 components

fsi, fo, (fsi + fo) , (fs - fo)

But,

sfi = (fo + If)

$\therefore$ fo, (fo + If) (2Fo + If) & If

This is unwanted slg at is known as 'Image frequency'

5] Double spotting : Double spotting is said to have happened when same station gets picked up at two different nearby points at the receiver's end.

It is due to poor front end and selectivity of the $RX^r$

Double spotting may mask the weak original channel s/g by the strong spot s/g.

to reduce the Double spotting, the selectivity of the $RX^r$ has to be improved. It can also give if frequencies.